Final YOU GOT THIS DEIRDRE Flashcards
Characteristics of a vitamin
Exogenous supply is required
* Needed in small amounts
* Distinct from sugars, fats and proteins in regard
to structure and function
* Perform at least one essential biochemical
function in the body
* When lacking in the diet, a characteristic
deficiency disease develops
* Vitamins are organic
– Primary distinction from minerals (which are inorganic)
What are the functional groups of micronutrients?
Group I: control type II steroid hormones (Iodine, Vit A, Vit D, Calcium, Vit K, Phosphorus and Fluoride)
Group II: role in oxidant defence
(Vit E, Selenium, Vit C, Niacin, Riboflavin, Copper, Zinc, Manganese)
Group III: enzyme cofactors (Thiamin, Niacin, Riboflavin, Vit B6, Folate, Vit B12, Biotin, Pantothenic
acid)
Group IV: Iron, copper, and zinc-related divalent cations
Group I
These micronutrients control cellular function through type II
steroid receptors
* Not all act directly on steroid hormone receptors (only the
bioactive forms of Vit A, Vit D, and iodine do)
– But we can’t talk about Vit D without talking about calcium,
phosphorus, and Vit K since they are all involved in bone metabolism
Iodine used to make T3 hormone, which regulates synthesis
of proteins that control a person’s basal metabolic rate
- Vit A precursors are converted to retinoids, which regulate
night vision, epithelial differentiation, and gene expression - Vit D precursors are converted to calcitriol, which regulates
calcium levels in the body
Steroid hormone receptors
intracellular protein receptors that
need to bind a ligand to become a
functional (active) transcription
factor
Two types
1 - cytosolic
2 - nuclear
Two types of steroid hormone receptors
Type 1 Receptors are Cytosolic
– Respond to steroid hormones
like estrogen, testosterone,
progesterone, glucocorticoids,
and mineralcorticoids
– We won’t discuss these
- Type 2 Receptors are Nuclear
– Respond to steroid and non-steroid ligands, like thyroid hormone, retinoic acid, and
calcitriol
Iodine in diet
Iodine content of most foods is low
* Iodine is an inorganic mineral that is highly water soluble
– It’s found in higher concentrations in coastal populations compared to
mountainous regions
– Seafood has high concentrations (especially sea greens)
* In North America, most of the iodine consumed comes
through “salt fortified with potassium iodine”
– Iodized salt contains 0.03mg iodine per g of salt
As of 2007…29% of the world population live in areas of iodine deficiency.
How and where is iodine absorbed?
Dietary iodine can be bound to amino acids or found free
* In our gastrointestinal tract, iodine (I) is rapidly converted to iodide (I- , its ionic form) and absorbed
– Most is absorbed in the stomach, and a bit in the small intestine
In blood, free I- circulates and can enter all tissues, but most accumulates in the thyroid gland (uptaken by Na+/I- symporter)
T3 and T4 transport and functions
All tissues depend on
thyroid hormones (T3 and T4)
rather than iodine itself
* Once T3 and T4 are made,
they are released into blood
and transported by specific
carrier proteins (albumin,
transthyretin, etc.)
* 50× more T4 in blood
compared to T3, BUT T3 is 100× more potent
- T3 half-life < T4
- T3 interacts with the thyroid hormone receptor (THR)
- T3 and T4 production
regulated by TSH (thyroid stimulating hormone)
When T3 blood
levels are low, the
hypothalamus
signals to the
pituitary to release
TSH
T4 vs. T3
50× more T4 in blood
compared to T3, BUT T3 is100× more potent
* T3 half-life < T4
How are thyroid hormones made?
In the colloid, iodide is quickly oxidized to form a free radical (I●)
* The thyroglobulin (THG) protein is produced in the thyroid cell and then released into the colloid (lumen)
– THG is a tyrosine-rich (Tyr) protein
- The iodide radical “attacks” the Tyr residues in THG in the colloid, which causes a cross-linking between tyrosine residues
- Thyroid cell proteases hydrolyze THG, releasing fragments that correspond to T3 and T4 hormones like thyroxine and/THG-3’, 5’, 3, 5-tetraiodothyronine
Thyroid hormone functions
Thyroid hormones influence how your body stores and
uses energy (i.e., affects metabolism)
* Thyroid hormones control such things as:
– Breathing
– Heart function
– Nervous system function
– Body temperature
– Cholesterol level
– Energy balance
– Brain development
– Moisture in the skin
– Menstruation
(PRETTY MUCH EVERYTHING)
Examples of genes activated by T3
ATPases (pump Na+ and Ca2+ out of cells), which increases metabolic rate
* Na+ (muscle contraction, neuron
firing)
* Ca2+ (signaling events)
– Growth hormone (anabolic effects)
Processes regulated by T3
Adipose Tissue –> lipolysis
Muscle –> contraction
Bone –>promotes making bone
Heart –> increases heart rate
GIT –> stimulates nutrient digestion
Vitamin A - what is it?
Vitamin A is a general term used to refer to a group of compounds known as retinoids
- lipophobic - handled like other lipids
– Major forms of Vitamin A in the body are retinol, retinal, retinoic acid, & retinyl esters
The alcohol form (retinol) was first identified and then carotenes were recognized
as the plant form of vitamin A
plant provitamin
a plant provitamin is the form of a vitamin produced by plants that, when consumed by animals, is converted into the bioactive vitamin
ex. carotenes are precursors for Vit A
Carotenes
(provitamin A) are precursors for Vitamin A
– Carotenes are a type of carotenoid, which are pigments produced in plants (e.g. β-carotene, α-carotene, etc.)
Retinyl esters
Retinyl esters (rich in milk, eggs)
– Retinyl ester = Retinol + fatty acid
– Retinyl esterase cleaves the fatty acid
from retinol (necessary for absorption)
retinyl esterase
Pancreatic
Retinyl esterase cleaves the fatty acid from retinol (necessary for absorption)
β-carotene and fates
plant form of Vit A
it is directly absorbed into enterocyte
Depending on the Vit A status of a person, β-carotene has 2 fates within the intestinal enterocyte:
- Converted into a retinyl ester (retinyl palmitate)
- Incorporated “as is” into chylomicrons
how is beta carotene converted to retinyl ester?
b-carotene gets converted to retinal w 15,15 DO enzyme
retinal converted to retinol
retinol gets converted to retinyl palmitate (retinyl ester)
retinyl palmitate is incorporated into chylomicron
Where does most vit A end up?
Most ends up at the liver in a chylomicron remnant
– β-carotene can be packaged into VLDL
and sent for storage in adipose tissue.
– Retinyl esters (e.g., retinyl palmitate) are
stored in hepatic stellate cells
hypercarotenosis
b-carotene from liver to VLDL and to peripheral tissues (mainly adipose)
In the liver, what happens to retinyl esters?
retinyl palmitate and esters are converted to retinol in liver by retinyl esterase enzyme
Retinyl esters are stored in hepatic stellate cells until needed
* When needed, liver retinyl esterase removes the FA, releasing retinol
which binds to retinol binding protein (RBP) and is secreted into blood
* Liver RBP synthesis depends on a person’s Vit A status
then they are sent out into blood bound to RBP
- low levels of RBP stimulate hepatic retinyl esterase!!
RBP
Retinol-RBP levels are key!
- they are what tell us about Vit A status
– Low levels stimulate hepatic retinyl esterase which converts retinyl ester to retinol
15, 15’ carotenoid dioxygenase
In the intestine, β-carotene can be
broken down into all trans retinal
(aldehyde)
– Enzyme responsible for this is
15,15’-carotenoid dioxygenase
all trans retinal can be converted to all trans retinol and then converted to retinyl ester to be sored
all trans retinal can also be converted to retinoic acid directly
Retinol production
Retinol (alcohol) production
– All trans retinal can be converted into all trans retinol by retinol dehydrogenase
– The retinol can act like a detergent, which is unsafe for a cell. So, retinol is converted into a retinyl ester (safer for the cell)
– Retinyl esters have no direct function in the body
retinol dehydrogenase
converts all trans retinal to all trans retinol
Why is retinol converted to retinyl ester?
The retinol can act like a
detergent, which is unsafe for a
cell. So, retinol is converted into a
retinyl ester (safer for the cell)
– Retinyl esters have no direct
function in the body.
The retinoid cycle
Vit A and Night vision!
(aka the retinoid vidual cycle)
see slide 24 of 32 and draw out process
11-cis retinal can cross to photoreceptor (rod cell) bount to interphotoreceptro RBP where it can be added to RHODOPSIN (very sensitive pigment) conversion of RHODOPSIN to OPSIN results in neuronal signalling and vision
opsin can be combined w all trans retinal to make all trans retinol or can be regenerated to rhodopsin in cycle
Rod cells
Rod cells use rhodopsin
to absorb light (greyish
purple colour)
* Opsin + 11-cis retinal
combine to become
rhodopsin, which is light-
sensitive
* When light hits
rhodopsin, it reforms all-
trans retinol
Retinoic Acid (RA) Signaling
Retinol-RBP brings retinol to cells, which can then be converted into RA
* RA goes to nucleus where it binds and activates both the retinoic acid receptor (RAR) and the retinoid X
receptor (RXR) transcription factors
* These complexes homo- and hetero-dimerize with other nuclear hormone receptors (NHR), creating a
huge number of possible combination of transcription factors, which allows the regulation of gene
expression
ex. is for increasing growth hormone and decreasing collagenase
Vit A deficiency and epithelial differentiation
Affects stem cell differentiateion
When vit A present - normal cell differentiation
- mucus secretion and keratinization
When vit A deficient
Keratinization, but little mucous secretion
not enough mucus to form a good protective layer, so viruses and bacteria can penetrate
poor differentiated function
Keratin
Keratin is a
major protein
in all epithelial
cells whose
expression is
regulated by
retinoic acid.
What are the 5 consequences of Vit A deficiency
- Night blindness (lack of rhodopsin)
– Reversible, one of the first signs of Vit A deficiency
– Associated with Bitot’s spots, a buildup of keratin debris in the
conjunctiva of the eye - Impaired epithelial cell differentiation
– Can cause permanent blindness and life-threatening infections - Impaired growth (growth hormone not produced)
– Impacts bone development, tooth decay, etc. - Impaired fertility
– Decreased sperm formation, fetal resorption (early death of the embryo) - Fetal development defects
– Birth defects due to loss of control of differentiation
– Can occur with too little OR too much Vit A
Vit A dietary requirements
Retinol Activity Equivalents (RAE)
1 RAE = 1 μg dietary retinol = 12 μg dietary β-carotene = 24 μg other carotenes
RAE accounts for differences
in the biological activity of various carotenoids
No UL for B-carotene
Why no UL for B-carotene?
Carotenoids prevent
deficiency, but don’t cause toxicity.
– 15,15’-carotenoid
dioxygenase regulated by Vit A status. If you have enough Vit A, carotenoids aren’t converted to retinol
but are stored “as is”
Vit A dietary toxicity
Most severe consequence is liver cell death.
Retinyl esters are stored in the stellate cells of the liver and with
excess Vit A intake, the cells reach capacity.
Raw Vit A spills out
and the local hepatocytes become damaged and die.
Death from liver failure can result
Why you can’t eat a polar bear liver
Excessive intake of β-carotene is not toxic, but can cause
hypercarotenosis (skin turns yellow-orange)
Acutane
Accutane, an acne drug that contains 13-cis retinoic acid, was
shown to cause birth defects in women in the early months of
pregnancy
Taken off the general market due to lawsuits
proteinogenic AAs
In humans there are 21 proteinogenic AAs (includes selenocysteine)
- A “proteinogenic AA” refers to an AA that is incorporated into a protein
during translation
- All but selenocysteine are part of the standard genetic code
non-proteinogenic AAs
Non-proteinogenic AAs also exist (e.g., some neurotransmitters like GABA),
but these are not used to make protein
Two types of aas in body
Two types of amino acids in the body:
1. Standard Amino Acids:
– All are used to make protein
– 20 AAs are encoded in the genetic code
(except for selenocysteine)
2. Non-Standard Amino Acids:
– Many exist in the body, but they are rarely used to make
proteins
– Usually formed by post-translational modification of other AAs or as intermediates in the metabolic pathways of standard AAs
* For example, the GABA neurotransmitter is a metabolite of the amino acid glutamate
– We don’t talk about these in NUTR*3210 in any detai
Which configuration of aa is naturally occuring
L configuration of AAs is naturally occurring
*D configuration of AAs is made through
post-translational modifications
Zwitterions
At physiological pH, AAs are ionized
*Protonated amine group
*Deprotonated carboxyl group
*No overall charge (except R group)
*This increases polarity
Oligopeptide vs polypeptide vs protein
approx 50 aas in peptide chain = oligopeptide
over 50 = polypeptide
1 or more polypeptides = a biologically active protein
Denaturation of protein
A native protein corresponds to
a protein in its normal 3D
conformation.
* Proteins can be denatured in a
number of ways:
– e.g., heat, salt treatment,
detergents, pH (stomach acid).
* When a protein is denatured, it
loses its bioactivity.
* Denaturation affects 2°, 3°, and
4° structures (but not 1°)
ex. albumin is the protein in egg and when cooked it becomes opaque cause it’s been denatured
Essential amino acides
Essential AA (Indispensable)
– Not made by the body or can’t be made quickly enough to meet demands
* 9 AAs (lys, thr, iso, leu, met, phe, trp, val, and his)
IKTVLMFHW
Conditionally essential amino acids
Not normally required in the diet in a healthy individual, but become essential
under specific contexts. For example,
– A genetic problem:
– Phenylketonuria: an inborn error of metabolism whereby a person is unable to breakdown Phe into Tyr.
