Role of hepatic microsomal and purified cytochrome P-450 in one-electron reduction of two quinone imines and concomitant reduction of molecular oxygen

The possible role of cytochrome P-450 in one-electron reduction of quinoid compounds as well as in the formation of reduced oxygen species was investigated in hepatic microsomal and reconstituted systems of purified cytochrome P-450 and purified NADPH-cytochrome P-450 reductase using electron spin resonance (ESR) methods. Two compounds were selected as model compounds: N-acetyl-parabenzoquinone imine (NAPQI) and 3,5-dimethyl-N-acetyl-para-benzoquinone imine (3,5-dimethyl-NAPQI). Both compounds could be reduced by oxyhaemoglobin, the semiquinones formed were detectable by ESR and did not reduce molecular oxygen. Both NAPQI and 3,5-dimethyl-NAPQI underwent one-electron reduction in microsomal systems and in fully reconstituted systems of cytochrome P-450 and NADPH-cytochrome P-450 reductase under anaerobic and aerobic conditions. In both incubation systems the semiquinone formation was diminished under aerobic circumstances and concomitant reduction of oxygen occurred, leading to the formation of hydrogen peroxide and hydroxyl free radicals. Both the reduction of the quinone imines and the reduction of oxygen were found to be cytochrome P-450 dependent. Both activities of cytochrome P-450 may also be involved in the bioactivation of other compounds with quinoid structural elements, like many chemotherapeutic agents.