Contribution of aniline metabolites to aniline-induced methemoglobinemia.

Methemoglobinemia after aniline and certain aniline derivatives is thought to be mediated by toxic metabolites formed during the hepatic clearance of the parent compounds. However, three aniline metabolites--phenylhydroxylamine, 2-aminophenol, and 4-aminophenol--catalyze methemoglobin formation in erythrocyte suspensions and, hence, could contribute to methemoglobin formation in vivo after aniline. To determine the relative contributions of these aniline metabolites to aniline-induced methemoglobinemia in rats, we determined time courses of methemoglobinemia in rat erythrocyte suspensions and in rats after treatment with 2- and 4-aminophenol, phenylhydroxylamine, and aniline. The relative potencies for methemoglobin production in vitro after phenylhydroxylamine, 2-aminophenol, and 4-aminophenol were about 10:5:1, based on both peak and area of the methemoglobin versus time curve. Approximate minimum concentrations for observable methemoglobin formation in vitro from these compounds were 20, 50, and 200 microM, respectively. Compared with the in vitro data, the relative potencies of the aminophenols for methemoglobinemia in rats after intraperitoneal injections were reduced with respect to phenylhydroxylamine (to 100:4:1, respectively), apparently as a result of rapid in vivo clearance of the aminophenols. Subsequent experiments, in which the time courses of the aniline metabolites were determined in blood after toxic doses of aniline, demonstrated that only phenylhydroxylamine (measured as phenylhydroxylamine + nitrosobenzene) accumulated to blood levels exceeding the minimum concentration required for methemoglobin production in vitro. In addition, blood levels of phenylhydroxylamine remained in the toxic range throughout most of the methemoglobinemic response after aniline treatment. These data are consistent with phenylhydroxylamine being the sole mediator of aniline-induced methemoglobinemia in these rats.