Requirement for iron for the production of hydroxyl radicals by rat liver quinone reductase

NADPH-quinone reductase catalyzes the two-electron reduction of quinones such as menadione, and generally is considered to play a protective role against quinone-mediated toxicity. Recent studies have shown that reactive oxygen intermediates may be produced during metabolism of quinones by quinone reductase. Experiments were carried out to evaluate the effect of iron complexes on production of hydroxyl radical (·OH) when menadione was oxidized by a rat liver cytosolic fraction. Menadione-stimulated H₂O₂ production when added to the cytosol; dicoumarol, a potent inhibitor of quinone reductase, completely blocked this stimulation. Results were identical with either NADH or NADPH as reductant. In the absence of added iron, ·OH, assessed as oxidation of chemical scavengers, was not produced. Various ferric chelates, added to the cytosol in the absence of menadione, did not catalyze ·OH production. However, ·OH was produced in the presence of menadione with all ferric complexes evaluated except for ferric- desferrioxamine. Catalase, competitive scavengers and GSH inhibited ·OH production, as did dicoumarol. Superoxide dismutase inhibited with ferric- ATP, ferric-citrate, ferric-histidine or ferric ammonium sulfate as iron catalysts, but had no effect with ferric-EDTA or ferric-diethylenetriamine penta-acetic acid. Reduction of the ferric complexes was increased by menadione. NADH and NADPH were equally effective as cofactor for all these reactions. Metabolism of menadione in the presence of iron complexes caused inactivation of enzymes present in the cytosolic fraction such as glutamine synthetase and lactic dehydrogenase. These results indicate that metabolism of menadione by quinone reductase can lead to the production of ·OH in the presence of various ferric catalysts. Menadione-dependent production of H₂O₂ and reduction of the ferric catalyst appear to be responsible for the generation of ·OH in this system. Quinone reductase may not always serve a protective role against free radical damage, especially if iron is present.