Antioxidant Mechanisms of Vitamin E and β-Carotene

Summary

Vitamin E is an excellent trap for peroxyl radicals (ROO^•) and it is the major lipid soluble antioxidant present in mammalian cells. It therefore occupies a unique position in the arsenal of natural antioxidants providing protection against various diseases. Product studies of the reaction of α-tocopherol with peroxyl radicals suggest that the existence of an α-tocopherol regeneration mechanism is essential for maintaining the antioxidant viability of the vitamin. Evidence now exists that vitamin C may regenerate vitamin E in some tissues. Studies carried out with deuterium-labeled α-tocopherol have confirmed that turnover of vitamin E is very slow in neural tissue, the tissue most susceptible to the effects of a deficiency of vitamin E in humans.

There is much interest in β-carotene because it appears to have anticancer activity. It has been assumed, but not shown, that the anticancer activity is related to β-carotene’s antioxidant properties. Possibly, the superior ability of β-carotene to quench singlet oxygen is important but there is no evidence that singlet oxygen is generated in significant quantities in any organs except those exposed to light (i.e., skin and eye). Alternatively, β-carotene may trap peroxyl radicals, but in vitro studies show that it can do this only at low partial pressures of oxygen. This observation is difficult to reconcile with the epidemiological evidence indicating that the strongest association between dietary intake of β-carotene and reduced incidence of cancer occurs for the tissue exposed to the highest partial pressures of oxygen, i.e., the lung.