Oxidation of glutathione by the superoxide radical to the disulfide and the sulfonate yielding singlet oxygen

Abstract

The reaction of superoxide with reduced glutathione (GSH) was studied with two O⁷₂-producing systems: xanthine oxidase using xanthine or acetaldehyde as substrates and, secondly, quinol autoxidation. The capability of GSH to quench superoxide radicals was detected by lowered O⁷₂-mediated cytochrome c³⁺ reduction. The formation of the oxidation products, glutathione disulfide (GSSG) and glutathione sulfonate (the latter at levels of about 6–15% compared to GSSG), was dependent on the O⁷₂-production and was inhibited by superoxide dismutase. The presence of GSH together with an O⁷₂-producing system led to an extra uptake of oxygen, which was also depressed by superoxide dismutase. The observed O₂ uptake was accounted for by the formation of GSSG and GSO⁻₃ from GSH; the data are in accordance with a mechanism involving thiyl radicals. Low-level chemiluminescence measurement indicated the formation of excited oxygen species. The intensity of photoemission was dependent on the GSH concentration and on the O^−·₂ production rate. Chemiluminescence was inhibited by superoxide dismutase and also by glutathione peroxidase, but not by catalase or OH^· quenchers. Spectral analysis and the effects of 1,4-diazabicyclo[2.2.2]octane and sodium azide indicated the contribution of singlet molecular oxygen to the light emission. It is suggested that singlet oxygen results from an intermediate oxygen addition product such as a glutathione peroxysulphenyl radical.