* A build-up of Phe in the body causes intellectual disability
* The solution is to limit Phe in the diet and supplement with Tyr
Development of a disease:
– Liver disease (cirrhosis) impairs Phe and Met catabolism
* Tyr and Cys are synthesized from Phe and Met, respectively
* Tyr and Cys become indispensable in this context
Basic AAs
HKR
K
Essential
*Limiting in grain products
*Involved in the production
of carnitine, which is
important for fatty acid
metabolism
R
Preterm infants unable to
synthesize arginine
*Non-essential in healthy
adults
H
Essential
*Ring structure
*Used to produce
histamine (inflammation)
Acidic AAs
ACIDIC = DE (aspartate and glutamate
D
Non-essential
*Important for amino
acid catabolism
*Transaminated to
oxaloacetate (Krebs)
*A “source” of
nitrogen in the urea
cycle
E
Non-essential
*Important for amino
acid catabolism
*Transaminated to α-
ketoglutarate (Krebs)
*Used to produce
GABA
(neurotransmitter
can react with a basic group to produce asparagine and glutamine
Neutral AAs
N, Q, G, A
Asparagine,
(non-essential)
glutamine
(non-essential - Important in AA
catabolism because it
is an inter-organ
carrier of nitrogen (to
the liver & kidney))
Glycine
only non enantiomer
Non-essential
*No enantiomers
*Used primarily to produce
porphorin (a component of
heme, which is found in
hemoglobin)
alanine
Non-essential
*Important in AA
catabolism because it is
an inter-organ carrier of
nitrogen (to liver &
kidney)
*Important role in the
glucose-alanine cycle
Branched chain AA
LIV
All are essential
*Not catabolized in the liver, so high levels found in circulation
*Promote protein synthesis
*BCAA levels are high in protein supplements
Hydroxylated AA
ST
Ser is non-essential and Thr is essential
-OH group on side chain is important for post-translational
phosphorylation of proteins
Sulfur containing AA
Cysteine and Methionin
Cysteine
Non-essential
*Made from methionine
*“Spares” methionine when
cysteine consumed in the diet
*Used to form disulfide bonds
*Used in glutathione synthesis
(oxidant defence system)
Methionine
Essential
*1st step in the synthesis of all
proteins
*Methionine is limiting in legumes
Aromatic AAs
WYF P
Phenylalanine
*Essential
*Used to make
Tyrosine
Tyrosine
Non-essential
*“Spares” Phe
*Used to synthesize
neurotransmitters
Tryptophan
Essential
*Used to make
serotonin (mood)
*Used for niacin (Vit
B3) synthesis
Proline
Non-essential
*Important for
collagen production
(extracellular matrix)
*Aliphatic side chain
Post-translational modifications
Most proteins require some type of modification before
they are biologically functional
* PTMs take place in polypeptide chains, not free AA
* Phosphorylation (the addition of a phosphate group) by
kinase enzymes
- serine, threonine and tyrosine
Hydroxylation (creation of a new hydroxyl group)
- lysine and proline
Gamma-carboxylation
Iodination
ADP - ribosylation
Phosphorylation
Type of PTM
Phosphorylation (the addition of a phosphate group) by
kinase enzymes
– Serine-OH
– Threonine-OH
– Tyrosine-OH
requires phosphorus
Hydroxylation
Hydroxylation (creation of a new hydroxyl group)
– Lysine hydroxylysine (very important in elastin subunits,
needs copper; associated with aortic rupture) - copper dependent
– Proline hydroxyproline (very important in collagen subunits,
needs Vit C; associated with scurvy) - Vit C dependent
Gamma carboxylation
a tupe of PTM
Required for calcium homeostasis and blood clotting
– Certain proteins are modified to become Ca2+ binding proteins
– Another carboxyl group is added to glutamate
Vit K dependent
ex. glutamate-COO- to Glutamate w 2 COO-
Iodination
a type of PTM
Critical in the formation of thyroid hormones
– Crucial for regulation of the metabolic rate
– About 2 billion humans are iodine deficient
- iodine dependent
ADP - ribosylation
Type of PTM
Adding ADP-ribose to an acceptor protein
– Critical for DNA repair and regulation of protein function
– Dependent on Vit B3 (niacin)
– Niacin used to form NAD+. When NAD+ is broken down in the cell, ADP- ribose and nicotinamide are the products
niacin dependent
Vitamin D
The bioactive form of Vit D acts as a hormone (this is a “true”
steroid hormone)
– The bioactive molecule is made in one tissue (kidney) and acts on
other tissues
– Works with other hormones such as parathyroid and calcitonin
Vit D history (dogs)
While Vit A and iodine are deficiencies of the developing world,
Vit D is the most prevalent micronutrient deficiency in the
developed world
1919–deficiencies induced in dogs and reversed with cod liver oil
– Rediscovered the use of cod liver oil to prevent bone diseases
– Sunlight also used to prevent rickets, which indicated that fat soluble Vit D can also be synthesized by the body
Vitamin D sources
- Natural plant sources (provitamin D2)
Shitake mushrooms
* Not very active in plants, so not a great source of Vit D
*Ergocaliferol is less bioactive than cholecalciferol - Natural Animal Sources (Provitamin D3)
- fish, fish liver oils
- 7-dehydrocholesterol (7-D) is converted to cholecalciferol by sunlight (UVB and infrared)
- occurs in sebaceous glands of skin
However, it’s difficule to achieve good vid D status by consuming animals or plants
- Sunlight
- Vid D3 is made in the skin from 7-D
- Vit D3 binds to the vit D binding protein (DBP) and circulates around the body
Lumisterol and tachysterol
are made in the skin from 7-D, but are inactive molecules that are eventually lost as we shed skin cells
they can be converted back to 7-D
Melanin
Melanin in epidermis absorbs UV rays, which slows down Vit D3 production
- part of the problem for vid D deficiencies over time (migrating populations)
more melanin -> less ability to make vit D
Other vit D sources (not food or skin)
- Supplementation
Should use Vit D3
(cholecalciferol)
* Especially important for someone who spends lots
of time indoors - Fortification
Government initiative
* Milk and margarine
* Insufficient for health if this is a person’s only source of Vit D3
Vit D from skin
Vit D3!!
7-D is rapidly converted to Vit D3 in skin
– No risk of Vit D
toxicity from sun
– Vit D3 enters blood
bound to DBP (Vit D
bind protein) and can
go to the:
* Liver (conversion)
* Adipose (storage)
Vit D3 absorption from the diet
- Primarily absorbed
passively in the small
intestine
– Incorporated into
chylomicrons and
eventually ends up in
the liver
Vit D3 conversion in the liver
In the liver, the 25th carbon on Vit D3 is hydroxylated to form 25-OH D3 by the
enzyme 25-hydroxylase (a cytochrome p450 enzyme)
- Once produced, 25-OH D3 is secreted into blood bound to DBP
– This corresponds to the largest pool of 25-OH D3 in the body - If 25-OH D3 levels in the blood are low, this is the sign of Vit D deficiency (THIS
IS KEY!) - When calcium levels are low in the body, 25-OH D3 will be converted into an active molecule
How do we detect a vit D deficiency?
Once produced, 25-OH D3 is secreted into blood bound to DBP
– This corresponds to the largest pool of 25-OH D3 in the body
* If 25-OH D3 levels in the blood are low, this is the sign of Vit D deficiency (THIS
IS KEY!
Active Vit D3
High gradient of blood to intracellular Ca2+ is
essential
- Low blood Ca2+ is sensed by the parathyroid gland
– Releases PTH
(parathyroid hormone)
* PTH promotes uptake of 25-OH D3 / DBP complex into the kidney
- 1-hydroxylase converts
inactive 25-OH D3 into
active 1,25-(OH)2 D3 - Active form known as calcitriol
Calcitriol
= 1,25-(OH)2-D3
Produced in the kidneys
active form of vit D that is bound to DBP in blood and sent out into the body to activate intracellular signalling pathways
GENOMIC calcitriol signaling
Affects the GENOME of a cell - so must enter nucleus
Calcitriol enters cell and binds Vit D receptor (VDR) is in cell cytosol and then
– VDR is a type-2 steroid
hormone receptor
– Transcription factor that promotes calcium binding protein synthesis
– Calcium-binding proteins are activated by a Vitamin K
dependent PTM (post translational modification)
* γ-carboxylation
- CREATES Calcium binding proteins!!
NON-GENOMIC calcitriol signaling
does NOT affect Genome
Calcitriol binds cell surface receptors,
like MARRS (membrane-
associated rapid response steroid-binding protein)
– Activates intracellular
signalling cascades
– Very fast, no dependence on
Vit K
MARRS
membrane associated rapid response steroid-binding protein
on cell surface
- responds to calcitriol
- activates intracellular cascade that leads to protein phosphorylation and signalling events by opening membrane channels that allow Ca2+ to rush in
Responses to low blood calcium by calcitriol signalling
GENOMIC AND NON-GENOMIC RESPONSES
- VDR is activated and turns on the expression of genes coding for
calcium binding proteins (which require Vitamin K dependent γ-
carboxylation to become fully functional)
* Membrane receptors are activated (e.g., MARRS)
IN BONE
Elevated calcitriol and PTH work together to stimulate resorption of Ca2+ and phosphorus from bone
* Calcitriol causes an increase in the expression of RANK ligand (RANKL),
a cytokine
* Osteoblasts secrete RANKL that activates osteoclasts to degrade the bone matrix and release Ca2+ and P into the blood
IN INTESTINE
- Primary function is to increase absorption and reabsorption of Ca2+
* Calcitriol turns on the expression of genes that encode calcium binding proteins
RANKL
RANK ligand
Calcitriol causes an increase in the expression of RANK ligand (RANKL),
a cytokine
- Osteoblasts secrete RANKL that activates osteoclasts to degrade the bone matrix and release Ca2+ and P into the blood
Organs relevant to Vit D/Ca2+ story
Absorption (small intestine)
– Minerals need transporters for absorption by intestinal
enterocytes and entry into portal circulation
– Proper absorption of Ca2+ depends on the expression of Ca2+
binding proteins in enterocytes
Reabsorption (kidney)
– Small molecules like Ca2+ circulate in blood and eventually reach
the kidney, where they pass through the filter and can end up in
urine (unless reabsorbed)
– Reabsorption removes the molecules from the filtrate and gets
them back into the blood
Resorption (bone)
– Dissolving bone structure to release Ca2+ into the blood
* Osteoclasts resorb bone, osteoblasts build bone
* Balance between break-down and synthesis allows for bone maintenance, remodeling and repair
Hormones involved in Ca2+ control system
KEY POINT maintaining blood Ca2+ is more important than maintaining Ca2+ reserves
in bone (as seen by a release of bone calcium with pregnancy and lactation when
calcium is limited in the diet)
Hormones:
PTH (parathyroid hormone)
Calcitriol (active vit D)
Calcitonin
PTH
parathyroid hormone
Secreted by the parathyroid glands
* Serves to INCREASE blood calcium
– Promotes the production of calcitriol in kidney by activating the 1-hydroxylase enyzme
– Stimulates bone resorption by activating osteoclasts
– Maximizes tubular reabsorption of calcium in kidney
How does calcitriol affect blood Ca2+
VITAMIN D (Calcitriol)
* Serves to INCREASE blood calcium
– Stimulates Ca2+ resorption from bone
– Helps to increase absorption of Ca2+ from intestine
– Maximizes tubular reabsorption of Ca2+ in kidney
Calcitonin
Secreted by parafollicular cells in the thyroid
* Serves to DECREASE blood Ca2+ (e.g., in response to Ca2+ rebound)
– Suppresses tubular reabsorption of Ca2+ in kidney
– Inhibits bone resorption and facilitates remineralization
Vitamin D deficiency
Consequence of Vit D deficiency varies across the lifespan
Vit D deficiency in Infants = Rickets (poor mineralization)
– Bones don’t mineralize properly and can’t support the infant’s body weight
when they start walking (permanent and reversible only with surgery)
– This was seen in Britain during the industrial revolution
Vit D deficiency in Adolescents to Adult = Osteomalacia
– Bones become demineralized (can be reversed with supplementation)
– Bone fractures can occur more easily
*
Vit D deficiency in Middle-Aged to Elderly = Osteoporosis
– Normal part of aging (loss of both mineral and organic parts of bone)
– Diagnosed with bone density scans
– Difficult to reverse due to erosion of bone (holes in bone form)
Composition of normal bone
Mixture of solid (outer) and spongy (inner) parts
– Solid part: How much is mineral and how much is organic?
* Solid part is 60% mineral (Ca2+, P) and 40% organic (collagen)
* In babies, bones start as collagen and gradually become infused with minerals
* A Vit D deficiency concerns solid bone only, which changes the ratio of mineral to
collagen in the bone matrix
Osteoporosis and treatment
Osteoporosis Bone loss associated with aging
Worsened by chronic low Ca2+, Vit D, and/or Vit K intake
Fractures (e.g., hip) increase mortality in the elderly
Men generally have higher peak bone mass between 20-
30 yrs and lower bone loss
Treatments include dietary supplements or drugs that
affect bone formation/resorption
Changes for RDA for vit D
Health Canada tripled the RDA based on overwhelming scientific evidence
– Used input from research, stakeholders, and scientists
Tested in 5300 Canadians
(age 6-79 yrs)
-Very low levels of Vit D,
with some ethnicities
having lower levels than
others
Vit D surveys and human intervention studies suggest that ↑ Vit D3 improves bone mass;
decreases colon, prostate and breast cancers; diminishes MS, psoriasis, rheumatoid
arthritis; decreases hypertension & CVD; decreases diabetes; improves muscle strength
and motor nerve function in the elderly
Vit D Toxicity
- no risk from sun exposure since prod. of Vit D3 is limited by amounts of 7-D present in the skin Also produces the inactive lumisterol and tachysterol metabolites with prolonged sun exposure, which have no bioactivity
RARE but…
Can get much higher levels of circulating 25-OH D3 with high dietary
intake
– Vit D3 is absorbed and incorporated into chylomicrons and eventually
ends up in the liver where it is hydroxylated at position 25 and returned
to circulation as 25-OH D3 bound to DBP.
