Vitamin E: function and metabolism

Although vitamin E has been known as an essential nutrient for reproduction since 1922, we are far from understanding the mechanisms of its physiologi cal functions. Vitamin E is the term for a group of tocopherols and tocotri enols, of which alpha-tocopherol has the highest biological activity. Due t o the potent antioxidant properties of tocopherols, the impact of alpha-toc opherol in the prevention of chronic diseases believed to be associated wit h oxidative stress has often been studied, and beneficial effects have been demonstrated, Recent observations that the alpha-tocopherol transfer prote in in the liver specifically sorts out RRR-alpha-tocopherol from all incomi ng tocopherols for incorporation into plasma Lipoproteins, and that alpha-t ocopherol has signaling functions in vascular smooth muscle cells that cann ot be exerted by other forms of tocopherol with similar antioxidative prope rties, have raised interest in the roles of vitamin E beyond its antioxidat ive function, Also, gamma-tocopherol might have functions apart from being an antioxidant, It is a nucleophile able to trap electrophilic mutagens in lipophilic compartments and generates a metabolite that facilitates natriur esis, The metabolism of vitamin E is equally unclear, Excess a-tocopherol i s converted into alpha-CEHC and excreted in the urine, Other tocopherols, l ike gamma- and delta-tocopherol, are almost quantitatively degraded and exc reted in the urine as the corresponding CEHCs. All rac alpha-tocopherol com pared to RRR-alpha-tocopherol is preferentially degraded to alpha-CEHC. Thu s, there must be a specific, molecular role of RRR-alpha-tocopherol that is regulated by a system that sorts, distributes, and degrades the different forms of vitamin E, but has not yet been identified, In this article we try to summarize current knowledge on the function of vitamin E, with emphasis on its antioxidant vs. other properties, the preference of the organism fo r RRR-alpha-tocopherol, and its metabolism to CEHCs.