Mechanisms of oxygen activation by nitrofurantoin and relevance to its toxicity

Purified ferredoxin-(cytochrome c)-NADP⁺ oxidoreductase and xanthine oxidase were found to catalyse the reduction of nitrofurantoin to the free radical. Under aerobic conditions, the nitrofurantoin radical underwent autoxidation to regenerate the parent compound with the concomitant production of Superoxide and eventually hydrogen peroxide. The nitrofurantoin radical was also shown to react with hydrogen peroxide to generate a highly reactive species which was capable of oxidising methionine to ethylene. This active oxygen radical appeared to be identical with the crypto-OH radical, previously proposed as being formed from the analogous reaction of the methyl viologen radical with hydrogen peroxide [R.J. Youngman and E.F. Elstner, FEBS Lett. 129, 265 (1981)]. Catalase inhibited nitrofurantoin-dependent ethylene formation in both enzyme systems, whereas Superoxide dismutase was only inhibitory in the xanthine oxidase mediated reaction. Although the primary function of the respective enzyme systems is to generate the nitrofurantion radical, the xanthine oxidase reaction is markedly more complex than that of ferredoxin-(cytochrome c)-NADP⁺ oxidoreductase. The differences between the two enzyme reactions appear to be due to the endogenous autoxidation of xanthine oxidase. The aerobic activation of nitrofurantoin by xanthine oxidase involved the Superoxide anion as an intermediate, whereas the nitrofuran was directly reduced by ferredoxin-(cytochrome c)-NADP⁺ oxidoreductase without a requirement for active oxygen species.