Participation of cytochrome P-450 in nicotine oxidation

Modulation of the activation of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone by hamster liver microsomes to protein alkylating species

During incubation with hamster liver microsomes, the tobacco-specific N-nitrosamine, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) was metabolized by alpha-carbon hydroxylation, pyridine N-oxidation and carbonyl reduction. The modulation of the activation of NNK to intermediates, alkylating microsomal proteins, by some compounds related to tobacco smoke, P-450 inhibitors, thiol and alcohol compounds, has been determined. Binding of both the pyridyloxobutyl and the methyl moieties of NNK was inhibited by more than 40% by N'-nitrosonornicotine or N-nitrosodimethylamine. In contrast, nicotine and 3-acetylpyridine inhibited only the binding of the pyridyloxobutyl moiety. Binding of both moieties of NNK was not inhibited by its deactivation metabolite, the NNK-N-oxide. Whereas the P-450 inhibitors metyrapone, [2-(diethylamino)ethyl 2,2-diphenyl pentenoate]hydrochloride and piperonyl butoxide inhibit the binding of the pyridyloxobutyl moiety selectively, carbon monoxide inhibits the binding of both moieties. Metyrapone reduced alpha-carbon hydroxylation but not pyridine N-oxidation, suggesting that in hamster livers pyridine N-oxidation is not mediated by P-450-dependent monooxygenases. The binding of the pyridyloxobutyl and methyl moieties was partially dependent on the presence of NADPH in the incubation mixture. Glutathione had no effect on the metabolism of NNK but inhibited the enzymatic and non-enzymatic binding of its pyridyloxobutyl moiety. It is concluded that the binding of NNK-derived intermediates to cellular proteins is inhibited by various substances present in tobacco smoke and by inhibitors of P-450 monooxygenases. Only part of this binding is enzymatically mediated.

Cotinine effects on nicotine metabolism

BACKGROUND

Nicotine clearance and half-life are known to be significantly reduced in smokers compared to nonsmokers. Cotinine is the major primary metabolite of nicotine, and it accumulates in the body with regular smoking. Nicotine and cotinine appear to be metabolized by the same liver enzyme. Therefore we hypothesized that cotinine inhibits nicotine metabolism, resulting in slower nicotine clearance in smokers compared with nonsmokers.

METHODS

This was a crossover, randomized, double-blind, and placebo-controlled study. The subjects were 12 healthy nonsmoking volunteers. They received two intravenous infusions of deuterium-labeled nicotine-d2 and cotinine-d4 (0.5 micrograms/kg/min), once with oral cotinine treatment of 0.25 mg/kg twice a day and once with placebo. Nicotine and cotinine pharmacokinetic parameters were determined for each infusion.

RESULTS

During oral cotinine treatment, average plasma levels of cotinine ware 900 ng/ml, comparable to levels observed in some very heavy smokers. Cotinine had no effect on the disposition kinetics of nicotine-d2. The half-life of cotinine after low-dose cotinine-d4 infusion was comparable to that after high-dose cotinine described in previous studies. The half-life of labeled cotinine derived from nicotine was significantly longer than the half-life of cotinine administered as cotinine.

CONCLUSIONS

Cotinine is not responsible for the lower nicotine clearance observed in smokers. Our data suggest that the pharmacokinetics of low-dose cotinine in nonsmokers do not differ from those of high-dose in smokers, and therefore cotinine levels can be used quantitatively in environmental tobacco exposure. The longer half-life of cotinine derived from nicotine suggests that slow release of nicotine from tissues is responsible for the apparent long half-life of cotinine in nonsmokers exposed to environmental tobacco smoke.

Role of the lung in accumulation and metabolism of xenobiotic compounds--implications for chemically induced toxicity

