Trapping of Reactive Intermediates by Incorporation of 14C-Sodium Cyanide during Microsomal Oxidation

Gorrod, J. W.; Whittlesea, Catherine M. C.; Lam, S. P.

doi:10.1007/978-1-4684-5877-0_82

Cited by 49 publications

(30 citation statements)

References 20 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Cyanide does enter the active site of P450 enzymes and has been used by several investigators to trap nicotine iminium ion metabolites (Nguyen et al, 1979;Koop and Hollenberg, 1980;McCoy et al, 1986;Williams et al, 1990;Gorrod et al, 1991). KCN did seem to protect CYP2A13 from nicotine-mediated inactivation.…”

Section: Discussionmentioning

confidence: 99%

Inactivation of CYP2A6 and CYP2A13 during Nicotine Metabolism

Weymarn¹,

Brown

Murphy³

2005

J Pharmacol Exp Ther

View full text Add to dashboard Cite

Nicotine is the major addictive agent in tobacco. The primary catalyst of nicotine metabolism in humans is CYP2A6. However, the closely related enzyme CYP2A13 is a somewhat better catalyst. CYP2A13 is an extrahepatic enzyme that is an excellent catalyst of the metabolic activation of the tobaccospecific carcinogen 4-(methylnitrosamine)-1-(3-pyridyl)-1-butanone (NNK). Here we report that both CYP2A6 and CYP2A13 were inactivated during nicotine metabolism. Inactivation of both enzymes was dependent on NADPH and increased with time and concentration. Alternate substrates for CYP2A6 and CYP2A13 protected these enzymes from inactivation. Inactivation of CYP2A13 was irreversible upon extensive dialysis and seems to be mechanism-based. The K I of CYP2A13 inactivation by nicotine was 17 M, the rate of inactivation, k inact , was 0.1 min Ϫ1 , and the t 1/2 was 7 min. However, the loss in enzyme activity occurred after nicotine metabolism was complete, suggesting that a secondary or possible tertiary metabolite of nicotine may be responsible. H]Nicotine metabolism by CYP2A13 was monitored by radioflow high-pressure liquid chromatography during the course of enzyme inactivation; the major product was the ⌬ 1Ј(5Ј) iminium ion. However, cotinine was a significant metabolite even at short reaction times. The metabolism of the nicotine ⌬ 1Ј(5Ј) iminium ion to cotinine did not require the addition of aldehyde oxidase. CYP2A13 catalyzed this reaction as well as further metabolism of cotinine to 5Ј-hydroxycotinine, trans-3Ј-hydroxycotinine, and N-(hydroxymethyl)-norcotinine as enzyme inactivation occurred. Studies are on-going to identify the metabolite responsible for nicotinemediated inactivation of CYP2A13.Nicotine is the major addictive agent in tobacco, and individuals continue to smoke and use tobacco products to maintain plasma nicotine levels (Benowitz, 1988;Hukkanen et al., 2005). The plasma concentration of nicotine varies among tobacco users, due in part to differences in nicotine metabolism. The major catalyst of nicotine metabolism in humans is CYP2A6 (Hukkanen et al., 2005). However, the closely related enzyme CYP2A13 is a better catalyst (Bao et al., 2005;Murphy et al., 2005). Whether or not CYP2A13, an extrahepatic P450 expressed in the respiratory tract (Su et al., 2000), plays a significant role in nicotine metabolism in smokers is unknown. However, CYP2A13 is an excellent catalyst of the metabolic activation of the tobacco carcinogen NNK (Jalas et al., 2005;Wong et al., 2005b). NNK is only present in tobacco and tobacco smoke; therefore, exposure to NNK is always concurrent with nicotine exposure. NNK requires metabolic activation to exert its carcinogenic potential, and CYP2A13 is likely a key catalyst of activation in smokers. Therefore, any inactivation of CYP2A13 by nicotine will influence the metabolic activation of NNK, a potent lung carcinogen.Nicotine is primarily metabolized by 5Ј-oxidation, but 2Ј-oxidation and methyl-oxidation also occur ( Fig. 1) (Hecht et al., 2000;Hukkanen et al., 2005;Mu...

show abstract

Section: Discussionmentioning

confidence: 99%

Inactivation of CYP2A6 and CYP2A13 during Nicotine Metabolism

Weymarn¹,

Brown

Murphy³

2005

J Pharmacol Exp Ther

View full text Add to dashboard Cite

show abstract

“…For compounds whose reactive metabolites are not trapped by GSH, like dexamethasone, other trapping assays [such as iminium ion trapping with cyanide (Gorrod et al, 1991) and aldehyde trapping with semicarbazide (Xu et al, 2005), among others] should be conducted to help predict the structure of reactive metabolites.…”

Section: Evaluation Of Drug-induced Liver Injury By Covalent Bindingmentioning

confidence: 99%

Evaluation of the Potential for Drug-Induced Liver Injury Based on in Vitro Covalent Binding to Human Liver Proteins

