Enzymic oxidation of (-)-nicotine by guinea-pig tissues in vitro

Simultaneous quantification of tobacco alkaloids and major phase I metabolites by LC-MS/MS in human tissue

INTRODUCTION

Insurance agencies might request laboratories to differentiate whether a deceased has been a smoker or not to decide about refunding of his nonsmoker rate. In this context, the question on a solid proof of tobacco alkaloids and major metabolites in tissues came up. Currently, an appropriate assay is still lacking to analyze tissue distribution in smokers or nonsmokers. Nicotine (NIC), nornicotine (NNIC), anatabine (ATB), anabasine (ABS), and myosmine (MYO) are naturally occurring alkaloids of the tobacco plant; most important phase I metabolites of NIC are cotinine (COT), norcotinine (NCOT), trans-3'-hydroxycotinine (HCOT), nicotine-N'-oxide (NNO), and cotinine-N-oxide (CNO). An analytical assay for their determination was developed and applied to five randomly selected autopsy cases.

METHODS

Homogenates using 500 mg aliquots of tissue samples were analyzed by liquid chromatography/tandem mass spectrometry following solid phase extraction. The method was validated according to current international guidelines.

RESULTS

NIC, COT, NCOT, ABS, ATB, and HCOT could be detected in all tissues under investigation. Highest NIC concentrations were observed in the lungs, whereas highest COT concentrations have been found in the liver. MYO was not detectable in any of the tissues under investigation.

CONCLUSIONS

The assay is able to adequately separate isobaric analyte pairs such as NIC/ABS/NCOT and HCOT/CNO thus being suitable for the determination of tobacco alkaloids and their phase I metabolites from tissue. More autopsy cases as well as corresponding body fluids and hair samples will be investigated to differentiate smokers from nonsmokers.

Nicotine, its metabolism and an overview of its biological effects

Nicotine is a naturally occurring alkaloid found in many plants. The principal sources of nicotine exposure is through the use of tobacco, nicotine containing gum and nicotine replacement therapies. Nicotine is an amine composed of pyridine and pyrrolidine rings. It has been shown that nicotine crosses biological membranes and the blood brain barrier easily. The absorbed nicotine is extensively metabolized in the liver to form a wide variety of metabolites including nicotine N'-oxide and cotinine N'-oxide. These are the products of mixed function oxidase system. Nicotine is also converted to some biologically important compounds during harvesting. Among these are the nitrosamines specific to tobacco. Nicotine has been shown to affect a wide variety of biological functions ranging from gene expression, regulation of hormone secretion and enzyme activities. The objective of this study was to overview the biological effects and metabolism of nicotine.

Nicotine metabolism, human drug metabolism polymorphisms, and smoking behaviour

Large interindividual differences occur in human nicotine disposition, and it has been proposed that genetic polymorphisms in nicotine metabolism may be a major determinant of an individual's smoking behaviour. Hepatic cytochrome P4502A6 (CYP2A6) catalyses the major route of nicotine metabolism: C-oxidation to cotinine, followed by hydroxylation to trans-3'-hydroxycotinine. Nicotine and cotinine both undergo N-oxidation and pyridine N-glucuronidation. Nicotine N-1-oxide formation is catalysed by hepatic flavin-containing monooxygenase form 3 (FMO3), but the enzyme(s) required for cotinine N-1'-oxide formation has not been identified. trans-3'-Hydroxycotinine is conjugated by O-glucuronidation. The uridine diphosphate-glucuronosyltransferase (UGT) enzyme(s) required for N- and O-glucuronidation have not been identified. CYP2A6 is highly polymorphic resulting in functional differences in nicotine C-oxidation both in vitro and in vivo; however, population studies fail to consistently and conclusively demonstrate any associations between variant CYP2A6 alleles encoding for either reduced or enhanced enzyme activity with self-reported smoking behaviour. The functional consequences of FMO3 and UGT polymorphisms on nicotine disposition have not been investigated, but are unlikely to significantly affect smoking behaviour. Therefore, current evidence does not support the hypothesis that genetic polymorphisms associated with nicotine metabolism are a major determinant of an individual's smoking behaviour and exposure to tobacco smoke.

