A novel ring oxidation of 4- or 5-substituted 2H-oxazole to corresponding 2-oxazolone catalyzed by cytosolic aldehyde oxidase.

The ring oxidation of 2H-oxazole, or C2-unsubstituted oxazole, to 2-oxazolone, a cyclic carbamate, was observed on various 4- or 5-substituted oxazoles. Using 5-(3-bromophenyl)oxazole as a model compound, its 2-oxazolone metabolite M1 was fully characterized by LC/MS/MS and NMR. The reaction mainly occurred in the liver cytosolic fraction without the requirement of cytochrome P450 enzymes and co-factor NADPH. Investigations into the mechanism of formation of 2-oxazolone using various chemical inhibitors indicated that the reaction was primarily catalyzed by aldehyde oxidase and not by xanthine oxidase. In addition, cytosol incubation of 5-(3-bromophenyl)oxazole in the medium containing H218O led to the 18O incorporation into M1, substantiating the reaction mechanism of a typical molybdenum hydroxylase. The rank order of liver cytosols for the 2-oxazolone formation was mouse>monkey>>rat and human liver cytosol, whereas M1 was not formed in dog liver cytosol. Since the reaction was observed with a number of 4- or 5-substituted 2H-oxazoles in mouse liver cytosols, 2H-oxazoles represent a new substrate chemotype for ring oxidation catalyzed by aldehyde oxidase.