Kinetics of NAD(P)H:quinone oxidoreductase I (NQO1) inhibition by mitomycin C in vitro and in vivo.

The bioreductive activation of the antitumor quinone mitomycin C (MMC) by NAD(P)H: quinone oxidoreductase 1 (NQO1) is complicated by the ability of MMC to also act as a mechanism-based inhibitor of NQO1 in a pH dependent manner. Inhibition of NQO1 by MMC has been studied in purified enzyme preparations and in cultured cells but has not determined in vivo. In the studies presented here, NQO1 activity was measured in mouse tissues following treatment with MMC or the potent mechanism-based human NQO1 inhibitor 5-methoxy-1,2-dimethyl-[(4-nitrophenoxy)methyl]indole-4,7-dione (ES936). NQO1 activity was significantly decreased at 1, 2, and 4 h following MMC (10 or 20 mg/kg) treatment in kidney and lung but was unchanged in brain, heart, liver, and bladder. ES936 (1 mg/kg) treatment led to a significant and much more potent inhibition of NQO1 in all murine tissues analyzed except for bladder. To extrapolate these in vivo results from mice to humans, the species-specific kinetics of NQO1 inactivation by MMC was determined in vitro using mouse, rat, and human recombinant NQO1 proteins. Results showed the inactivation kinetics of mouse and human proteins by MMC were similar. Treatment of human and murine endothelial cells with MMC or ES936 showed similar inhibition of NQO1 activity. The aforementioned results clearly demonstrate that MMC can serve as a substrate for NQO1 in vivo; however, the metabolism resulting in enzyme inactivation is possibly tissue-specific. Furthermore, the kinetic similarities for inactivation between murine and human forms of NQO1 show these results are apropos to clinical use of MMC.