Metabolism - Exam #2, Part 2 Flashcards
1
Q
What makes up Pantothenic Acid?
A
Beta-alanine and pantoic acid linked by a PEPTIDE BOND/AMIDE LINKAGE
2
Q
What is the history of Pantothenic Acid?
A
- Originally B5;
- R.J. Williams isolated in 1931; structure in 1939; also C.V. Elvehjem and T.H. Jukes;
- Williams named “everywhere” as “pantos” in Greek → the vitamin is found widely in foods → Deficiencies are unlikely;
- F. Lipmann won the Nobel prize in 1957 for his discoveries that coenzyme A facilitated biological acetylation reactions
3
Q
What are the supplement forms of Pantothenic Acid?
A
Calcium pantothenate or as pantethenol (alcohol form)
4
Q
What is Coenzyme A?
A
-Coenzyme, notable for its role in the synthesis and oxidation of fatty acids, and the oxidation of pyruvate in the citric acid cycle;
-Thio that can form thirsters with carboxylic acids and act as an acyl carrier;
CoA biosynthesis requires cysteamine, pantothenate, and adenosine triphosphate
5
Q
What are the structural components of Coenzyme A?
A
-Beta-Alanine + Pantoic Acid = Pantothenic Acid;
…
6
Q
hat are the FOOD sources of Pantothenic Acid?
A
- ALMOST ALL plant and animal foods;
- VERY GOOD sources are meats, egg yolk, yogurt, legumes, whole-grain cereals, potatoes, mushrooms, broccoli and avocados;
- FREE or BOUND in food
7
Q
How is majority of Pantothenic Acid found in food?
A
-85% as coenzyme A (CoA) → CoA DEGRADED to pantothenic acid by phosphatases and pyrophosphatases
8
Q
How is Pantothenic Acid ABSORBED?
A
-Mainly in JEJUNUM by a sodium-dependent active multivitamin transporter (SMVT) with typical ingestion;
-50% of pantothenic acid is absorbed with NORMAL intakes
-HIGH dietary doses there is PASSIVE absorption;
(absorption DROPS when higher amounts are ingested – don’t need the excess)
9
Q
Why is the transporter called “multivitamin”?
A
- The transporter is SHARED with biotin and lipoic acid;
- Carrier competition!
10
Q
What is the form of Pantothenic Acid in the blood?
A
- FREE in BLOOD PLASMA;
- Higher amounts are in the RED BLOOD CELLS
11
Q
How is Pantothenic Acid taken into TISSUES?
A
The uptake into TISSUES occurs by SMVT (sodium-dependent multivitamin transporter)
12
Q
What happens to Pantothenic Acid once it enters the cells?
A
- Found within CELLS and leads to CoA;
- Highest concentrations in liver, adrenal glands, kidney, brain, and heart = very metabolically tissues
13
Q
What compounds inhibit the synthesis of CoA from Pantothenic Acid?
A
-Acetyl CoA;
-Malonyl CoA;
-Propionyl CoA;
and other acyl chain CoAs
(Products of rxn preventing the initial conversion b/c it is not needed)
14
Q
What is the main function of CoA?
A
- Functions as carriers of acetyl and acyl groups;
- •4’-phosphopantetheine is the prosthetic group responsible for its ability to act as acyl carrier protein
15
Q
How does 4’-phosphopantetheine and CoA function as acyl carriers?
A
- Forming THIO ESTERS (substitute S for an O) with carboxylic acids;
- Thioester – compounds with the functional group C-S-CO-C; Product of esterification between a carboxylic acid and a thiol → the best-known derivative being acetyl-CoA.
16
Q
How is CoA used in metabolism of CHO, Fat, and PRO?
A
-CoA as acetyl CoA holds the central position in the transformation of energy;
→ Key step to the entrance into the TCA cycle and glycolysis metabolism and NO MORE carb synthesis
17
Q
Pantothenic acid, thiamin, riboflavin, and niacin work together in what reactions of the TCA cycle?
A
-Oxidative decarboxylation of pyruvate;
And
-Oxidative decarboxylation of alpha-ketoglutarate to succinic acid
18
Q
What SYNTHESIS reaction utilize CoA?
A
- Synthesis of cholesterol, bile acids, ketone bodies, fatty acids, and steroid hormones (used for fatty acid oxidation);
- Phospholipid and sphingomyelin require CoA for synthesis from phosphatidic acid and sphingosine
19
Q
What is the function of CoA relating to protein modification?
A
- Posttranslational acetylation or acylation of PROTEINS (also some with sugars and drugs);
- EX: Acetylation of histone proteins;
- Can activate or inactivate;
- Acetylation – adds and acetyl functional group = CH3CO
20
Q
How is Pantothenic Acid EXCRETED?
A
- Excreted in the URINE, small amounts in feces → NO METABOLITES in urine and feces have ever been identified;
- Excretion ranges generally from between 1 to 8 mg/day
21
Q
What is the AI for Pantothenic Acid?
A
- AI = 5mg/day for all adults
- This amount replaces losses in urinary excretion;
- Excretion of less than 1 mg/day considered poor status;
- No nationally recored of average intake
22
Q
What is “Burning Feet Syndrome”?
A
- Numbness of toes and burning of the feet; warmth makes worse and cold improves; other symptoms are vomiting, fatigue, weakness, restlessness, and irritability;
- Any diseases that affect absorption put a person at risk of deficiency, usually deficiency occurs with multiple nutrients
23
Q
What is the UL for Pantothenic Acid?
