Abstract

The metabolism of furazolidone by rat liver and Escherichia coli was characterized in vitro under aerobic and anaerobic incubation conditions. Rat liver 9000g supernatant rapidly metabolized 14C-furazo-lidone to more polar metabolites in the presence or absence of oxygen when NADPH was provided as a cofactor. At least five polar radiolabeled metabolites were detected in these incubations by high pressure liquid chromatography. Moreover, a significant (30-40%) proportion of the total radiolabeled metabolites remained tightly associated with liver protein despite repeated organic solvent extractions of the tissue. The major solvent-extractable metabolites produced under aerobic and anaerobic incubation conditions were isolated and analyzed by mass spectrometry. The mass spectra indicated that these derivatives possessed the same chemical structure. Subsequently, this metabolite was unequivocally identified as 3-(4-cyano-2-oxobutylideneamino)-2-oxazolidinone, an end product of reductive metabolism of the nitro group of furazolidone. The formation of the reduced metabolite under aerobic conditions indicated that this metabolic pathway was markedly less sensitive to oxygen than many previously studied nitroreduction reactions catalyzed by mammalian enzymes. This NADPH-dependent, oxygen-insensitive nitroreductase activity was further localized to the microsomal fraction of rat liver. E. coli also rapidly metabolized furazolidone (FZN) to a complex series of metabolites, including the reduced cyano metabolite, under both aerobic and anaerobic conditions. Sonic lysis of the bacteria released an NADPH-dependent, oxygen-insensitive nitroreductase which converted FZN to the cyano metabolite and other unidentified derivatives. The complete reduction of FZN by the solubilized bacterial enzyme was strongly inhibited by the addition of the thiol nucleophile glutathione to the incubation medium.