– Very high dietary levels can cause hypercalcemia, leading to a possible
calcification of soft tissues
* People with Vit D intake > 10,000 IU (or more) / day for several
months experience toxicity (hypercalcemia) and acute kidney injury.
Vitamin K
- a type I vitamin
Vitamin K named after the Danish word “koagulation”, which means coagulation (so Vit K important for bone formation & blood coagulation)
Obtained through consumption of green leafy plants (as phylloquinone)
– However, phylloquinone is sensitive to light and heat
* Vit K is also made by gut bacteria, in the form of menaquinone
menaquinone
Vit K is also made by gut bacteria, in the form of menaquinone
- unsaturated side chain
passive uptake into colonocytes
deficiency is very rare due to this production
Why do newbors have poor vit K status?
Newborn infants have poor Vit K status. Why?
– Little Vit K in mother’s milk and babies aren’t eating leafy plants yet
– Babies also haven’t developed their colonic (gut) bacteria, so no menaquinone
– Babies given Vit K via a heel prick at birth (lasts 3-6 months
What happened to baby chickens when fed a low cholesterol diet?
Baby chickens fed a low-fat and cholesterol-free diet bled excessively and their blood took a long time to clot. When Vit K was added to their diet, the problems disappeared.
– Dam and Doisy won the Nobel prize in Medicine in 1943 for this discovery
Phylloquinone
Vit K in PLANTS
- saturated side chain
- requires no digestion
Vit K digestion and absorption
hylloquinone requires no digestion
– Incorporated into micelles and absorbed in the small intestine
– Absorbed via NPC1L1 apical transporter (also involved in cholesterol absorption)
* Menaquinones produced by bacteria in the large intestine
– Passive uptake into colonocytes
* Both are incorporated into lipoproteins (e.g., chylomicrons) and delivered to
various tissues around the body
Vitamin K cycle
Dietary intake (or from bacteria)
Vit K (quinone) is the starting molecule - inactive form
- Vit K quinone is reduced by quinone reductase and NAD(P)H - electron donor to Vit K hydroquinone (active f0rm)
- Vit K hydroquinone (active) / reduced form now passes its electrons onto something such as in gamma-glutamyl carboxylase reaction (glutamic acd to gamma-carbocyglutamic acd) via this electron donation adn addition of CO2
So hydroquinone acts as a reducing agent, and gets oxidized to vitamin K epoxide (inactive) / oxidized form of Vit K - Vit K epoxide (inactive/oxidized) gets converted back to vit K quinone (starting point from step 1) via epoxide reductase
the cycle then continues
Warfin
- blocks Vit K cycle
- used in rat poison
- prevents coagulation
- causes excessive bleeding
- blocks epoxide reductase enzyme
Vit K and γ-Carboxylation
a protein precursor with a glutamate side-chain undergoes a post translational modification reaction
VIt K hydroquinone (active vit K) donates its electrons and CO2 is also added to the protein with help of enzyme γ-glutamyl carboxylase
Now, the protein has another COOH group! (2 total), and a gamma-glutamyl carboxylase side chain
At phys. pH these groups are COO- and form Gla residues! /calcium binding protein
Gla residues
Produced via gamma-carboxylation and help of Vit K hydroquinone
Calcium binding residues on proteins w 2 COO- groups that bind Ca2+
Gla residues on blood clotting proteins bind Ca2+.
Ca2+ allows Gla-containing proteins to bind to phospholipids on membranes of blood platelets and endothelial cells
Susceptible populaitons for vit K deficiency
ewborn infants (injected with phylloquinone at birth)
* Little Vit K in breast milk
* Vit K can’t cross the placenta for delivery to the developing fetus
* Gut bacteria population not established
– People who take antibiotics chronically
* Antibiotics destroy the gut bacterial community
– People with malabsorptive illnesses (IBD, Crohn’s, pancreatitis)
Vit K toxicity
Virtually no instances of toxicity in adults
Vit K Deficiency symptoms
Deficiency symptoms related to role in γ-carboxylation
– Impaired blood clotting
* Possible hemorrhagic syndrome (mostly seen in newborns)
– Impaired activation of calcium binding proteins
- Accelerate development of osteoporosis (mostly seen in elderly adults)
Calcium in body
Represents ~40% of the body’s mineral mass
– 1-2% of human adult body weight corresponds to calcium
– Bones and teeth contain about 99% of the calcium
– The remaining 1% is critically important for signalling pathways
Calcium sources
Predominantly obtained from dairy products, but also high in sardines,
salmon, and some green leafy vegetables (e.g., spinach)
– Present in these foods as an insoluble salt
– Stomach acid creates soluble Ca2+
Calcium absorption
Absorption (about 25-30% of dietary calcium is absorbed) in the small
intestine
– Saturable, carrier-mediated, active transport
* Absorption regulated by calcitriol; Most calcium absorbed this way
– Diffusion via paracellular route is a secondary pathway for Ca2+ uptake
How is calcium transported around the body?
Calcium is transported around the body in various ways:
– ~40% bound to albumin
– ~10% found complexed with sulfate, phosphate, etc
– ~50% found in free (ionized) form
BONE calcium
BONE Calcium (99%)
– Minerals (calcium, phosphorus, fluoride, magnesium, potassium,
etc) make up hydroxyapatite (a crystal-like structure)
- functionas as a reservoir for calcium
If intra- and extra-cellular levels of Ca2+ drop, bone will
be sacrificed otherwise death occurs.
Intra and extracellular calcium functions
INTRA- and EXTRACELLULAR Calcium (1%)
– Ionized calcium is active and used for:
* Blood clotting (formation of “Gla” residues on coagulation proteins)
* Skeletal muscle contraction (release of calcium stores)
* Nerve potential (acting through ion channels)
* Intracellular signalling pathways
– e.g., Ca2+ activates PLA2, which cleaves arachidonic acid from
phospholipids to produce eicosanoids
TRPV5/6
is an apical transporter of calcium on enterocyte
most calcium is absorbed through here to get into intestinal cell
Calbindin - D
Is like a ferry for the Calcium in the enterocyte
- produced in the enterocyte
it picks up the calcium after it’s been absorbed through TRPV5/6 and then brings it to the Ca2+ transporter on basolateral side where it’s absorbed ito blood
- calcitriol (active Vit D3) receptor is located in nucleus of enterocyte and calcitriol has positive regulatory effect of the expression of calbindin D3 in the enterocyte
- It uses vit K to bind calcium/can only bind Ca2+ thanks to vit K
- Trpv5/6 and calbindin expression decrease naturally with age
Ca2+ concentrations in relation to enterocyte
[Ca2+] Lowest in intestinal cell (nM)
[Ca2+] higher outside cell in intestinal lumen (mM)
this makes it easy to rush in!
and [Ca2+] high in blood
(mM)
Intracellular calcium
Most is present in mitochondria and the endoplasmic reticulum
– Released from stores in response to an extracellular signal (e.g.,
receptor binding) to ultimately promote an intracellular response (e.g., gene expression, neurotransmission, etc.)
Extracellular calcium
- IN BLOOD!
Maintained at a very constant level; ~10,000× the concentration of
intracellular calcium***
– Nearly identical to the concentration of phosphorus
Bone calcium
The majority (around 99%) of the calcium in the body is in the bone and teeth
– In bone, 99% is in mineral phase (hydroxyapatite), and 1% is in a pool
that can readily exchange with extracellular calcium
Changes to Ca2+ RDA
Most recent changes
related to UL?
↑ UL in children
↓ UL in adults (51-70 yrs)
↓ UL in >70yrs to prevent
developing kidney stones
Factors affecting calcium absorption?
Caffeine ↓
Some fibres ↓
Magnesium & Zinc ↓
PTH (Vit D) ↑
Pregnancy & lactation ↑
Ca2+ deficiency
DEFICIENCY
* Profoundly affects bone and muscle
Bone
* Inadequate mineralization in bone
– Rickets in children (commonly associated with Vit D deficiency)
– Osteomalacia in adults (and increases risk of developing Osteoporosis)
Muscle
* Tetany
– A condition characterized by involuntary muscle contractions
* Evidence for association with hypertension
- Evidence for association with colon cancer (current research
supports association in high-risk populations)
Ca2+ toxicity
Constipation, bloating, and/or gas
* Hypercalcemia
– Kidney stones
Phosphorus
Widely distributed in food, so deficiency and toxicity are very rare;
however, very important in physiology (2nd most abundant mineral in body)
- Found in both animal products (as phosphorus) and in grains (as phytic
acid)
Phosphorus absorption
Most phosphorus is absorbed in the small intestine in its ionic form (PO4-3)
- Passive diffusion (primary method)
- Saturable, carrier-mediated, active transport (NaPi cotransporter)
– 50-70% of the phosphorus in foods is absorbed
– Absorption inhibited by magnesium, aluminum, & calcium (cause they’re phosphate binders)
Phosphorus in the body
Primarily transported in the blood as organic phosphate (e.g., incorporated
into phospholipids)
- Found largely in bone (hydroxyapatite), but also in molecules key for metabolism (ATP, DNA, RNA, cAMP, etc.)
- Plays a key role in protein phosphorylation (common PTM in proteins)
Fluoride
Fluoride present in the body in trace amounts; not essential
* Community water fluoridated (with ~1 ppm or ~1mg/L) for the past 60 years
due to the inverse relationship between fluoride intake and dental caries
– water not fluoridated in Guelph because groundwater contains low levels of
naturally occurring fluoride
* Absorption in stomach by passive diffusion (nearly 100% efficiency)
* Transported in the body as ionic fluoride or bound to plasma proteins
* Major function related to effects on mineralization of teeth and bones
– Increases resistance of enamel to acid demineralization by forming fluoroapatite
(protective layer)
- Deficiency?
– Increased incidence of tooth decay - Toxicity?
– Fluorosis (mottling of the teeth); a cosmetic problem
– Tolerable upper limit in adults is 10mg/day
Deficiency and toxicity of fluoride
Deficiency?
– Increased incidence of tooth decay
* Toxicity?
– Fluorosis (mottling of the teeth); a cosmetic problem
Group II micronutrients
Micronutrients Involved in Oxidant Defense
Vitamin E, Selenium, Vitamin C
(Sulfur amino acids, Niacin, Riboflavin,
Iron/Zinc/Copper)
ROS
Reactive oxygen species
ROS produced as a by-product of the
ETC when proper electron flow fails
(~1% “leakage”)
*Occurs in an O2-rich environment,
where oxygen can react with electrons
*Radicals have unpaired electrons
*Mutations in SOD cause Lou Gehrig’s
*H2O2 converted to H2O by glutathione
peroxidase and catalase (selenium dependent enzymes)
Normally, O2 would react with 4 electrons to make 2 H2O
BUT SOMETIMES…
1. O2 reacts with 1 electron from mitochondrial leaked e-, producing superoxide anion radical (modest reactivity)
- superoxide anion radical (modest reactivity)
reacts with another electron (from Cu+, Zn2+ - metal cofactors that act as electron donors) and superoxide dismutase enzyme (SOD) to make hydrogen peroxide (modest reactivity - no unpaired electrons, pretty stable) - Hydrogen peroxide reacts with an electron (such as Fe2+ that’s accumulated –> fenton reactions) to produce OH. hydroxyl radical that is incredibly reactive and attacks macromolecules (DNA, lipids) leads to cancer, cell death
Vitamin E
Vitamin E is actually a general term that describes 8
structurally-related compounds (known as vitamers)
– 4 tocopherols
* Have saturated side chains with 16 carbons
– 4 tocotrienols
* Have unsaturated side chains with 16 carbons
* Vitamers in both classes (α, β, γ, δ)
* Only α-tocopherol has significant activity in the body
* No inter-conversion of vitamers in animals
– For example, β-tocotrienol cannot be converted into α-tocopherol
* All are found naturally in foods
- The word “tocopherol” is derived from Greek words:
– Tokos = “childbirth” & Phero = “to bear or bring forth”
– This was based on work showing that rats couldn’t reproduce when
Vitamin E was absent from the die
Tocopherols vs. tocotrienols
are Vitamin E vitamers
4 tocopherols
* Have saturated side chains with 16 carbons
– 4 tocotrienols
* Have unsaturated side chains with 16 carbons
* Vitamers in both classes (α, β, γ, δ)
* Only α-tocopherol has significant activity in the body
Vitamer
are chemical compounds that have a similar molecular structure, each
showing vitamin activity to some extent.
4 Tocopherols
Vitamin E
Saturated side chain (phytyl tail)
- Nomenclature used to describe # and position of ring methyl (CH3)
groups - α isoform has the most methylated ring
- The hydroxyl group is the antioxidant site!
