convertible in the human and thus do not behave the same metabolically.
Synthetic forms of α-tocopherol are present in fortified foods and in vitamin supplements. Vitamin E supplements are sold as esters of either the natural RRR- or the synthetic mixture (all rac-) forms of α-tocopherol. Because α-tocopherol has three asymmetric carbon atoms, it has eight possible stereoisomers, seven of which are only found in synthetic preparations. Synthetic vitamin E, all rac-α-tocopherol (historically and incorrectly labeled dl-α-tocopherol) (Horwitt, 1976),1 is produced by coupling trimethylhydroquinone with isophytol; it contains all eight stereoisomers in equal amounts (Figure 6-2). Four of the stereoisomers are in the 2R-stereoisomeric form (RRR-, RSR-, RRS-, and RSS-α-tocopherol) and four are in the 2S-stereoisomeric form (SRR- SSR-, SRS-, and SSS-α-tocopherol). Although RRR-α-tocopherol is the most biologically active of the eight stereoisomers in rats, the other 2R-stereoisomers generally have a higher activity than the 2S stereoisomers (Weiser and Vecchi, 1982; Weiser et al., 1986).
The naturally occurring stereoisomer is RRR-α-tocopherol (historically and incorrectly labeled d-α-tocopherol) (Horwitt, 1976). RRR-α-Tocopherol can be derived by methylating γ-tocopherol isolated from vegetable oil. This is labeled “natural source” vitamin E when marketed.
Esterification of the labile hydroxyl (OH) group on the chromanol ring of vitamin E prevents its oxidation and extends its shelf life. This is why esters of α-tocopherol are often used in vitamin E supplements and in fortified foods. In apparently healthy humans,
The original international standard for vitamin E, dl-α-tocopheryl acetate (one asymmetric carbon atom in the 2 position on the chromal ring, ambo-α-tocopheryl acetate) is no longer commercially available. It was synthesized from natural phytol and was a mixture of two stereoisomers of α-tocopherols, RRR-α-tocopheryl acetate and SRR-α-tocopheryl acetate (Horwitt, 1976). For practical purposes at the time, the activity of 1 mg of dl-α-tocopheryl acetate was defined as equivalent to one IU of vitamin E. The dl-α-tocopheryl acetate of commerce currently available is synthesized from synthetic isophytol, has eight stereoisomers, and is labeled as dl-α-tocopheryl acetate. However, it is more accurately called all rac-α-tocopheryl acetate (AIN, 1990; IUPAC, 1974) because it contains three asymmetric carbon atoms in the 2, 4', and 8' positions (2RS, 4'RS, 8'RS-α-tocopherol). The all rac and ambo-α-tocopheryl acetates were shown to have the same biological activity in rats (Weiser et al., 1986).