Characterization of hamster liver nicotine metabolism--II. Differential effects of ethanol or phenobarbital pretreatment on microsomal N and C oxidation

Variables affecting nicotine metabolism

Nicotine metabolism is exceedingly sensitive to perturbation by numerous host factors. To reduce the large variations and discrepancies in the literature pertaining to nicotine metabolism, investigators in future studies need to recognize and better control these host factors. Recent advances in the understanding of nicotine metabolism have suggested new approaches to elucidating underlying mechanisms of certain toxic effects associated with cigarette smoking.

Metabolism of S-nicotine in noninduced and aroclor-induced rats

The urinary excretion of nicotine and its metabolites in noninduced and Aroclor-induced male and female rats has been determined following intravenous administration of 2'-[14C]-labeled S-nicotine at a dose of 4.6 mumol/kg. Complete recovery of the administered radioactivity was achieved: 95% in urine and 4% in feces over 96 h and 1% remaining in the body. More than 40 nicotine metabolites were found by radio-HPLC; 19 were identified including the cis/trans-diastereomers of nicotine-N'-oxide and 3'-hydroxycotinine. The urinary metabolite profile and excretion kinetics of nicotine and its metabolites were significantly different between noninduced and Aroclor-induced rats. The major urinary nicotine metabolite in the noninduced rat was cis-nicotine-N'-oxide. In the Aroclor-induced rat, cotinine metabolites were the major metabolites found. Sex differences were found for the urinary nicotine metabolite profile, mainly expressed in the excretion of cis-nicotine-N'-oxide, 29% in the male and 17% in the female noninduced rat, and the excretion of cotinine, 5% in the male and 12% in the female noninduced rat. High stereoselectivity was found for the formation of the cis/trans-diastereomers of nicotine-N'-oxide as well as of 3'-hydroxycotinine, the stereoselectivity being more pronounced in male rats.

Metabolism of nicotine by hepatocytes

The profile of nicotine metabolites produced by freshly isolated hepatocytes from rats, hamsters, guinea pigs, mice and humans was investigated after a 30-min exposure to nicotine ([2-14C]pyrrolidine). Large species differences occurred in the extent of nicotine metabolism; these ranged from 95% metabolism in guinea pig hepatocytes to only 30% metabolism in human and rat hepatocytes. The spectrum of metabolites formed also varied widely in different species. In hepatocytes from obese human subjects, nicotine was metabolized most extensively in smokers, least in nonsmokers, and to an intermediate degree in exsmokers, suggesting that cigarette smoking enhances the rate of nicotine metabolism. Pretreatment of all nonhuman species studied with phenobarbital and beta-naphthoflavone and with Aroclor in rats produced distinctive inductive patterns. Phenobarbital pretreatment of nonsmokers for 2 days prior to liver biopsy doubled the extent of nicotine conversion to cotinine by their hepatocytes. Rat and hamster hepatocytes exhibited sex and stereoselectivity differences in nicotine metabolism. Collectively, these studies indicate that hepatocytes offer some advantages over in vivo systems in investigating certain aspects of nicotine metabolism.

Biochemical and immunochemical evidence for the induction of an ethanol-inducible cytochrome P-450 isozyme in male Syrian golden hamsters

The effects of ethanol and of phenobarbital pretreatment on hamster microsomal metabolism of aniline and p-nitrophenol have been investigated. Hydroxylation of both compounds was increased over 2-fold by ethanol pretreatment, whereas phenobarbital pretreatment had little effect on either activity. Ethanol pretreatment had no effect on the specific content of total cytochrome P-450, while phenobarbital pretreatment increased the specific content 1.6-fold. Comparison of the specific activities for aniline hydroxylation and p-nitrophenol hydroxylation of individual microsomal samples from control, ethanol-pretreated and phenobarbital-pretreated animals showed a high degree of correlation (r2 = 0.98) consistent with the involvement of the same site for catalysis of these two compounds. Antibody to rabbit ethanol-inducible cytochrome P-450 (isozyme 3a) inhibited over 80% of the aniline (high affinity) and p-nitrophenol hydroxylase activities of microsomes from ethanol-treated hamsters. A comparison of the antibody-inhibitable rates for both hydroxylase activities with microsomes from untreated, ethanol- or phenobarbital-pretreated hamsters suggested that an isozyme homologous to rabbit isozyme 3a (hamster cytochrome P-450alc) was induced in hamsters about 3.5-fold by ethanol and was unaffected by phenobarbital. The induction of hamster cytochrome P-450alc was confirmed by immunoblot analysis of hamster microsomes. A single protein with a molecular weight of approximately 54,000 was recognized by antibody to the rabbit isozyme. Quantification of the immunoblots demonstrated that the hamster isozyme was increased about 3-fold, in good agreement with the induction determined by a comparison of the antibody-inhibitable rates. The results indicated that, although there was no change in the total spectrally observable cytochrome P-450, there was a marked change in the distribution of the isozymes of cytochrome P-450, with an increase in the alcohol-inducible form after 28-day ethanol consumption by chow-fed hamsters. This isozyme can be readily monitored by either high-affinity aniline or p-nitrophenol hydroxylation or by Western immunoblot analysis and appears to be the ethanol-inducible form of cytochrome P-450 in hamsters.