- Transport-mediated uptake in the
small intestine (e.g., NPC1L1) - Natural α-tocopherol is RRR (3 chiral carbons)
– Fits into the binding pocket of the
tocopherol transfer protein (TTP) - the only one that can get sent out into the body cause it’s the only one that can fint!!
4 Tocotrienols
Vitamin E
Unsaturated side chain
(phytyl tail)
* Nomenclature rules the same as previous slide
* Lower levels in food
compared to tocopherols
- Transport-mediated
uptake in the small
intestine (e.g., NPC1L1) - Tocotrienols have ANTI-OXIDANT ABILITY IN THE LIVER ONLY
(not converted to α-tocopherol or able to fit in the TTP binding pocket)
Vit E sources
Food sources: nuts, seeds, vegetable oils, avocado
– Sensitive to food preparation & storage (e.g. roasting nuts ↓ Vit E levels)
– Mostly obtained from plants because Vitamin E accumulates in adipose tissue in animals (and we don’t normally eat the fat)
RDA for vit E
RDA based on only α-tocopherol
– Determined with tests that examined the hemolysis (breakdown) of red blood cells in the
presence of dilute H2O2
– (>20% RBC hemolysis means there is a Vit E deficiency)
RDAs can be reported as mg or IU
Upper limit for Vit E
UL = 1,000 mg / day (above this amount will cause increased bleeding)
– However, gastrointestinal problems can be seen at lower levels than the UL
Vitamin E deficiencies
Vitamin E deficiencies are very rare, but can occur in:
– Pre-mature infants (kept in oxygen-rich incubators, which increases oxidative stress), who
are then at risk for hemolytic anemia
– People with fat malabsorption disorders or gallbladders removed, which can reduce
absorption of dietary fats
– People with genetic defects in lipoproteins or TTP, which prevents transport of α-tocopherol
around the body
Digestion, Absorption and Metabolism of Vit E
Requires bile acids for emulsification
and incorporation into micelles
- Absorbed in small intestine by transporters (e.g., NPC1L1)
- Packaged into chylomicrons
- Chylomicron remnants, containing
Vit E, are taken up by the liver - Liver makes TTP, which is needed to
get α-tocopherol into VLDL
– Other vitamers can help a bit with
anti-oxidant activity in the liver only,
but they are quickly degraded - No specific “storage” organ for Vit E,
but most of it accumulates in lipid
droplets in adipose tissue
TTP
tocopherol tranfer protein is the thing that carries RRR-a-tocopherol around in the VLDL in the body
is needed to
get α-tocopherol into VLDL
– Other vitamers can help a bit with
anti-oxidant activity in the liver only,
but they are quickly degraded
Liver makes TTP
Lines of Defense of antioxidants
- GSH peroxidase
- Vitamin E
- FA peroxidase
GSH peroxidase
is the first line of defencse when fighting against the production of ROS
= glutathione peroxidase
- it donates an electron to hydrogen peroxide to produce 2 H2O
- it’s a selenoprotein (only functions thanks to selenium in it’s structure)
How and where does vitamin E act to defend against ROS
it donates an electron (via its OH group) to PUFA peroxy radical
this still produces PUFA hydroperoxide
BUT it prevents another PUFA from donating the electron to produce more PUFA free radicals
TOC(OH) –> TOC(O*)
FA peroxidase
Is the third line of defense against the formation of ROS
= fatty acid peroxidase
- it with glutathione 2GSH –> GSSG
helps convert PUFA hydroperoxide to PUFA alcohol
What is the PUFA cycle
- Initiation - Hydroxyl radical steals an electron from a PUFA (polyunsaturated fatty acid) to produce a PUFA free radical
- PUFA free radical very reactive and short lived
- there’s a collapse of the douple bond structure and it goes to react with O2 to produce PUFA peroxy radical
theres no time to mount defense cause it’s so short lived - Propagation - PUFA peroxy radical is less reactive and longer lived - it can steal an electron from another PUFA to produce another PUFA free radical in this vicious cycle and then becomes PUFA hydroperoxide which
- PUFA hydroperoxide is super chemically unstable, but not a radical - it’s broken down to short chain aldehydes which kills cell! i think?
Vit E further metabolism
Vit E donates its electron from OH group to PUFA peroxyradical to produce PUFA hydroperoxide
Vit E radical (lost it’s electron) is relatively stable and
2/3 are vit E dimers excreted by bile
1/3 are quinone (single ring) and excreted through urine
Selenium
Plants incorporate selenium from the
soil into methionine and cysteine amino acids instead of sulfur
– Therefore, selenium content in food is
determined by selenium levels in the soil
SelenoAA are absorbed in the small intestine by amino acid transporters
and travel freely in the blood
The body uses selenocysteine
Selenium deficiency
China and Africa (low Se in soil)
– Keshan disease (cardiomyopathy from
cell damage by free radicals)
Selenium toxicity
Selenium toxicity is rare:
– Selenosis – chronic consumption of lots
of brazil nuts which are rich in selenium
can lead to hair and nail loss (most common symptoms)
GSH and selenoproteins
= GLUTATHIONE
Glutathione
(GSH) is the
major
intracellular
reducing agent.
(it is a tripeptide)
Two important selenoproteins involved in oxidant defense:
– Glutathione peroxidase = 1st line of defence against lipid peroxidation
– Fatty acid peroxidase = 3rd line of defence against lipid peroxidation
- Both use glutathione (GSH) as a substrate, which helps to protect cells against oxidative damage
- Selenoproteins work with GSH (which acts as the reducing agent)
- The reaction site of these proteins is where the selenium is in their structure
Structure of glutathione is crucial, where glutamate and cysteine amino acids are linked through a gamma-carbon
– This gamma peptide bond is resistant to cellular proteases
Note that glutathione itself does not have selenium in its structure
How glutathione functions
GSH is a single
electron donor, so
2 GSHs are
needed per
reaction
H2O2 –> H2O
via GSH peroxidase
2GSH (red.) –> GSSG (ox.)
GSH is oxidized
and reacts with
another GSH to
form GSSG
same reaction occurs when FA peroxidase converting PUFA hydroperoxide to PUFA alcohol
How is glutathione regenerated
NADPH (+ H+) donates electrons to GSSG to become NADP+ and and regenerate 2 GSH
via glutathione reductase (riboflavin) Vit B12
Pentose phosphate pathway (hexose
monophosphate shunt) regenerates NADPH
(which requires niacin)
glutathione in a healthy cell
A healthy cell has
>90% GSH and
<10% GSSG
– High cellular
levels of GSSG
indicative of
high oxidative
stress
Vitamin C
ascorbic acid
At physiological pH, Vit C is known as “ascorbate”
Exists as both D- and L-isomers
– The L-isomer is biologically active in humans
Uronic acid pathway
not active in humans
way of synthesizing Vit C from glucose
Glucose (or galactose) -> gulonolactone –> ascorbic acid (vit C)
via gulonolactone oxidase)
Many mammals can synthesize Vit C from glucose, except:
– Humans, primates, fruit bats, guinea pigs, and some birds
– This is because we lack the specific enzyme gulonolactone oxidase
Primary sources of vit C
Primary sources of Vitamin C are fruits and vegetables
* Vitamin C is very sensitive to heat, light, oxidation, and alkaline solutions
Vit C absorption
Vitamin C doesn’t require digestion prior to absorption
* Uptake via sodium-dependent vitamin C (SVCT) 1 and 2 transporters in the
small intestine (feedback mechanism exists)
– 70-90% dietary Vit C is absorbed
- There appears to be a maximum amount of Vit C that can be absorbed
- Found in circulation primarily in “free form” (i.e., not bound to a protein)
dehydroascorbic acid
oxidized form of Vit C
another biologically active form
Foods contain mostly
ascorbic acid, but can have small amounts of the oxidized form
(dehydroascorbic acid)
Functions of Vit C
Involved in several
biological processes, such as:
– Collagen synthesis
– Tyrosine synthesis
– Neurotransmitter synthesis
- Vit C acts primarily as a
reducing agent in these
processes
– “2 electron donor”
(via its OH groups)
Vit C concentrations are
high in white blood cells, as well as in many tissues
post translational modification of procollagen
procollagen w a proline residue undergoes PTM where prolyl hydroxylase adds an OH group to proline (proline-OH)
Pro-OH is then exported to extracellular space and pro-OH is what allows the collagen molecules to stick tg and make collagen WALL
role of Vit C in collagen formation
ascorbic acid donates electrons to 2-prolyl hydroxylase-Fe3+ (ferric inactive) which activates the enxyme to 2 prolyl hydroxylase-Fe2+ (ferrous active)
that active enxyme donates its electron then to proline and is what converts proline to proline-OH on pro collagen, activating collagen
so basically, Vit C indirectly donates an electron to form proline-OH
Vitamin C and Oxidant Defense
The role of Vitamin C in oxidant defense is unclear, but
the following evidence suggests a possible role:
– We just learned about the reactions of lipid peroxidation, with
free radicals “stealing” electrons from PUFA in cell membranes.
If Vit C is around, there is some evidence that this happens to a
lesser extent. This is seen by reduced levels of lipid peroxidation
products being measured in the urine
.
– In white blood cells, where we have a lot of oxygen radicals,
there are higher levels of Vit C. Coincidence?
– With a Vit C deficiency, there is some increase in GSSG levels
(oxidized dimer form that is inactive) and reduced GSH levels
(the active form)
Vit E RDA
RDA: The goal is to maximize tissue concentrations
and minimize urinary excretion
higher in men than women
RDA is increased in pregnant and lactating women to support
mother and fetus/infant
UL for vit C
> 2 g/d, above this amount increases risk for digestive
problems (diarrhea) and kidney stones
Signs of Vit C deficiency
curvy (plasma Vit C levels < 0.2 mg/dL)
* If you consume 10 mg Vit C per day, signs of scurvy would
develop in 1 month
– Increased risk of hemorrhages (skin, follicles, gums)
– Increased hair loss, loose teeth
– Swollen joints, poor wound healing
**due to problems producing hydroxyproline (i.e. collagen)
Micronutrients Group III
micronutrients that act as enzyme cofactors
Niacin, thiamine, riboflavin, B6, folate, B12, biotin, pantothenic acid
Niacin
= Vitamin B3
- type III micronutrient
Discovered through the study of the condition pellagra in humans and a similar
condition, called black tongue, in dogs
– Niacin was considered the “anti-black tongue” factor
The term “niacin” is a generic term and used interchangeably with Vitamin B3.
In plant foods, Vitamin B3 is predominantly found as nicotinic acid, which is the provitamin.
In animal-derived foods, Vitamin B3 is commonly found as:
– Nicotinamide
– Nicotinamide adenine dinucleotide (NAD)
– Nicotinamide adenine dinucleotide phosphate (NADP)
Sources of niacin
Dietary sources for niacin? Most fish, meats, breads and cereals, coffee and tea
– In coffee, trigonelline is converted to nicotinic acid by heat (i.e., coffee bean roasting
Niacin can also be produced in the liver from the amino acid tryptophan (but only
about 1/60th of tryptophan is converted into nicotinamide)
Digestion and Absorption of Niacin
Digestion of NAD and NADP is required before absorption
– Hydrolyzed by the glycohydrolase enzyme to release free nicotinamide
- Nicotinic acid and nicotinamide are absorbed a bit in the stomach, but most
is absorbed in the small intestine through facilitated diffusion - In plasma, niacin circulates primarily as free nicotinamide
In plasma, niacin circulates primarily as
free nicotinamide
Both nicotinic acid and nicotinamide can cross cell membranes by simple diffusion in most tissues, except in…
kidney and red blood cells (where it is
carrier-mediated)
Main function of niacin
Once NAD(P) is synthesized, niacin is essentially trapped within the cell. In
their reduced forms, the main functions are:
– NADH: the transfer of electrons to ETC
– NADPH: a reducing agent in biochemical pathways
nicotinic acid vs. nicotinamide
nicotinic acid is the provitamin B3 form found in plants
nicotinamide is vit B3 form found in animal foods
Both nicotinic acid and nicotinamide are precursors for NAD in the body
NAD(P) production steps
Two steps (from Nicotinic acid):
1. Convert the acid to an amide
2. Build into a dinucleotide
structure:
nucleotide structure contains a
sugar (ribose), a nitrogenous base
and a phosphate
(NAD or NADP)
Cellular levels of NAD and NADP
NAD+ (oxidized)»_space; NADH
NADPH (reduced)»_space; NADP+
opposing roles of NAD+ and NADP+ in the cell
NAD+ reduced to NADH in:
– Glycolysis, Kreb’s cycle, B-
oxidation
– Role in CATABOLISM
NADP+ reduced to NADPH in
the hexose monophosphate
shunt and used for:
– Fatty acid synthesis,
glutathione regeneration, etc.
– Role in ANABOLISM
But both are 2 electron acceptors
Over 200 enzymes require NAD(H) or NADP(H) as cofactors
Corn and niacin
Corn/Maize
– Contains significant amounts of niacin, but it’s bound and not absorbed
– Also deficient in Tryptophan
– Use of lime (from limestone, not the fruit) helps to release niacin in corn
How do we release niacin from corn?
