Abstract
N-Aralkylated derivatives of 1-aminobenzotriazole are well-established, mechanism-based inhibitors of cytochrome P450 (CYP or P450). In this study, the kinetics of inactivation of CYP2B-dependent 7-pentoxyresorufin O-depentylation (PROD) and CYP1A-dependent 7-ethoxyresorufin O-deethylation (EROD) activities by enantiomers ofN-(α-methylbenzyl)-1-aminobenzotriazole (αMB) were compared. The racemic mixture (±)-αMB, as well as the enantiomers (−)-αMB and (+)-αMB, produced a time-, concentration-, and NADPH-dependent loss of PROD and EROD activity in hepatic microsomes from phenobarbital-treated guinea pigs. The rates of PROD inactivation by (−)-αMB were significantly faster than for (+)-αMB. Consistent with this, the derived maximal kinact was also significantly greater for (−)-αMB than for (+)-αMB (0.49vs. 0.35 min−1). In contrast, the concentrations required for the half-maximal rate of inactivation (Ki ) were equivalent for (−)-αMB and (+)-αMB, whereas the degree of competitive inhibition of PROD activity was greater for (+)-αMB. No significant differences were found among (−)-αMB, (+)-αMB, and (±)-αMB with respect to mechanism-based inactivation (kinact = 0.18, 0.16, and 0.17 min−1, respectively) or competitive inhibition of EROD activity. No differences were found for the maximal extent of PROD or EROD inhibition or the loss of spectral P450 after an extended 30-min incubation with the inhibitors. We conclude that mechanism-based inactivation of guinea pig CYP2B, but not CYP1A, isozymes by αMB occurs in a stereoselective manner, most likely as a result of a difference in the balance between metabolic activation and deactivation for the αMB enantiomers.
Footnotes
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Send reprint requests to: Dr. John R. Bend, Department of Pharmacology and Toxicology, Medical Sciences Building, Room 275, University of Western Ontario, London, Ontario, Canada N6A 5C1.
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This work was supported by the Medical Research Council of Canada (Grant MT9972 to J.R.B.) and by the Academic Development Fund, University of Western Ontario. C.J.S. is the recipient of an Ontario Graduate Scholarship.
- Abbreviations used are::
- P450 or CYP
- cytochrome P450
- ABT
- 1-aminobenzotriazole
- BBT
- N-benzyl-1-aminobenzotriazole
- αMB
- N-(α-methylbenzyl)-1-aminobenzotriazole
- PROD
- 7-pentoxyresorufin O-depentylation
- EROD
- 7-ethoxyresorufin O-deethylation
- Received November 26, 1997.
- Accepted March 20, 1998.
- The American Society for Pharmacology and Experimental Therapeutics
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