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NFS382 > Vitamin C > Flashcards

Vitamin C Flashcards

1
Q

What is vitamin C?

A

water soluble
aka ascorbic acid (mostly in food), ascorbate (Body)

food: mostly ascorbic acid, small amounts in oxidized form (dehydroascorbic acid)

most animals can syn. vitamin C
(except guinea pigs, primates, fruit bats, some birds)

Presence of critical compound in citrus fruits and fresh vegetables has been known for centuries

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2
Q

How is ascorbic acid synthesized?

A

start w/ D-glucose
-our body: L and D isomer
only D isomer biologically active in humans

human lack Glunoactone oxidase

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3
Q

How is ascorbic acid synthesized?

A

D-glucose –> G6P –> D-glucoronate 1-phosphate
–phosphatase–> D-glucuronate –> redox rxn –> L-gulonte –aldonolactonase–> L-gulonolactone –Gulonolactone oxidase–> 2-keto-L-gulomolactone –Spontaneous –>L-ascorbate

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4
Q

What is the interconversion of ascorbic acid and dehydroascorbic acid?

A

Ascorbic acid lose proton and electron–> Ascorbyl radical –>lose proton and electron–> dehydroascorbic acid

–>interconverted in body (requires ability to be oxidized)
(dehydroascorbate reductase)
(2 GSH –> GSSG)

can regenerate absorbic acid if 2 radical –> 1 ascorbic acid, 1 dehydroascorbic acid

glutathione to reform ascorbic acid

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5
Q

Vitamin C RDA?

A

infants: AI: no data
0-6 months: based on human milk

Males >19 years: near maximal neutrophil conc. (max. tissue conc. to provide antioxidant protection and minimal urine losses)

  • Special RDA for smokers
  • recent RCTs suggest 200 mg may be the optimal amount of vit C for most adults
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6
Q

What are some sources of vitamin C?

A

any juice sold in Canada has to be fortified with vit C

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7
Q

What is the rate of absorption of vitamin C?

A

normal intakes: absorption efficiency 70-90%
-decrease w/ increase intakes
(16% absorbed at high intakes vs. 98% at low intakes)

does not seem to differ b/w supp. forms (e.g. calcium ascorbate, sodium ascorbate)

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8
Q

What is the absorption process of Ascorbate (not dehydroascorbic acid)?

A

throughout small intestine

  • uptake occurs by at least 2 different Na-dependent vitamin C transporter
  • SVCT1 (main carrier) has a higher capacity for ascorbate than SVCT2
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9
Q

How is dehydroascorbate absorbed?

A

dehydroascorbate (oxidized ascorbate) is absorbed by GLUT1 and GLUT3 transporters (on all cell types), facilitated diffusion

  • better absorbed than ascorbate (use Na-dependent carriers)
  • once absorbed in enterocytes–> rapidly reduced back to ascorbate using glutathione (GSH)
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10
Q

How is vitamin C transported in our bodies?

A

ascorbic acid is exported from enterocytes via anion channels into the EC fluid
–> enter plasma via capillaries (systemic circulation)

-ascorbic acid is transported in blood: mostly in free form

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11
Q

What is the average plasma and tissue conc. of vitamin C in the body?

A

Plasma conc.: 0.4-1.7 mg/dL

Tissue conc.: typically exceed plasma conc.

  • highest: brain, eyes, adrenal glands
  • WBCs have up to 80x more than plasma (immune funcition, antioxidant)

body content 2 g

water soluble but have body storage

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12
Q

How is vitamin C uptake by cells? (cellular uptake)

A

SVCT1 and SVCT2 involved in uptake of ascorbic acid in many tissues (e.g. liver and kidney)

GLUT transporters involved in uptake of dehydroascorbate

transporters on enterocytes is also transporters on other tissues

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13
Q

What are the main functions of Vit. C?

A

Maintenance of metalloenzyme redox state:

  • vitamin C functions as reducing agent
  • Maintains iron and copper atoms in a reduced state within metalloenzyme
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14
Q

How does vit.C maintain metalloenzyme redox state? Collagen synthesis

A

Lysyl hydroxylase and proline hydroxylase: iron-dependent enzymes

  • catalyze hydroxylation of lysine and proline residues in collagen
  • Ascorbate is required to regenerate oxidized (Fe3+) iron back to the reduced state (Fe2+)**
  • critical for the formation and maintenance of skin, bone, tendons, cartilage, and dentine
  • critical for formation of scar tissue and wound healing

**important in creating structure of collagen

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15
Q

How is vitamin C involved in the hydroxylation of peptide-bound proline and lysine?

A

Ferrous iron reduced back to ferric iron as
ascorbate–>dehydroascorbate
lysine –>hydroxylysine

Ascorbate acts as a reducing agent to convert the oxidized iron atom (Fe3+) back to its reduced state (Fe2+) in the enzyme lysyl hydroxylase and prolyl hydroxylase–incorporate one atom of oxygen in the hydroxyl group of the product and other in succinate

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16
Q

How is vitamin C involve in phenylalanine hydroxylase?

A

vitamin C catalyze the first step in breakdown of phenylalanine

  • Fe-dependent enzyme
  • Ascorbate regenerates tetrahydrobiopterin: a co-substrate for the rxn
  • phenylalanine: precursor for tyrosine synthesis
  • phenylalanine removal is important for the prevention of phenylketonuria and brain damage
17
Q

How is vitamin C involve in depomaine monooxygenase?