The mammalian lung is exposed to and affected by many airborne and bloodborne foreign compounds. This review summarizes the role of lung in accumulation and metabolism of xenobiotics, some of which are spontaneously reactive or are metabolically activated to toxic intermediates. The specific architectural arrangement of mammalian lung favors that so-called pneumophilic drugs are filtered out of the blood and are retained within the tissue as shown in particular for amphetamine, chlorphentermine, amiodarone, imipramine, chlorpromazine, propranolol, local anaesthetics, and some miscellaneous therapeutics. There is strong evidence that intrapulmonary distribution activity and regulation of drug-metabolizing enzymes in lung is distinct from liver. This review focuses on the metabolic rate of selected compounds in lung such as 5-fluoro-2'-deoxyuridine, local anesthetics, nicotine, benzo(alpha)pyrene, ipomeanol, 4-methylnitrosamino-1-(3-pyridyl)-1-butanone. It is widely accepted that the formation of radical species is a key event in the pneumotoxic mechanisms induced by bleomycin, paraquat, 3-methylindole, butylhydroxytoluene, or nitrofurantoin. Finally, methodological approaches to assess the capacity of lung to eliminate foreign compounds as well as biochemical features of the pulmonary tissue are evaluated briefly.

The metabolism of alicyclic amines to reactive iminium ion intermediates

The evidence implicating the formation of iminium ions as reactive intermediates in the metabolism of alicyclic amines has been reviewed. The mechanism of formation of iminium ions and their conversion to alpha-carbonyl compounds or demethylated amines is discussed. The use of a simple cyanide trapping technique for iminium ions has been demonstrated to monitor a large number of alicyclic drugs for iminium ion formation. The possible role of iminium ions in the pharmacology and toxicology of alicyclic amines is considered.

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.

Nicotine metabolism by rat hepatic cytochrome P450s

Many kinds of cytochrome P450s were purified from rat hepatic microsomes, and their role in the metabolization of nicotine in a reconstituted system examined. Of four phenobarbital-inducible P450s, P450 2B1 had the highest nicotine oxidation activity and P450 2B2 showed a low rate of nicotine oxidation, whereas P450 2C6 and 3A2 had no detectable activity toward nicotine. Among eleven other purified cytochrome P450s tested, P450 2C11 had high nicotine oxidation activity and P450 1A2 and 2D1 showed low catalytic activity toward nicotine. The other cytochrome P450s, P450 1A1, 2A1, 2A2, 2C7, 2C12, 2C13, 2E1 and 4A1, had no detectable nicotine oxidation activity. Based on these results, participation of cytochrome P450s in nicotine metabolism in human and animal livers is discussed.

Nicotine metabolism in isolated perfused lung and liver of phenobarbital- and benzoflavone-treated rats

The kinetics of nicotine elimination was investigated in isolated perfused lung and liver of phenobarbital (PB)- and 5,6-benzoflavone (BF)-pretreated rats. The estimated kinetic parameters demonstrated a high nicotine elimination rate in rat lung approaching the capacity of liver when both organs were in an uninduced state. The concentration-time profiles of cotinine as the main metabolite were almost identical for isolated lung and liver. In both organs the cotinine plasma concentrations reached a plateau level after 60 min of perfusion. Pretreatment of rats with 5,6-benzoflavone did not affect the rate of nicotine elimination and cotinine formation either in the lung or in the liver. Phenobarbital treatment, however, induced nicotine clearance in lung approximately 2-fold. This effect is quantitatively lower than the PB-related 8-fold induction of hepatic nicotine elimination observed in a previous study. The present results also indicate that the turnover of cotinine is markedly enhanced after PB induction. The elimination half-lives and clearance values for cotinine as the substrate were approximately 10-fold increased in rat liver after PB pretreatment. Thus, an important contribution of extrahepatic tissues to nicotine metabolism in rats has to be assumed. Moreover, since cotinine elimination is significantly increased after PB induction it is questionable whether cotinine plasma concentrations can further be used as suitable parameter for nicotine consumption.

Increased hepatic nicotine elimination after phenobarbital induction in the conscious rat

Elimination parameters of [14C]nicotine in conscious rats receiving nicotine (0.3 mg/kg) either intravenously or orally were studied. The oral availability of unchanged nicotine, derived by comparison of the respective areas under the concentration vs time curves (AUC), was 89%, indicating low hepatic extraction ratios of about 10%. Pretreatment of rats with phenobarbital (PB) markedly increased hepatic first-pass extraction of nicotine. The oral availability of unchanged nicotine in plasma dropped to 1.4% of the corresponding values obtained from PB-treated rats receiving nicotine iv. After PB pretreatment, the clearance of iv nicotine was increased approximately twofold over controls, much less than the observed more than ninefold increase of hepatic first-pass extraction. It is assumed that extrahepatic metabolism contributed significantly to the rapid removal of nicotine from the plasma. The elimination of cotinine, originating from nicotine administered either po or iv, was significantly increased by PB pretreatment, as determined by the ratio of corresponding AUCs. The pattern of nicotine metabolites in urine also indicated an increase in the rate of cotinine metabolic turnover. The amount of norcotinine in the organic extract of urine paralleled PB microsomal enzyme induction. The ratio between urinary concentrations of the normetabolite and cotinine correlated strongly with the PB-induced state of rat liver. This may be a suitable indicator of PB-inducible hepatic cytochrome P450 isoenzyme(s). Since smoking habits in man are feedback-regulated by nicotine plasma concentrations, a similar increase of nicotine elimination by microsomal enzyme induction in man may be of relevance for tobacco consumption.