NIXTAMALIZATION
Use of lime (from limestone, not the fruit) helps to release niacin in corn
* A process known as nixtamalization
* Lime is alkaline and causes the breakdown of hemicellulose in corn, releasing niacin
* Used by Indigenous peoples in food practices, but not initially in Europe or Africa
Niacin deficiency
leads to PELLAGRA
- the 4 Ds:
dermatitis, dementia, diarrhoea, death
- most symptoms are reversible
Niacin RDA
RDA recommendations include the small
amount of niacin produced from tryptophan
(Trp)
– Therefore, we consider Niacin Equivalents (NE)
– NE = mg preformed niacin + mg Trp/60
– 14/16mg per day NE adult women/men
Riboflavin
Vitamin B2
a type III micronutrient
Riboflavin stems from “ribo” (ribose-like side chain) + “flavus” (yellow
colour
FAD FMN are coenzymes derived from ribofflavin
Sources of Vit B2
aka riboflavin
Rich in foods of animal origin
– Milk, milk products, meat, etc. (Source of free riboflavin)
– Other foods contain flavins, found as either flavin mononucleotide (FMN) or
flavin adenine dinucleotide (FAD), which are essentially bound riboflavin
– Riboflavin is degraded with sunlight
– Fun Fact: this is why milk is no longer widely sold in glass bottles
Absorption of riboflavin
Riboflavin that is bound to proteins must be released prior to its absorption
– This is done by HCl (protein denaturation) in the stomach
- Free riboflavin is absorbed from the gut lumen by an active transport
mechanism known as the riboflavin transporter 2 (RFT2)
Riboflavin transport and storage:
Riboflavin, FAD, FMN are transported in the body bound to proteins (in particular albumin)
- Riboflavin is stored a little bit in the body (in tissues like the liver, kidney, and heart), but extra riboflavin is generally excreted in urine
– Sufficient amounts in the body to last 2-6 weeks when riboflavin is no longer consumed in the diet
Where are FMN and FAD made?
FMN and FAD are made in cells:
– FMN and FAD are involved in redox reactions
– FAD is the primary form of riboflavin in the body (60-95%)
– Production is positively regulated by T3 hormone, which
increases the activity of the flavokinase enzyme
What is the primary form of riboflavin?
FAD is the primary form of riboflavin in the body (60-95%)
Metabolism of riboflavin
This is how riboflavin becomes FMN and FAD
1) Riboflavin is converted to Riboflavin PO4 (flavin mononucleotide = FMN)
via an ATP dependent reaction involving enzyme flavokinase and Mg2+ or Mn2+
(this step activates in and makes it a nucleotide)
2) Riboflavin PO4 / flavin mononucleotide / FMN is converted to FAD by ATP dependent reaction using FAD synthetase (Mg2+ or Mn2+) (FAD considered a dinucleotide = FMN + adenine + sugar + P)
riboflavin’s role in redox reactions
- riboflavin functions in many biochemical pathways ex. b-oxidation, and Kreb’s Cycle
- delivery of electrons to ETC
Glutathione reductase
GSSG + NADPH –> 2GSH +NADP+
basically, NADPH transfers its electrons to FAD, reducing it to FADH2 and getting oxidized to NADP+
riboflavin (FAD) is bound within the glutathione reductase enzyme
FADH2 then passes its electrons to GSSG to reduce it to GSH (regenerates glutathione)
FMN & FAD function similar to NAD(P) in electron transfer
* The primary difference is that FMN
and FAD are typically bound to the active site of an enzyme
* To regenerate glutathione, FAD
accepts 2 electrons from NADPH, temporarily becoming FADH2
* Reforming glutathione requires
both niacin (NADP) and
riboflavin (FAD
Riboflavin deficiency
Ariboflavinosis
– Signs of deficiency appear after ~3-4 months
– Deficiency relatively common when dietary intake is insufficient
because riboflavin is continuously excreted in the urine
– No clear riboflavin deficiency has been characterized because
typically occurs with other vitamin deficiencies.
Reversible!
– Cracked and red lips, inflammation of the lining of mouth and
tongue, mouth ulcers, cracks at the corners of the mouth.
WHich populations are at risk of a riboflavin deficiency
Populations “at risk” for a deficiency?
– People with hypothyroidism or thyroid disease, because they can’t make thyroid hormones to activate flavokinase
– Chronic alcoholism, which reduces riboflavin digestion &
absorption
– People who are lactose intolerant (reduced consumption of dairy foods and beverages)
No UL for riboflavin
Thiamine
Vit B1
Need for thiamine first discovered in the 1800s when birds fed a diet of
cooked polished rice developed neurological problems (now known as beriberi)
– Polished rice has had the husk, bran and germ removed
In plants, the thiamine provitamin exists in a free form
* In animals, thiamine exists in phosphorylated form (known as thiamine
pyrophosphate or TPP) – this is the active form
– Phosphate group must be removed to allow for its absorption
Thiamine absorption
In animals, thiamine exists in phosphorylated form (known as thiamine
pyrophosphate or TPP) – this is the active form
– Phosphate group must be removed to allow for its absorption
- Absorption occurs in the small intestine
– Typically controlled by thiamine transporters - In plasma, TPP is found in free form or bound to albumin
TPP production
thiamine (plant provitamin) is converted by addition of 2 phosphates (from ATP–> AMP) to thiamine pyrophosphate / TPP (the active form)
active form has a carbanion (acts as the point of sword)
Thiamine role in energy metabolism
Thiamine is an important cofactor for many enzymes, such as:
1. - Pyruvate dehydrogenase (pyruvate acetyl CoA)
2. - Alpha-ketoglutarate dehydrogenase (Kreb’s cycle)
These enzymes also require riboflavin (FAD), niacin (NAD), and
pantothenic acid (CoA) as cofactors
- Critical in the movement of sugars
and amino acids into energy metabolism pathways
Thiamine is also required in the non-
oxidative phase of the hexose monophosphate shunt (transketolase
step –> we did not discuss this
enzyme in the course), thus playing a
role in the synthesis of nucleotide
precursors
What are the 3 enzymes in PDH complex
3 enzymes in PDH complex:
* Pyruvate dehydrogenase (PDH)
* Dihydrolipoyl transacetylase
* Dihydrolipoyl dehydrogenase
- PDH bound TPP stabs the carbonyl carbon of pyruvate and it’s a decarboxylation/ looses CO2 group
producing an intermediate - Co-ASH is added to this intermediate in a transacetylation reaction (brings CoA from pantothenic acid) to produce acetyl Co-A!
this step also uses FAD and NAD+ to NADH in a dehydrogenation!
Micronutrients:
* Pyruvate dehydrogenase needs Thiamine
* Transacetylase needs Pantothenic acid
* Dihydrolipoyl dehydrogenase needs
Riboflavin (FAD) and Niacin (NAD)
Thiamine deficiency
Thiamine deficiency interferes with critical energy metabolism pathways (pyruvate and alpha-ketoglutarate dehydrogenase complexes)
– Ultimately prevents ATP production and acetyl-CoA synthesis
– Causes an accumulation of pyruvate, lactate, and alpha-ketoglutarate in
blood
causes 3 types of beriberi (dry, wet and acute)
Beriberi
caused by a thiamine deficiency
3 TYPES:
- Dry beriberi
– Predominantly in adults due to chronic low thiamine intake
– Muscle weakness, affects the nervous system - Wet beriberi
– Predominantly in children and young adults
– More severe than dry beriberi and affects the cardiovascular system - Acute beriberi
– Occurs primarily in infants
– Anorexia, vomiting, lactic acidosis, and eventually death if left untreated
Who is susceptible to a thiamine deficiency?
Who is susceptible to a thiamine deficiency?
– Chronic alcoholism
– People depending on polished (white) rice as a major source of food
Thiamine UL
HAHA GOT YOU
THERE IS NONE!
Pantothenic Acid
Vitamin B5
EVERYWHERE! so no jameson supplement :( lol
Derived from the Greek word pantos, which means “everywhere”
– Present everywhere (found in virtually all plant & animal foods); therefore
deficiency is next to impossible
– Little to no toxicity associated with dietary and supplemental pantothenic acid
- Occurs ubiquitously in foods in both free and bound forms
– Most pantothenic acid in food occurs as part of coenzyme A (CoA)
Vitamin B5 absorption
aka pantothenic acid is
Absorbed in the jejunum by passive diffusion
- Found free in blood
- Uptake into tissues occurs through the sodium-dependent multivitamin
transporter (SMVT) - Precursor in the synthesis of Coenzyme A (CoA)
Pantothenic acid in energy metabolism
Essential in energy metabolism
(formation of acetyl CoA), allowing Kreb’s cycle to take place.
*Cysteine required to make CoA (which is the active SH group)
*Important component of acyl-carrier
protein in the fatty acid synthase complex
*Energy production
*Formation of acetyl-CoA
- Pantothenic acid is converted to 4-phosphopantetheine (active form in fatty acid synthesis)
- this requires phosphate from ATP and cysteine!!
CO2 is extruded - 4-phosphopantetheine is converted to Coenzyme A (CoA, CoASH) upon donation of AM from ATP! Releases PPi
Coenzyme A/ CoA is active form in oxidative reactions
involved in
- energy production
- formation of acetyl-CoA
What is 4-phophopantothene??
its an intermediate in the conversion of pantothenic acid to CoA
but it’s the active form in fatty acid synthesis
- Important component of acyl-carrier
protein in the fatty acid synthase complex
Biotin! (and sources)
Vitamin B7
Discovered in 1931 during experiments examining the cause of
“egg white injury”
– Eating raw eggs led to hair loss, dermatitis, etc. This led to the discovery
of avidin, a biotin-binding protein
Sources of Biotin:
1. Biotin is made by intestinal bacteria
* although not enough is made to meet the needs of humans
2. Widely found in foods bound to proteins
Biotin absorption
aka vit B7 absorption
Must be removed from proteins prior to its absorption
– Proteolysis by pepsin in the stomach breaks down proteins and
releases biotin
- Free biotin is absorbed to near completion
– Alcohol inhibits biotin absorption
Circulates in blood in the free form (~80%), with a bit bound to
albumin
Biotin RDA
GOTCHA again! no RDA cause biotin is so prevalent!! (its made in intestinal bacteria)
Biotin-mediated carboxylation
Involves 2 reactions!!
RXN #1
we load a bicarbonate/HCO3- onto biotin with help of ATP –> ADP
/ carboxylate the N in biotin ring structure
RXN#2
we transfer the carboxyl group to another molecule/ substrate (such as pyruvate to oxaloacetate (carboxylated substrate))
and this regenerates the OG biotin
Biotin structure
Two rings (with N and S) and a side chain with a carboxyl group
- The carboxyl group interacts with the NH2 group of a lysine side chain in an enzyme (e.g., pyruvate
carboxylase)
the enzyme i s joined by a peptide bond to biotin at the lysine residue
3 key reactions involving biotin:
- Pyruvate carboxylation
(production of oxaloacetate) - Malonyl CoA formation
- Conversion of propionate into glucose (important in
ruminants)
How many active sites on pyruvate carboxylase enzyme?
Pyruvate carboxylase
requires a biotin cofactor to be active.
- Two active sites on the enzyme!
- The biotin is anchored to a lysine side chain, allowing it to “reach” into the carboxylation reactive site (active site #1), where it is carboxylated.
- The cofactor can then
swing to the active site #2, bringing the carboxyl
= group into close proximity with the substrate (in this example, pyruvate)
proprionate to glucose
Conversion of propionate (a SCFA( into glucose
(important in ruminants)
its the main pathway for making glucose in ruminants and involves
BIOTIN!!
this pathway feeds into citric acid cycle like its a type of like gluconeogenesis
Folate
Vit B9
Folate and Vit B12 were discovered during the search to cure megaloblastic anaemia, a problem that was first described in the late 1800s
- Folate is a generic term that refers to both natural folates in food and the synthetic form
used in supplements and fortified foods called folic acid - Folic acid refers to the oxidized form of the vitamin found in fortified foods and supplements (100%
bioavailable)
– Folate refers to the reduced form of the vitamin found naturally in foods (~50% bioavailable)
Folate is the precursor for tetrahydrofolate (THF) and 5-methyl THF
- folate is important for production fo nucleic acid precursors and several amino acids as well as in methylation reactions so its really important in bone marrow and the developing fetus
Folate vs. Folic acid
“folate” and “folic acid” are structurally different (see next slide)
– Folic acid refers to the oxidized form of the vitamin found in fortified foods and supplements (100%
bioavailable)
Only 1 glutamate residue
“pteroylmonoglutamate”
– Folate refers to the reduced form of the vitamin found naturally in foods (~50% bioavailable)
Multiple glutamate residues
“pteroylpolyglutamate”
Folate vs. Folic Acid
(You don’t need to
memorize these
structures)
Folate digestion
Natural folates have multiple glutamate residues (i.e., polyglutamates)
which must be removed for absorption in its simplest form (monoglutamate)
– Polyglutamate hydrolase removes the glutamate residues
- This enzyme is sensitive to alcohol and inhibitors naturally present in certain foods (legumes,
lentils, etc.)
– Folic acid is already a monoglutamate structure, so no digestion is required
polyglutamate hydrolase
is the enzyme that removes the glutamate residues on folate to be absorbed
Folate absorption
Once the extra glutamate residues have been removed, folate/follic acid is absorbed through a proton-coupled folate transporter (PCFT) in the small intestine
In the intestine, most natural folates and folic acid are converted into the bioactive compound 5-methyltetrahydrofolate (5-methyl THF)
- In plasma, you mostly detect 5-methyl THF and a bit of folate (more on this
later)
– Generally transported in blood bound to proteins, such as albumin - 5-methyl THF is the bioactive form of folate
5-methyl THF
the bioactive form of folate
DFE
dietary folate equivalent
Total DFE =
μg food folate +
(1.7 × μg folic acid)
the 1.7 takes into account the bioavailability of folate
DFE is much higher in pregnant women and lactating women
there is no DFE for infants
folate UL
is not that much higher than the RDA!!
