Metabolic characterization of the major human small intestinal cytochrome P450S

Human small intestine epithelial cells (enterocytes) provide the first site for cytochrome P450 (CYP)-catalyzed metabolism of orally ingested xenobiot ics. CYP3A4 is the major form of CYP expressed in enterocytes and CYP2C is also expressed at a significant level. In this study, we further characteri zed the expression of CYP3A4 and CYP2C in human enterocytes and their inter individual variations by examining the metabolic activities from 10 individ uals. CYP3A4 in human jejunum microsomes, as determined by 6 beta -testoste rone hydroxylase activity, varied from 0.36 to 2.46 nmol/ min/mg. The appar ent average K-m and V-max values from two representative individuals were 5 4 mM and 3.2 nmol/min/mg, respectively. CYP2C9 and CYP2C19 in human jejunum microsomes, as determined by diclofenac 4'-hydroxylase and mephenytoin 4'- hydroxylase activities, varied over an 18-fold range (7.3-129 pmol/min/mg) and 17-fold range (0.8-13.1 pmol/min/mg), respectively. The mean apparent K -m for diclofenac 4'-hydroxylase was 9.9 muM, whereas the apparent mean K-m for S-mephenytoin 4'-hydroxylase was 79.3 muM. The mean intrinsic clearanc e (V-max/K-m) was approximately 130-fold greater for diclofenac 4'-hydroxyl ase than for mephenytoin 4'-hydroxylase. The metabolic activities of CYP2C9 and CYP2C19 were confirmed by inhibition by sulfaphenazole for CYP2C9 and ticlopidine for CYP2C19. In addition, CYP2C9 activities did not correlate w ith CYP3A4 activities, while CYP2C19 activities had a significant but poor correlation with those of CYP3A4. Thus the major CYP activities in human en terocytes have large interindividual variabilities that are not strongly re lated.