Abstract
Genetic polymorphisms of CYP2A6 gene are known as a causal factor of the interindividual differences in nicotine metabolism. We found three novel CYP2A6 alleles. The CYP2A6*18A allele has a single nucleotide polymorphism (SNP) of A5668T (A1175T, Y392F) in exon 8. The CYP2A6*18B allele has synonymous SNPs of G51A (G51A), T5684C (T1191C), and T5702C (T1209C) in addition to A5668T (A1175T, Y392F). The CYP2A6*19 allele has the SNPs of A5668T (A1175T, Y392F), T6354C (intron 8), and T6558C (T1412C, I471T) as well as the conversion with the CYP2A7 sequence in the 3′-untranslated region, in which the latter two changes correspond to CYP2A6*7. Ethnic differences in the frequencies of these alleles were observed between whites, African-Americans, Japanese, and Koreans. Wild or variant CYP2A6 (CYP2A6*18, CYP2A6*19, and CYP2A6*7) were expressed in Escherichia coli. For coumarin 7-hydroxylation and 5-fluorouracil formation from tegafur, the Km values were increased, and Vmax values were decreased in CYP2A6.18 compared with those in CYP2A6.1, resulting in decreased clearance to 50 and 35% of that of the wild type, respectively. The Km and Vmax values for nicotine C-oxidation were both increased, resulting in no change of clearance. In CYP2A6.19, the effects on the coumarin 7-hydroxylation and 5-fluorouracil formation (increased Km and decreased Vmax) were prominent, resulting in decreased clearance to 8% of those of the wild type. For nicotine C-oxidation, the Km and Vmax values were both decreased, resulting in decreased clearance to 30% of that of the wild type. The changes of the kinetics in CYP2A6.19 were similar to those in CYP2A6.7. In vivo nicotine metabolism was evaluated in whites (n = 56) and Koreans (n = 40). Although the CYP2A6*18 and CYP2A6*19 alleles were found only heterozygously, a subject with CYP2A6*7/CYP2A6*19 showed a lower cotinine/nicotine ratio of the plasma concentration compared with homozygotes of the CYP2A6*1A, supporting the in vitro results that the CYP2A6*19 allele leads to decreased enzymatic activity.
Footnotes
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This study was supported in part by a grant from Japan Health Sciences Foundation with Research on Health Science Focusing on Drug Innovation, by an SRF grant for biomedical research in Japan, and by Philip Morris Incorporated.
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Article, publication date, and citation information can be found at http://dmd.aspetjournals.org.
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doi:10.1124/dmd.105.004994.
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ABBREVIATIONS: P450, cytochrome P450 enzyme; SNP, single nucleotide polymorphism; 5-FU, 5-fluorouracil; PCR-RFLP, polymerase chain reaction-restriction fragment length polymorphism; PCR, polymerase chain reaction; bp, base pair(s); AS-PCR, allele specific-polymerase chain reaction; NPR, NADPH-P450 reductase; HPLC, high-performance liquid chromatography.
- Received April 7, 2005.
- Accepted May 10, 2005.
- The American Society for Pharmacology and Experimental Therapeutics
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