A
- NO UL → no reports of adverse effects for oral intake of pantothenic acid;
- Intakes up to 10 g/day for 6 weeks caused no problems, higher doses have been associated with intestinal distress including diarrhea
24
Q
How was Folate and B12 discovered?
A
- By Mitchell et al in 1941;
- Resulted from the need to find a cure for macrocytic or megablastic anemia that was a problem in the late 1870s and 1880s → eating LIVER cured this condition as well as other vitamin deficiency disorders
25
What is the Folic Acid form?
- SYNTHETIC OXIDE form (refers to REMOVAL of all but one glutamic acid residue) of the vitamin in fortified foods and supplements and is RARE IN NATURE;
- Can only have ONE glutamic acid to make it Folic Acid
26
What is the Folate form?
- Folate is the reduced form (means contains 2-10 glutamic acid residues) that is found NATURALLY in food;
- Folate = generic name for this vitamin → folate and folic acid are NOT exactly interchangeable (Folate is reduced; Folic Acid is oxidized);
- Latin folium means “leaf” and folate from Italian means “foliage"
27
What is the function of the Folate COENZYMES?
-Folate COENZYMES function in the acceptance and transfer of 1-carbon units in the synthesis, interconversion and modification of nucleotides, amino acids, and other key cellular components
28
What makes Folate a vitamin?
- the ENZYME for coupling the pteridine (pterin) to the para-aminobenzoic acid (PABA) to form the pteroic acid component of folate is NOT present in human bodies;
- LACK of synthetic ENZYME to make folate in the body makes it essential
29
What is included in the structure of Folate?
- Folate is made up of PTERIDINE, which is conjugated by a methylene group (—CH2—) to PABA, forming Pteroic Acid;
- The carboxy group (—CO—) of PABA is peptide bound to the amino group (—NH—) of glutamic acid to form the monoglutamate form of folate=
- Pteridine + PABA + Glutamic Acid = Pteroylmonoglutamic Acid (FOLTATE)
30
What are the THF derivatives of Folate?
- 5-and-10-formly THF (O=CH);
- 5-formimino THF (-Hc=NH-);
- 5,10-methenyl THF (=CH-);
- 5,10-methylene THF (-CH2);
- 5-methyl THF (-CH3)
31
What are the GOOD natural food sources of Folate?
- Mushrooms, green vegetables (spinach, brussels sprouts, broccoli, asparagus, turnip and collard greens, okra), peanuts, legumes (lima, pinto, kidney), lentils, fruits (strawberries and oranges) and their juices, and liver;
- Like other water soluble vitamins can LOSE the vitamins with COOKING as destroyed by heat and oxidation
32
What is the fortification of Folate?
-FORTIFICATION of flours, grains and cereals with folic acid began in 1998 as 140 micrograms folic acid per 100 g of products!!
-Some juices are also fortified
→ Greatly improved folate nutrition and lessened Neural Tube Defects
33
What are the forms of Folate within Foods?
Food has multiple glutamic acid residues (REDUCED forms)=
- 5-methyl tetrahydrofolate (THF);
- 10-formyl THF;
- but over 150 forms of folate have been reported
34
What is the bioavailability of Folate?
- Folate bioavailability from foods varies from 10 to 98% → 50% is usually assumed for a mixed diet;
- Folic acid (the most oxidized form) as a supplement is ~100% absorbed on an empty stomach, in foods ~85% available
35
What form of Folate is easiest to absorb?
MONOGLUTAMATE form;
| -Two folypoly γ-glutamyl carboxypeptidases (FGCP) that REMOVE glutamates = Enzymes
36
What are the enzymes that allow Folate to be absorbed as Monoglutamate?
- Enzymes in the JEJUNUM;
- Pteroylpolyglutamate hydrolases (hydrolysis) or conjugases (deconjugation);
- One is free and the other in the membrane of mucosal cells (brush border) and latter one is zinc-dependent;
- Found in pancreatic juice and bile
37
What can reduce the absorption of Monoglutamate Folate?
- Chronic alcohol ingestion and conjugase inhibitors in several foods REDUCE absorption;
- Conjugase inhibitors DO NOT affect synthetic folic acid
38
How does forms of folate enter the intestinal CELLS?
-Monoglutamate form of folate and 5-methyl tetrahydrofolate (THF) are transported into intestinal cells in DUODENUM and UPPER JEJUNUM by the proton-coupled folate transporter (PCFT)
39
What occurs when there is a defect of PCFT?
Hereditary folate malabsorption
40
What happens to folate in CELLS before it enters the portal blood?
REDUCED from dihydrofolate (DHF) to THF by cytosolic NADPH-dihydrofolate reductase
41
How does Folate enter the portal blood?
- Folate binding proteins;
- Transported across the cell and the THF is transported across the basolateral membrane into PORTAL blood by carrier-dependent active transport probably by multidrug resistance protein (MRP)
42
What happens to Folate when absorbed by the LIVER?
- Taken in by PCFT also;
- ONLY Monoglutamates can cross cells;
- GLUTAMATES are added to trap folates by folypoly-glutamate synthetase (Polyglutamyl form);
- To leave cells, hydolases remove the glutamates
43
What are the forms of folate found in the LIVER?
THF, 5-methylTHF, and 5- or 10-formyl THF:
- About half body folate is found in liver that supplies actively proliferating cells;
- Less metabolically active tissues return folate monoglutamates back to the liver for redistribution
44
What forms of folate are found in Systemic Plasma?
