Characterization of human small intestinal cytochromes P-450.

Human small intestine epithelial cells (enterocytes) provide the first site for cytochrome P-450 (CYP)-catalyzed metabolism of orally ingested xenobiotics. The CYP composition of enterocytes could thus affect the potential toxicity or therapeutic efficacy of xenobiotics by modifying systemic uptake. We have characterized human enterocyte CYP composition to enable assessment of its functional roles. An isolation method for enterocytes from human small intestine was developed using EDTA buffer-mediated elution. Villous enterocytes were isolated in high yield, separated from crypt cells. Reverse transcriptase-polymerase chain reaction of total RNA from enterocytes revealed that CYP1A1, 1B1, 2C, 2D6, 2E1, 3A4, and 3A5 mRNA were expressed, but only CYP2C and 3A4 were detectable by Western immunoblotting in enterocyte microsomes from 10 human small intestines, whereas CYP1A1 was weakly detectable in two of eight intestines tested. Microsomal protein content decreased markedly along the small intestine from the duodenum to the ileum, whereas total CYP content and CYP3A4 erythromycin N-demethylase activity increased slightly in progressing from the duodenum to the jejunum and then decreased markedly toward the ileum. Levels of CYP3A4 and 2C protein did not decrease in concert as a function of length along the intestine distally. Maximal CYP content for the 10 intestines varied from 0.06 to 0.18 nmol/mg microsomal protein and maximal CYP3A4 erythromycin N-demethylase activity varied from 0.30 to 0.76 nmol/min/mg microsomal protein. In conclusion, CYP3A4 is the major form of CYP expressed in human small intestine enterocytes, CYP3A5 expression was not detected, CYP2C and, in some intestines, CYP1A1 were expressed. The highest metabolic activity occurred in the proximal intestine.