Hepatic and pulmonary microsomal metabolism of naphthalene to glutathione adducts: factors affecting the relative rates of conjugate formation

Abstract

Earlier studies demonstrating marked differences in the profile of polar metabolites formed during incubations of glutathione, naphthalene and microsomes from target (lung) and nontarget (liver and kidney) tissues of the mouse suggested that the formation of a particular reactive metabolite may be the underlying basis for the highly organ selective toxicity of this hydrocarbon. The studies reported here were done to characterize more fully the microsomal metabolism of naphthalene to 1,2-dihydro-1,2-dihydroxynaphthalene and to three glutathione-derived conjugates that were separated by high-pressure liquid chromatography. The microsomal formation of polar naphthalene metabolites was linear with time and microsomal protein; the relative proportions of each of the metabolites remained relatively stable over the range of time and protein concentrations studied. The rate of formation of naphthalene glutathione adducts, but not the dihydrodiol, was dependent upon the amount of 100,000 X g supernatant protein added. Addition of lung cytosol to liver microsomal incubations or liver cytosol to lung microsomal incubations altered the overall rate of conjugate formation but not the relative proportions of each of the three conjugate peaks. Epoxide hydrolase induction by dietary butylated hydroxyanisole or inhibition by cyclohexene oxide altered the rate of hepatic microsomal formation of naphthalene dihydrodiol in the expected manner and increased the production of conjugate peak 2. Butylated hydroxyanisole or cyclohexene oxide failed to alter the rate of formation of conjugate peak 1 or 3. Addition of piperonyl butoxide or SKF 525-A to hepatic microsomal incubations markedly decreased covalent binding of naphthalene metabolites but only slightly decreased glutathione adduct formation. Dihydrodiol formation was increased by both inhibitors. Phenobarbital or 3-methylcholanthrene pretreatment produced a marked increase in the pulmonary microsome-catalyzed formation of all four polar naphthalene metabolites. In comparison, phenobarbital increased the rates of formation of the dihydrodiol, conjugate peaks 1 and 2 but not 3 in hepatic microsomes. 3-Methylcholanthrene increased the rate of formation of the dihydrodiol and conjugate peak 2 but not 1 or 3. These studies indicate that the predominant formation of conjugate peak 2 in lung microsomal incubations in comparison to liver microsomal incubations is due to the regio- or stereoselectivity of naphthalene metabolism by cytochrome P-450 monooxygenases or epoxide hydrolases but not by the glutathione transferases.