- THF or
- 5-methylTHF and 10-formylTHF as monoglutamates;
- They are FREE (1/3) in equilibrium or (2/3) BOUND;
- Bound to either albumin, alpha-2 macroglobulin or folate binding protein (a soluble form of the membrane folate receptor);
- Reduced folate carriers may also transport folate in systemic plasma and may be the folate binding protein
45
What folate is found in red blood cells?
- RBCs have MORE of folate forms than plasma;
- All is taken up before they mature in their development;
- B/c mature RBCs cannot take up folate forms;
- Folate in RBCs is a sign long term folate status (2 to 3 months)
46
How does folate enter other tissues besides the liver?
- Transport into CELLS is by PCVT and by folate receptor α, β, or ϒ;
- Inside cells, glutamates are added and make polyglutamyl form by folypolyglutamyl synthetase to trap;
- To leave HYDROLASES similar to those found in intestine convert back to monoglutamyl form;
- Addition of glutamates is ATP-dependent
47
What other purpose does the addition of glutamates to folate in the cell/tissues serve?
- POLYglutamate form is better substrate for metabolic enzymes;
- Found in both the cytoplasm mitochondria for reactions
48
What is the main function of Folate?
Accept and donate ONE CARBON UNIT
49
What is the metabolic role of 10-formyl THF?
Folate transfers formate as 10-formyl THF for PURINE synthesis
50
What is the metabolic role of 5,10-methylene THF?
- Folate transfers formaldehyde as 5,10-methylene for PYRIMIDINE synthesis;
- Folate receives formaldehyde for SERINE degradation/GLYCINE synthesis;
- Folate receives formaldehyde for GLYCINE degradation;
- Folate receives formaldehyde for GLYCINE synthesis
51
What is the metabolic role of 5-methyl THF?
-Folate provides a METHYL group (1 carbon) for METHIONINE synthesis
52
What is the metabolic role of 5-formimino THF?
-Folate receives a formimino group in HISTIDINE degradation
53
What is the "methyl trap" of folate interconversion?
5-methyl THF cannot be converted to 5,10-methylene THF due to Vitamin B12 DEFICIENCY;
- MTHFR is B12-dependent for the remethylation of homocysteine to methionine;
- Leads to MEGALOBLASTIC Anemia and due to impaired folate metabolism
54
What other condition can cause a "methyl trap" beside B12 deficiency?
-Mutation in MTHFR associated with birth defects and hyperhomocysteinemia = eleveated Homocysteine in the blood
55
How can HIGH can high folate levels overcome B12 deficiency and prevent megaloblastic anemia?
--
56
What are the red blood cell characteristics of megaloblastic anemia?
- Immature stage of development;
| - Still have nuclei and are slightly larger than normal red blood cels
57
What causes Microcytic hypochromic anemia?
-Iron deficiency
58
Folate is involved in the metabolism of what AMINO ACIDS?
- Histidine;
- Serine;
- Glycine;
- Methionine
59
What reaction in Histidine catabolism can be used to measure a Folate deficiency?
- Intermediate interconversion of Formiminoglutamate to Glutamate;
- Changes THF to 5-formimino THF;
- Gives a Histidine Load and measures FIGLU in the Urine (will have high concentrations)
60
How is Glycine synthesized from Serine in the body?
- Reversible rxn;
- Enzyme = serine hydroxymethyltransferase catalyses transformation using THF;
- Generates methylene THF and glycine.
61
How is Glycine primarily degraded?
- Glycine Cleavage System;
| - Leads to the degradation of glycine into ammonia and CO2 with the use of NAD+
62
How is Glycine synthesized from Choline degradation?
- Requires folate and goes through Betaine (trimethylglycine);
- Betaine is a NEUTRAL compound due to having both a + and - functional group
63
What other benefit has been show of Betaine?
- Reduction of the levels of homocysteine and folate supplementation appears to increase betaine levels;
- Involved in the alternative conversion of homocysteine to methionine with homocysteine transferase
64
Why is Folate essential for cell division?
- Essential for purine and pyrimidine synthesis;
- Acts as a substrate/reactant in the formation of dTMP (nucleotide monomer for DNA synthesis);
- dTMP is absolutely necessary for DNA synthesis
65
What are the two enzymes that are responsible for the formation of dTMP from 5,10-methylene THF?
1.Thymidylate synthetase = 5,10-methylene THF to DHF
and
2. Dihydrofolate reductase = DHF to THF
66
What is the overall reaction for the generation of dTMP?
5,10-methylenetetrahydrofolate + dUMP ← → dihydrofolate + dTMP
67
What is Methotrexate?
- ANTIFOLATE drug → binds to active site of dihydrofolate reductase and is used as chemotherapeutic drug, to treat rheumatoid arthritis, psoriasis, Crohn’s to prevent synthesis of actively dividing cells;
- Prevents METABOLISM of Folic Acid;
- Side affects mimic that of folate deficiency;
- High intakes or supps with Methotrexate can calm side effects w/o altering drug effects
68
What tissues are affected by Folate deficiency?
- All tissues that turn over regularly;
- May be related to cancer as DNA repair is impaired;
- Will reflect different functions in cytoplasm and mitochondria
69
What is Cobalamin (B12)
Generic term for a group of compounds called CORRINOIDS with a CORRIN nucleus
70
What makes up a Corrin Nucleus?
