Objectives: Our objectives were to determine the content of cytochrome P450 (CYP) 3A4, CYP3A5, and P-glycoprotein and to measure CYP3A4-dependent catalytic activity in paired human small intestinal and liver specimens.
Methods: Samples of duodenum or proximal jejunum and liver wedge biopsy specimens were obtained from 15 patients undergoing a gastrointestinal operation. Enterocytes were isolated from the intestinal samples. The contents of CYP3A4, CYP3A5, and P-glycoprotein and CYP3A4-mediated catalytic activities were determined in homogenized enterocyte and liver samples.
Results: The CYP3A4 protein content was about 3 times (P <.01) and the P-glycoprotein content about 7 times (P <.0001) higher in the enterocyte homogenates than in the liver homogenates. CYP3A5 protein was detected in all samples, but the levels were too low in most cases to allow quantification. The 2 cases with a quantifiable hepatic CYP3A5 content had the CYP3A5*1/*3 genotype; all other cases were homozygous for the CYP3A5*3 allele. No intraindividual correlations between the intestine and liver with respect to CYP3A4 content, P-glycoprotein content, or the measured catalytic activities were present. Values for the maximum rate of metabolism (V(max)) of verapamil N-dealkylation (formation of D-617) and N-demethylation (formation of norverapamil) activities correlated with the CYP3A4 protein content in both organs.
Conclusions: This work demonstrated a much higher content of both CYP3A4 protein and P-glycoprotein in enterocytes isolated from human duodenal or jejunal mucosa than in paired specimens of liver tissue. These results lend support to the view that biotransformation in the gut wall substantially contributes to the overall first-pass metabolism of many CYP3A4 substrates. Furthermore, the high content of P-glycoprotein on the apical surface of enterocytes supports the theory that this efflux transporter may act in concert with CYP3A4 to limit oral drug bioavailability. Finally, these results indicate that neither CYP3A4 nor MDR1 (P-glycoprotein) is coordinately regulated in the liver and intestine.