Formation of glutathione conjugates during oxidation of eugenol by microsomal fractions of rat liver and lung.

Rat hepatic and pulmonary microsomes catalyzed the formation of at least three distinct glutathione conjugates with eugenol (4-allyl-2-methoxyphenol). These three conjugates were identical with the products obtained from the chemical reaction of synthetic eugenol quinone methide and glutathione. The microsomal reaction was dependent on NADPH and oxygen and was inhibited by cytochrome P450 inhibitors such as metyrapone, 2-diethylaminoethyl-2,2'-diphenylvalerate (SKF 525-A), alpha-naphthoflavone and piperonyl butoxide. The enzyme responsible for eugenol oxidation was inducible with 3-methylcholanthrene but not phenobarbital pretreatment. The rate of formation of conjugates was not affected by the presence of glutathione-depleted cytosol which contained active glutathione transferase, even at low glutathione concentrations, suggesting that conjugation occurs nonenzymatically with an electrophilic metabolite of eugenol. Covalent binding to microsomal protein was observed using [3H]eugenol. Cumene hydroperoxide catalyzed the formation of these same glutathione conjugates via the formation of a quinone methide-like intermediate which was detected by spectroscopic means. Our results suggest that eugenol is oxidized by cytochrome P450 to a reactive quinone methide intermediate which can then covalently modify protein or conjugate with glutathione.