- Macrocyclic ring (BIG ring) made of FOUR reduced pyrrole rings linked together;
- In the center of the corrin is an atom of COBALT;
- Attached to the cobalt is a group defining each type of Cobalamin
71
What are the various groups attached to Cobalt to make different Cobalamin forms?
- CN = Cyanocobalalmin;
- OH = Hydroxycobalamin;
- H2O = Aquocobalamin;
- NO2 = Nitrotocobalamin;
- 5'deoxyadenosyl = 5'deoxadenosylcobalamin;
- CH3 = Methylcobalamin
72
What are the ACTIVE COENZYME forms Cobalalmin?
-5’-deoxyadenosyl-cobalamin (adenosyl);
OR
-Methylcobalamin;
-Other forms can be converted to coenzyme forms
73
What is Vitamin B12?
-Cyanocobalamin – the most common and widely produced of the chemical compounds that have vitamin activity as vitamin B12;
Vitamer of B12 because body can convert cyanocobalamin to ANY one of the active coenzyme forms of B12
74
When was B12 discovered?
- LAST discovered;
- 1948 isolated by Smith and Riskes et al;
- Structure by Hodgkin;
- 1926, Minot and Murphy discovered that consumption of LIVER could help correct pernicious anemia
75
What is Pernicious Anemia?
- B12 deficiency;
- Body can't make enough healthy red blood cells because it doesn't have enough vitamin B12;
- Due to Lack of intrinsic factor for B12 metabolism;
- Causes Megolobalastic anemia
76
What are the FOOD sources of B12?
- ONLY sources are animal products;
- vegans at risk for vitamin B12 deficiency;
- Only microbes can synthesize;
- Meat and meat products, poultry, fish, shellfish, eggs (yolk) mainly in form of adenosyl- and hydroxocobalamin;
- Plants DO NOT use, would only be from nitrogen-fixing bacteria
77
What are the Supplemental forms of B12?
- Cyanocobalamin;
| - Hydroxocabalamin
78
What is the FIRST step of B12 digestion?
- Ingested cobalamins must be RELEASED from proteins/polypeptides in foods;
- Pepsin and HCl perform this release in the STOMACH denature and release
79
What then binds B12 after the action of HCl?
- R protein (protects vit from acid):
- Found in saliva and gastric juice, binds to the vitamin B12 prior to or after its release from food;
- Complex leaves stomach into the duodenum
80
What happens with B12 digestion in the DUODENUM?
-The R protein is digested
by pancreatic proteases;
-FREE vitamin (all forms) then binds to the intrinsic factor (IF) that was produced by gastric parietal cells, but is not catabolized by proteases
81
How is B12 finally absorbed?
-Receptors in the ILEUM, especially the distal third, (called cubulin, IF factor or cubam) bind the
IF-vitamin B12 complex and take in the whole thing
82
What are the absorption receptors for the IF-B12 complex?
- Ileum receptors = cubulin, IF factor or cubam;
- Amnionless (protein) helps cubulin bind to membrane to bind to IF-B12;
- Megalin (protein) might help transport this complex into the cell
83
What are R proteins?
-Cobalophilins or haptocorrins (HCs)
84
What results from Pancreatic Insufficiency with B12 digestion?
- Prevents release of B12 from R protein;
- Reduced bicarbonate release into duodenum, so acids aren't neutralized;
- oAcidic environment FAVORS BINDING of R protein to B12 rather than IF
85
What is Zollinger-Ellison Syndrome?
- Increased gastric acid secretion;
| - Causes a LOWER pH in the small intestine and this PREVENTS release of the vitamin from R protein
86
Why are older people recommended to consume B12 fortified foods or supplements?
- Cyanocobalamin does NOT decrease with age;
- 10% to 30% of older people have GI changes that limit absorption of food-bound forms of the vitamin (release from food hampered)
87
What limits the release from food with age?
-Decrease in gastric acidity so not released from binding in food;
-Atrophic gastritis treatable with antibiotics or achlorhydria;
or
-Overgrowth of bacteria that compete for uptake of vitamin B12;
-Might be increased autoimmune in aging and destruction of cells that produce IF (intrinsic factor)
88
How is B12 absorbed in to cells?
- IF-mediated transport is saturated at 1.5 to 2.0 micrograms of vitamin B12;
- 1% to 3% is absorbed by diffusion especially with large doses;
- Absorption ranges from 11% to 65%, decreasing as intake increases;
- 50% absorption is generally assumed;
- Enterohepatic circulation (already within the body) is important as this contributes from 3-8 micrograms daily
89
What happens to B12 once in the cells?
Within the mucosal cells, vitamin B12 is RELEASED from IF
90
What are the forms of coenzyme forms of B12?
- 60% to 80% -- methylcobalamin;
| - 20% --adenosylcobalamin
91
What is B12 bound to in the blood?
- Bound in blood to either TCI, TCII or TCIII;
| - When bound = HOLOTRANSCOBALAMINS
92
What are the Holotranscobalamins?
-Form of R proteins:
•TCII is the main one initially;
•Later 80% is bound to TCI (haptocorrin) and may be circulating storage and cells take on by non specific receptors
•TCIII may return the vitamin to the liver
93
How is B12 taken into TISSUES?
- Receptor dependent as ALL tissues have receptors for TCII;
- TCII-cobalamin complex is taken up by receptor-mediated endocytosis;
- Then fuses with lysosomes to DEGRADE TCII and free the vitamin
94
What is the mutation TC766C>G?
- Genetic mutations in TCII exist;
- Arginine to proline and reduces protein’s ability to bind the vitamin;
- 20% of population is homozygous for GG with elevated homocysteine;
- Women appear most vulnerable
95
How much B12 is stored in the body?
