Reductive activation of mitomycin C by NADH:cytochrome b5 reductase.

Mitomycin C requires bioreduction in order to exert its cytotoxic action. Activation of mitomycin C to an electrophile was equally supported by NADPH and NADH in EMT6 tumor cell sonicates under hypoxia. No alkylation was observed under aerobic conditions. Purified NADH:cytochrome b5 reductase catalyzed the reduction of mitomycin C with a Km of 23 microM at pH 6.6. At pH 7.6, the rate of enzymatic reduction of mitomycin C was 61% of that at pH 6.6. NADH:cytochrome b5 reductase catalyzed the activation of mitomycin C to alkylating metabolites under both hypoxic and aerobic conditions, with alkylation being 1.5 times greater in hypoxia. Dicumarol at 100 microM inhibited the NADH:cytochrome b5 reductase-catalyzed reduction of mitomycin C by 24% and by 57% at 300 microM. The degree of inhibition of the enzyme by dicumarol was the same at both pH 6.6 and 7.6. NADH:cytochrome b5 reductase exhibited a small but measurable NADH-oxidase activity, which was unaffected by 300 microM dicumarol. These findings demonstrate that (a) NADH:cytochrome b5 reductase can metabolically activate mitomycin C and (b) dicumarol is capable of inhibiting this enzymatic activity.