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10.1.9 Vitamin C (Ascorbic Acid)
Vitamin C is L ascorbic acid, an enediol lactone which is oxidized to dehydroascorbic acid. It is reminiscent of glucose, from which it is derived in the majority of mammals. However, in primates, including humans, and a number of other animals – e.g. guinea pigs, some bats, birds, fishes, and invertebrates – the absence of the enzyme L-gulonolactone oxidase prevents this synthesis. Active vitamin C is ascorbic acid itself, a donor of reducing equivalents and antioxidant. When ascorbic acid acts as a donor of reducing equivalents, it is oxidized to dehydroascorbic acid.
Structure of Ascorbic acid Functions
1. Ascorbic acid is required for hydroxylation of proline to hydroxyproline in collagen synthesis.
2. In the degradation of tyrosine, the oxidation of p-hydroxyphenylpyruvate to homogentisate requires vitamin C, which may maintain the reduced state of copper necessary for maximal activity. The subsequent step is catalyzed by homogentisatedioxygenase, which is ferrous iron-containing enzyme that also requires ascorbic acid.
3. In the synthesis of epinephrine from tyrosine, it is required at the dopamine Þ-hydroxylase step.
4. In bile acid formation, it is required at the initial 7 ß-hydroxylase step.
5. The absorption of iron is significantly enhanced by the presence of vitamin C.
6. Ascorbic acid may act as a general water soluble antioxidant and may inhibit the formation of nitrosamines during digestion.
Deficiency: A lack of ascorbic acid in the diet causes a condition known as ‘scurvy’, which is characterized by hemorrhages under the skin and in other tissue; and swollen and spongy gums from which the teeth are easily dislodged or may fall out. The cells of the body concerned in the formation of bone and the enamel and dentine of teeth lose their normal functional activity in the absence of ascorbic acid. Scurvy in infants is associated with great tenderness and pain in the lower limbs together with changes in the bone structure which are not found in adult scurvy.
RDA
Infants :35mg Children :40mg Adults :45mg Pregnant women :60mg Lactating women :80mg
Sources
Richest sources :Amla Good sources :Citrus fruits, tomatoes, green pepper, raw cabbage, guava Fair sources :Milk, liver, green leafy vegetables apple, banana, jack fruit etc. |