UV-B-Induced Photooxidation of Vitamin E in Mouse Skin

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Abstract
Topically applied α-tocopherol (α-TH, vitamin E) inhibits UV-B (290−320 nm) photocarcinogenesis, yet α-TH undergoes rapid photooxidation by UV-B in vitro. To examine the effect of UV-B on α-TH in vivo, we studied the fate of α-TH in UV-B-irradiated mouse skin. [14C]-α-TH was applied to mouse skin at various times prior to UV-B irradiation. UV-B irradiation for 1 h at a dose rate of 2.6−2.9 J m-2 s-1 resulted in consumption of 40−60% of the applied dose and formation of oxidation products. The major product fraction formed in UV-B-irradiated mice treated topically with α-TH contained an α-TH dihydroxy dimer and its two-electron oxidation product, a spirodimer. Products previously identified as being derived from photochemical or peroxyl radical scavenging reactions of α-TH were also observed, including α-tocopherolquinone (α-TQ), α-tocopherolquinone 2,3-epoxide (α-TQE 1), α-tocopherolquinone 5,6-epoxide (α-TQE 2), and 8a-(hydroperoxy)epoxytocopherones. These results indicate that topically applied α-TH is extensively oxidized in skin and suggest that α-TH photoproducts may be involved in the observed effects of topically applied vitamin E in UV-B-irradiated skin.