Vitamin E: action, metabolism and perspectives

Natural vitamin E includes four tocopherols and four tocotrienols. RRR-alph a -tocopherol is the most abundant form in nature and has the highest biolo gical activity. Although vitamin E is the main lipid-soluble antioxidant in the body, not all its properties can be assigned to this action. As antiox idant, vitamin E acts in cell membranes where prevents the propagation of f ree radical reactions, although it has been also shown to have pro-oxidant activity. Non-radical oxidation products are formed by the reaction between alpha -tocopheryl radical and other free radicals, which are conjugated to glucuronic acid and excreted through the bile or urine. Vitamin E is trans ported in plasma lipoproteins. After its intestinal absorption vitamin E is packaged into chylomicrons, which along the lymphatic pathway are secreted into the systemic circulation. By the action of lipoprotein lipase (LPL), part of the tocopherols transported in chylomicrons are taken up by extrahe patic tissues, and the remnant chylomicrons transport the remaining tocophe rols to the liver. Here, by the action of the "alpha -tocopherol transfer p rotein", a major proportion of alpha -tocopherol is incorporated into nasce nt very low density lipoproteins (VLDL), whereas the excess of alpha -tocop herol plus the other forms of vitamin E are excreted in bile. Once secreted into the circulation, VLDL are converted into IDL and LDL by the action of LPL, and the excess of surface components, including alpha -tocopherol, ar e transferred to HDL. Besides the LPL action, the delivery of alpha -tocoph erol to tissues takes place by the uptake of lipoproteins by different tiss ues throughout their corresponding receptors. Although we have already a su bstantial information on the action, effects and metabolism of vitamin E, t here are still several questions open. The most intriguing is its interacti on with other antioxidants that may explain how foods containing small amou nts of vitamin E provide greater benefits than larger doses of vitamin E al one.