Ultraviolet light-induced generation of vitamin E radicals and their recycling. A possible photosensitizing effect of vitamin E in skin.
Vitamin E (alpha-tocopherol) is the major lipid-soluble chain-breaking antioxidant of membranes. Its UV-absorbance spectrum (lambda max 295 nm) extends well into the solar spectrum. We hypothesize that in skin alpha-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 alpha-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) alpha-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 alpha-tocopheroxyl (chromanoxyl) radicals providing for vitamin E (chromanol-alpha-C6) recycling; (iii) recycling of UV-induced alpha-tocopheroxyl radicals depletes endogenous antioxidant pools (accelerates ascorbate oxidation); (iv) beta-carotene, a non-reducing antioxidant, is not active in alpha-tocopherol recycling, and its UV-dependent depletion is unaffected by vitamin E.
Kagan
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V
Department of Molecular and Cell Biology, University of California, Berkeley 94720.
Witt
E
E
Goldman
R
R
Scita
G
G
Packer
L
L
D000818:Animals; D000975:Antioxidants; D001205:Ascorbic Acid / Q000037:antagonists & inhibitors; D002338:Carotenoids / Q000037:antagonists & inhibitors; D004578:Electron Spin Resonance Spectroscopy; D005609:Free Radicals; D008081:Liposomes / Q000528:radiation effects; D000432:Methanol; D051379:Mice; D008812:Mice, Hairless; D008954:Models, Biological; D009381:Neoplasms, Radiation-Induced; D010100:Oxygen / Q000378:metabolism; D010713:Phosphatidylcholines; D011836:Radiation Tolerance / Q000187:drug effects*; D012867:Skin / Q000187:drug effects / Q000528:radiation effects*; D012878:Skin Neoplasms / Q000209:etiology; D013535:Suspensions; D008063:Thioctic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D014466:Ultraviolet Rays; D014810:Vitamin E / Q000737:chemistry / Q000494:pharmacology / Q000528:radiation effects*; D014867:Water; D019207:beta Carotene
D000975:Antioxidants; D005609:Free Radicals; D008081:Liposomes; D010713:Phosphatidylcholines; D013535:Suspensions; D019207:beta Carotene; D014867:Water; D014810:Vitamin E; D002338:Carotenoids; D008063:Thioctic Acid; C015144:dihydrolipoic acid; C017251:1,2-oleoylphosphatidylcholine; D001205:Ascorbic Acid; D010100:Oxygen; D000432:Methanol