Vitamin E.

The term vitamin E is used to describe eight lipophilic, naturally occurring compounds that include four tocopherols and four tocotrienols designated as alpha-, beta-, gamma-, and delta-. The most well-known function of vitamin E is that of a chain-breaking antioxidant that prevents the cyclic propagation of lipid peroxidation. Despite its antioxidant function, dietary vitamin E requirements in humans are limited only to alpha-tocopherol because the other forms of vitamin E are poorly recognized by the hepatic alpha-tocopherol transfer protein (TTP), and they are not converted to alpha-tocopherol by humans. In attempts to gain a better understanding of vitamin E's health benefits, the molecular regulatory mechanisms of vitamin E have received increased attention. Examples of these mechanisms include: (1) the role of the hepatic alpha-TTP in preferentially secreting alpha-tocopherol into the plasma, (2) phase I and phase II metabolism of vitamin E and the potential impact for drug-vitamin E interactions, and (3) the regulation of biliary excretion of vitamin E by ATP-binding cassette protein(s). It is expected that the continued studies of these regulatory pathways will provide new insights into vitamin E function from which additional human health benefits will evolve.