Rabbit nasal cytochrome P-450 NMa has high activity as a nicotine oxidase

Rabbit nasal olfactory and respiratory microsomes demonstrate high activity toward [3H]-(S)-nicotine, with specific activities of 22.2 and 6.5 nmol/min/mg protein, respectively. The major metabolite produced is (S)-nicotine delta 1'; 5'-iminium ion, with lesser amounts of nornicotine and the N'-oxide. Reconstitution of the rabbit nasal microsomal system with cytochromes P-450 NMa and NMb indicated that only P-450 NMa has significant activity toward nicotine, and the metabolite profile and turnover are similar to that observed with nasal microsomes. The low Km (35 microMs) and high Vmax (28 min-1) suggest that a significant portion of inhaled nicotine is metabolized by nasal tissues in the rabbit.

Formation of two major nicotine metabolites in livers of guinea pigs

Using antibody against NADPH-cytochrome P-450 reductase and several effectors of cytochrome P-450 and FAD-containing monooxygenase, we investigated nicotine metabolites formed by these two enzymes. When [3H]nicotine was metabolized by the combination of liver microsomes of guinea pigs and partially purified aldehyde oxidase, three distinct spots corresponding to nicotine, cotinine and nicotine-1'-oxide were observed on fluorograms of thin-layer chromatography. Antibody against NADPH-cytochrome P-450 reductase inhibited the formation of cotinine but not nicotine-1'-oxide. Metyrapone and n-octylamine inhibited the cotinine formation, while methimazole prevented the formation of nicotine-1'-oxide. These results show that microsomal electron transport systems participate in the formation of nicotine-1'-oxide and strongly suggest the involvement of FAD-containing monooxygenase in the formation of nicotine-1'-oxide.

Eightfold induction of nicotine elimination in perfused rat liver by pretreatment with phenobarbital

Elimination of nicotine by isolated rat livers was increased eightfold after pretreatment with phenobarbital (PB) as an inducer of cytochrome P-450 while it was only marginally influenced after pretreatment with 5,6-benzoflavone (BF) as an inducer of cytochrome P-448. Initial rates of cotinine formation were enhanced in the same order of magnitude in PB-induced livers. The 14C-nicotine-derived radioactivity excreted into bile within 2 h ranged between 6 -17% of the dose with only 2.7 fold higher values after PB pretreatment compared to controls.

Cytochrome P-450-dependent nicotine oxidation by liver microsomes of guinea pigs. Immunochemical evidence with antibody against phenobarbital-inducible cytochrome P-450

When guinea pigs were treated with phenobarbital (PB), the specific activity of liver microsomal nicotine oxidase increased by 42%. PB-inducible cytochrome P-450 (PB-P-450) was purified to homogeneity from liver microsomes of PB-treated guinea pigs. Purified PB-P-450 catalyzed nicotine oxidation when reconstituted with NADPH-P-450 reductase and phospholipid system. Antibody prepared against the purified PB-P-450 formed single precipitation lines with both purified PB-P-450 and microsomal components in livers of PB-treated guinea pigs, and both precipitation lines fused. The antibody against PB-P-450 strongly inhibited nicotine oxidation in the reconstituted system. The antibody also inhibited liver microsomal nicotine oxidase activities in PB-treated and untreated guinea pigs by about 30% and less than 5% respectively. About 45% of total P-450 in liver microsomes of PB-treated guinea pigs was precipitated by the antibody. These results show that PB-P-450 participates in liver microsomal nicotine oxidation in PB-treated guinea pigs but not in untreated control animals.