Hydroxylation of 4,4'-methylenebis(2-chloroaniline) by canine, guinea pig, and rat liver microsomes

An in vitro microsomal mixed function oxidase enzyme system was used to study the phase I metabolism of 4,4'-methylenebis(2-chloroaniline) (MBOCA) by dog, guinea pig, and rat liver. TLC with color development and autoradiography, and HPLC with detection by UV absorbance and radioactivity flow monitoring were utilized to isolate metabolites. Reference standards of the N-oxidized metabolites were prepared by oxidation of MBOCA with 3-chloroperoxybenzoic acid and structures confirmed by mass spectrometry and proton NMR. These were utilized to identify the N-hydroxy and nitroso metabolites of MBOCA isolated from the microsomal incubations by comparison of their HPLC retention times and mass spectra. The structure of the o-hydroxy metabolite (ring, ortho to the amine) isolated from the microsomal incubations was elucidated by mass spectrometry and proton NMR. N- and o-hydroxylations of MBOCA were shown to increase with incubation time, microsomal protein, substrate, and NADPH concentration, and were inhibited by 2,3-dichloro-6-phenylphenoxyethylamine, an inhibitor of the microsomal mixed function oxidase enzyme system. Guinea pig liver microsomes oxidized MBOCA to the N-hydroxy metabolite predominantly, whereas the dog liver formed predominantly the o-hydroxylated metabolite, with significant amounts of the hydroxylamine as well. The rat liver formed lesser amounts of the N- and o-hydroxylated metabolites, but larger numbers of other polar compounds.