Tocopherol metabolic functions

A specific role for vitamin E in a required metabolic function has not been found. In addition to its direct antioxidant effects, a-tocopherol has been reported to have specific molecular functions. 

Tocopherols (vitamin E) occur in many food substances that are consumed as part of the normal diet. The daily nutritional requirement has not been clearly defined but is estimated to be 3.0-20.0 mg. Absorption from the gastrointestinal tract is dependent upon normal pancreatic function and the presence of bile. Tocopherols are widely distributed throughout the body, with some ingested tocopherol metabolized in the liver excretion of metabolites is via the urine or bile. Individuals with vitamin E deficiency are usually treated by oral administration of tocopherols, although intramuscular and intravenous administration may sometimes be used. 

The chemistry and biochemistry of the tocopherylquinones and tocopherols has been recently reviewed in some detail. The first recognized function of vitamin E was as an antisterility factor for the laboratory rat. However, it is now known to have multiple functions in vivo. For example, it is required for the maintenance of structural and functional integrity of skeletal, cardiac, and smooth muscle and, in some animals, the peripheral vascular system. Tocopherols also function as intracellular antioxidants, particularly with respect to the stabilization of ingested fats and perhaps products arising in the metabolic synthesis and degradation of lipids. It has also been proposed that the tocopherylquinones may have some functional role in electron transport systems. 

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. 

This article reviews the chemistry of the tocopher-ols their dietary sources, absorption, transport, and storage and their metabolic function. In addition, the potential role of dietary or supplemental tocopherol intake in the prevention of chronic disease and possible mechanisms for observed protective effects are discussed. Finally, a summary of the assessment of tocopherol status in humans, intake requirements, and an overview of the safety of high intakes is provided. 

Some sprays include vitamins such as tocopherols (vitamin E) or panthenol, which is metabolized in the skin to become pantothenic acid, a B vitamin. Since hair does not metabolize ( It s dead, Jim ), these sprays perform the functions of antioxidants (tocopherols). In other words, they add shine and moisture (panthenol) rather than perform their normal vitamin roles. Moisture helps prevent damage during combing. 

Although many in vitro studies on the antioxidative property of food constituents have been reported, little is known about the biological functions of dietary antioxidants in vivo, except for several well-known antioxidants such as tocopherols, (i-carotene, and ascorbic acid. Because the bioavailability of food constituents is limited by their digestibility and metabolic fate, an oral administration trial of a dietary antioxidant is favored to evaluate its biological function. 

Harata N, Iwasaki Y (1995) Evidence for early blood-brain btirrier breakdown in experimental thiamine deficiency in the mouse. Metab Brain Dis 10(2) 159-174 Harper CG (1983) The incidence of Wernicke s encephalopathy in Australia A neuropathological study of 131 cases. J Neurol Neurosurg Psychiatry 46 593-598 Harper CG, Butterworth RF (1997) Nutritional and metabolic disorders. In Graham DI, Lantos PL (eds) Greenfield s neuropathology. Arnold, London, pp 601-655 Hayton SM, Kriss T, Wase A, Muller DP (2006) Effects on neural function of repleting vitamin E-deflcient rats with alpha-tocopherol. J Neurophysiol 95(4) 2553-2559 Hayton SM, MuUer DP (2004) Vitamin E in neural and visual function. Ann N Y Acad Sd 1031 263-270... 

Tada, H., Ishii, H., and Isogai, S. (1997) Protective effect of D-alpha-tocopherol on the function of human mesangial cells exposed to high glucose concentrations. Metabolism 46,779-84. 

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. 

Metabolic sparing of ascorbic acid by flavonoids. The fourth possible mechanism for the flavonoid-induced increases in tissue ascorbic acid is that flavonoids, by taking over some of the biological functions of the acid, thereby reduce the metabolic demand for the vitamin and permit its increased accumulation in the tissues. The inability of flavonoids to prolong the life of scorbutic guinea-pigs [43,46] implies that if substitution occurs then it must be in an area other than that normally associated with the development of scurvy — namely, the defective hydroxylation of the collagen molecule precursor. A possible corollary of this is that ascorbic acid is a multifunctional vitamin, with functional substitution by flavonoids possible only in certain metabolic areas. Such partial substitution is not unknown in nutrition the partial replacement in animals of the vitamin E function of tocopherols by antioxidants is a case in point. 

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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