Fig. 4.8 Structures of compounds with vitamin E activity. All molecules comprise a chromanol ring system and a 16C phytyl side chain. Tocopherols have a saturated side chain; tocotrienols contain three unsaturated bonds in the side chain. Analogues are named a, p, y and S-tocopherols or tocotrienols as follows:
a: R1 = CH3; Rz = CH3; R = CH: P: R1 = CH3; R2 = H; R3 = CH3 y: R1 = H; R2 = CH3; R3 = CH3 S: R1 = H; R2 = H; R3 = CH3
specific tissue vitamin E binding proteins, but research into this topic is still in its infancy.
Until the last decade of the twentieth century, almost all interest in vitamin E was concerned with its putative role as a natural antioxidant. a-Toco-pherol is present in the lipid bilayers of biological membranes (Section 6.5) and may play a structural role there. However, its main function is thought to be the prevention of the oxidation of the unsatu-rated lipids present in the membrane (Section 2.3.4 and Box 4.3). The products of lipid peroxidation can cause damage to cells if the oxidative process is not kept in check (Box 4.3). Such damage appears to be exacerbated in animals fed diets deficient in vitamin E. a-Tocopherol primarily acts as a terminator of the lipid peroxidation chain reactions by donating a hydrogen atom to a lipid radical (Section 2.3.4). The resulting tocopheryl radical is relatively unreactive and unable to attack adjacent unsatu-rated fatty acids because the unpaired electron becomes delocalized in the aromatic ring structure. Experiments in vitro demonstrate that ascorbic acid (vitamin C) is capable of regenerating the anti-oxidant form of vitamin E. If this were to occur in vivo, it would allow one vitamin E molecule to scavenge many lipid radicals, but such a process has not been demonstrated conclusively in vivo.
It is a generally held view that dietary requirements should be considered in relation to the PUFA content of the diet rather than in absolute amounts. A ratio of vitamin E to linoleic acid of about 0.6 mgg"1 is generally recommended. In general, those vegetable oils that contain high concentrations of PUFA are sufficiently rich in vitamin E to give adequate protection.
There is now accumulating evidence that vitamin E may also play other biological roles that do not necessarily involve its antioxidant function. A possible structural role in the maintenance of cell membrane integrity has already been mentioned. Some observations imply diverse roles in immune
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