Molecular enzymology of the reductive bioactivation of hypoxic cell cytotoxins

The hypoxic cell cytotoxins SR 4233, benznidazole (Benzo), and CB 1954 were readily reduced by anaerobic mouse liver microsomes in vitro to their respective amino or single N-oxide derivatives. The reactions were inhibited in air and required reduced cofactors, particularly NADPH. The rates of reductive bioactivation were markedly different for each drug, with SR 4233 much greater than CB 1954 greater than Benzo. Using purified cytochrome P-450 reductase (P-450 reductase) and an inhibitory antibody to this enzyme, we demonstrated that P-450 reductase was involved in the reductive bioactivation of all 3 compounds. It had a minor role in SR 4233 reduction, but a more important involvement in CB 1954 metabolism to its 4-amino metabolite. Using carbon monoxide, a specific inhibitor of cytochrome P-450 (P-450), we demonstrated that P-450 was involved in both SR 4233 and Benzo reduction. P-450 had a major role both in SR 4233 conversion to SR 4317 and in the latter steps of Benzo amine formation. Purified xanthine oxidase was shown to reduce SR 4233 and Benzo in vitro, but cytosolic aldehyde oxidase activity was only detectable with Benzo as substrate. Characterizing the relative participation of the various reductases in tumor versus normal tissues may allow a more rational selection and application of hypoxic cell cytotoxins in cancer therapy.