- About 2 to 4 mg of the vitamin is STORED mainly (~50%) in LIVER and can be stored for even years;
- More like a fat soluble vitamin
96
What are the 2 enzymatic reactions require the coenzyme forms of B12?
- One requires methylcobalamin and the other adenosylcobalamin;
1. Resynthesis of methionine from homocysteine (with Folate)
2. Oxidation of L-methylmalonyl-CoA.
97
How does B12 deficiency cause Megaloblastic Macrocytic Anemia?
- Like folate deficiency;
- Occurs when vitamin B12 is deficient causes methyl-folate trap;
- Folate stays in the 5-methylTHF form and homocysteine increases;
- Leads to a higher risk for atherosclerosis;
- Can be MASKED by folate supps.
98
What neurological problems come from B12 deficiency?
- 75-90% deficient people;
- Tingling and numbness (paresthesia) in extremities, abnormal gait, loss of concentration, loss of coordination, memory loss, disorientation, swelling of myelinated (insulated) nerve fibers, psychosis and possibly dementia
99
What causes the neurological problems?
- DEMYLINATION of nerves in the central nervous system - nerves coated in MYELIN;
- May be from lack of SAM needed for methylation reactions for maintenance of myelin, or an imbalance in cytokines and growth factors
100
What is Pernicious Anemia?
- B12 deficient;
- Autoimmune disease in which PARIETAL CELLS of the stomach responsible for secreting intrinsic factor are DESTROYED;
- IF is crucial for the normal absorption of B12, so a lack of intrinsic factor, causes a vitamin B12 deficiency;
- Nothing to do with Folate;
- Pernicious refers to the neurological damage
101
How is Folate excreted?
- Intact or metabolized to N-acetyl paraaminobenzoyl glutamate is excreted in the urine;
- Also enterohepatic recirculation of folate;
- Folate of microbial origin can be high in feces (colonic microbes)
102
How is B12 excreted?
- ~0.1% loss/day bound to R protein in bile;
| - Only excreted in urine if injected amount is so much that exceeds binding capacity of blood
103
What is the RDA for Folate?
- Adults – 400 micrograms per day of Dietary Folate Equivalents (DFEs) = fortification/supps and foods;
- Reduce the risk of Neural Tube Defects for women of childbearing age;
- Too much fortification could mask a B12 deficiency, especially in the elderly
104
What are DFEs?
-One DFE is equal to 1 microgram of food folate or 0.6 microgram of folic acid from a supplement or fortified food consumed with a meal or 0.5 micrograms of folic acid from a supplement taken without food
105
What is the RDA for B12?
- Adults – 2.4 micrograms per day;
| - People older than 50 are told to consume most of their requirement from FORTIFIED foods or SUPPS
106
What is the UL/toxicity for Folate?
- Concern of masking B12 deficiency;
| - UL = 1,000 micrograms/day from fortified foods and supps. (not natural)
107
What is the UL/toxicity for B12?
- NO toxicity;
- No benefits of large doses for those with adequate Vitamin status;
- Large doses are effective for people that CANNOT ABSORBD the vitamin by the normal process → 1 to 3% absorbed by passive diffusion especially with pharmacological doses
108
How is Folate status assessed?
3 indicators:
- Erythrocyte folate (<6.8 ng/ml)
- Plasma homocysteine
* Remember low vitamin B12 can affect these measures
109
How is B12 status assessed?
- Based on normal hematological status and serum levels → concentration in serum or plasma reflects BOTH intake and stores;
- Lower limit of adequate is 170-250 pg/ml
- In the short-term serum levels may be maintained at the expense of tissue stores, low serum levels reflects long-term;
- High serum and urine methylmalonic acid is favored, but not enough data at this time
110
Who discovered B6?
- Gyorgy showed role in curing rat acrodynia (skin lesions);
- 1930s distinguished from other B vits;
- Lepkovsky obtained crystalline structure from plants 1938;
- Strucutre by Harris and Folkers, 1939;
- Snell synthesized pyridoxal (PL) and pyridoxamine (PM) as other natural forms of the free vitamin
111
What does B6 deficient Infants develop?
- Epileptiform seizures and dermatitic lesions;
| - Occured in 1950s due to heat treatment of formula and formation of pyridoxine-lysine (not active)
112
What is the functional COENZYME form of B6?
- Pyridoxal 5’-phosphate (PLP);
| - Umbreit discovered in 1944
113
What are the derivatives of B6?
1. Pyridoxine (PN)
2. Pyridoxial (PL)
3. Pyridoxamine (PM)
4. Pyridoxine phosphate (PNP)
5. Pyridoxial phosphate (PLP)
6. Pyridoxamine phosphate (PMP
114
What are the 3 vitamins of B6 found in foods?
* All interchangleable and can make coenzyme;
1. PN/PNP is in PLANTS and can be conjugated as pyridoxine glucoside;
2. PL/PLP and PM/PMP in ANIMAL products
115
What are the best sources of B6?
- EXCELLENT sources are meats, whole-grains, vegetables, some fruits (bananas), and nuts;
- Fortified cereals are a good source;
- Supps = pyridoxine hydrochloride
116
How can B6 be LOST from foods?
- Food matrix affects bioavailability;
- Can be lost with processing;
- Prolonged high heating as in sterilization (sterilization of infant formula) and canning can destroy;
- Lost with milling and refining of grains
117
How is B6 absorbed into MUCOSAL CELLS?
