Respiratory effects and ascorbate reactions with misonidazole and other recently developed drugs

A number of nitro compounds behave as electron acceptors with ascorbate in a non-enzyme-catalyzed reaction and with NADPH as electron donor in an enzyme-catalyzed reaction with liver homogenate. In both systems, the oxygen consumption is conveniently monitored by a Clark-type oxygen electrode. The oxygen consumption by S9 homogenate was found to be influenced by the electron affinity of the nitro compound. Detailed studies with misonidazole as the electron acceptor demonstrated that the oxygen consumption was influenced by protein and drug concentrations. Various inhibitors of electron transfer reactions, such as rotenone, antimycin A, and sodium azide, had no effect on misonidazole-stimulated oxygen utilization. However, cyanide was found to be stimulating in this system and this may be due to the inhibition of enzymes such as catalase and superoxide dismutase known to be present in S9. The oxygen consumption by S9 did not appear to be due to a hydroxylating reaction because metyrapone, a known inhibitor of this reactions, had no effect. Methional, a hydroxyl radical scavenger, was inhibitory as was glutathione. Clelands reagent had no effect. Additional studies with thiol-reactive agents suggest that a protein thiol may be important for the enzyme-catalyzed reaction between NADPH, misonidazole, and oxygen.