Short Communication Inhibitory Effects of Neurotransmitters and Steroids on Human CYP2A6

Human CYP2A6 catalyzes the metabolism of nicotine, cotinine, and coumarin as well as some pharmaceutical drugs. CYP2A6 is highly expressed in liver and, also, in brain and steroid-related tissues. In this study, we investigated the inhibitory effects of neurotransmitters and steroid hormones on CYP2A6 activity. We found that coumarin 7-hydroxylation and cotinine 3 -hydroxylation by recombinant CYP2A6 expressed in baculovirus-infected insect cells were competitively inhibited by tryptamine (both Ki 0.2 M), serotonin (Ki 252 M and 167 M), dopamine (Ki 49 M and 22 M), and histamine (Ki 428 M and 359 M). Cotinine formation from nicotine was inhibited by tryptamine (Ki 0.7 M, competitive), serotonin (Ki 272 M, noncompetitive), dopamine, noradrenaline, and adrenaline (Ki 11 M, 54 M, and 81 M, uncompetitive). Estrogens (Ki 0.6–3.8 M), androgens (Ki 60–149 M), and corticosterone (Ki 36 M) also inhibited cotinine formation, but coumarin 7-hydroxylation and cotinine 3 -hydroxylation did not. Nicotine5 (1 -iminium ion formation from nicotine was not affected by these steroid hormones, indicating that the inhibition of cotinine formation was due to the inhibitory effects on aldehyde oxidase. The nicotine5 (1 -iminium ion formation was competitively inhibited by tryptamine (Ki 0.3 M), serotonin (Ki 316 M), dopamine (Ki 66 M), and histamine (Ki 209 M). Thus, we found that some neurotransmitters inhibit CYP2A6 activity, being related with interand intraindividual differences in CYP2A6-dependent metabolism. The inhibitory effects of steroid hormones on aldehyde oxidase may also contribute to interindividual differences in nicotine metabolism. Cytochrome P450 (P450) enzymes are of great importance and interest because they catalyze the metabolism of both endogenous compounds and xenobiotics. The substrates for the P450s include a variety of clinically used drugs, environmental pollutants, carcinogens, steroid hormones, fatty acids, and bile acids. Studies on the inhibition of P450 are useful to predict drug efficacy, drug interaction, and drug toxicity. Because many endogenous compounds are substrates of P450s, they may inhibit drug metabolism catalyzed by P450s. Previous studies reported that human CYP1A2 (Agúndez et al., 1998), CYP2C9 (Gervasini et al., 2001), CYP2D6 (Martı́nez et al., 1997), and CYP3A (Martı́nez et al., 2000) activities were inhibited by neurotransmitters and their precursors. The inhibition would be of particular importance, since these P450s are expressed in brain and are involved in the metabolism of psychoactive drugs or associated with Parkinson’s disease. In addition, it has been reported that androgens activate or inhibit the enzymatic activities catalyzed by CYP3A4 (Nakamura et al., 2002). Since endogenous compounds are always found in vivo, the inhibitory effects of endogenous compounds may contribute to the intraindividual and interindividual differences in P450 activity. Human CYP2A6, first purified as a coumarin 7-hydroxylase (Yun et al., 1991), catalyzes the metabolism of pharmaceutical agents such as tegafur, losigamone, letrozole, and valproic acid. CYP2A6 is a major enzyme responsible for the metabolism of nicotine to cotinine (Nakajima et al., 1996b) and, further, to trans-3 -hydroxycotinine (Nakajima et al., 1996a). Furthermore, it can metabolically activate aflatoxin B1 and tobacco-specific nitrosamines such as 4-methylnitrosoamino-1-(3-pyridyl)-1-butanone and N-nitrosodiethylamine (Nakajima et al., 2002). CYP2A6 is predominantly expressed in liver and also in brain (Miksys and Tyndale, 2004). Although the systemic clearance of nicotine is due to the metabolism in liver, CYP2A6 expressed in human brain contributes to the local metabolism of nicotine (Miksys and Tyndale, 2004; Yamanaka et al., 2005). In addition, CYP2A6 is also expressed in steroid-related tissues such as adrenal gland, testis, ovary, and breast (Nakajima et al., 2006b). Recent studies reported that CYP2A6 catalyzes the 16 hydroxylation of estradiol (Lee et al., 2003) and progesterone 6 hydroxylation (Niwa et al., 1998), although the activities were much lower than those of other P450s. This background prompted us to investigate the inhibitory effects of neurotransmitters, their precursors, and steroid hormones on CYP2A6 activity. Materials and Methods Materials. Tryptamine, serotonin, melatonin, tyrosine, dopamine, histamine, corticosterone, testosterone, indole-3-acetic acid, and tryptophol were purchased from Wako Pure Chemical Industries (Osaka, Japan). Tryptophan, L-DOPA, noradrenaline, adrenaline, progesterone, 4-androstene-3,17-dione (androstenedione), 5 -androstane-3 ,17 -diol (androstanediol), estrone, estradiol, estriol, coumarin, 7-hydroxycoumarin, nicotine, cotinine, and caffeine were obtained from Sigma-Aldrich (St. Louis, MO). trans-3 -Hydroxycotinine was kindly provided by Dr. William S. Caldwell (R. J. Reynolds Tobacco This study was supported in part by a grant from the Smoking Research Foundation in Japan. Article, publication date, and citation information can be found at http://dmd.aspetjournals.org. doi:10.1124/dmd.106.014084. ABBREVIATIONS: P450, cytochrome P450; HPLC, high-performance liquid chromatography. 0090-9556/07/3504-508–514$20.00 DRUG METABOLISM AND DISPOSITION Vol. 35, No. 4 Copyright © 2007 by The American Society for Pharmacology and Experimental Therapeutics 14084/3192453 DMD 35:508–514, 2007 Printed in U.S.A. 508 at A PE T Jornals on A ril 2, 2017 dm d.aspurnals.org D ow nladed from