Comparison of the arachidonic acid and NADPH-dependent microsomal metabolism of naphthalene and 2-methylnaphthalene and the effect of indomethacin on the bronchiolar necrosis

Abstract

Naphthalene and 2-methylnaphthalene cause a highly organo- and species-selective lesion of the pulmonary bronchiolar epithelium in mice. Naphthalene- but not 2-methylnaphthalene-induced pulmonary bronchiolar injury is blocked by prior administration of the cytochrome P-450 monooxygenase inhibitor piperonyl butoxide, thus suggesting that metabolism by enzymes other than the P-450 monooxygenases may be important in 2-methylnaphthalene-induced lung injury. Since many of the polycyclic aromatic hydrocarbons are metabolized by the prostaglandin endoperoxide synthetase system and because detectable xenobiotic metabolizing activity has been associated with the prostaglandin synthetases in the Clara cell, the studies reported here were done to compare NADPH- versus arachidonate-dependent metabolism of naphthalene and 2-methylnaphthalene in vitro and to determine whether indomethacin, a potent inhibitor of prostaglandin biosynthesis, was capable of blocking the in vivo toxicity of these two aromatic hydrocarbons. The NADPH-dependent metabolism of naphthalene and 2-methylnaphthalene to covalently bound metabolites in lung or liver microsomal incubations occurred at easily measurable rates. Renal microsomal NADPH-dependent metabolism of either substrate was not detected. The formation of covalently bound naphthalene or 2-methylnaphthalene metabolites was dependent upon NADPH and was inhibited by the addition of reduced glutathione, piperonyl butoxide, and SKF 525A. Covalent binding of radioactivity from [ 14C]2-methylnaphthalene also was strongly inhibited by incubation in a nitrogen atmosphere or at 2°. The arachidonic acid-dependent formation of reactive metabolites from naphthalene or 2-methylnaphthalene was undetectable in microsomal incubations from lung, liver or kidney. Indomethacin, 1 hr before and 6 hr after the administration of 300 mg/kg naphthalene or 2-methylnaphthalene, failed to block the pulmonary bronchiolar injury induced by these aromatic hydrocarbons. These studies suggest that the major enzymes involved in the metabolic activation of naphthalene or 2-methylnaphthalene in vitro are the cytochrome P-450 monooxygenases and that cooxidative metabolism by the prostaglandin synthetases appears to play little role in the formation of reactive metabolites in vitro.