Effects of nicotine and cotinine on bovine theca interna and granulosa cells

The purpose of this study was to determine if nicotine or cotinine inhibits steroidogenesis in the ovarian follicle. Theca interna and granulosa cells were isolated from bovine follicles, cultured with nicotine or cotinine for 24h, and culture media were assayed for androstenedione or estradiol. Treatment of theca interna with 6, 60, and 600 micro M nicotine decreased (P<or=0.002) production of androstenedione to 55, 53, and 24% of control levels, respectively. Levels of androstenedione in theca interna treated with cotinine were not different from control values. In granulosa cells, nicotine inhibited production of estradiol at the highest dose tested. Treatment with 600 micro M nicotine decreased (P<or=0.001) estradiol concentration to 12% of control values, attributable to a general cytotoxic effect. Cotinine had no effect on estradiol production by granulosa cells. These results provide novel evidence for inhibitory effects of nicotine on androgen production by theca interna.

Nicotine enantiomers and oxidative stress

Nicotine affects a variety of cellular processes ranging from induction of gene expression to secretion of hormones and modulation of enzymatic activities. The objective of this study was to characterize the toxicity of nicotine enantiomers as well as their ability to induce oxidative stress in an in vitro model using Chinese hamster ovary (CHO) cells. Colony formation assay has demonstrated that (-)-nicotine is the more toxic of the enantiomers. At 6 mM concentrations, (-)-nicotine was found to be approximately 28- and 19-fold more potent than (+)-, and (+/-)-nicotine (racemic), respectively. Results also indicated that the toxicity of (+/-)-nicotine is higher than that of (+)-nicotine. (-)-Nicotine at a 10 mM concentration substantially decreased glutathione (GSH) levels (46% decrease). In addition, a 3-fold increase in malondialdehyde (MDA) level was evident in cells after exposure to 10 mM (-)-nicotine. Increased lactate dehydrogenase (LDH) activities in the media demonstrated that cellular membrane integrity was disturbed in nicotine treated cells. In the presence of superoxide dismutase (SOD) and catalase (CAT), the LDH activities returned to control value in 24 h with all concentrations of (-)-, (+)-, and (+/-)-nicotine. The decreases in LDH activities in the presence of the radical scavenging enzymes SOD and CAT suggest that membrane damage may be due to free radical generation.

Nicotine, but not cotinine, has a direct toxic effect on ovarian function in the immature gonadotropin-stimulated rat

PMSG-primed and hCG-triggered rat ovaries were exposed to nicotine and its major metabolite, cotinine, using in vitro and in vivo experimental approaches. In vivo, a dose-dependent reduction in oocytes within the fallopian tube was noted in nicotine treated rats (6.25 ng/g animal weight, P < 0.001). Serum estradiol concentrations were also reduced in rats receiving nicotine (P < 0.04). There were no significant differences in weight gain. Cotinine had no effects. In vitro, nicotine also caused a dose-dependent reduction in oocytes in the collection chamber (P < 0.0001). Estradiol levels in nicotine-treated perfusions were reduced and reached statistical significance at 7 h (P < 0.003). The in vitro fertilization rate was reduced for nicotine-treated perfusions exposed to 1.43 pg/mL of nicotine (P < 0.001). Cotinine had no effect in vitro. We conclude that nicotine inhibits ovulation, estradiol production, and fertilization both in vivo and in vitro in rat models of ovulation. Cotinine did not affect these parameters. These effects of nicotine are notably independent of nicotine's known effect on the midcycle gonadotropin surge.

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.

