ULTRAVIOLET LIGHT-INDUCED GENERATION OF VITAMIN-E RADICALS AND THEIR RECYCLING - A POSSIBLE PHOTOSENSITIZING EFFECT OF VITAMIN-E IN SKIN

Vitamin E (αtocopherol) is the major lipid-soluble chain-breaking antioxidant of membranes. Its UV-absorbance spectrum (λmax 295 nm) extends well into the solar spectrum. We hypothesize that in skin αtocopherol may absorb solar UV light and generate tocopheroxyl radicals. Reduction of tocopheroxyl radicals by other antioxidants (e.g. ascorbate, thiols) will regenerate (recycle) vitamin E at the expense of their own depletion. Hence, vitamin E in skin may act in two conflicting manners upon solar illumination: in addition to its antioxidant function as a peroxyl radical scavenger, it may act as an endogenous photosensitizer, enhancing light-induced oxidative damage. To test this hypothesis, we have illuminated various systems (methanol-buffer dispersions, liposomes and skin homogenates) containing αtocopherol or its homologue with a shorter 6-carbon side chain, chromanol-alpha-C6 with UV light closely matching solar UV light, in the presence or absence of endogenous or exogenous reductants. We found that: (i) a-tocopheroxyl (chromanoxyl) radicals are directly generated by solar UV light in model systems (methanol-water dispersions, liposomes) and in skin homogenates; (ii) reducing antioxidants (ascorbate, ascorbate + dihydrolipoic acid) can donate electrons to αtocopheroxyl (chromanoxyl) radicals providing for vitamin E (chromanol-alpha-C6) recycling; (iii) recycling of UV-induced αtocopheroxyl radicals depletes endogenous antioxidant pools (accelerates ascorbate oxidation); (iv) βcarotene, a non-reducing antioxidant, is not active in αtocopherol recycling, and its UV-dependent depletion is unaffected by vitamin E. © 1992 Informa UK Ltd All rights reserved: reproduction in whole or part not permitted.