Metabolic reduction of 4-nitroquinoline N-oxide and other radical-producing drugs to oxygen-reactive intermediates.

Abstract

Summary This study was undertaken to investigate some of the reaction products that result from the metabolism of 4-nitroquinoline N -oxide (4-NQO), various nitrofurans, and menadione in mammalian cells and to determine how different metabolic conditions affect these reactions. The nitrofurans, 4-NQO, and menadione, stimulate oxygen consumption of Ehrlich cells in suspension in both the absence and the presence of KCN. In all cases glucose was required for maximal stimulation of oxygen consumption. Our data suggest that the increased consumption of oxygen is due to the production of radical intermediates. Evidence for the radical intermediate of 4-NQO was obtained by measuring the reduction of intracellular ferricytochrome c + c 1 in the presence or absence of inhibition of early steps of the cytochrome chain. Blockage of the cytochrome oxidase by KCN, which also inhibits catalase and superoxide dismutase, resulted in the transfer of the radical anion electron to oxygen with the production of oxygen superoxide and peroxide. Peroxide production also occurred with menadione and nitrofurazone. The production of the oxygen superoxide radical and peroxide was also demonstrated with isolated microsomes in the presence of 4-NQO, nitrofurazone, and menadione. Neither cell suspensions nor isolated microsomes reduced the nitrofurans to stable intermediates in the presence of oxygen; however, the reduction of 4-NQO to the corresponding hydroxylamino derivative was demonstrated spectrophotometrically in aerobic suspensions of both Ehrlich and V79 cells. The reduction was found to be dependent on reducing substrates and was stimulated by KCN and by anoxia. In addition, the 4-NQO-mediated killing of aerobic V79 cells was found to be dependent on cell density and on temperature, indicating that metabolic activation of the drug is necessary for its cytotoxicity.