Bioactivation of mitomycin C by xanthine dehydrogenase from EMT6 mouse mammary carcinoma tumors.

Abstract

Mitomycin C is an antineoplastic antibiotic requiring bioactivation to an alkylating species or to an intermediate capable of generating oxygen radicals for its toxic effect. The enzymes responsible for the in vivo activation of mitomycin C have been proposed to include NADPH-cytochrome-c reductase, DT-diaphorase, and xanthine oxidase. In this study, xanthine dehydrogenase, an enzyme structurally similar to xanthine oxidase, was assessed for its ability to activate mitomycin C. Partially purified xanthine dehydrogenase, from EMT6 mouse mammary tumors, was investigated for its ability to bioactivate mitomycin C under both aerobic and hypoxic conditions. We conducted this analysis by measuring mitomycin C-induced oxygen consumption, alkylating potential, and mitomycin C consumption and metabolite formation as determined by high-pressure liquid chromatography analysis. Bioactivation of mitomycin C by xanthine dehydrogenase under both aerobic and hypoxic conditions gave rise to the formation of a metabolite, 2,7-diaminomitosene. Formation of this metabolite and alkylating ability were greater under hypoxic than under aerobic conditions and were increased when the pH was decreased from 7.4 to 6.0. Mitomycin C consumption was the same under both aerobic and hypoxic conditions and was independent of pH. Oxygen consumption studies showed that xanthine dehydrogenase-activated mitomycin C consumed oxygen at a much lower rate than xanthine oxidase-activated mitomycin C. Xanthine dehydrogenase-activated mitomycin C appears to be a good alkylating species but a relatively poor generator of reactive oxygen when compared with xanthine oxidase activation under aerobic conditions. Xanthine dehydrogenase may play an important role in the bioactivation of mitomycin C to an alkylating species under both aerobic and hypoxic conditions.