TY - JOUR T1 - CYP2A6 AND CYP2B6 ARE INVOLVED IN NORNICOTINE FORMATION FROM NICOTINE IN HUMANS: INTERINDIVIDUAL DIFFERENCES IN THESE CONTRIBUTIONS JF - Drug Metabolism and Disposition JO - Drug Metab Dispos SP - 1811 LP - 1818 DO - 10.1124/dmd.105.006254 VL - 33 IS - 12 AU - Hiroyuki Yamanaka AU - Miki Nakajima AU - Tatsuki Fukami AU - Haruko Sakai AU - Akiko Nakamura AU - Miki Katoh AU - Masataka Takamiya AU - Yasuhiro Aoki AU - Tsuyoshi Yokoi Y1 - 2005/12/01 UR - http://dmd.aspetjournals.org/content/33/12/1811.abstract N2 - Nornicotine is an N-demethylated metabolite of nicotine. In the present study, human cytochrome P450 (P450) isoform(s) involved in nicotine N-demethylation were identified. The Eadie-Hofstee plot of nicotine N-demethylation in human liver microsomes was biphasic with high-affinity (apparent Km = 173 ± 70 μM, Vmax = 57 ± 17 pmol/min/mg) and low-affinity (apparent Km = 619 ± 68 μM, Vmax = 137 ± 6 pmol/min/mg) components. Among 13 recombinant human P450s expressed in baculovirus-infected insect cells (Supersomes), CYP2B6 exhibited the highest nicotine N-demethylase activity, followed by CYP2A6. The apparent Km values of CYP2A6 (49 ± 12 μM) and CYP2B6 (550 ± 46 μM) were close to those of high- and low-affinity components in human liver microsomes, respectively. The intrinsic clearances of CYP2A6 and CYP2B6 Supersomes were 5.1 and 12.5 nl/min/pmol P450, respectively. In addition, the intrinsic clearance of CYP2A13 expressed in Escherichia coli (44.9 nl/min/pmol P450) was higher than that of CYP2A6 expressed in E. coli (2.6 nl/min/pmol P450). Since CYP2A13 is hardly expressed in human livers, the contribution of CYP2A13 to the nicotine N-demethylation in human liver microsomes would be negligible. The nicotine N-demethylase activity in microsomes from 15 human livers at 20 μM nicotine was significantly correlated with the CYP2A6 contents (r = 0.578, p < 0.05), coumarin 7-hydroxylase activity (r = 0.802, p < 0.001), and S-mephenytoin N-demethylase activity (r = 0.694, p < 0.005). The nicotine N-demethylase activity at 100 μM nicotine was significantly correlated with the CYP2B6 contents (r = 0.677, p < 0.05) and S-mephenytoin N-demethylase activities (r = 0.740, p < 0.005). These results as well as the inhibition analyses suggested that CYP2A6 and CYP2B6 would significantly contribute to the nicotine N-demethylation at low and high substrate concentrations, respectively. The contributions of CYP2A6 and CYP2B6 would be dependent on the expression levels of these isoforms in any human liver. The American Society for Pharmacology and Experimental Therapeutics ER -