Distinct reduction of nitrofurans and metronidazole to free radical metabolites by Tritrichomonas foetus hydrogenosomal and cytosolic enzymes.

Anaerobic Tritrichomonas foetus hydrogenosomes supplemented with pyruvate and CoA effectively reduce nitrofurans and metronidazole to their respective anion free radicals. Addition of purified ferredoxins from Clostridium pasteurianum or Spinacia oleracea to these preparations causes a great stimulation of metronidazole reduction, but does not affect nitrofuran reduction. A similar stimulatory effect of ferredoxin on metronidazole reduction, but not on nitrofuran reduction, is observed in incubations containing purified NADPH:ferredoxin oxidoreductase from S. oleracea. NADH is less effective than pyruvate as a reducing cofactor for metronidazole and nitrofuran reduction by the hydrogenosomes, and these activities are not modified by the addition of ferredoxins. In contrast to the results observed with hydrogenosomes, the T. foetus soluble fraction supplemented with NADH or NADPH is able to reduce nitrofurans, but not metronidazole. Under aerobic conditions, the anion free radical metabolites generated from metronidazole and nitrofurans are oxidized, resulting in catalytic superoxide anion formation as detected by spin-trapping experiments. Oxygen consumption and H2O2 formation by T. foetus hydrogenosomes and NADPH:ferredoxin oxidoreductase are also stimulated by nitrofurans and high concentrations of metronidazole. Addition of ferredoxin enhances metronidazole-stimulated, but not nitrofuran-stimulated, oxygen consumption and H2O2 formation in both systems. These results support the role of air oxidation as a detoxification reaction of the metronidazole anion radical and the involvement of ferredoxin in its formation. On the other hand, redox cycling of nitrofurans with formation of high steady state concentrations of oxygen-derived radicals might be of toxicological significance.