A physiologically based pharmacokinetic model for nicotine disposition in the Sprague-Dawley rat

A physiologically based pharmacokinetic (PBPK) model was developed to describe the disposition of nicotine in the Sprague-Dawley (SD) rat. Parameters for the model were either obtained from the literature (blood flows, organ volumes) or determined experimentally (partition coefficients). Nicotine metabolism was defined in the liver compartment by the first-order rate constants KNC and KNP which control the rate of nicotine metabolism to cotinine and "polar metabolites" (PM), respectively. These rate constants were estimated by optimizing the model fit to pharmacokinetic data obtained by administering an intraarterial (S)-[5-3H]nicotine bolus of 0.1 mg/kg to 6 rats. Model simulations that optimized for the appearance of cotinine in plasma estimated KNC and KNP to be 75.8 and 24.3 hr-1, respectively. Use of these constants in the model allowed us to accurately predict nicotine plasma kinetics and the fraction of the dose eliminated by renal (8.5%) and metabolic (91.5%) clearance. To validate the model's ability to predict tissue kinetics of nicotine, 21 male SD rats were administered 0.1 mg/kg (S)-[5-3H]nicotine intraarterially. At seven time points following treatment, 3 rats were euthanized and tissues were removed and analyzed for nicotine. Model-predicted nicotine tissue kinetics were in agreement with those determined experimentally in muscle, liver, skin, fat, and kidney. The brain, heart, and lung exhibited nonlinear nicotine elimination, suggesting that saturable nicotinic binding sites may be important in nicotine disposition in these organs. Inclusion of saturable receptor binding expressions in the mathematical description of these compartments resulted in better agreement with the experimental data. The Bmax and KD estimated by model simulations for these tissues were brain, 0.009 and 0.12; lung, 0.039 and 2.0; and heart, 0.039 nmol/tissue and 0.12 nM, respectively. This PBPK model can successfully describe the tissue and plasma kinetics of nicotine in the SD rat and will be a useful tool for pharmacologic studies in humans and experimental animals that require insight into the plasma or tissue concentration-effect relationship.

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.

New mammalian metabolites of sparteine

Sparteine is reportedly metabolized in mammals with the formation of an N-oxide which undergoes dehydration to delta 2 and delta 5-dehydrosparteine. In our studies male Sprague-Dawley rats were found to metabolize sparteine and alpha-isosparteine to lupanine and alpha-isolupanine respectively in vivo. Metabolic conversion of sparteine in vitro in the presence of microsomal and 9000 x g supernatant fractions of the rat liver homogenate did not produce detectable lupanine. The in vivo studies were conducted by pretreating rats with inducers and inhibitors of microsomal enzymes. Inducers did not increase levels of lupanine in the rat urine but a significant decrease was observed in the presence of the inhibitor SFK 525A. Disulfiram reduced lupanine levels in the urine. The bioconversion of sparteine to lupanine appears to be mediated by microsomal enzymes and may proceed via an aldehyde intermediate. The conversion of sparteine to lupanine may parallel the mammalian metabolism of nicotine to cotinine.

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.

Characterization of hamster liver nicotine metabolism. I. Relative rates of microsomal C and N oxidation

A high pressure liquid chromatographic procedure has been developed for the determination of the two principal N- and C-oxidation products of nicotine in hamster liver subcellular fractions. Advantage was taken of the fact that cyanide ion forms a stable adduct with the microsomal metabolite that is the precursor of cotinine. The rate and extent as well as the sensitivity of inhibition were similar for cotinine, the 5'-cyanonicotine adduct, and an as yet unidentified microsomal metabolite which is presumed to be the initial microsomal metabolite on the pathway to cotinine formation. The rates of nicotine-N'-oxidase and nicotine-5'-hydroxylase activities exhibited differential response to inhibitors as well as differential susceptibility to proteolytic digestion. Data are presented which indicate that low levels of nornicotine contamination in stock nicotine resulted in the artifactual formation of methylcyanonornicotine adduct. No evidence consistent with the formation of nornicotine by isolated microsomes was obtained.

