Vitamin C - The Vitamers of C and Their Biological Significance

The Vitamers of C and Their Biological Significance

The name vitamin C always refers to the -enantiomer of ascorbic acid and its oxidized forms. The opposite -enantiomer called -ascorbate has equal antioxidant power, but is not found in nature, and has no physiological significance. When -ascorbate is synthesized and given to animals that require vitamin C in the diet, it has been found to have far less vitamin activity than the -enantiomer. Therefore, unless written otherwise, "ascorbate" and "ascorbic acid" refer in the nutritional literature to -ascorbate and -ascorbic acid respectively. This notation will be followed in this article. Similarly, their oxidized derivatives (dehydroascorbate, etc., see below) are all -enantiomers, and also need not be written with full sterochemical notation here.

Ascorbic acid is a weak sugar acid structurally related to glucose. In biological systems, ascorbic acid can be found only at low pH, but in neutral solutions above pH 5 is predominantly found in the ionized form, ascorbate. All of these molecules have vitamin C activity, therefore, and are used synonymously with vitamin C, unless otherwise specified.

The biological role of ascorbate is to act as a reducing agent, donating electrons to various enzymatic and a few non-enzymatic reactions. The one- and two-electron oxidized forms of vitamin C, semidehydroascorbic acid and dehydroascorbic acid, respectively, can be reduced by the body by glutathione and NADPH-dependent enzymatic mechanisms. The presence of glutathione in cells and extracellular fluids helps maintain ascorbate in a reduced state.