This is important
cause exceeding the UL supports the development of cancer!/increases cancer risk! (related to Vit B12)
Very easy to exceed
the UL if taking
supplements and
eating fortified foods
Cobalamin
= Vit B12!
BIGGEST VITAMIN
Vit B12 was the last vitamin to be
discovered
- Generic term for a group of compounds called corrinoids
(because of a corrin nucleus that contains cobalt, which is very rare)
– Most complex vitamin structure and is
unique in that it contains a metal ion.
- Only bacteria produce Vit B12
- Vit B12 is present in animal products, like meat, poultry and
eggs
– Strict vegetarians/ vegans are at risk of
a deficiency - No plant provitamin B12 exists
Vit B12 deficiency causes:
A “functional” deficiency for folate (the two work together)
– Neurological problems
Vitamin B12 absorption
In the stomach, a specific binding
protein is secreted from the gastric
lining, known as intrinsic factor (IF)
* The Vit B12-IF complex goes to a
receptor in the small intestine
* Complex broken down in the enterocyte. Vit B12 is absorbed, while IF is released back into the intestinal lumen
- Vit B12 is stored in the liver and can undergo enterohepatic circulation
- High storage in the liver, so deficiencies can take years to surface
Intrinsic factor
a specific binding
protein is secreted from the gastric
lining of the stomach that helps in Vit B12 absorption
The Vit B12-IF complex goes to a receptor in the small intestine
* Complex broken down in the
enterocyte. Vit B12 is absorbed, while
IF is released back into the intestinal
lumen
Two causes leading to a Vit B12 deficiency
Two causes leading to a Vit B12 deficiency:
- Not enough in your diet – in strict vegans (resolved with megadoses of Vit B12)
- Improper absorption due to defects in Intrinsic Factor
Deficiencies shown in blood in the form of megaloblastic anemia (discussed later)
Single carbon metabolism
Inside the intestinal enterocytes, most natural folate and folic acid are methylated to the N5-
methyl tetrahydrofolate (5-methyl THF) form and released into the blood stream.
When taken up by cells, the N5-methyl group is removed by methionine synthase thanks to the
Vitamin B12 (cobalamin) cofactor.
The cobalamin temporarily becomes methyl-cobalamin. (it accepts methyl from N5-methyl THF
Consequently, the cell now has free tetrahydrofolate (THF), which is important in a wide variety of
reactions
THF can pick up a methyl group from an
intracellular methyl donor (e.g. serine or choline) to form N5N10-methylene THF. Two things can
happen to N5N10-methylene:
1) dUMP to dTMP to dNTPs
2) irreversible reaction
converted to N5-methyl-THF. This secondary pathway is thought to exist as a means for
the cell to funnel extra methyl groups back to the SAM cycle
Key players in the single carbon metabolism
-Folate (N5-methyl THF or N5,N10-
methylene THF)
-Vit B12 (“prosthetic” group)
-S-adenosyl methionine (SAM)
Two things can
happen to N5N10-methylene THF:
1) The major outcome is that the N5N10-methylene THF passes the methyl group to dUMP,
forming dTMP, which is required for DNA synthesis. The enzyme coordinating this is known as thymidylate synthase. Losing a methyl group allows THF to be reformed, which can then pick up another intracellular methyl group from another serine (for example), and the cycle
continues.
2) An alternate minor outcome is that N5N10 undergoes an irreversible reduction reaction where
it is converted to N5-methyl-THF. This secondary pathway is thought to exist as a means for
the cell to funnel extra methyl groups back to the SAM cycle.
Folate deficiency concequences
DNA synthesis and cell division slow down, and this has a major impact in tissues which depend on rapid cell division, including;
- bone marrow; leading to megaloblastic anaemia (few, large red blood cells, overstuffed with
haemoglobin) - developing fetus; improper neural tube closure, defects such as spina bifida
and just impaired DNA synthesis and repair: uracil misincorporation, megaloblastic anemia
Only reaction that can metabolize N5-methyl THF
is methionine synthase - Vit B12 dependent
When someone is Vit B12 deficient,
N5-methyl THF is “trapped” and
can’t be converted to THF
SAM cycle
Occurs in precence of Vit B12
basically end goal is to methylate a substrate, be it DNA, or what not
methyl-cobalamin (methylated B12 after having accepted that methyl group from N2-methyl THF) donates the methyl to homocysteine, producing methionine and converting itself back to cobalamin
methionine then gets converted to S-adenosyl-methionine (SAM) which donates its methyl to a substrate like DNA to become S-adenosylhomocysteine
important for like:
- phosphatidylcholine
epinephrine
creatine
DNA metabolism
drug methylation
drug metabolism
The Methyl Folate
Trap
When someone is Vit B12 deficient,
N5-methyl THF is “trapped” and
can’t be converted to THF
Only reaction that can metabolize N5-methyl THF is methionine
synthase (Vit B12 dependent)
The problem? Large doses of folate (from supplements) can hide
a Vit B12 deficiency. 5× the RDA for folate overwhelms the ability to form N5-methyl THF in the gut, so you get more folate going to the liver where it is converted into THF
(this will bypass the trap)
- However, there is still a problem with the SAM cycle!
Neural tube defects
Folate is critically important in dividing cells, which
supports the production of specialized cells that form
the neural tube.
This takes place during early pregnancy, so folate
deficiency leads to severe birth defects (neural tube defects; NTD). In the embryo, the neural tube is the CNS, which becomes the brain and spinal cord.
NTD is an opening of the spinal cord or brain
Closed vs. open NTD (open more common): the brain or spinal cord are exposed
Vitamin B6
Initially found while trying to understand dermatitis in rats
* Exists as 6 vitamers (which are interchangeable and
comparably active)
– Pyridoxine (plant provitamin), pyridoxal, pyridoxamine
– All can be found phosphorylated and unphosphorylated
We’ve already seen Vit B6 (transamination reactions use pyridoxal phosphate; PLP)
PLP is the main form found in blood, bound to albumin
* High levels of Vit B6 are found in the muscle
What is the main form of Vit B6 in blood
pyridoxal phosphate; PLP
found bound to albumin
Vit B6 subtypes
Pyridoxine (plant provitamin), pyridoxal, pyridoxamine
– All can be found phosphorylated and unphosphorylated
* All isomers are found in foods
Vit B6 absorption
Vit B6 is dephosphorylated prior to absorption
– Passive diffusion in the jejunum
* PLP is the main form found in blood, bound to albumin
* High levels of Vit B6 are found in the muscle
Pyridoxine activation
this is Vit B6!!
Conversion occurs primarily in the
liver
Pyridoxine (alcohol) is the plant provitamin B6
its converted to pyridoxal (aldehyde)
pyrodoxal is then converted to pyrodoxal phosphate (PLP!!) /it’s phosphorylated and this is the active form and used as a co-enzyme
PLP
= pyridoxal phosphate
the active form of Vit B6
it’s a co-enzyme
PLP is a cofactor for many
enzymes, including
aminotransferases
* Also involved in the 1st step in
porphyrin (heme) synthesis
– *uses glycine and succinyl CoA from
the Kreb’s cycle
– Heme is incorporated into hemoglobin,
myoglobin, as well as cytochromes in
ETC
Synthesis of neuroactive amines
(epi & norepinephrine, serotonin,
histamine, GABA)
– Relationship with serotonin is why
you’ll find Vit B6 is sometimes sold to
“treat mood disorders”
Vit B6 deficiency
rare
but causes microcytic anemia
Micronutrient Deficiencies & Anaemia
Stem cells in bone marrow are differentiated in the
presence of erythropoietin (EPO)
- Normally, differentiation
causes red blood cells to shrink, nucleus is removed, and able to carry O2 - Process is very sensitive to micronutrient deficiencies, which leads to anaemia
3 types of anaemia and their causes
3 types of anaemia
- Megaloblastic anaemia
- def. in folate, vit B12
- impaired DNA synthesis and RBC production - Hemolytic anaemia
- vit E, selenium, cysteine def.
- oxidant stress - Microcytic anaemia
- vit. B6 def.
- impaired porforin - problem making heme
Megaloblastic anemia
Megaloblastic anemia (problem with DNA synthesis)
- RBCs are too big but too few, become stuffed with hemoglobin.
- In this anemia, there is a deficiency of folate and B12, which ultimately impairs the activity of the enzyme thymidylate synthase, which is required for DNA synthesis. As a result, RBCs can’t be made in the initial stages
Hemolytic anemia
Hemolytic anemia (problem with oxidative damage)
- There are not enough RBCs due to premature destruction of the cells.
- This is due to a deficiency in antioxidant nutrients like Vitamin E and selenium, as well as cysteine (which is required for glutathione). The high level of oxygen in RBCs encourages the production of free
radicals. When antioxidant nutrients are deficient, there is a lot of lipid peroxidation of the membrane which ends up destroying RBCs
Mycrocytic hypochromic anemia
Problem w protein production
Microcytic hypochromic anemia (problem with protein production)
* RBCs are small and pale due to a deficiency of Vit B6 affecting the
synthesis of porphyrin. Without adequate porphyrin, you don’t have normal synthesis of hemoglobin and the cells are very pale. They are also very small because the hemoglobin is what gives them bulk
Group 4 micronutrients
essential trace minerals
iron, copper and zinc
Iron sources
Iron is found in a lot of different foods at low levels
– Rich in liver, meats, and plant sources (e.g., leafy green vegetables,
fruits, nuts)
- In foods, iron can be found in one of two forms:
- Heme (within the porphyrin ring of hemoglobin and myoglobin) (from animals)
- Non-heme (from plants)
Iron Functions in body
Functions of iron in the body include:
- Oxygen transport (i.e., important for hemoglobin and myoglobin)
- Redox reactions – is an active component of the electron transport chain (iron sulfur centers and cytochrome heme proteins)
- Iron metalloenzymes
iron in body
Only two states of iron are stable in the aqueous environment of the body and in food:
Fe3+ (ferric, oxidized) and
Fe2+ (ferrous, reduced)
RDA iron
unique cause higher in women by more than double due to menses and even higher in pregnant women to cover the needs of the fetus
How much of iron is absorbed
Between 10-18% of the iron ingested is absorbed
– How much is absorbed will depend on a person’s iron status
Absorption of non-heme iron
HCl and proteases cleave non-heme iron from food components in the stomach to release mostly ferric (Fe3+) iron
– The acidic environment of the stomach converts
most Fe3+ into ferrous iron (Fe2+)
– Any remaining Fe3+ is reduced into Fe2+ by a
reductase enzyme in the small intestine
– Fe2+ is taken up into intestinal cells by the divalent metal transporter 1 (DMT1)
Heme iron absorption
Released from hemoglobin/myoglobin by proteases in the stomach and small intestine
– Heme (porphyrin ring) is taken up in the small
intestine by heme carrier protein 1 (HCP1)
– Inside intestinal cells, the heme porphyrin ring is broken down by heme oxygenase, releasing Fe2+
and protoporphyrin
Fate of iron in intestinal cell
in non-heme iron…
Fe2+ is taken up into intestinal cells by the divalent metal transporter 1 (DMT1)
In heme iron…
Heme (porphyrin ring) is taken up in the small
intestine by heme carrier protein 1 (HCP1
Inside intestinal cells, the heme porphyrin ring is broken down by heme oxygenase, releasing Fe2+
and protoporphyrin
In both cases -
Fe2+ is either used in intestinal cells (i.e., functional), stored in intestinal cells (as ferritin) or transported into
blood (via ferroportin)
Ferroportin
HCP1
Heme oxygenase
Chelators
Chelators are small organic compounds that form a complex with a metal ion
– This can affect iron absorption
– If the iron-chelate is soluble, then absorption is enhanced
– If the iron-chelate is insoluble, then absorption is inhibited
Factors influencing iron absorption
Chelators are small organic compounds that form a
complex with a metal ion
– This can affect iron absorption
– If the iron-chelate is soluble, then absorption is enhanced
– If the iron-chelate is insoluble, then absorption is inhibited
Iron absorption enhancers
Examples of enhancers: Vitamin C, and some evidence for specific soluble fibres (pectin).
– Vitamin C acts as a reducing agent that helps convert Fe3+ into Fe2+ (which is more readily absorbed)
Example of inhibitor of iron absorption
Examples of inhibitors: Polyphenols (in tea and coffee), oxalic acid (in spinach), and insoluble fibres.
– Coffee just after a meal reduces iron absorption by ~50%.
– Oxalic acid binds with iron (and other metals too), preventing its
absorption
Iron transportation in body
ron is transported in the blood in the ferric
(Fe3+) form bound to transferrin, which is a
carrier protein.
It’s important that iron is bound by a carrier (instead of being free)?
– Unbound ferrous (Fe2+) iron has a high redox activity and can readily lose an electron,
increasing free radical production (e.g., H2O2 to OH●)
Major storage sites for iron:
liver, bone
marrow, and spleen
Iron metabolism steps
1) In intestinal lumen, iron is reduced from Fe3+ to Fe2+ by reductase enzyme
Fe2+ is then absorbed by enterocyte and into blood
2) In blood, Fe2+ is oxidized by ceruloplasmin to attach Fe3+ to transferrin
transferrin-Fe3+ can now bind transferrin receptor on target tissue and it gets taken in to cell via receptor mediated endocytosis
3) in target tissue cell, Fe3+ is reduced by NADH, FADH2 or vit C
now Fe2+ can go:
to iron sulfur centres
iron metalloenxymes
heme to make cytochromes, hemoglobin or myoglobin
OR
transferrin-Fe3+ can pass Fe3+ directly on to ferritin-Fe3+ for storage!! (key storage of iron - readily available) and ferritin-Fe3+ can create a complex w other ferritins and denatured proteins (less accessible/available)
Iron status in the blood
Iron status in
blood
High iron = when
>40% of transferrin
is bound with iron.
