Relative roles of metal ions and singlet oxygen in UVA-induced liposomal lipid peroxidation

Unilamellar liposomes prepared from phosphatidylcholine were exposed to relatively low doses (36-72 kJ m(-2)) of UVA to study the relative role of metal ions and singlet oxygen. Electron spin resonance (ESR) spectroscopy showed that singlet oxygen was produced during UVA radiation of liposomes and that the signal was enhanced when liposomes and the buffer were prepared in D2O. However, in liposomes and buffer prepared in deionized water (dH(2)O), UVA-induced lipid peroxidation (measured as formation of thiobarbituric acid reactive substances, TBARS) was strongly dependent on adventitious metal ions because formation of TBARS was inhibited similar to 80% by inclusion of 10 mM EDTA or desferrioxamine (desferal) during 1-h UVA radiation (72 kJ m(-2)). Dimethysulfoxide (DMSO) and mannitol had no effect and NaN3 and histidine only slightly inhibited TBARS, suggesting that singlet oxygen is a minor factor in UVA-induced liposomal peroxidation. To further elucidate the relative roles of singlet oxygen and metal ions, Fe(III) was added to liposomes during UVA irradiation, which resulted in markedly enhanced formation of TBARS. Singlet oxygen was detected at UVA doses of 72 and 108 kJ m(-2) by N,N-dimethyl-p-nitrosoaniline bleaching test carried out in D2O but not in dH(2)O. Formation of TBARS was completely inhibited by 10 mM EDTA and desferal but was unaffected by either 50 mM DMSO, 50 mM mannitol, 50 mM NaN3, 4 mM tryptophan or 0.1 mM beta-carotene. The present study demonstrates that, although singlet oxygen is produced by UVA radiation, its role in the formation of TBARS appears to be relatively minor compared with that of metal ions in unilamellar liposomes irradiated with physiological doses of UVA. (C)1999 Elsevier Science Inc. All rights reserved.