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.

Isolation and analysis of N-oxide metabolites of tertiary amines: quantitation of nicotine-1'-N-oxide formation in mice

To investigate the formation and elimination of nicotine-1'-N-oxide (NNO) in mice treated with a single injection of nicotine, sensitive and selective methods were developed to quantitate this polar and heat-labile metabolite. The compound was isolated from tissue homogenates as a dodecyl sulfate ion pair with C18 extraction cartridges and analyzed on an amino bonded-phase high-performance liquid chromatographic column with a mobile phase consisting of isopropanol-water. Overall recoveries of NNO were 64-76% from biological media. Several methods of detection were evaluated; radiolabeling was necessary to achieve the sensitivity required for pharmacokinetic studies in mice. The cis and trans isomers of NNO were separated on a Partisil PAC column and enzymatic selectivity was evaluated for the formation of these isomers in mice.

The metabolism of tertiary amines

The metabolism of tertiary amines is mediated primarily by cytochrome P-450 and MFAO, leading to alpha-C oxidation and N-oxidation, respectively. We have discussed how lipophilicity, basicity, steric hindrance, and stereochemistry can effect the outcome of metabolism as well as species, sex, and age. The proposed oxidation of tertiary amines to iminium ions by cytochrome P-450 may explain the isolation of various intramolecular and cyanide-trapped metabolites. N-oxides may represent a smaller percentage of the overall in vitro metabolism of tertiary amines due to the postmortem inactivation of MFAO. In addition N-oxide reducing enzymes present in vivo and in vitro may influence the extent of N-oxide formation. In general, definite conclusions about substrate requirements have been difficult to formulate because of the numerous biological and physical parameters affecting the outcome of metabolism. More singularly directed research on a single species of animal and a wide variety of substrates or vice versa would greatly increase our understanding of the potential metabolism of tertiary amine xenobiotics.

Toxic alkaloids and their interaction with microsomal cytochrome P-450 in vitro

Studies on the binding spectra of certain alkaloids with rat liver microsomes revealed that brucine, scopolamine and strychnine are type I compounds, whereas boldine, emetine, nicotine, reserpine and sanguinarine show type II binding. In contrast, colchicine and solanine failed to produce any measurable binding spectra. In vitro incubation of colchicine, nicotine or scopolamine with microsomal suspensions and NADPH resulted in demethylation of these alkaloids, while the incubation of boldine, brucine, emetine, reserpine, sanguinarine or solanine showed little or no dealkylation reaction. Furthermore, the effect of these alkaloids on the in vitro microsomal metabolism of a drug, benzphetamine, has also been studied.

N-demethylation of nicotine and reduction of nicotine-1'-N-oxide by Microsporum gypseum

Several microorganisms were examined for their abilities to convert S-nicotine into nornicotine. Five microorganisms including Microsporum gypseum (ATCC 11395) produced nornicotine and three unknown metabolites. M. gypseum efficiently reduced nicotine-1'-N-oxide to nicotine, but no nornicotine was obtained when the N-oxide was used as substrate.