- Iron remains in
intestinal cells as
ferritin
Low iron = when
<15% of transferrin
is bound with iron.
- Iron transported by
ferroportin, where it
is picked up by
transferrin
transferrin
the thing that transfers iron in the blood
Transferrin actually has two binding sites for 2 Fe3+ = diferric transferrin
via a reaction w ceruloplasmin, transferrin can bind Fe3+
WHat is Hepcidin
a protein that prevents iron absorption by inhibiting ferroportin transporter on basolateral side of enterocyte, preventing iron from entering blood
Liver senses diferric transferrin levels
* High levels cause production and
secretion of hepcidin, which inhibits
ferroportin
does not let iron be absorbed if transferrin levels over 40%
Why is it so important to control iron absorption
Important to control
absorption because
iron is not excreted
from the body.
Iron’s role in hematopoiesis
Haematopoiesis is the formation of blood cellular components (e.g.,
iron needed for red blood cells)
* Iron is critical due to its presence in heme, which enables oxygen
transport to tissues (hemoglobin), oxygen storage within a tissue
(myoglobin), and transport of electrons through ETC (cytochromes)
BASICALLY… in plasma, iron is transported as Fe3+ bound to transferrin, can go to:
BONE marrow - where it is reduced Fe2+ and used to make heme and be incorporated into red blood cells
In LIVER - where it can be stored as ferritin-Fe3+ –> accessible
or hemosiderin-Fe3+ (less acceble)
RBCs circulate and eventually are degraded in spleen by reticuloendothelial cells - releasing hemoglobin and subsequently iron back to blood
heme synthesis
Porphyrin synthesis starts with glycine & succinyl-CoA (from pantothenic acid)
and requires vitamin B6
(which is why a B6 deficiency can lead to anemia)
Ferrous iron (Fe2+) is then added to porphyrin to produce heme
which can now be used to make hemoglobin, cytochromes or myoglobin
Examples of heme-dependent enzymes
(1) Catalase (an enzyme in our oxidant defense system)
Converts H2O2 into H2O
(2) Thyroid peroxidase
Addition of iodides to thyroglobulin protein
Populations succeptible to iron deficiency:
Worldwide problem (~30%), esp. premenopausal women, infants, adolescents
Deficiencies most often seen in four groups:
1) Infants and young children
2) Adolescents in their early growth spurts
3) Females during childbearing years
4) Pregnant women
Symptoms of iron deficiency
General symptoms: fatigue, pallor, weakness, hair loss, irritability, brittle or grooved nails, impaired immune function
Short term consequences of iron deficiency in fetal development/early childhood
Lower test scores on mental development
* Lower test scores on motor development
* Some improvements in test scores after
treatment with iron.
– In other words, some of the outcomes of iron deficiency may be permanent
Iron toxicity
Toxicity (eventually causes liver failure)
* Hemochromatosis (increased iron absorption)
* Hemosiderosis (iron deposition in tissues)
* Can be treated with iron chelators and blood-letting
Microcytic anemia
Copper sources
Content varies widely in foods, and is sensitive to how food is produced and
handled
* Good sources of copper include oysters & shellfish, whole grains, beans,
nuts, potatoes, organ meats, and dark leafy greens
Enzymes that copper functions with
Copper functions as a component of many important enzymes, including:
- Ceruloplasmin (necessary for iron metabolism)
- Cytochrome c oxidase (key enzyme in the electron transport chain)
- Superoxide dismutase (necessary for oxidant defense system)
- Dopamine Monoxygenase (key for norephinephrine production)
- Tyrosinase (involved in melanin formation, i.e., pigments)
Iron role
Acts as an enzyme cofactor in many redox reactions
Copper absorption and transport
Cu2+ bound to amino acids
– HCl and pepsin release
Cu2+
- Cu2+ must be reduced to
Cu1+ to be absorbed by enterocyte
– Most is absorbed in the small
intestine
* Specific transporters exist
Enhancer Chelators
– E.g., Vit C
Inhibitor Chelators
– High levels of metal ions in
the diet promote the
production of
metallothionein, which
binds all metal ions (e.g.,
copper) and prevents their
absorption
- Absorbed copper is
transported to the liver
bound to protein (albumin), where it
gets incorporated into
ceruloplasmin
ceruloplasmin
is a protein produced by the liver that transports Cu+ around tissues and helps w the metabolism of iron
Copper deficiency
General symptoms? Anaemia, hypopigmentation
of skin, bone abnormalities, thrombosis, etc.
Can be treated with copper supplements
some people got menke’s disease/kinky hair disease but that’s cause they born w genetic mutation that causes body to not be able to absorb enough copper
Menke’s Disease
Menke’s Disease (aka Menke’s Kinky Hair Syndrome)
X linked genetic disease (i.e., genetic mutation in the ATP7A gene on X chromosome)
Inborn error of metabolism in which the body cannot absorb enough copper
Low serum copper and ceruloplasmin
Copper Excess
WILSON’S DISEASE!!
Symptoms?
Drooling
Slurred speech
Problems swallowing
Problems walking
Cognitive impairment
Treatments?
Low copper diet
Chelation therapy
Zinc therapy
Causes
Wilson disease is caused by a buildup of
copper in the body. Normally, excess
copper is excreted in bile. But people who
have Wilson disease cannot do this, due to
a mutation in the ATP7B gene. When the
copper storage capacity of the liver is
exceeded, copper starts to accumulate in
other organs – including the brain, kidneys,
and eyes.
Zinc and sources
ound in all organs, tissues, and body fluids (typically as Zn2+)
* Sources:
– Oysters contain more zinc per serving than any other food, but red meat
and poultry provide most of the zinc in the North American diet. Other
good food sources include beans, nuts, crab, lobster, whole grains,
fortified breakfast cereals, and dairy products
– Found complexed with nucleic acids and protein
Main functions of zinc:
Main functions:
1. Zinc-containing metalloenzymes - more than 200 known at present and
involved in all metabolic pathways
* Provides structural integrity (stabilizes tertiary structure of a protein,) and/or
* Plays a role at reaction site of an enzyme
2. Oxidant defense system
3. DNA binding (formation of zinc fingers)
examples of zinc-dependent enzymes
Examples of zinc-dependent enzymes:
carboxypeptidase (protein digestion),
superoxide dismutase (oxidant defense),
polyglutamate hydrolase (folate digestion),
nucleic acid synthesis (DNA and RNA polymerase)
Digestion and absorption of zinc
Absorption:
– Like iron and copper, zinc must be released from amino acids before absorption
* Done by HCl in the stomach & digestive enzymes in the small intestine
– Zinc can be absorbed via two mechanisms: 1) carrier-mediated (ZIP4; primary
mechanism) and
2) simple diffusion
– Absorption decreases when a person’s zinc status is high (feedback regulation
reduces ZIP4 levels in the intestine)
CHelators that affect zinc absorption
Enhancer chelators
* Organic acids, Prostaglandins
Inhibitor chelators
* Antacids, phytic acid, oxalic acid
* Metallothionein induced by high dietary zinc in intestinal cells (see Copper and
Absorption Transport slide) is also induced by high dietary copper, so a high copper
diet can cause a zinc deficiency
In enterocytes, zinc can be:
- Used locally
– Sequestered by binding with metallothionein (and then eventually lost)
– Secreted into circulation and transported bound to albumin (first destination is
the liver, then other tissues)
Zinc fingers and DNA binding
Major role regulating gene
expression
* Zinc fingers
– Describes the shape of regions
of transcription factor proteins
that interact with DNA
– Interactions between zinc and
side chains of histidine &
cysteine
– Enables basic amino acids to
interact with DNA
Zinc RDA
(higher RDA in men to support testosterone function)
(highest RDA in pregnant women to support fetal development)
Zinc deficiency
Common in humans (especially older adults, vegetarians, & children)
- Poor absorption can be caused by phytic acid in grains
- In children, zinc deficiency can slow growth (inadequate cell division),
poor wound healing, delayed sexual maturation (deficient testosterone
synthesis), and impaired taste (insufficient taste proteins).
Zinc toxicity
Zinc toxicity leads to neurological problems, numbness, metallic taste, nausea, etc.
– Causes a copper deficiency due to the activation of metallothionein
Protein digestion in body general
Mouth
– No enzymatic digestion
– Mechanical breakdown
- Stomach
– HCl in gastric juice
– Pepsin (endopeptidase) - Pancreas
– Pancreatic juice containing
zymogens (inactive
digestive proenzymes) - Small Intestine
– Zymogens are activated
– Enzymes break-down
peptides
– Absorption of AAs
Protein digestion in the stomach
Stomach produces “gastric juice”
– HCl is secreted from parietal cells; its release is triggered by gastrin, acetylcholine, and histamine
– Pepsin is secreted as pepsinogen, which is an inactive zymogen
HCl
Secreted from parietal cells
its release is triggered
by gastrin, acetylcholine, and histamine
HCl has two functions:
1. Denatures proteins - disrupts H-bonds and electrostatic bonds
2. Activates pepsin
Pepsin
is secreted as pepsinogen, which is an inactive zymogen
Active in an acidic pH, inactive at a neutral pH
– HCl causes a conformational change in pepsinogen, allowing it to then autoactivate itself
– Pepsin is an endopeptidase, i.e., in other words, it cleaves peptide bonds within a polypeptide chain
– Generates mostly oligopeptides and some free AAs
Protein Digestion in Small Intestine
Pancreas secretes zymogens and proenzymes: trypsonogen, chymotrypsinogen, proelastase, procarboxypeptidase A & B
these are secreted into the small intestine and
trypsinogen is activated by enteropeptidase (located in brush border) to trypsin which activates all the other zymogens
theres also aminopeptidase (an ezokinase) and yeah the polypeptides are broken into free aas and small peptides
Which enzymes break basic, neutral aliphatic and large neutral amino acids
ENDOPEPTIDASES
break basic - trypsin
neutral aliphatic - elastase
large neutral amino acids - pepsin, parapepsin, chymotrypsin
Which enzymes break the amino and carboxy tails off a polypeptide
EXOPEPTIDASES
amino-peptidases break amino tail off
carboxy-peptidase - break carboxy tail off
AA absorption
Most AAs are absorbed in upper small intestine
- Two ways AAs are absorbed
– Facilitated diffusion
(doesn’t require Na+ or ATP, will not concentrate against gradient)
– Active transport (>60% of AAs are absorbed this way)
* Sodium-dependent transporters (indirect ATP requirement) - cause they go through PEPT1 = peptide transporter 1 which is on intestinal cell membrane like uses an Na+ gradient which is why Na+ has to be actively transported back out
- requires ATP
- concentrate against gradient
What does evidence suggest about amino acid absorption
Essential AAs may be absorbed faster than non-essential AAs
– Competition for absorption exists between AAs
– Free AAs have no absorptive advantage (i.e., protein supplements) over AAs in foods
AAs used in small intestine
AAs are either transported out of the intestinal cell or used directly within the enterocyte for:
1. Synthesis of new protein
2. Energy
*Estimates indicate 30-40% of essential AAs are used in the small intestine
What is glutamine used in intestinal enterocytes for?
- Generate energy for the cell
- Stimulate cell proliferation (to replace shed enterocytes)
- Increase synthesis of heat shock proteins (chaperones)
- Drive mucus production, which helps to prevent bacterial translocation
Lever role in AA metabolism
The liver is very effective at taking
up AAs from circulation. they travel from the small intestine to the liver through the portal vein
Liver uses ~20% of the AAs to:
- Make new proteins/enzymes
- Albumin and other transport proteins
- Make peptide hormones
Liver catabolizes the remaining
80% of AAs, where:
- NH3 sent to the urea cycle
- Carbon skeleton sent to Kreb’s
cycle (for energy) or used for
gluconeogenesis or lipogenesis
BCAAs are not taken up by liver,
and instead are anabolic signals
for tissues like muscle
BCAAs are not taken up by the liver, and instead are anabolic signals for tissues like muscle
4 aspects of protein quality
AA composition
Digestibility
Presence of toxic factors
Species consuming the protein
AA composition
is a measure of quality for the protein
Any protein that provides all essential AA is considered “high quality”.
Animal protein > plant protein. For example, grains are limiting in lysine, legumes are limiting in sulfur-containing AA (methionine).
Digestibility
a measure of protein quality
Some proteins are more digestible than others. More digestible, means
higher quality. Animal protein > plant protein. Some materials, like hair, have a great amino acid balance but are indigestible.
Presence of toxic factors in protein
measure of quality
think soy
Less toxic factors means higher quality. Animal protein > plant protein. Plants contain thousands of phytochemicals. For example, soybeans contain inhibitors that interfere with trypsin, thus preventing protein digestion.
Species consuming the protein
Is one of the aspects of protein quality
Humans, pigs and chickens have similar protein needs. Ruminants have bacteria in the rumen that can make all AAs, so none are considered essential (remember that ruminants can use low quality protein sources).
PER
way of assessing protein quality
Official method in Canada for the evaluation of protein quality
- With this method, young rats are fed a diet for 4 weeks. The diet has all nutrients present at adequate levels except for protein, which is included at 10% kcal of the diet
- 10% protein is lower limit for health. If there is anything wrong with the protein source, the growth of rats will be impaired
- Rats are weighed at the beginning and end of the 4 weeks.
