Inhibitition of Human Cytochrome P450 2E1 by Nicotine, Cotinine, and Aqueous Cigarette Tar Extract in Vitro

Authors

Terry R. Van Vleet, Utah State University
David W. Bombick, R. J. Reynolds Tobacco Co., EMT Division
Roger A. Coulombe Jr., Utah State UniversityFollow

Document Type

Article

Journal/Book Title/Conference

Toxicological Sciences

Volume

64

Issue

2

Publisher

Oxford University Press

Publication Date

2001

First Page

185

Last Page

191

Abstract

Cigarette smoke is a complex mixture containing, among other chemicals, pyridine alkaloids and N-nitrosamines. Carcinogenic tobacco-specific N-nitrosamines, N-nitrosodimethylamine (NDMA) and 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), are both activated by cytochrome P450 (CYP) 2E1 in rats. Previous reports indicate that nicotine and the main nicotine metabolite, cotinine, reduce the mutagenicity of both NNK and NDMA in Salmonella typhimurium. To study the mechanism of this effect, we examined inhibition of CYP 2E1 activity, as assessed by p-nitrophenol (pNP) hydroxylation, by nicotine, cotinine, and an aqueous cigarette tar extract (ACTE) in human 2E1-expressing microsomes. At all substrate concentrations (0–1.25 mM) nicotine was a significantly more potent inhibitor of CYP 2E1 activity compared to cotinine. Estimated Ki values for nicotine and cotinine (both at 10 mM) were 13 mM (2 mg/ml) and 308 mM (54 mg/ml) respectively. The Ki for ACTE was 0.2 mg/ml at a concentration of 0.32 mg/ml. This rank order for inhibition was also seen when the data was expressed as IC50. When compared on a mass/vol basis, ACTE was a significantly more potent CYP 2E1 inhibitor relative to nicotine and cotinine. Double-reciprocal plots indicated that nicotine and ACTE inhibited by a competitive, while cotinine inhibited CYP 2E1 by an uncompetitive mechanism. Although the contribution of nicotine to ACTE-mediated 2E1 inhibition is probably modest, pyridine alkaloid-mediated CYP 2E1 inhibition is a possible mechanism for the observed inhibition of NNK and NDMA mutagenicity by nicotine and cotinine in vitro.

Comments

Originally published by Oxford University Press. Publisher's PDF and HTML fulltext available through remote link.

Recommended Citation

Van Vleet, T.R., Bombick, D.W., and R.A. Coulombe, Jr. (2001) Inhibtition of Human Cytochrome P450 2E1 Activity by Nicotine, Cotinine, and Aqueous Cigarette Tar Extract in vitro. Toxicological Sciences 64, 185-191.