In vitro metabolic characteristics of cytochrome P-450 2A6 in Chinese liver microsomes.
Abstract
AIM: To investigate the metabolic characteristics of cytochrome P-450 C YP2A6 in
human liver microsomes.
METHODS: Cytochrome P-450 enzyme activities were measured by biochemical assays.
Xenobiotics were employed to observe their effects on CYP2A6 in vitro. The
kinetics of coumarin 7-hydroxylase was determined, and the correlation between
CYP2A6 and UDP-glucuronosyltransferase (UGT) was analyzed.
RESULTS: CYP2A6 activities of human liver microsomes were from 0.47 to 4.14
micromol . min-1 . g-1, with a 8.8-fold variation. The Km and Vmax of CYP2A6
ranged from 0.25 to 1.56 micromol/L and 1.41 to 8.70 micromol . min-1 . g-1,
respectively. CYP2A6 activity was markedly inhibited (> 50 %) by pilocarpine,
diethyldithio carbamic (DDC), and rifampicin, the IC50 was 5.31 micromol/L,
156.35 micromol/L, and 38.81 micromol/L, respectively. alpha-Naphthoflavone,
sulfaphenazole, troleandomycin (TAO), ketoconazole, phenobarbital, prednisolone,
and azithromycin had little or no effects on coumarin 7-hydroxylation. A
significant correlation was observed between CYP2A6 and UGT2 (r = 0.9453, P <
0.05).
CONCLUSION: CYP2A6 activity and kinetics exhibited a considerable variation in
human liver microsomes in vitro, and a significant correlation was existed
between CYP2A6 and phase II enzyme UGT2. Not only pilocarpine, CYP2A6 specific
inhibitor, but also rifampicin and DDC inhibited CYP2A6 activity selectively.
Keywords:
human liver microsomes.
METHODS: Cytochrome P-450 enzyme activities were measured by biochemical assays.
Xenobiotics were employed to observe their effects on CYP2A6 in vitro. The
kinetics of coumarin 7-hydroxylase was determined, and the correlation between
CYP2A6 and UDP-glucuronosyltransferase (UGT) was analyzed.
RESULTS: CYP2A6 activities of human liver microsomes were from 0.47 to 4.14
micromol . min-1 . g-1, with a 8.8-fold variation. The Km and Vmax of CYP2A6
ranged from 0.25 to 1.56 micromol/L and 1.41 to 8.70 micromol . min-1 . g-1,
respectively. CYP2A6 activity was markedly inhibited (> 50 %) by pilocarpine,
diethyldithio carbamic (DDC), and rifampicin, the IC50 was 5.31 micromol/L,
156.35 micromol/L, and 38.81 micromol/L, respectively. alpha-Naphthoflavone,
sulfaphenazole, troleandomycin (TAO), ketoconazole, phenobarbital, prednisolone,
and azithromycin had little or no effects on coumarin 7-hydroxylation. A
significant correlation was observed between CYP2A6 and UGT2 (r = 0.9453, P <
0.05).
CONCLUSION: CYP2A6 activity and kinetics exhibited a considerable variation in
human liver microsomes in vitro, and a significant correlation was existed
between CYP2A6 and phase II enzyme UGT2. Not only pilocarpine, CYP2A6 specific
inhibitor, but also rifampicin and DDC inhibited CYP2A6 activity selectively.