- DEPHOSPHORYLATED form very rapidly;
- Range ~ 61% to 92% or avg. 72%;
- Alkaline phosphatase and other phosphatases at the intestinal brush border hydrolyze the phosphorylated forms of PN, PL, and PM = ALLOW ABSORPTION;
- High doses phosphorylated and pyridoxine glucoside will be absorbed
118
How do CELLS utilize B6?
- PN, PL and PM are mainly NOT METABOLIZED by the intestinal CELLS;
- Released into the portal blood
119
How is are the forms of B6 utilized in the LIVER?
- Absorbed by Passive diffusion;
- Majority is converted to PLP;
- Major form in systemic blood and bound to ALBUMIN;
- Binding proteins in the liver protect PLP from hydrolysis
120
How do do B6 derivatives enter TISSUES?
- RBCs take up the PLP and it is bound to HEMOGLOBIN;
| - Other tissues take up only NON-phosphorylated forms so alkaline phosphatase converts PLP to PL
121
How is PL converted to PLP in CELLS?
-Pyridoxine kinase phosphorylates PL to PLP → this enzyme is found in most tissues; most tissues LACK the PMP and PNP FMN-dependent oxidase;
The oxidase is found mainly in LIVER and INTESTINE and to lesser extent in some other tissues
122
What is the storage of B6?
- 5 to 10% in liver
- 75 to 80% in muscle
- Storage in body ~40-185 mg
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What is the main functions of B6?
1. PLP, the COENZYME form, is associated with >100 enzymes = AMINO ACID metabolism;
2. NON-coenzyme role in GENE EXPRESSION;
3. Glycogen metabolism
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What does PLP form as a coenzyme during amino acid metabolism?
-Schiff base = has a functional group with a C-N double bond with the N attached to an aryl or alkyl, NOT hydrogen
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What effect does the PLP have on the amino acid during metabolism?
- Covalent bonds of an alpha amino acid are made MORE REACTIVE (labile) by its binding to PLP containing enzymes;
- All depends on which enzyme as far as which group is attacked
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What types of reactions take place with the alpha amino acids?
- Transamination;
- Decarboxylation
- Transulfhydration;
- Desulfhydration;
- Dehydration (elimination)/deamination;
- Cleavage;
- Racemization,
- Synthesis
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B6 and Transamination?
Step 1. Transfers from enzyme to amino acid and then back to enzyme;
Step 2. Formation of a new amino acid and reformation of PLP-enzyme
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B6 and Dehydration/Deamination
- Dehydratase enzyme removes water and amino group;
| - Requires PLP
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B6 and Decarboxylation
- Removal of the carboxyl group (COO-) from an amino acid or other compound;
- EX: GABA synthesis from the amino acid glutamate= Glutamate decarboxylase is PLP-dependent;
- EX: Serotonin synthesis (from tryptophan) and degradation = decarboxylase is PLP-dependent;
- EX: Arginine, proline, histidine, and glutamate metabolism.;
- EX: Selenium metabolism
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B6 and Cleavage Reaction
- Removal of a hydroxymethyl group from serine by hydroxymethyl-transferase;
- Hydroxymethyl group of serine is transferred to THF and GLYCINE is formed
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How do bacteria utilize B6?
- Bacteria need PLP for the interconversion of D- and L-amino acids;
- Enzymes = Racemases (isomerase enzymes that alter stereochemistry);
- EX: Serine Racemase – an enzyme which generates D-serine from L-serine.
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What are some other reactions that require PLP?
- First step of synthesis of heme by (delta) δ-aminolevulinic acid synthetase = combines Glycine and Succinyl-CoA;
- Condensation reaction to form sphingolipid;
- Niacin synthesis from tryptophan;
- Carnitine and taurine synthesis
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How does B6 function in Glycogen metabolism?
- PLP-dependent glycogen phosphorylase DEGRADES glycogen by one glucose to produce glucose-1-phosphate;
- PLP in MUSCLE is bound to glycogen phosphorylase
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What is the NON-coenzyme role of B6?
-Bind to DNA and interact with steroid hormone receptors and other transcription factors to modulate their binding to regulatory regions of DNA = Gene Expression
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What is the metabolite of B6 that is excreted?
- 4-pyridoxic acid (PIC);
- Produced by PL acted on by either NAD-dependent aldehyde dehydrogenase or FAD-dependent aldehyde oxidases found in LIVER and KIDNEY;
- Excreted in the URINE and indicates RECENT INTAKE and not stores;
- Ingestion of large doses (100 mg) of PN results in urinary excretion of PN and 5-pyridoxic acid
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What is the RDA for B6?
-Adult men and women ages 19 to 50 = 1.3 mg/day
•Men older than 51 = 1.7 mg/day
•Women older than 51 = 1.5 mg/day
*Based on plasma PLP of at least 20 nmol/L;
**Elderly higher because of possible acceleration of PLP hydrolysis and oxidation of PL to 4-pyridoxic acid → also at risk of deficiency because of low intakes
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What are the symptoms of B6 deficiency?
- RARE;
- Symptoms from lack of enzyme functions → hypochromic, microcytic anemia or hyperhomocysteinemia
- Presence of acetaldehyde from alcohol metabolism enhance the vitamin’s DEGRADATION
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What vitamins INCREASE the dietary need for B6?
- Corticosteroids
- Oral contraceptives
- Penicillamine (autoimmune diseases and Wilson’s disease [copper not excreted into bile]);
- Isoniazid (tuberculosis)
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What is the UL for B6?
