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Original Article

Ultraviolet Light-Induced Generation of Vitamin E Radicals and Their Recycling. a Possible Photosensitizing Effect of Vitamin E IN Skin

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Pages 51-64 | Received 02 Oct 1991, Published online: 07 Jul 2009
 

Abstract

Vitamin E (α-tocopherol) 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 α-tocopherol 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 α-tocopherol 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 α-tocopheroxyl (chromanoxyl) radicals providing for vitamin E (chromanol-alpha-C6) recycling; (iii) recycling of UV-induced α-tocopheroxyl radicals depletes endogenous antioxidant pools (accelerates ascorbate oxidation); (iv) β-carotene, a non-reducing antioxidant, is not active in α-tocopherol recycling, and its UV-dependent depletion is unaffected by vitamin E.

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