Usui¹,

Mise²,

Hashizume³

et al. 2009

Drug Metab Dispos

140

123

View full text Add to dashboard Cite

ABSTRACT:Prediction of idiosyncratic drug-induced liver injury (DILI) is difficult, and the underlying mechanisms are not fully understood. However, many drugs causing DILI are considered to form reactive metabolites and covalently bind to cellular macromolecules in the liver. The objective of this study was to clarify whether the risk of idiosyncratic DILI can be estimated by comparing in vitro covalent binding (CB) levels among 12 positive compounds (acetaminophen, alpidem, bromfenac, carbamazepine, diclofenac, flutamide, imipramine, nefazodone, tacrine, ticlopidine, tienilic acid, and troglitazone) for DILI and 12 negative compounds (acetylsalicylic acid, caffeine, dexamethasone, losartan, ibuprofen, paroxetine, pioglitazone, rosiglitazone, sertraline, theophylline, venlafaxine, and zolpidem). After incubation with human liver microsomes in the presence of NADPH, there was a large overlap in the distribution of CB amounts between the positive and negative groups. On addition of UDP-glucuronic acid (UDPGA) as a cofactor for glucuronidation, the CB levels of bromfenac and diclofenac were increased. With addition of nucleophilic glutathione (GSH), values for most compounds were decreased. However, separation of the two groups on the basis of CB could not be improved by UDPGA or GSH. Furthermore, CB with human hepatocytes also failed to discriminate positive from negative compounds. Therefore, the CB amount alone is not sufficient for risk assessment of DILI. In contrast, when the CB amount was multiplied by the maximum daily dose, which may reflect maximum hepatic exposure, the two groups did become discriminated. Taken together, our findings suggest that the combination of CB amount and daily dose can estimate the risk of idiosyncratic DILI.

show abstract

“…For nicotine and phencyclidine, cyanide reacts at the ␣-position to form the cyanoamine (Hoag et al, 1987;Shigenaga et al, 1988). Cyanide adducts have been observed for several other amine drugs when incubated with liver microsomes (Gorrod et al, 1991). In addition, for nicotine iminium ion, the adduct formed between hydroxide ion and the iminium ion is also observed at pH greater than 7.5 (Brandange and Lindblom, 1979).…”

Section: Discussionmentioning

confidence: 97%

Metabolism, Pharmacokinetics, and Excretion of the 5-Hydroxytryptamine_1BReceptor Antagonist Elzasonan in Humans

Kamel¹,

Obach²,

Colizza³

et al. 2010

Drug Metab Dispos

View full text Add to dashboard Cite

ABSTRACT:The metabolism, pharmacokinetics, and excretion of a potent and selective 5-hydroxytryptamine 1B receptor antagonist elzasonan have been studied in six healthy male human subjects after oral administration of a single 10-mg dose of [ 14 C]elzasonan. Total recovery of the administered dose was 79% with approximately 58 and 21% of the administered radioactive dose excreted in feces and urine, respectively. The average t 1/2 for elzasonan was 31.5 h. Elzasonan was extensively metabolized, and excreta and plasma were analyzed using mass spectrometry and NMR spectroscopy to elucidate the structures of metabolites. The major component of drug-related material in the excreta was in the feces and was identified as 5-hydroxyelzasonan (M3), which accounted for approximately 34% of the administered dose. The major human circulating metabolite was identified as the novel cyclized indole metabolite (M6) and accounted for ϳ65% of the total radioactivity. A mechanism for the formation of M6 is proposed. Furthermore, metabolism-dependent covalent binding of drug-related material was observed upon incubation of [ 14 C]elzasonan with liver microsomes, and data suggest that an indole iminium ion is involved. Overall, the major metabolic pathways of elzasonan were due to aromatic hydroxylation(s) of the benzylidene moiety, N-oxidation at the piperazine ring, N-demethylation, indirect glucuronidation, and oxidation, ring closure, and subsequent rearrangement to form M6.

show abstract

Trapping of Reactive Intermediates by Incorporation of 14C-Sodium Cyanide during Microsomal Oxidation

Cited by 49 publications

References 20 publications

Inactivation of CYP2A6 and CYP2A13 during Nicotine Metabolism

Inactivation of CYP2A6 and CYP2A13 during Nicotine Metabolism

Evaluation of the Potential for Drug-Induced Liver Injury Based on in Vitro Covalent Binding to Human Liver Proteins

Metabolism, Pharmacokinetics, and Excretion of the 5-Hydroxytryptamine_1BReceptor Antagonist Elzasonan in Humans

Contact Info

Product

Resources

About

Trapping of Reactive Intermediates by Incorporation of 14C-Sodium Cyanide during Microsomal Oxidation

Cited by 49 publications

References 20 publications

Inactivation of CYP2A6 and CYP2A13 during Nicotine Metabolism

Inactivation of CYP2A6 and CYP2A13 during Nicotine Metabolism

Evaluation of the Potential for Drug-Induced Liver Injury Based on in Vitro Covalent Binding to Human Liver Proteins

Metabolism, Pharmacokinetics, and Excretion of the 5-Hydroxytryptamine1BReceptor Antagonist Elzasonan in Humans

Contact Info

Product

Resources

About

Metabolism, Pharmacokinetics, and Excretion of the 5-Hydroxytryptamine_1BReceptor Antagonist Elzasonan in Humans