Vitamin E

The term vitamin E describes a family of eight antioxidants, a-, P-, y-, and 5-tocopherol and a-, P-, y-, and 5-tocotrienol, which are synthesized only by plants. Tocopherols and tocotrienols are characterized by a substituted hydroxylated ring system (chromanol ring). However, tocotrienols have an unsaturated side chain, while tocopherols have a saturated side chain with three asymmetric carbon atoms (Figure 21.2). Out of eight possible tocopherol stereoisomers, only the ...-stereoisomer occurs naturally.12

Unless otherwise noted, the vitamin E supplements discussed in this chapter contain either natural RRR-a-tocophcrol (also known as J-a-tocopherol) or synthetic a -tocopherol (all rac-a-tocopherol or Jl-a-tocopherol), which contains equal amounts of all eight stereoisomers. Only ... -a-tocopherol and the other 2. -stereoisomers of a-tocopherol are selectively maintained in plasma,13 suggesting that all rac-a-tocopherol is only half as potent as ... -a-tocopherol. For this reason, the only forms of vitamin E that meet the most recent RDA of 15 mg/d for adults are the naturally occurring RRR-a-tocopherol and the other three synthetic 2R-stereoisomers of a-tocopherol.14

Vitamin E is a lipid-soluble antioxidant present in cell membranes and lipoproteins. All forms of vitamin E (tocopherols and tocotrienols) are capable of inhibiting lipid peroxidation because they react with lipid peroxyl radicals much faster than these radicals can react with polyunsaturated fatty acids (PUFA) to propagate the chain reaction of lipid peroxidation.15 For example, lipid peroxidation is inhibited when a-tocopherol donates a phenolic hydrogen (electron) to the lipid peroxyl radical, resulting in the formation of a lipid hydroperoxide and an a-tocopheroxyl radical.16 The potentially dangerous lipid hydroperoxide can be enzymatically reduced to an alcohol. The a-tocopheroxyl radical is relatively stable and may undergo one of several reactions: (1) reduction back to a-tocopherol by a co-antioxidant such as vitamin C; (2) reaction with another radical to form nonradical products; (3) further oxidation to form a-tocopherol quinone; or (4) in the absence of co-antioxidants, during in vitro lipoprotein oxidation, the a-tocopheroxyl radical may oxidize PUFA through a process termed tocopherol-mediated peroxidation (TMP).11 However, TMP has not been conclusively demonstrated in vivo. While most research has focused on its role as an antioxidant, other biological activities of vitamin E have been identified that are not necessarily related to its antioxidant activity, including protein kinase C inhibition by oc-tocopherol and regulation of gene expression by several forms of vitamin E.17

The function of y-tocopherol in humans is presently unclear. Because y-tocopherol is initially absorbed in the same manner as a-tocopherol, small amounts are detectable in blood and tissue. However, a-tocopherol is preferentially incorporated into VLDL (very low density lipoprotein) by the hepatic a-tocopherol transfer protein (oc-TTP), while y-tocopherol and other forms of vitamin E

FIGURE 21.2 Structures of the eight naturally occurring forms of vitamin E. Tocopherols are in the Reconfiguration, and tocotrienols are in the R-configuration.

are rapidly degraded and eliminated through biliary and urinary excretion. The selective incorporation of a-tocopherol into lipoproteins by a-TTP explains why human plasma concentrations of y-tocopherol are generally 4 to 10 times lower than those of a-tocopherol despite the fact that y-tocopherol is the most abundant form of vitamin E in the U.S. diet.18 Due to a lack of one of the electron-donating methyl groups on its chromanol ring, y-tocopherol is a slightly less potent electron donor (antioxidant) than a-tocopherol and, therefore, a slightly less potent inhibitor of lipid peroxidation. However, this structural difference makes y-tocopherol more efficient at trapping electrophiles, such as RNS associated with inflammation.19 Recent evidence that y-tocopherol or its metabolites may have important physiological functions, in addition to observations that high doses of a-tocopherol decrease plasma and tissue levels of y-tocopherol, have led some scientists to call for additional research on the effects of y-tocopherol on human health.20

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