DIFFERENT CONTRIBUTIONS OF CYTOCHROME-P450 2C19 AND 3A4 IN THE OXIDATION OF OMEPRAZOLE BY HUMAN LIVER-MICROSOMES - EFFECTS OF CONTENTS OF THESE 2 FORMS IN INDIVIDUAL HUMAN SAMPLES

Omeprazole 5-hydroxylation and sulfoxidation activities were determine d in liver microsomes of different humans whose levels of individual f orms of cytochrome P450 (P450 or CYP) varied. Correlation coefficients between omeprazole 5-hydroxylation activities (when determined at a s ubstrate concentration of 10 mu M) and S-mephenytoin 4'-hydroxylation and testosterone 6 beta-hydroxylation activities were found to be 0.64 and 0.67, respectively, in liver microsomes of 84 human samples exami ned. Omeprazole sulfoxidation activities in these human samples were c orrelated with testosterone Gp-hydroxylation activities (r = 0.86). Om eprazole 5-hydroxylation by liver microsomes of a human sample that co ntained relatively high levels of CYP3A4 and low levels of CYP2C19 wer e inhibited very significantly by ketoconazole and anti-CYP3A4 antibod ies, although a human sample having high in CYP2C19 and low in CYP3A4 was found to be sensitive toward fluvoxamine and anti-CYP2C9 antibodie s. Sulfaphenazole (at 100 mu M) did not affect the omeprazole 5-hydrox ylation and sulfoxidation catalysed by human liver microsomes. Both re combinant human CYP2C19 and CYP3A4 enzymes had activities for omeprazo le 5-hydroxylation, with low K-m and high V-max values for the former enzyme and high K-m and low V-max values for the CYP3A4. These results suggest that contributions of CYP2C19 and CYP3A4 in the omeprazole 5- hydroxylation depend upon the ratio of these two P450 levels in human liver microsomes. Omeprazole 5-hydroxylation activities of different h uman samples were found to be related to predicted values calculated f rom the kinetic parameters of recombinant enzymes and the levels of li ver microsomal CYP2C19 and CYP3A4 enzymes. Finally, when recombinant h uman CYP2C19 and CYP3A4 were mixed at levels found in different human samples, relatively similar profiles of omeprazole oxidation by the re combinant and microsomal enzyme systems were determined by analysis of high-performance liquid chromatography. These results suggest that bo th CYP2C19 and CYP3A4 are involved in the 5- oxidation of omeprazole ( at a substrate concentration of 10 mu M) in human liver microsomes and that contributions of these P450 enzymes depend on the compositions o f CYP2C19 and CYP3A4 in liver.