Vitamin A and carotenoids as antioxidants in a physiological context.

Under selected conditions, vitamin A and carotenoids can both accept and donate electrons, and carotenoids can also quench singlet oxygen. Thus both sets of compounds can theoretically participate in a biological antioxidant network. Under physiological conditions, vitamin A esters are transported and stored in a lipid matrix that contains other antioxidants, and retinol and its active metabolites are largely bound in clefts of specific retinoid-binding proteins. Thus, vitamin A seems to be protected in vivo by other antioxidants and proteins rather than protecting other molecules. Carotenoids are largely distributed in lipoproteins, membranes, and the lipid phases of intracellular structures, usually together with vitamin E. Carotenoids can interact with other antioxidants in vitro, but whether they play similar significant roles in vivo is not clear. Nonetheless, some genetic conditions and precancerous lesions respond to carotenoids, and the dietary intake of carotenoids has been associated with a reduced risk of several chronic diseases. Carotenoids seem to act per se in such systems rather than by their conversion into vitamin A.