The chemistry of vitamin E is rather complex because there are eight structurally related forms— four tocopherols (a, 7, and ¿) and four tocotrie-nols (a, /3, 7, and ¿)—that are synthesized from homogentisic acid and isopentenyl diphosphate in the plastid envelope of plants. The structures of a-, 7-, and ¿-tocopherols are shown in Figure 1. a-Tocopherol is methylated at C5, C7, and C8 on the chromanol ring, whereas the other homologs (/3, 7, and ¿) have different degrees of methylation (Figure 1). Tocopherols have a saturated phytyl side chain attached at C2 and have three chiral centers that are in the R configuration at positions C2, C41, and C81 in the naturally occurring forms, which are given the prefix 2R, 41R, and 81R (designated RRR). The members of the tocotrienols are unsatu-rated at C31, C71, and C111 in the isoprenoid side chain and possess one chiral center at C2 in addition to two sites of geometric isomerism at C31 and C71. Vitamin E biological activity is expressed as mg RRR-a-tocopherol equivalents (a-TE) whenever possible. The activity of RRR-a-tocopherol is 1. The activities of RRR-^-, RRR-7-, and RRR^-tocopherol are 0.5, 0.1, and 0.03, respectively.
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