Food consumption is carefully monitored
PER = gain in body mass (g) / total protein intake (g)
ex. Rat gains 10 g and eats 14g
PER = 10g/14g=0.71
At 10% “perfect protein” you get 2g of rat growth per gram intake
***(whole egg is optimal to get a value of 2.0
Pros and cons of PER
Protein Efficiency Ratio (PER)
PROs
- Simple
*Cheap
*Very sensitive to
AA balance,
digestibility, toxic
factors
CONS
- Rats are not
humans
*Growth, not
maintenance
You don’t know
WHY a protein is
poor quality
Chemical score
second way of assessing protein quality
the test protein is chemically digested into free amino acids
- these are then quantified by chromatography, and mathematically compared to the composition of whole egg protein (reference)
CS = abundance of first limiting AA in test protein / abundance of same AA in whole egg
x 100
Pros and cons of chemical score
PROS:
*Simple and cheap
*Identifies the limiting
AA in the food
*Used to optimize
feeds by mixing
different sources of
protein
CONS:
*Doesn’t account for
digestibility (e.g. hair)
or toxins
*Assumes whole egg
is an ideal protein
Nitrogen balance (protein quantity)
Nitrogen balance is a measure of N intake (diet) and N loss (urine, feces,
sweat)
Nitrogen Balance (NB) = Nitrogen Intake – Nitrogen Loss
- During growth, pregnancy, and times of tissue repair (NB > 0)
- When you don’t have enough protein (NB < 0)
– The problem is exacerbated with poor protein quality because body proteins
are used as a source of essential AA (in other words, body proteins are broken
down to “free up” essential AA, ultimately leading to a loss of function)
– NB < 0 is seen in people with serious tissue injuries, wasting diseases like
sarcopenia, and long-term fasting
- For most adults, NB = 0 (people are generally in balance)
- Problems with poor protein quality may be overcome with high protein
quantity (commonly observed in developed countries like USA)
When are protein requirements the highest?
Higher protein
requirements in:
*Infancy, childhood,
and in teenagers
*During pregnancy and
lactation
NOTE: Recommendations for
protein requirements based on
ANIMAL sources of protein.
Plant sources may be less
digestible due to differences in the
nature of protein and other
components (fibre). If these
recommendations used plant
sources of protein, the values
would be higher
Abnormal protein intake overview
What happens when you consume too much or too little protein?
1)Excessive Intake
- High protein diets (Atkins, South Beach)
- Protein Supplementation
2)Deficient Intake
- Deficient in both protein quantity and energy / overall malnutrition
- Deficient in only protein quantity
High Protein Diets
High protein diets are very popular.
* Typically, high protein diets are low in carbs.
* 3 common high protein diets:
– Atkins Diet (most criticized) (C:F:P = 3:64:33)
- South Beach diet
- The Zone Diet
Atkins Diet
Atkins Diet (most criticized) (C:F:P = 3:64:33)
- a high protein diet
* Different phases where macronutrient content varies.
* CHO intake very low, while fat and protein intake very high (~ 30% protein).
* Criticized because no attention to type of CHO or fat consumed
South beach diet
South Beach diet (C:F:P = 30:40:30)
a high protein diet
* Different phases where macronutrient content varies.
* For CHO intake there is an emphasis on low GI foods.
* Protein is consistent throughout the various phases (~30%)
The Zone Diet
The Zone Diet
* Not really a high protein diet, rather a balanced diet.
Evaluation / clinical results of high protein diet
Short term weight loss is comparable to other diet approaches.
– Some studies show improved insulin sensitivity with high protein as
compared to high CHO diets (probably due to reduced burden on the pancreas to generate insulin to deal with CHO).
– Conflicting results with respect to effects on cardiovascular health. A moderate increase in protein appears to be cardioprotective, but high protein may be a concern in the long-term.
– People with kidney diseases should avoid high-protein diets
Protein supplements
Does weight lifting change the RDA for protein (0.8g protein / kg body weight / day)?
- Protein supplements are widely used by
athletes. - Supplements help to ensure that the correct
balance of AAs are delivered to the muscle. - HOWEVER, this would be the same if a person ate a high quality protein (eggs, meat, fish).
- Most protein supplements deliver high levels of BCAAs. BCAAs are rapidly absorbed and delivered to the muscle.
- Protein supplements and aging
– Anabolic response of the muscle to a
protein meal gradually diminishes after 40
years of age
– Can be improved with protein supplements
Marasmus
overall malnutrition basically
Protein and Energy Deficiency
* Marasmus (Protein/Calorie malnutrition).
– Very low intake of a balanced diet with
around 8-10% protein (so just a bit below
what is needed).
- Because everything is in balance, the body
switches to starvation mode.
– Well organized utilization of body fuel
stores allows survival, eventually leading
to a complete loss of body fat which
causes a wrinkled appearance to the skin
- Adults cope with Protein/Calorie malnutrition better than children.
- Characterized by a complete loss of body fat
and muscle, peeling skin, uneven pigmentation
Kwashiorkor
Protein deficiency
* Pronounced “kwash – ee – or –kor”
* Diet has sufficient Calories, but is deficient in protein
* Only 1-2% protein in the diet
* Typically seen in developing countries where
agriculture is key to the diet
* High CHO foods (e.g., tuber cassava)
– When a child is weaned from mother’s breast milk (very
balanced source of nutrients) to cassava porridge (no
protein or fat), problems can emerge
– Lots of CHO, but no protein to metabolize or transport
nutrients
* Patients are characterized by a enlarged abdomen,
‘burns’ on the skin and diarrhea
Why the enlarged abdomen for pepes with Kwashiorkor
Decreased plasma proteins causes an osmotic imbalance in the gut (edema), leading to a swelling of the gut
– Liver is enlarged due to the inability to export fat from the liver (can’t make VLDL)
Reactions of protein catabolism
See slide 21 of proteins part 2
Fate of NH4+ from AA catabolism DIFFERENCES IN FED VS. FASTED STATES
NH4+ is toxic and needs to be transported safely between organs
3 differences between fed and fasted states:
- FASTED state involves the formation of both glutamine and alanine, while the FED state is mostly glutamine.
- FED state involves both the liver and kidneys. (while FASTED only kidneys)
- FED state mainly involves the excretion of the amino group as urea, whereas the FASTED state mainly involves the excretion of the amino group as ammonium (NH4+) directly.
Why is there a difference btw fed and fasted state?
Catabolizing an α-ketoacid leads to the production of bicarbonate (HCO3-)
Bicarbonate is a weak base that reacts with a H+ (if this happens, no change in pH)
However, the fed state promotes a mild alkalosis!
High dietary protein intake increases amino acid catabolism, which can increase HCO3-
(because H+ is used up).
This can increase pH a bit (to ~7.8)
α-ketoacids enter the Kreb’s cycle in all cells, resulting in CO2 production;
however, @ physiological pH the CO2 is actually HCO3-
So why don’t we die when we eat a high protein diet (i.e., the fed state)?
- The liver converts the amino group to urea in a process that consumes HCO3-.
- Catabolism of sulfur-containing AA produces a bit of sulphuric acid to neutralize pH.
Why the difference between fed and fast?
With a long-term fast / starvation, minor amounts of protein are catabolized to release glucogenic amino acids (for gluconeogenesis)
- However, remember that the primary source of energy is TAG from adipose tissue
- The breakdown of TAG leads to the production of acidic ketone bodies.
So, long-term fasting promotes a mild acidosis (pH can drop to ~7.0)!
- This is also known as nutritional ketosis
Products of TAG breakdown (long hydrocarbon chains) are not very water
soluble. The liver converts these long hydrocarbons into small soluble ketone bodies (which the brain can use for energy during starvation).
Why don’t we die during a long term fast
long-term fasting promotes a mild acidosis (pH can drop to ~7.0)!
- This is also known as nutritional ketosis
When AAs are catabolized in a fasted state, the amino group is brought directly to the kidney (thus bypassing the urea cycle where HCO3- is used up).
This means that HCO3- produced in the Kreb’s cycle can be used to neutralize the weak acidosis state caused by ketone bodies.
Important AAs in nitrogen metabolism
Glutamate
– Incredibly important in AA catabolism
– It is a common end product of transamination reactions
* α-ketoacid for glutamate is alpha-ketoglutarate
Aspartate
– Donates an amino group in the urea cycle
* α-ketoacid for aspartate is oxaloacetate
Alanine
– Inter-organ nitrogen carrier
* Muscle to the liver
* α-ketoacid for alanine is pyruvate
Glutamine
– Most abundant AA in the body
– Inter-organ nitrogen carrier (goes to liver & kidney)
– Can donate an amino group to other reactions
4 reactions move nitrogen from catabolized
protein between organs for excretion
1) transamination
2) oxidative deamination
3) a) glutamine production
b) glutamate regeneration
4) Urea cycle
Transamination
reactions move nitrogen from catabolized
protein between organs for excretion
TRANSAMINATION: transfer of an amino group to an AA carbon skeleton
(i.e., α-ketoacid) –> catalyzed by “aminotransferases”
–> most AA undergo transamination (except lysine, proline, and threonine)
DRAW GENERAL TRANSAMINATION REACTION from slide 26 on proteins part 4
TRANSAMINATION, cont.
* Bi-directional reactions
* Active in all tissues
* Always produces an AA (usually glutamate) and α-ketoacid
* At least 1 transaminase exists for each AA, with each using
glutamate/alpha-ketoglutarate as one of the pairings
most abundant aminotransferases in the liver
Most abundant aminotransferases in the liver:
* Glutamate pyruvate transaminase (GPT) (also known as ALT)
* Glutamate oxaloacetate transaminase (GOT) (also known as AST)
GPT converts
a-ketoglutarate and alanine to
glutamate and pyruvate
GOT converts
alpha ketoglutarate and aspartate to
glutamate and oxaloacetate
Oxidative deamination
- glutamate is the main AA to undergo oxidative deamination since it is the main product of transamination
Amino group is released from the glutamate backbone
Reaction favours the formation of alpha-ketoglutarate
Process is very active in all tissues in the body
REACTION
glutamate gets converted to alpha ketoglutarate + NH4+
via glutamate dehydrogenase enzyme and reduction of NAD+ to NADH
How NH4+ is handled depending on the tissue
In extrahepatic tissue (EHT), the
NH4+ is used in the synthesis of
glutamine
In the liver, NH4+ is used for urea
synthesis
In kidneys, NH4+ is excreted directly
as is into urine
Glutamine prduction
Formation of glutamine (primary inter-organ nitrogen carrier)
* Muscle produces ~90% of the glutamine found in the body
SO MOSTLY HAPPENING IN MUSCLE
Glutamate is converted to glutamine via glutamine synthetase
NH4+ group is added
ATP –> ADP + Pi
H2O is removed
Glutamine is the most abundant aa
In the fed state, primarily travels
to liver
In the fasted state, primarily travels
to kidney
Glutamate regeneration
Opposite reaction to last slide; however, a different enzyme is required
SO MOSTLY HAPPENING IN LIVER AND KIDNEY after glutamine has inter-organ transferred
- Releases amino group from the glutamine side chain (i.e., deamination)
Glutamine is converted back to glutamate via the addition of H2O and excretion of NH4+ and the enzyme GLUTAMINASE
- Active in liver in the fed state (amino group used for urea synthesis)
- Active in the kidney during fasting (amino group secreted as NH4+
Urea cycle
Toxic NH4+ is converted to less toxic urea in the liver
- Urea transported to kidney for excretion
NH4+ and HCO3- are added to carbarnyl phosphate via ATP hydrolysis reaction
producing citrulline
CItruline condenses with aspartate and donates amino group to produce arginine
Fed state: 80-90% of urinary N will be in form of urea
NH4+ from oxidative deamination and glutamate regeneration from glutamine
Aspartate condenses with citrulline and donates an amino group
Urea cycle uses HCO3-, thereby preventing
alkalosis
Urea cycle requires energy (ATP)
Defects in any of the enzymes in the urea cycle leads to developmental neurotoxicity
due to a build-up of NH4+ in the body.
Cahill cycle
aka the Glucose-Alanine Cycle
SPECIFIC TO MUSCLE
glucose is converted to pyruvate (releasing 2 atp)
pyruvate is converted to alanine e via GPT transamination
= pyruvate + glutamate –>
alanine + a-ketoglutarate
alanine then acts as an interorgan transporter and can go through blood to LIVER
to undergo another transamination –> being reconverted to pyruvate (and glutamate so NH4+ is excreted via oxidative deamination)
pyruvate that was produced can now be converted to glucose (via gluconeogenesis - consuming 6ATP) and released by liver to muscle where it can be used as energy or like redo cycle (pyruvate to alanine, alanine to liver, alanine to pyruvate)
WHat happens to α-ketoacids
When the body’s energy sources are low, protein is broken down.
AA catabolism releases
(1) Amino group (NH4+), and
(2) α-ketoacid.
α-ketoacids can be formed by:
Deamination – removal of amino group from AA. The carbon skeleton that remains is the α-ketoacid (mostly seen with glutamate).
Transamination – transfer of an amino group from an AA to an α-ketoacid. In the process, the “donating” AA becomes an α-ketoacid and the “receiving” α-ketoacid becomes an AA.
α-ketoacids contain KETONE and CARBOXYLIC ACID functional groups.
Ketogenic –> a degraded AA that can be converted into Acetyl CoA
Glucogenic –> a degraded AA that can be converted into Glucose
