Aerobic Reductive "Activation" of 5-Nitro-2-Furaldehyde Semicarbazone by Rat Liver Xanthine Dehydrogenase

Abstract

5-Nitrofurans are synthetic antibacterial agents. In general, nitrofurans have been shown to be toxic and mutagenic to cultured mammalian cells and carcinogenic in rodents. The possibility that human exposure to nitrofurans may be causing genetic damage or cancer has stimulated research directed towards elucidating the metabolism and mechanism of action of these compounds. A comprehensive understanding of the molecular basis of nitrofuran action may also be useful for comprehending the mechanism of action of other aryl and heterocyclic nitro compounds. It is known that enzymatic reduction of nitrofurans to reactive but uncharacterized metabolites that damage DNA constitutes an important "activation" step in both bacteria and hypoxic mammalian cells. However, since the known mammalian enzymes having nitroreductase activity are reported to be strongly inhibited by molecular oxygen, the relation of reductive activation to the DNA-damaging effects of nitrofurans in intact animals or in aerobic cultured cells is unclear. In rodents the liver is a major site of nitrofuran reduction in vivo. Net reduction of 5-nitro-2-furaldehyde semicarbazone (nitrofurazone) by rat liver homogenate was found to be relatively insensitive to oxygen when compared to net nitroreduction by milk xanthine oxidase. Intermediates generated in the aerobic nitroreduction bound tightly and probably covalently to protein. The nitroreductase in the rat liver preparation was identified as xanthine oxidoreductase by its apparent MW, substrate specificity and inhibition by allopurinol. Xanthine oxidoreductase is known to function in vivo as xanthine dehydrogenase (D form) which is converted to xanthine oxidase (0 form) during purification and storage. The 0 form is considered to be the major cytosolic nitroreductase and its activity is strongly inhibited by oxygen in vitro. Net nitroreduction by the D form has not been studied previously. In the rat liver preparation the bulk of the aerobic nitroreductase activity was associated with the D form of xanthine oxidoreductase during chromatography on CM cellulose, heat conversion of D form to 0 form and chemical interconversion of D form to 0 form and back to D form. Thus, net reduction of nitrofurazone by xanthine dehydrogenase is considerably less sensitive to inhibition by oxygen than is net nitroreduction by rat liver or milk xanthine oxidase. The ability of xanthine dehydrogenase to reduce nitrofurazone aerobically to highly reactive species in vitro suggests that this enzyme may play a role in a nitroreductive process which contributes to the mutagenic and carcinogenic action of nitrofurans and other nitroheterocyclic and nitroaromatic compounds in vivo. On the other hand, the nitroreductase activity of xanthine dehydrogenase in non-target tissues may, in some cases, decrease the amount of nitrocompound available in target tissues and hence play a "protective" role.