The kinetics of the aerobic reduction of nitrofurantoin by NADPH-cytochrome P-450 (c) reductase

In vivo studies have indicated that mammalian organisms can reduce nitro aromatic compounds. This reduction occurs primarily in liver. On the other hand, nitro reductase activity is not observed in vitro for liver preparations under the aerobic conditions found in mammalian tissues. Studies from our laboratory suggest that this inhibition is due to the rapid reoxidation of the first reduction product, the nitro aromatic anion radical. We now find that physiological concentrations of detergent-solubilized microsomal NADPH-cytochrome P-450 (c) reductase can reduce nitrofurantoin aerobically. The rate is about 14% of the anaerobic rate of metabolism. This reduction is second-order in both substrate and enzyme, in agreement with the concept that it proceeds through the initial formation of the 5-nitrofuran amino radical. This radical then disproportionates to form the nitroso and reform the parent drug. These data indicate that the rate of disproportion of the anion radical is comparable to the rate of the reoxidation of the radical by oxygen. Furthermore, since these results demonstrate rapid nitro reduction an aerobic mammalian system, they lend support to the hypothesis that the nitroaromatic carcinogens and radiosensitizers are metabolically activated in vivo by reduction to their nitroso and hydroxylamine derivatives.