First reports about a nutritional factor required to maintain pregnancy in rats appeared in the early 1920s. This factor was named vitamin E. a-tocopherol is the naturally occurring compound with the highest vitamin E activity (A). It has three chiral centers (at 2', 4', 8' in the figure) at which the methyl groups are in R-configuration. According to IUPAC, the correct name is therefore 2R,4R,8R-a-tocopherol or short RRR-a-tocopherol. Naturally occurring a-tocopherol is usually accompanied by small amounts of p-, y-, and 5-tocopherol, which differ by the number and positions of the methyl groups attached to the ring. Additionally, there are naturally occurring toco-trienoles with three additional double bonds in the side chain. Lastly, various synthetic or half-synthetic a-tocophe-rols are always mixtures of various stereoisomers. The most common form of synthetic vitamin E consists of eight stereoisomers, and has a 12.5% RRR-a-tocopherol content. The mixture is called all-rac-a-tocopherol. IUPAC recommends the use of the biochemical nomenclature (in analogy to vitamin A): vitamin E comprises all tocopherols and tocotrienoles that have the same qualitative biological activity as RRR-a-toco-pherol. However, quantitatively, their biological effects vary greatly. All naturally occurring tocopherols/tocotrie-noles exhibit less than 50 % of the activity of RRR-a-tocopherol (= 100%), all-rac-a-tocopherol 74%, and all-rac-a-tocopherol acetate, which is frequently used in medications and is esterified for increased stability, 67 %. In order to account for these various degrees of vitamin E activity, nutrient tables use the term a-tocopherol equivalents (a-
TE), for purposes of which the effect of RRR-a-tocopherol is equivalent to 100%. Vitamin E in medications and in pure compounds is counted in international units (IU) or USP (United States Pharmacopeia).
Vitamin E is absorbed in the intestine together with lipids after any toco-pheryl esters have been hydrolyzed by lipases or mucosal esterases (B). The average absorption rate is 30 %, with a-tocopherol and its esters being better absorbed than the other forms. The absorbed vitamin E is rapidly transported to the liver; only small amounts are released from the chylomicrons by LPL on the surface of endothelial cells. Vitamin E incorporated in VLDL is released back into the bloodstream. There, a dynamic equilibrium exists between all lipoprotein fractions due to a rapid interchange. Uptake into the target cells occurs either through release by endothelium-based LPL or via receptor-mediated LDL endocyto-sis.
Vitamin E is stored in fatty tissue and muscle, preferentially as RRR-a-toco-pherol. Recently, an explanation for this was found in a cloned human gene that encodes for a hepatic a-tocopherol transfer protein. It causes RRR-a-toco-pherol to be preferentially incorporated into VLDL, while other forms of vitamin E are quickly excreted via bile fluid.
A. Chemistry and Biological Activity
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