Cyp2a6 is a principal enzyme involved in hydroxylation of 1,7-dimethylxanthine, a main caffeine metabolite, in humans.
In a caffeine test previously performed with healthy Japanese volunteers, we found that the CYP1A2 index defined as urinary {5-acetylamino-6-amine-3-methyluracil (AAMU)+1-methylxanthine (1X)+1-methyluric acid (1 U)}/1,7-dimethyluric acid (17 U) was affected by the whole deleted allele of CYP2A6 (CYP2A6*4). Since the high value of the CYP1A2 index could be caused by a low urinary concentration of 17 U, we postulated that CYP2A6 was responsible for the 1,7-dimethylxanthine (17 X) metabolism to generate 17 U (17 X 8-hydroxylation). Thus, the role of CYP2A6 in the 17 X 8-hydroxylation was fully examined in the present study. Among 10 isoforms of human cytochrome P450 (CYP1A2, CYP2A6, CYP2B6, CYP2C8, CYP2C9, CYP2C19, CYP2D6, CYP2E1, CYP3A4, or CYP3A5) expressed in Escherichia coli cells, CYP2A6 and CYP1A2 showed high catalytic activities for the 17 X 8-hydroxylation. The 17 X 8-hydroxylase activities significantly associated with coumarin 7-hydroxylase activities (r=0.67, p<0.01) in liver microsomes from 17 individuals, but not with ethoxyresorufin O-deethylase activities. Tranylcypromine, an inhibitor of CYP2A6, reduced the 17 X 8-hydroxylase activities of human liver microsomes. The 17 X 8-hydroxylase activities of CYP2A6.7, CYP2A6.10, and CYP2A6.11 expressed in E. coli cells were 12, 13, and 22% of that of CYP2A6.1, respectively. The 17 X 8-hydroxylase activities were found to be low in liver microsomes from individuals possessing the deletion or mutations in the CYP2A6 gene. Based on these data, we conclude that CYP2A6 is a main 17 X 8-hydroxylase and that the catalytic activities for the 17 X 8-hydroxylation are reduced by the genetic polymorphisms of the CYP2A6 gene.
Laboratory of Drug Metabolism, Graduate School of Pharmaceutical Sciences, Hokkaido University, Sapporo 060-0812, Japan.
Miyuki
M
Kimura
Hiroshi
H
Yamazaki
Masaki
M
Fujieda
Kazuma
K
Kiyotani
Goro
G
Honda
Junji
J
Saruwatari
Kazuko
K
Nakagawa
Takashi
T
Ishizaki
Tetsuya
T
Kamataki
D000328:Adult; D001189:Aryl Hydrocarbon Hydroxylases / Q000235:genetics* / Q000378:metabolism*; D002110:Caffeine / Q000378:metabolism*; D002478:Cells, Cultured; D019388:Cytochrome P-450 CYP1A2 / Q000378:metabolism; D065637:Cytochrome P-450 CYP2A6; D004926:Escherichia coli / Q000201:enzymology / Q000235:genetics; D005260:Female; D005838:Genotype; D006801:Humans; D006900:Hydroxylation; D008297:Male; D008862:Microsomes, Liver / Q000201:enzymology; D006899:Mixed Function Oxygenases / Q000235:genetics* / Q000378:metabolism*; D009251:NADPH-Ferrihemoprotein Reductase; D013806:Theophylline / Q000378:metabolism*
D002110:Caffeine; D013806:Theophylline; D006899:Mixed Function Oxygenases; D001189:Aryl Hydrocarbon Hydroxylases; C585710:CYP2A6 protein, human; D019388:Cytochrome P-450 CYP1A2; D065637:Cytochrome P-450 CYP2A6; D009251:NADPH-Ferrihemoprotein Reductase; C021183:1,7-dimethylxanthine