Vitamin C

Vitamin C is an electron donor, and this property accounts for its known and postulated functions. As an antioxidant, or reducing agent, the vitamin sequentially donates two electrons from the C2-C3 double bond. The first intermediate, formed by the loss of one electron, is the unstable free radical semidehydroascorbic acid. This intermediate is relatively unreactive and does not interact with other compounds to form potentially harmful free radicals, and can be reversibly reduced to ascorbate. Semidehydroascorbic acid undergoes further oxidation to form the more stable product dehydroascorbic acid (DHA) (Fig. 1), which can be reduced back to ascorbate by glutathione or by three distinct enzymatic reduction reactions.[2,3] If not reduced, DHA undergoes ring rupture and is irreversibly hydrolyzed to 2,3-diketogulonic acid. The latter is metabolized to xylose, xylonate, lyxonate, and oxalate, which is a clinically significant end product of vitamin C metabolism.