Vitamin C

1
Vitamin C

• Ascorbic acid (ascorbate) 6 carbon - Most
  mammals can synthesize it from glucose
  exception primates and guinea pigs
• Reason lack of enzyme gulonolactone oxidase
• For humans L isomer is the active one

2
Absorption, Transport and Storage-1

• Absorption generally by sodium-dependent, active
  transport
• Before absorption, some ascorbate oxidized to
  dehydroascorbate which is absorbed more than
  ascorbate
• Absorption inversely related with intake levels -
  16 absorption when intake is 12 g (so only 1.9
  g absorbed) to 98 absorption when intake is 20
  mg (so 19.6 mg absorbed)
• Typical daily intakes from food are about 20 -
  120 mg, and almost 80 to 95 gets absorbed

3
Absorption, Transport and Storage-2

• Absorbed ascorbate transported free in plasma
  and is easily able to get into most tissues but
  specific tissue concentrations exist
• Highest in adrenal and pituitary (350 mg/100 g
  tissue). Then liver, spleen, heart, kidneys least
  amounts is muscle and red blood cells
• Tissue levels of ascorbate 3 10 times higher
  than plasma levels
• Intakes of gt200 mg/day ? steady state plasma
  levels
• So 200 mg/day is practical upper limit for
  vitamin C intake

4
Functions of Vitamins C

• Collagen synthesis
• Carnitine synthesis
• Tyrosine metabolism
• Neurotransmitter synthesis
• Antioxidant activity

5
Collagen Synthesis

• Two hydroxylation reactions necessary for
  collagen synthesis (triple-helix formation) need
  vitamin C
• Proline and lysine residues hydroxylated by
  proline hydroxylase and lysine hrdoxylase enzymes
• Both enzymes have an Fe2 cofactor which gets
  converted to Fe3 (gets oxidized)
• Lysine a-KG O2 ? Hydroxylysine Succinate
  CO2
• Proline a-KG O2 ? Hydroxyproline Succinate
  CO2
• Ascorbate helps as a reducing agent by converting
  the Fe3 back to Fe2 in the two enzymes
• So vitamin C helps in normal development of bone,
  cartilage and the dentine also tissue required
  for wound healing

6
Carnitine Synthesis

• Ascorbate needed for 2 reactions in carnitine
  synthesis
• First is a hydroxylation reaction whereby
  trimethyllysine ? 3-hydroxytrimethyllysine
• Second is the reaction which produces carnitine
• Both above reactions (c.f. collagen synthesis)
  whereby an Fe2 gets converted to Fe3 (gets
  oxidized) and ascorbate helps as a reducing agent
  by converting the Fe3 back to Fe2

7
Tyrosine metabolism

• Tyrosine synthesized from the essential a.a.
  phenylalanine
• Synthesis depends on hydroxylation of
  phenyalanine via phenylanaine hydroxylase enzyme
  (which is Fe dependent). Needs tetrahydrobiopterin
  .
• Acorbate helps regenerate tetrahydrobiopterin
  from dihydrobiopterin