Tocopherols metabolism, regulation

Mooradian AD, Haas MJ, Wadud K. Ascorbic acid and alpha-tocopherol down-regulate apolipoprotein A-I gene expression in HepG2 and Caco-2 cell lines. Metabolism. 2006, 55 159-167. 

Most vitamins function either as a hormone/ chemical messenger (cholecalciferol), structural component in some metabolic process (pantothenic acid), or a coenzyme (phytonadi-one, thiamine, riboflavin, niacin, pyridoxine, biotin, folic acid, cyanocobalamin). At least one vitamin has more than one biochemical role. Vitamin A as an aldehyde (retinal) is a structural component of the visual pigment rhodopsin and, in its acid form (retinoic acid), is a regulator of cell differentiation. The precise biochemical functions of ascorbic acid and a-tocopherol still are not well defined. 

Vitamin E is the most potent fat-soluble antioxidant in human plasma. Although vitamin E was first discovered in 1922, its metabolic function remains an enigma. There are eight different molecular forms with vitamin E antioxidant activity, yet the body preferentially retains a-tocopherol. This preference for a-tocopherol has led the Eood and Nutrition Board in its 2000 Dietary Reference Intakes (DRIs) for vitamin E to recommend that only a-tocopherol, not the other forms, meets human requirements for vitamin E. Moreover, only a-tocopherol is recognized by the hepatic a-tocopherol transfer protein (a-lT P). This protein regulates plasma a-tocopherol concentrations and genetic abnormalities in the protein (or its absence) leads to vitamin E deficiency in humans. 

A variety of lipid molecules take part in diverse aspects of metabolism and its control. Polyunsaturated fatty acids and their metabolites have been discussed above. Others are the fat-soluble vitamins, retinol (vitamin A) and tocopherol (vitamin E) (Chapters 5 and 8). Sterols, such as cholesterol, regulate membrane function and act as precursors for a range of molecules with diverse metabolic activities cholecalciferol (vitamin D), which is metabolized further to hydroxylated derivatives that regulate calcium metabolism and other aspects of cellular function (Chapters 5 and 7) bile acids, which are involved in lipid absorption (Chapters 4 and 7) and steroid hormones (Chapter 7). 

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