Metabolism of norcocaine, N-hydroxy norcocaine and cocaine-N-oxide in the rat

1. The metabolism of [3H]norcocaine, N-hydroxy[3H]norcocaine and cocaine-N-oxide has been investigated in rats after i.v. injection. 2. The biological t 1/2 of norcocaine (dose 2 mg/kg i.v.) in plasma, liver and brain were 0.4, 1.6, 0.5 h, respectively and the compound was not detectable in the central nervous system 6 h after injection. The % dose of norcocaine excreted unchanged in urine and faeces in 96 h were 0.7 and 1.0, respectively. Benzoylnorecgonine, norecgonine, norecgonine methyl ester and an unidentified compound were excreted in urine. 3. The biological t 1/2 of N-hydroxynorcocaine (5 mg/kg i.v.) in brain and plasma were 0.3, 1.6 h respectively and only 1.3 and 1.6% of dose were excreted unchanged in urine and faeces in 96 h. N-Hydroxybenzoylnorecgonine and N-hydroxynorecgonine methyl ester were the major urinary metabolites. N-hydroxynorcocaine was not metabolized to norcocaine in vitro by liver microsomes. Doses of greater than 7.5 mg/kg i.v. resulted in death of rats by cardiorespiratory arrest. 4. Cocaine-N-oxide (50 mg/kg i.v.) yielded ecgonine-N-oxide methyl ester as its major metabolite; other minor metabolites were cocaine (0.5%), norcocaine (1%), benzoylecgonine, ecgonine, ecgonine-N-oxide, along with minor amounts of unmetabolized compound. Lethality of cocaine-N-oxide (100 mg/kg i.v.) was possibly due to metabolism to norcocaine and cocaine.

Reduction of levopropoxyphene N-oxide to propoxyphene by dogs in vivo and rat liver microsomal fraction in vitro

1. When l-propoxyphene-N-oxide was given orally to dogs, reduction occurred in vivo resulting in substantial plasma levels of l-propoxyphene. 2. When l-propoxyphene-N-oxide was given intravenously, near peak plasma levels of propoxyphene occurred in 10 minutes. This suggests that reduction is occurring at least in part in mammalian tissue rather than in gut flora. 3. Levopropoxyphene oxide was also readily reduced anaerobically in a rat liver microsomal fraction. 4. Results from this study explain our early observation that while l-propoxyphene-N-oxide is demethylated in vivo, demethylation does not occur in vitro when the oxide is incubated in air with liver homogenate. Thus demethylation of the oxide can occur only after reduction of the oxide to the tertiary amine. While reduction occurs readily in vivo it is completely inhibited by oxygen present in the usual microsomal incubation. 5. These studies further confirm that the N-oxide is not an intermediate in the demethylation of propoxyphene.

In vitro metabolism of 1-phenyl-2-(n-propylamino) propane (N-propylamphetamine) by rat liver homogenates

In vitro incubation of (+/-)-N-(n-propyl)amphetamine (NPA) with the 12000 g supernatant fraction of rat liver hmogenate resulted in the formation of two N-oxygenated products identified as N-hydroxy-1-phenyl-2(n-propylamino)propane and N-[(1-methyl-2-phenyl) ethyl]-1-propanamine N-oxide by g.l.c., g.l.c.-m.s. and t.l.c. Amphetamine, phenylacetone,...

A reappraisal of the stereoselective metabolism of nicotine to nicotine-1'-N-oxide

1. The cis and trans 1'-N-oxide metabolites of (2'R)-(+)-nicotine have the absolute configuration (1'S; 2'R) and (1'R; 2'R), respectively, and not the reverse as previously published. 2. Reinterpretation of metabolic data in the light of this reassignment reveals that N-oxidation of nicotine leads preferentially to the (1'R)-N-oxide, with little dependence on the configuration of the 2'-centre. 3. It is proposed that (2'S)-(-)-nicotine and (2'R)-(+)-nicotine bind to the same enzymic site by two distinct modes of binding; each of these modes involves the more basic centre (in this case the pyrrolidine ring) as the governing binding moiety.

Enzymic reduction of aromatic hydrocarbon epoxides by the microsomal fraction of rat liver

1. The oxidation of aromatic hydrocarbons to arene oxides and the reduction of these oxides to the parent hydrocarbons are both catalysed by enzymes in the microsomal fraction of rat liver. A suggested name for the enzyme concerned in the reduction of these epoxides is 'epoxide reductase'. 2. 'Epoxide reductase' is NADPH-dependent and is inhibited by oxygen. 3. Preliminary investigations suggest that the enzyme is specific for both 'K-region' and 'non-K-region' arene oxides.