- UL = 100 mg to minimize the risk of NEUROPATHY;
- Causes sensory and peripheral neuropathy with symptoms of unsteady gait, paresthesia, impaired tendon reflexes;
- Also may get degeneration of dorsal root ganglia in the spinal cord, loss of myelination, and degeneration of sensory fibers in peripheral nerves → Severe nerve damage with B6 toxicity!!
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What have pharmacological doses of B6 been used to treat?
Hyperhomocysteinemia, carpal tunnel syndrome, premenstrual syndrome, depression, muscular fatigue, and paresthesia (tingling or numbness of the feet and hands)
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How is B6 status assessed?
-Plasma PLP concentrations are thought to be the BEST indicator of TISSUE stores
oDEFICIENCY = Levels 30 nmol/L
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When was Biotin discovered?
- 1931, “egg white injury”, which consisted of hair loss, dermatitis, and neuromuscular problems;
- 1940s structure determined and chemically synthesized;
- once called vitamin H for the German word “haut” means “skin”;
- Has been referred to as vitamin B7
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What is "Egg White Injury"?
- Caused by AVIDIN;
- A protein in egg whites that forms one of the strongest covalent bonds in nature;
- Cooking DENATURES avidin and so cooked egg whites are safe for biotin nutrition;
- Denaturing prevents the Avidin from binding biotin and making unavailable
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What are the food sources of Biotin?
Widely distributed in foods = liver, soybeans, egg yolk, cereals, legumes, and nuts are good sources;
-In many foods biotin is BOUND to lysine in proteins or = Biocytin (AKA: biotinyllysine)
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What makes up the structure of Biotin?
-TWO rings;
-A Ureido ring;
and;
-Thiophene ring connected to a valeric acid side chain;
-Valeric Acid – pentanoic acid, is a straight-chain alkyl carboxylic acid with the chemical formula C5H10O2
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What is "Egg White Injury"?
- Caused by AVIDIN;
- A protein in egg whites that forms one of the strongest covalent bonds in nature;
- Cooking DENATURES avidin and so cooked egg whites are safe for biotin nutrition;
- Denaturing prevents the Avidin from binding biotin and making unavailable
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What are the food sources of Biotin?
Widely distributed in foods = liver, soybeans, egg yolk, cereals, legumes, and nuts are good sources;
-In many foods biotin is BOUND to lysine in proteins or = Biocytin (AKA: biotinyllysine)
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What makes up the structure of Biotin?
-TWO rings;
-A Ureido ring;
and;
-Thiophene ring connected to a valeric acid side chain;
-Valeric Acid – pentanoic acid, is a straight-chain alkyl carboxylic acid with the chemical formula C5H10O2
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What is the hydrolysis of proteins and protein-bound biotin?
proteins broken down into component amino acids
150
How are Biotinyl peptides digested?
-HYDROLYZED by proteases and peptidases
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How is Biocytin further digested?
- HYDROLYZED by biotinidase which is found on the intestinal brush border and secreted in pancreatic and intestinal juices;
- Some biocytin can be absorbed by peptide carriers and HYDROLYZED by biotinidase in plasma and in most body cells
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What is the hydrolysis of proteins and protein-bound biotin?
proteins broken down into component amino acids
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What is BIOTINIDASE?
- Enzyme that hydrolyzes protein-bound biotin and frees the vitamin;
- Present in most body cells;
- At ACIDIC pH it separates biotin from lysine or peptides
- At ALKALINE pH, the enzyme gets biotinylated and generates FREE lysine;
- Enzyme also removes biotin from histone proteins
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What is Biotinylation?
-Covalently attaching biotin to a protein → This is how it binds to AVIDIN to cause “egg white injury”
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What occurs with a Biotinidase (enzyme) efficiency?
- DEFICIENCY discovered in 1983;
- Autosomal recessive inborn error of metabolism;
- Without biotinidase (enzyme) biotin deficiency develops (secondary deficiency);
- Symptoms = seizures, ataxia, skin rash, alpopecia, and acidosis;
* *CAN’T metabolize and free biotin for use
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How is Biotin absorbed into the mucosal cells?
- FREE biotin is absorbed in the JEJUNUM, but some is absorbed in the ileum → Greater number of carriers in the jejunum;
- Physiological (normal) doses uses carrier mediated and sodium dependent;
- Pharmacologic (HIGH) doses use passive diffusion;
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What are the Biotin carrier proteins into TISSUES?
1. INTESTINE and LIVER = Sodium-dependent multivitamin transporter (SMVT) → Also transports pantothenic acid and lipoic acid;
2. Most tissues is solute carrier gene family 19 (SLC19A3) that also transports thiamin
3. Leukocytes, in addition to SLC19A3, have monocarboylate transporter 1 (MCT1)
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What is the form of Biotin found in the PLASMA?
~80% is found FREE in PLASMA;
- Smaller amounts are bound to alpha and beta globulins;
- And also biotinidase
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What are the functions of Biotin?
- Coenzyme covalently bound to enzymes, especially for caboxylase runs;
- Also functions in non-coenzyme roles possibly in cell proliferation and gene expression
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What is Pyruvate Carboxylase (Biotin-dependent coenzyme)?
- Converts pyruvate to oxaloacetate (OAA)
- Activated by Acetyl CoA;
- If the cell has a SURPLUS of ATP along with acetyl CoA, the OAA is used for gluconeogenesis;
- If the cell is DEFICIENT in ATP, the OAA enters TCA cycle and condenses with acetyl CoA for catabolism of the acetyl CoA and energy generation
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What are the Biotin-dependent enzymes of Holocarboxylases?
