Generation of superoxide by the microsomal mixed-function oxidase system.

Abstract

The microsomal mixed-function oxidase (MFO) system catalyzes the oxidation of numerous endogenous and exogenous compounds using NADPH as its electron donor. During its oxidation of drug substrates and even in the absence of substrates, this electron transport system has been demonstrated to produce O2·, H2O2, and excess H2O. We have investigated the ability of several purified forms of cytochrome P450 to produce O2· when incubated with purified NADPH cytochrome P450 reductase. Addition of cytochrome P450 isozymes to the purified reductase resulted in increased rates of O2· formation, assessed using either EPR spin trapping techniques or acetylated cytochrome c reduction. Conditions necessary for optimal rates of O2· production were similar to those previously shown to be necessary for optimal reconstitution of MFO activity (phospholipid, detergent-solubilized reductase, cytochrome P450:reductase ratios of greater than 3:1). Cytochrome P450d addition to incubations of reductase resulted in greater rates of O2· production than either cytochrome P450c or P450b, especially at equimolar concentrations of cytochrome P450 isozymes and reductase. The contribution of cytochrome P450d to microsomal O2· generation was addressed by investigating O2· production by microsomes isolated from isosafrole-treated rats.