Abstract
Human cytochrome P-450 1A1 (CYP1A1) is located primarily in extrahepatic tissues. To begin the characterization of this enzyme in the small intestine, we screened a bank of 18 human small intestinal microsomal preparations for CYP1A1 catalytic [(7-ethoxyresorufinO-deethylase (EROD)] activity and protein content. Although EROD activity was below detectable limits in 12 of the preparations, 6 exhibited measurable activity (1.4–123.5 pmol/min/mg), some exceeding that for 2 human liver microsomal preparations (11.0 and 26.4 pmol/min/mg). This variation was not due to variable quality of the preparations because each sample displayed readily detectable CYP3A4 catalytic activity and immunoreactive protein. We inadvertently found that intestinal EROD activity was inhibitable by ketoconazole at a concentration commonly believed to selectively inhibit CYP3A4. The possibility that CYP3A4 metabolizes 7-ethoxyresorufin was excluded because there was no correlation between intestinal CYP3A4 catalytic and EROD activity, and cDNA-expressed human CYP3A4 exhibited no EROD activity. Moreover, CYP1A1 immunoreactive protein was most abundant in the three intestinal preparations with the highest EROD activities, and the mean apparent Ki of ketoconazole observed for these three preparations (40 nM) was essentially identical with that for cDNA-expressed human CYP1A1 (37 nM). In summary, there is large interindividual variation in CYP1A1 expression in human small bowel, and ketoconazole is not a selective CYP3A4 inhibitor in in vitro metabolism studies involving intestinal tissue obtained from some individuals. These observations raise the possibility that in vivo drug interactions involving ketoconazole could result from CYP1A1 inhibition in the intestine in some individuals.
Footnotes
-
Send reprint requests to: Paul B. Watkins, M.D., Department of Internal Medicine, 1500 E. Medical Center Dr., 2150 Cancer Center and Geriatrics Center, Ann Arbor, MI 48109-0930. E-mail: pwatkins{at}umich.edu
-
This work was supported in part by a National Research Service Award (to M.F.P.) GM19034, from the National Institute of General Medical Sciences.
- Abbreviations used are::
- CYP
- cytochrome P-450
- EROD
- 7-ethoxyresorufin O-deethylase
- HG
- human small intestinal microsomes
- HL
- human liver microsomes
- Received September 30, 1998.
- Accepted December 21, 1998.
- The American Society for Pharmacology and Experimental Therapeutics
DMD articles become freely available 12 months after publication, and remain freely available for 5 years.Non-open access articles that fall outside this five year window are available only to institutional subscribers and current ASPET members, or through the article purchase feature at the bottom of the page.
|