Physiology Dietary Sources and Requirements

D A Bender, University College London, London, UK © 2005 Elsevier Ltd. All rights reserved.

Ascorbic acid is a vitamin (vitamin C) for only a limited number of species: man and the other primates, bats, the guinea pig, and a number of birds and fishes.

In other species ascorbic acid is not a vitamin, but is an intermediate in glucuronic acid catabolism, and its rate of synthesis bears no relation to physiological requirements for ascorbate. Species for which ascorbate is a vitamin lack the enzyme gulonolac-tone oxidase (EC 1.11.3.8) and have an alternative pathway for glucuronic acid metabolism.

Ascorbic acid functions as a relatively nonspecific, radical-trapping antioxidant and also reduces the tocopheroxyl radical formed by oxidation of vitamin E. It has a specific metabolic function as the redox coenzyme for dopamine ^-hydroxylase and peptidyl glycine hydroxylase, and it is required to maintain the iron of 2-oxoglutarate-dependent hydroxylases in the reduced state.

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