Glutathione-mediated redox cycling of alloxan. Mechanisms of superoxide dismutase inhibition and of metal-catalyzed OH. formation.

The mechanism of the reaction between alloxan and GSH has been studied in the presence and absence of superoxide dismutase. Excess GSH reduced alloxan to dialuric acid, which underwent subsequent autoxidation, thus establishing a redox cycle in which O2 and GSH in excess of the alloxan concentration were consumed. The major reaction products were H2O2 and GSSG. At each cycle, a small fraction of the alloxan reacted with GSH to form a 305 nm-absorbing adduct that gradually accumulated. In the presence of SOD, alloxan was reduced by GSH, but increasing concentrations of GSH progressively inhibited redox cycling as shown by decreased rates of O2 uptake and GSH oxidation. With GSH: alloxan or dialuric acid molar ratios of greater than 8-10:1, redox cycling was almost completely suppressed. A mechanism based on known reactions of GSH and dialuric acid is proposed. Alloxan and GSH, with an iron chelate present as catalyst, caused the hydroxylation of salicylate, an indicator of hydroxyl radical production. Hydroxylation was inhibited by catalase but not by superoxide dismutase, and it is attributed to the Fenton reaction in which the ferric catalyst is reduced by dialuric acid.