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Isoflavones Inhibit Nicotine C-Oxidation Catalyzed by Human CYP2A6

From Drug Metabolism and Toxicology, Division of Pharmaceutical Sciences, Graduate School of Medical Science, Kanazawa University, Kanazawa, Japan (Dr Nakajima, Ms Itoh, Ms Yamanaka, Ms Fukami, Dr Yokoi); Department of Legal Medicine, Dokkyo University School of Medicine, Mibu, Tochigi, Japan (Dr Tokudome); and Department of Biochemistry, Graduate School of Medical Science, Kanazawa University, Kanazawa, Japan (Dr Y Yamamoto, Dr H Yamamoto).

The inhibitory effects of isoflavones (daidzein, genistein, and glycitein) on human cytochrome P450 (CYP) 2A6 activities were investigated. Daidzein, genistein, and glycitein uncompetitively inhibited nicotine C-oxidation catalyzed by recombinant CYP2A6 expressed in baculovirus-infected insect cells with Ki values of 1.3 ± 0.3 µM, 0.7 ± 0.2 µM, and 5.2 ± 0.8 µM, respectively, but not coumarin 7-hydroxylation. Effects of the intake of soy isoflavones on in vivo nicotine metabolism were investigated with 7 healthy Japanese homozygotes of CYP2A6^*1. The cotinine/nicotine ratio of the plasma concentrations 2 hours after chewing 1 piece of nicotine gum under the basal condition (after abstaining from soy foods for 1 week) was 8.8 ± 2.6 (4.4-11.4). The ratio was significantly (P< .05) reduced to 6.7 ± 1.6 (4.0-8.2) after consumption of a soy isoflavone supplement (60 mg of total isoflavones/d) for 5 days. The authors found that isoflavone contained in soy products significantly decreased nicotine metabolism.

Key Words: Cytochrome P450 • CYP2A6 • isoflavones • coumarin • nicotine metabolism

Address for reprints: Miki Nakajima, PhD, Drug Metabolism and Toxicology, Division of Pharmaceutical Sciences, Graduate School of Medical Science, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan; e-mail: nmiki{at}kenroku.kanazawa-u.ac.jp.

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