Superoxide, hydrogen peroxide and singlet oxygen in hematoporphyrin derivative-cysteine, -NADH and -light systems

Abstract

Hematoporphyrin derivative and light in the presence of cysteine or glutathione were found to convert oxygen to superoxide and hydrogen peroxide at pH < approx. 6.5, while at pH > 6.5 no superoxide or hydrogen peroxide production was observed. However, at pH values greater than 6.5 the rate of oxygen consumption increased. This rate paralleled the acid dissociation curve of the cysteine thiol group and is consistent with the chemical quenching of 1O 2 by cysteine. The superoxide and hydrogen peroxide formation observed below pH 6.5 appeared not to be related to the singlet oxygen production of hematoporphyrin derivative. In addition, superoxide and hydrogen peroxide production was observed with hematoporphyrin derivative and light in the presence of NADH, both above and below pH 6.5. Direct detection of singlet oxygen luminescence at 1268 nm in the hematoporphyrin derivative-light system ( 2H 2O as solvent) revealed an apparent linear increase in the singlet oxygen emission intensity as the p 2H was raised from 7.0 to 10.0. Azide efficiently quenched this observed emission. In addition, at p 2H 7.4, 1 mM cysteine resulted in a 40% reduction of the singlet oxygen luminescence, while at p 2H 9.4 the signal was quenched by over 95% (under the experimental conditions employed). In total, we interpret these results as consistent with the chemical quenching of 1O 2 by the ionized thiol group of cysteine.