1. Pyruvate carboxylase;
2. Acetyl-CoA carboxylase
3. Propionyl-CoA carboxylase;
4. Beta-methylcrotonyl-CoA carboxylase;
* Holocarboxylase synthetase attaches to Biotin in 2 steps
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What occurs with a MUTATION to Holocarboxylase Synthetase?
*Enzyme that attaches biotin to dependent enzymes;
-Metabolic problems;
-TWO active sites on the enzymes:
1. For generation of the carboxybiotin enzyme
2. Transfers the activated carbon dioxide to a reactive carbon on the substrate = carboxylation rxn to ADD. CO2 to a compound;
The long flexible chain (Valeric Acid) allows the swinging between the active sites
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What is Pyruvate Carboxylase (Biotin-dependent coenzyme)?
- Converts pyruvate to oxaloacetate (OAA)
- Activated by Acetyl CoA;
- If the cell has a SURPLUS of ATP along with acetyl CoA, the OAA is used for gluconeogenesis;
- If the cell is DEFICIENT in ATP, the OAA enters TCA cycle and condenses with acetyl CoA for catabolism of the acetyl CoA
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What happens in the beginning of the FED state containing carbs?
- Glucose is catabolized to acetyl-CoA;
- ATP will be low and some pyruvate is diverted to OAA to handle acetyl CoA from dietary glucose to produce ATP;
- ATP increases, the OAA is diverted to a greater extent to gluconeogenesis and less of the acetyl CoA is catabolized and more of it is used for fatty acid synthesis;
- Coupled with stimulation of acetyl CoA carboxylase
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What activates Acetyl-CoA carboxylase?
- Activated by citrate/isocitrate;
- Citrate increases in concentration in the well-fed state and is an indicator of a plentiful supply of acetyl CoA → The way to get ACETATE out of the mitochondria
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What inactivates Acetyl-CoA carboxylase?
-INACTIVATED by phosphorylation of the enzyme and long-chain acyl CoA;
-Glucagon, epinephrine and insulin regulates this enzyme via changes in phosphorylation state;
= Add -PO4
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What is Propionyl-CoA carboxylase?
-Important for the catabolism of isoleucine, threonine and methionine
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What is Beta-Methylcrotonyl CoA ?
- Enzyme that catalyzes the metabolism of Leucine;
- If biotin is DEFICIENT beta-methylcrotonyl CoA is shunted to another pathway that produces 3-hydroxyisovaleric acid (3-HIA);
- Increased 3-HIA and decreased biotin in the urine are signs of biotin deficiency
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What are the NON-Coenzyme functions of Biotin?
- Functions are mediated through biotin attaching to histones;
- DNA is wrapped around histones as nucleosomes;
- Attachment of biotin to histones is mediated by HOLOCARBOXYLASE SYNTHETASE and BIOTINIDASE;
- Increased biotinylation correlates with increased cell proliferation compared to quiescent cells (dormant, inactive)
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What are the metabolites from Biotin degradation?
1. Bisnorbiotin converted to Tetranorbiotin by Beta-oxidation of side-chain;
2. Biotin sulfoxide converted to Biotin sulfone by Oxidation of Sulfur
* *All excreted in the Urine
171
How are Biotin Holocarboxylases broken-down?
1. (Biotin + attached enzymes) are DEGRADED by PROTEASES to give biotin oligopeptides
2. Biotin oliogopeptides are converted to biocytin (biotin + lysine);
3. Then biocytin is converted to free biotin and lysine by biotinidase
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How is Biotin excreted?
-Some biotin is excreted in the urine as biotin, some reused, and some degraded;
-Usually there is NO DEGRADATION of the RING structure, but DEGRADATION of the VALERIC ACID side chain;
-CAN be oxidation of the sulfur in the biotin RING → Lose electrons and become a radical/reactive;
Women that SMOKE seem to have accelerated biotin catabolism (breakdown)
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What are the metabolites from Biotin degradation?
1. Bisnorbiotin converted to Tetranorbiotin by Beta-oxidation of side-chain;
2. Biotin sulfoxide converted to Biotin sulfone by Oxidation of Sulfur
* *All excreted in the Urine
174
What is the requirement for Biotin?
- AI for adults = 30 micrograms/day
- Some biotin SYNTHESIZED by bacteria is absorbed, but not enough
- Biotin deficiency in humans is rare, but can happen especially with genetic problems
175
What causes Biotin deficiency?
```
1. Eating too many raw egg whites (Aviding binding to vitamin);
Most common=
2. Various GI problems
3. Chronic excessive alcohol consumption
4. Anticonvulsant drug therapy
```
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How are defects of Biotinidase and Holocarboxylase Synthetase treated?
- Biotinidase defects can be helped with 5 to 10 mg/day;
| - Holocarboxylase Synthetase defects can be helped with 40 to 100 mg/day
177
Is Biotin considered toxic?
- NO toxicity of biotin has been reported so no UL;
- Use of 100 mg/day for genetic disorders of biotin enzymes and uses of biotin in hair and skin conditioning have not caused any problems
178
How is Biotin status assessed?
- Urinary assessment of biotin and other compounds are BEST assessment methods – better than plasma biotin
- Normal urinary 3-hydroxyisovaleric acid are <0.2 micromoles/mg of creatinine
