Abstract
We have previously shown racial differences in propranolol kinetics, with the largest differences appearing to be in its 4-hydroxylation. The purpose of this study was to identify and confirm the cytochrome P450 enzymes (CYP) with propranolol 4-hydroxylase activity, describe their enzyme kinetics, and determine whether there were racial differences in their catalytic activity. Eleven human recombinant, expressed CYPs were screened, but only CYP1A2 and CYP2D6 possessed propranolol 4-hydroxylase activity. Subsequent studies were conducted in human liver microsomes, including correlation, inhibition, enzyme kinetics, and racial comparison studies. Significant correlations were noted between propranolol 4-hydroxylation and ethoxyresorufin-O-deethylation (marker of CYP1A2 activity), with marked improvement in the correlations when CYP2D6-mediated propranolol 4-hydroxylation was inhibited with quinidine. Inhibition studies showed that quinidine inhibited approximately 55% of propranolol 4-hydroxylation and furaphylline (CYP1A2-selective inhibitor) inhibited about 45% of propranolol 4-hydroxylation. Median (range) parameter estimates of (S)-4-hydroxypropranolol [(S)-HOP] formation were a Vmax value of 307 (165–2397) and 721 (84–1975) pmol/mg of protein/60 min for CYP1A2 and CYP2D6, respectively, and a Km value of 21.2 (8.9–77.5) and 8.5 (5.9–31.9) μM for CYP1A2 and CYP2D6, respectively. CYP1A2- and CYP2D6-mediated propranolol 4-hydroxylation was about 70 and 100% higher (P < .05 for both), respectively, in African-Americans compared with Caucasians. In summary, we found that both CYP1A2 and CYP2D6 catalyze formation of 4-hydroxypropranolol and that both enzymes exhibited racial differences in this reaction. The observed racial differences in drug metabolism may have relevance to drug efficacy, toxicity, or carcinogen activation for CYP1A2 or CYP2D6 substrates.
Footnotes
-
Send reprint requests to: Julie A. Johnson, Pharm.D., Department of Pharmacy Practice, University of Florida, Box 100486, Gainesville, FL 32610-0486. E-mail:Johnson{at}cop.ufl.edu
-
↵1 This work was supported by National Institutes of Health Grant GM54297, Cancer Center CORE Grant CA21765, and Grants N01-DK-9-2310 and CA51001; a Center of Excellence Grant from the State of Tennessee; and American Lebanese Syrian Associated Charities (ALSAC). This research was presented in part at the 100th Annual Meeting of the American Society for Clinical Pharmacology and Therapeutics, March 18–20, 1999, San Antonio, TX.
-
↵2 Present address: University of Florida College of Pharmacy, Gainesville, FL.
-
↵3 Present address: Center for Neurologic Diseases, Brigham and Women's Hospital and Harvard Medical School, Boston, MA.
- Abbreviations:
- HOP
- 4-hydroxypropranolol
- CYP
- cytochrome P450
- HLM
- human liver microsome
- EROD
- ethoxyresorufinO-deethylation
- Received March 13, 2000.
- Accepted May 25, 2000.
- The American Society for Pharmacology and Experimental Therapeutics
JPET 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.
|