A

NE is generated from hydroxylation of the side chain on dopamine
-requires 8 Cu atoms

Dopamine–dopamine monooxygenase–>NE
(Cu1+ –> Cu2+)

ascorbate–>dehydroascorbate
–> Cu2+ –> Cu1+

18
Q

How is vitamin C involved in tryptophan hydroxylase?

A

catalyze the rate-limiting step in the synthesis of the neurotransmitter serotonin

Fe-dependent enzyme

ascorbate helps regenerate tetrahydrobiopterin from dihydrobiopterin (cosubstrate in the rxn)

19
Q

How is vitamin C involved in glycine alpha-amidating monooxygenase?

A

contains a Cu atom

  • ascorbate maintains the Cu in the reduced state
  • amidates glycine and other terminal AAs
  • important for synthesis of:
  • cholecystokinin
  • calcitonin
  • thyrotropin
  • oxytocin, vasopressin
  1. vit C functions as a reducing agent to convert copper that has become oxidized during the rxn back to a reduced (Cu1+) form
  2. the enzyme cleaves the carboxyl-terminal residue on the peptide substrate
    - residue is released as glyoxylate
  3. many of the amidated peptides are active hormones, hormone-releasing factors, or neurotransmitters
20
Q

How is vitamin C involved in microsomal xenobiotic metabolism?

A

microsome: artifact of cell lysis: tiny spheres made form scrambles endoplasmic reticulum
- liver microsomes metabolized xenobiotics
- rxns meant to make compounds more water soluble–> facilitate excretion
- often involves hydroxylation rxns catalyzed by iron-dependent monooxygenases or cyt.p450 mixed-function oxidases

require reducing agents like vit.C to maintain reduced state of Fe-atoms

21
Q

What is the antioxidant activity of vitamin C?

A

Ascorbate is an antioxidant: reverse oxidation
-vit C’s 2 hydroxyl groups and carbonyl group facilitate its ability to act as hydrogen/electron donor

  • reacts w/ wide variety of ROS and nitrogen species to donate an electron in the form of a hydrogen ion
  • acts in aq. envi/ in blood as well as in cells
  • can regenerate other antioxidants (ex. vit E and glutathione)
22
Q

How is vitamin C excreted?

A

vit C is primarily excreted by kidneys

  • SVCT1 carriers resorb vit.C from renal tubule filtrate
  • SVCT1 is saturable and will only reabsorb vit.C to a particular threshold
  • above this level: all additional vit.C is excreted
  • level met when blood level reach 1.3-1.8 mg/dL

at vit.C intakes ~500 mg: all vit.C is usually excreted

23
Q

What are some nutrients that interact with vitamin C?

A

vit. C enhances absorption of non-heme iron by reducing iron to Fe2+
- Fe2+ more soluble than Fe3+ (ferrous)
- vit.C benefit thought max. at 75 mg/d

interact w/ both iron and copper through those enzymes both functioning for

24
Q

How is the level of vitamin C assessed?

A

plasma vit.C: < 0.2 mg/dL possibly deficient

vit. C content in WBCs (leukocytes):
- better reflection of tissue stores
- 10 microgram/10^8 cells/less = deficient

smokers require more

25
Q

What are some symptoms of vitamin C deficiency?

A

Scurvy

  • normal plasma conc. range from 0.6-0.8 mg/dL
  • manifested when total body vit.C pools fall below 300 mg and plasma conc. drops tp < 0.2 mg/dL

-may take 8-9 months to manifest in vit.C replete individuals
(~1 month in ppl w/ marginal vit.C status)
-symptoms resolve with 10 mg vit.C/day (typically take 100-500 mg)

26
Q

What are the major symptoms of scurvy?

A

bleeding gums, loose and decaying teeth
petechiae: small red skin discolorations ude to raptured small bvs
sublingual hemorrhages
easy bruising impaired wound and fracture healing
joint pain
hyperkeratosis of hair follicles

27
Q

What is the prevalence of VM supplement consumption?

A

% of all age group:
21-44% taking some sort of vit.C supp.
most commonly consumed supp. at certain age

28
Q

What is the prevalence of inadequacy among children 1-3 yr?

A

low prevalence of inadequacy among children (1-3 yr)
-kids dont rly need supp.

as kids grow older–> more deficiency observed

29
Q

What is the toxicity of vitamin C?

A

daily intake up to 2 g/day show no signs of toxicity
-side effects include: abdominal pain and osmotic diarrhea–> metabolism of unabsorbed vit.C by bacteria
(**UL determination)

-individuals w/ hemahromatosis (abnormally high iron body store) and those with renal disorders–>risk of adverse effects of high levels of vit.C intake

high intakes (>250 mg/d) have been associated w/ false negative results for stool and gastric occult blood

30
Q

What are some vitamin C supplements on the market?

A

contain 10X more vit.C than RDA

-normal person will not absorb much from supp.

31
Q

What are some possible disease prevention roles of supplemental vit.C?

A

Common cold: no

CvD prevention: biologically plausible but no

cancer prevention: biological plausible
some evidence

NFS382 (15 decks)

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