The human intestinal cytochrome P450 "pie"

Abstract

Cytochromes P450 (P450s) 3A, 2C, and 1A2 constitute the major "pieces" of the human liver P450 "pie" and account, on average, for 40, 25, and 18%, respectively, of total immunoquantified P450s (J Pharmacol Exp Ther 270:414-423, 1994). The P450 profile in the human small intestine has not been fully characterized. Therefore, microsomes prepared from mucosal scrapings from the duodenal/jejunal portion of 31 human donor small intestines were analyzed by Western blot using selective P450 antibodies. P450s 3A4, 2C9, 2C19, and 2J2 were detected in all individuals and ranged from 8.8 to 150, 2.9 to 27, <0.6 to 3.9, and <0.2 to 3.1 pmol/mg, respectively. CYP2D6 was detected in 29 individuals and ranged from <0.2 to 1.4 pmol/mg. CYP3A5 was detected readily in 11 individuals, with a range (average) of 4.9 to 25 (16) pmol/mg that represented from 3 to 50% of total CYP3A (CYP3A4 + CYP3A5) content. CYP1A1 was detected readily in three individuals, with a range (average) of 3.6 to 7.7 (5.6) pmol/mg. P450s 1A2, 2A6, 2B6, 2C8, and 2E1 were not or only faintly detected. As anticipated, average CYP3A content (50 pmol/mg) was the highest. Excluding CYP1A1, the remaining enzymes had the following rank order: 2C9 > 2C19 > 2J2 > 2D6 (8.4, 1.1, 0.9, and 0.5 pmol/mg, respectively). Analysis of a pooled preparation of the 31 donor specimens substantiated these results. In summary, as in the liver, large interindividual variation exists in the expression levels of individual P450s. On average, CYP3A and CYP2C9 represents the major pieces of the intestinal P450 pie, accounting for 80 and 15%, respectively, of total immunoquantified P450s.

FIG. 1

Western blot showing the specificity of the anti-CYP2C19 polyclonal antibody after a typical exposure for the detection of CYP2C9 and CYP2C19 (A) or a prolonged exposure (B). Except for the recombinant CYP2Cs, which represent equimolar amounts (200 fmol), the amount of recombinant P450 represents the highest amount used for the generation of the calibration curves for quantification (P450s 1A1, 2D6, 2J2, 3A4, and 3A5) or detection (P450s 2A6, 2B6, and 2E1) in human intestinal microsomes: 100, 80, 100, 600, 400, 80, 150, and 40 fmol, respectively. Each human liver microsomal (HLM) preparation represents 2 µg of microsomal protein. HL-150 was known to contain CYP3A5.

FIG. 2

Western blots of pooled (n = 31, except where noted) human intestinal microsomal preparations (HIM) showing the presence or absence of various P450s. The amount of recombinant enzymes (standards) ranged from 100 to 12.5 (CYP1A1), 80 to 10 (CYP2A6), 150 to 18.8 (CYP2B6), 600 to 75 (CYP2C9), 200 to 25 (CYP2C19), 80 to 10 (CYP2D6), 40 to 5 (CYP2E1), 100 to 12.5 (CYP2J2), 600 to 75 (CYP3A4), and 400 to 50 (CYP3A5) fmol. The pooled HIM represent 40 (P450s 1A1, 2A6, 2B6, 2C9, 2C19, 2D6, 2E1, and P2J2), 5 (CYP3A4), or 10 (CYP3A5) µg of microsomal protein. Each human liver microsomal (HLM) preparation represents 2 (CYP2E1), 5 (P450s 2A6, 2D6, 3A4, and 2J2), 10 (1A1, 2C9, 2C19, and 3A5), or 20 (CYP2B6) µg of microsomal protein. HL-150 was known to contain CYP3A5.

FIG. 3

The average human proximal small intestinal and hepatic P450 pies. The percent contributions of individual P450 enzymes are based on total immunoquantified P450 content. The values for the liver pie were derived from Shimada et al.(1994).