Toxicity and effects of 2,6-di-tert-butyl-4-methylphenyl N-methylcarbamate (terbutol) on hepatic cytochrome P450 in F344 rats

The subacute toxicity and effects of 2,6-di-tert-butyl-4-methylphenyl N-methylcarbamate (terbutol) on hepatic microsomal cytochrome P450 (P450) were investigated in male and female F344 rats. Rats were given 0.25, 0.5, and 1.0% terbutol for 28 days. Liver weights of male and female rats increased at all dose levels. The compound did not affect activity or amount of serum biochemical markers related to hepatic damage. The concentrations of terbutol in rat serum were less than 0.1 muM, and its major metabolites in serum were 2,6-di-teat-butyl-4-carboxyphenyl N-methyl-carbamate and 2,6-di-tert-butyl-4-carboxyphenol. In male rats, P450 and cytochrome b(5) (b(5)) contents, and NADPH cytochrome c reductase (fp2) activity in liver microsomes were increased about 2-fold by 1% terbutol administration for 7 to 28 days. Among the P450-dependent monooxygenase activities in liver microsomes, 7-benzyloxyresorufin-O-debenzylase (BROD) activity was greatly increased by 100-fold, and 7-ethoxyresorufin-O-deethylase (EROD), 7-ethoxycoumarin-O-deethylase (ECOD), and aminopyrine-N-demethylase (APND) activities were elevated 2- to 3-fold. 7-Methoxyresorufin-O-deethylase (MROD), erythromycin-N-demethylase (EMND), estradiol 2-hydroxylase (ED2H), chlorzoxazone 6-hydroxylase (CZ6H), and lauric acid omega -hydroxylase (LAOH) activities were unchanged. For the activities of testosterone hydroxylation, testosterone 16 beta -hydroxylase (T16BH) activity was markedly increased by 30-fold, and testosterone 6 beta -hydroxylase (T6BH) and testosterone 7 alpha -hydroxylase (T7AH) activities were slightly elevated. Testosterone 2 alpha -hydroxylase (T2AH) activity was not affected. Terbutol 4-methylhydroxylase (T4MH) activity was increased 9-fold by 1% terbutol. In an immunoinhibition study, T4MH activity in liver microsomes from 1% terbutol-treated rats was decreased about 50% by polyclonal anti-rat CYP2B1, whereas polyclonal anti-rat CYP2A1 and CYP2C11 did not affected the activity. These results indicate that terbutol increased CYP2B subfamily in rat liver microsomes, and that the compound did not cause serious hepatic damage.