| Vitamin C is a reducing agent
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While most of the animal kingdom synthesizes vitamin C, the human species does not. Vitamin C, ascorbic acid , is an essential nutrient in human beings, the higher primates, the guinea pig and fruit-eating bats. In all other animals, a specific pathway exists for its synthesis. The synthetic pathway and structure of vitamin C are shown in Figure 10.10. Vitamin C is labile: it is easily destroyed by oxygen, metal ions, increased pH, heat and light. Vitamin C serves as a reducing agent and its active form is ascorbic acid, which is oxidized during the transfer of reducing equivalents to dehydroascorbic acid (which also can act as a source of the vitamin). Vitamin C participates in the synthesis of collagen and adrenaline, steroidogenesis, degradation of tyrosine, bile acid formation, absorption of iron and in bone mineral metabolism. The prime function of this compound is to maintain metal cofactors in their lower valence state, e.g. Fe2+ and Cu+. This is the case in its role in the synthesis of collagen where it is required specifically for the hydroxylation of proline (see Chapter 27).
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| The antioxidant role of vitamin C has, in recent years, achieved greater attention. There were suggestions that it may play a role in the prevention of atherosclerosis and cancer but no conclusive proof of this has been obtained.
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| Vitamin C deficiency causes scurvy and compromises immune function
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| Scurvy is related to a defective collagen synthesis associated with vitamin C deficiency. It is characterized by subcutaneous and other hemorrhages, muscle weakness, soft, swollen, bleeding gums, osteoporosis, and poor wound healing and anemia. The osteoporosis results from the inability to maintain bone matrix in association with demineralization. This latter aspect results in the appearance of Looser's zones on radiography, especially in the hands.
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| Except in the elder individuals, vitamin C deficiency resulting in the full clinical picture of scurvy is rare.
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| Milder forms of vitamin C deficiency are more common and the manifestation of such includes easy bruising and the formation of petechiae (small, pinpoint hemorrhages under the skin) both due to increased capillary fragility. Immune function is also compromised in mild vitamin C deficiency. This reduction in immuncompetence has been the basis for providing megadoses of the vitamin to prevent the common cold and also for the role in cancer prevention. No clear evidence exists, however, to substantiate these claims first made by Linus Pauling in 1970s. Vitamin C is certainly required for normal leukocyte function, and leukocyte vitamin C levels drop precipitously after stress related to either trauma or infection.
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| Citrus and soft fruits and growing points of vegetables are rich sources of vitamin C. There is no clear evidence that vitamin C taken in excess is toxic. Theoretically, since it is metabolized to oxalate, there is a risk of the development of renal oxalate stones in susceptible individuals. However, this has not been substantiated in practice.
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| Figure 10.10 Structure and synthesis of vitamin C (ascorbic acid). Note that the enzyme that converts gulonolactone to ascorbic acid is absent in man, higher primates, the guinea pig and the fruit-eating bat. |
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