UVB INDUCED PHOTOOXIDATION OF VITAMIN-E

The photochemistry of alpha-tocopherol (alpha-TH, vitamin E) may contr ibute to its inhibition of UVB (290-320 nm) photocarcinogenesis. Photo chemical reactions of alpha-TH were studied by monitoring the fate of alpha-TH in UVB irradiated liposomes and solution. Soy phosphatidylcho line (SPC) and dioleoylphosphatidylcholine (DOPC) liposomes were suppl emented with alpha-TH (1.0 mol % alpha-TH/phospholipid) and irradiated with UVB at a dose rate of 6.0 J m(-2) s(-1) for up to 90 min. alpha- TH was rapidly depleted in UVB irradiated liposomes. Oxidative damage, assessed by monitoring lipid peroxidation, was suppressed in SPC lipo somes until alpha-TH was depleted to 20% of initial levels. alpha-TH a lso was rapidly depleted by UVB irradiation in acetonitrile/H2O (4:1 v /v) solution. In SPC liposomes, products previously identified as mark er products for peroxyl radical scavenging by alpha-TH were observed, including alpha-tocopherol quinone, 5,6-epoxy-alpha-tocopherol quinone , and 2,3-epoxy-alpha-tocopherol quinone. These products also were for med in DOPC liposomes, which are resistant to lipid peroxyl radical fo rmation. In addition, an alpha-tocopherol dihydroxy dimer and several 8a-(hydroperoxy)epoxytocopherones were identified by HPLC and HPLC-MS. The dimer appears to result from recombination of photoinduced tocoph eroxyl radicals. Products associated with peroxyl radical scavenging ( quinones, epoxyquinones, 8a-(hydroperoxy)epoxytocopherones) and with U VB dependent production of tocopheroxyl radicals (dihydroxy dimer) als o were found when alpha-TH was oxidized by UVB in acetonitrile. Becaus e the acetonitrile contained no autoxidizable substrate, formation of peroxyl radical derived products may occur via intermediate tocopheron e peroxyl radicals. These results indicate that alpha-TH photooxidatio n proceeds via competing reactions of WB induced tocopheroxyl radicals .