Generation of oxidants in the near-UV photooxidation of human lens alpha-crystallin.

In this study we report on the generation of superoxide anion (O2-) and hydrogen peroxide in the near-UV irradiation of human lens alpha-crystallin by monochromatic light at 300 nm. Photolysis of human lens alpha-crystallin at 300 nm, at irradiances similar to those encountered in sunlight causes an alteration of protein tertiary structure, a loss of tryptophan fluorescence and increase of nontryptophan fluorescence. The nontryptophan fluorescence is likely to be due to the photooxidation of tryptophan to N-formylkynurenine (N-FK or related species), which is a good photodynamic sensitizer, has significant absorption at 300 nm, and can thus react via its triplet state with O2 to generate 1O2 or with reducing substrates (amino acids of the protein) to generate free radicals. The latter, in the presence of O2 can lead to the generation of O2- and H2O2. These species have been directly assayed in this study in photolyzed solutions of fetal, young and old human lens alpha-crystallin. The addition of superoxide dismutase (SOD) to the protein solution prior to photolysis increased the amount of H2O2 generated by 3- to 4-fold. This observation not only provides definitive evidence for the photogeneration of O2-, but also indicates that only a fraction of this species is transformed into H2O2 in the absence of SOD. Significant amounts of O2- and H2O2 were formed by 340 nm irradiation of old human lens alpha-crystallin, in which the basal level of N-FK is high. The role of 1O2 in these photoreactions has been studied by investigating the quenching effect of azide and the enhancing effects of D2O on the rate of loss of tryptophan fluorescence yield and the effect of azide on the rate of H2O2 generation.