Vitamin C



I.  Structure

  1. Interconversion of ascorbate and dehydroascorbate

II.  Functions

  1. Prevent or cure survy (every body function affected to some extent)
  2. Antioxidant and prooxidant activity
    1. Antioxidant
      1. React with free radicals formed during cellular metabolism to prevent them from attacking cellular constitutients
      2. To balance or set the redox potential of other cellular water-soluble substances
    2. Prooxidant
      1. Reduce transition metals (Cu+2 to Cu+1; Fe+3 to Fe+2)
      2. The reduced metals catalyze formation of free radicals including the hydroxyl radical which can initiate peroxidative chain reactions
  3. Collagen synthesis
    1. Two hydroxylation reactions requiring vitamin C are necessary for collagen synthesis
      1. Hydroxylation of selected proline residues on newly synthesized collagen alpha chains is necessary for the collagen molecule to aggregate into its triple-helix configuration
      2. Hydroxylation of lysine residues permits cross-linking, glycosylation and phosphorylation
      3. Hydroxylation of proline and lysine are catalyzed by dioxygenases
      4. During hydroxylation reactions, iron bound as a cofactor is oxidized
      5. Ascorbate reduces Fe+3 back to Fe+2
  4. Carnitine synthesis
    1. Reactions involving ascorbate are hydroxylations
      1. Almost identical to those for proline and lysine hydroxylation
      2. Vitamin C is the preferred reducing agent in carnitine synthesis
  5. Tryosine synthesis and catabolism
    1. Phenylalanine is hydroxylated via phenylalanine hydroxylase, an iron-dependent enzyme to form tryosine
      1. Reaction requires 02 and tetrahydrobiopterin which is converted to dihydrobiopterin
      2. Vitamin C is believed to regenerate tetrahydrobiopterin from dihydrobiopterin
    2. p-hydroxyphenylpyruvate is converted to homogentisate by copper dependent p-hydroxyphenylpyruvate hydroxylase
      1. Vitamin C is the preferred reductant
      2. Vitamin C is thought to protect the enzyme from inhibition by its substrate
    3. Homogentisate is converted to 4-maleylacetoacetate by homogentisate dioxygenase
      1. The enzyme is iron dependent
      2. Vitamin C is required
  6. Neurotransmitter synthesis
    1. Dopamine mono-oxygenase is Cu and ascorbate dependent
      1. Converts dopamine to norepinephrine
    2. Peptidylglycine alpha-amidating mono-oxygenase
      1. Amidated peptides from this reaction are active in hormones, hormone-releasing factors, or neurotransmitters
      2. Examples are gastrin releasing peptide cholecystokinin, corticotropin-releasing factor, gastrin, growth hormone releasing factor, oxytocin, and vasopressin
      3. If ascorbate is the favored reductant the required amidating enzyme, then the vitamin has an indirect but important role in many regulatory processes
  7. Cholesterol degradation for bile acid synthesis
    1. Shown to be diminished in vitamin C deficient guinea pigs
  8. Other functions of vitamin C
    1. Microsomal hydroxylation reactions of noncholesterol steroids and drugs
    2. Regulation of cellular nucleotide concentrations
    3. Roles in lipid metabolism
    4. 4Immune function
    5. Prevention of oxidative destruction of other vitamins
    6. Sulfation for proteoglycan synthesis

III.  Sources

  1. Citrus fruit, tomatoes, cabbage, sweet potatoes, white potatoes, many vegetables
  2. Cooking losses can be high

IV.  Metabolism

  1. Absorption and transport
    1. Absorption occurs primarily by active transport
      1. Ascorbate may be oxidized to dehydroascorbate which may be absorbed by passive diffusion
      2. Most absorption occurs in distal small intestine
      3. Degree of absorption decreases with increased intake
    2. Within intestinal cells, dehydroascorbic acid is reduced to ascorbic acid
      1. The enzyme dehydroascorbate reductase requires reduced glutathione (GSH)
      2. GSH is oxidized (GSSG)
    3. Factors which impair absorption of vitamin C
      1. Pectin and Zinc
      2. High iron concentration may oxidize vitamin C to other products without vitamin C activity
    4. Vitamin C is transported across the basolateral membrane of intestional cells by Na-independent carrier-mediated transport systems
    5. Absorbed ascorbate is transparted in plasma in the free form although some may bind to albumin
    6. Ascorbate moves freely into body cells
      1. Tissue concentrations usually exceed plasma concentrations by 3 to 10 times
      2. Tissue and plasma concentrations are related
      3. Both are related to intake until 90 mg/day is exceeded
  2. Excretion
    1. Vitamin C metabolites and excess ascorbate are excreted in urine
    2. When the body pool is low, kidney tubules can reabsorb ascorbic acid and dehydroascorbic acid
      1. When body pool is < 1500 mg, little or no ascorbic acid appears in urine; only its metabolites are excreted
      2. When body pool is > 1500 mg, ascorbate is excreted along with its metobolites

V.  Requirements

  1. Dietary requirement only for human and other primates, guinea pig, and a few other animals
  2. Most other animals meet their requirement through synthesis
  3. Recommended allowance
    1. Human
      1. Adult......60 mg/day
      2. Child......40 mg/day
      3. Infant.....30 mg/day
    2. Guinea pig.......16 mg/day
    3. Monkey............25 mg/kg body weight

VI.  Deficiency

  1. Vitamin C intakes below 10 mg/day may result in scurvy
  2. Total body pool of vitamin C falls below 300mg
  3. Symptoms of scurvy
    1. Bleeding gums
    2. Petechiae (small skin discolorations due to ruptured blood vessels
    3. Sub-lingual hemorrhages
    4. Easy bruising
    5. Impaired wound and fracture healing
    6. Joint pain
    7. Loose and decaying teeth
    8. Hyperkeratosis of hair follicles

VII.  Toxicity

  1. Toxicity is more likely with several large doses throughout the day than with one single dose
  2. Many potentially harmful effects are possible but frequency of recorded toxicity is very low
    1. Osmotic diarrhea
    2. Kidney stones (Vitamin C is metabolized to oxalate, a common constituent of kidney stone)
    3. Ascorbic acid increases monheme iron absorption
    4. Excess ascorbate excretion can interfere with a variety of clinical laboratory tests
    5. Scurvy like symptoms have been reported in some individuals on abrupt withdrawal of large intakes of vitamin C

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