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Research ArticleArticle

Triazolam Substrate Inhibition: Evidence of Competition for Heme-Bound Reactive Oxygen Within the CYP3A4 Active Site

Michael L. Schrag and Larry C. Wienkers
Drug Metabolism and Disposition January 2001, 29 (1) 70-75;
Michael L. Schrag
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Larry C. Wienkers
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Abstract

In human liver microsomes, triazolam is principally metabolized by CYP3A4 to form two metabolites, 1′-hydroxytriazolam (1′OHTz) and 4-hydroxytriazolam (4OHTz). The velocity of 1′OHTz formation was found to decrease at higher triazolam concentrations (>200 μM), indicative of “substrate inhibition”. Coincubation of [14C]triazolam with authentic metabolite standards of either 1′OHTz or 4OHTz up to 30 μM did not significantly inhibit the rate of [14C]1′OHTz formation. The effects of secondary compounds on triazolam oxidation were shown to be product-specific, producing either activation or inhibition depending on the triazolam metabolite monitored. When human liver microsomes were supplemented with exogenous human cytochrome b5, it was observed that substrate inhibition was attenuated and the resulting increase in 1′OHTz formation, relative to control (nonsupplemented) incubations, corresponded to a decrease in the ratio of 4OHTz to 1′OHTz. In contrast, when cofactor (e.g., 100 μM NADPH) was rate limiting, the metabolite ratio (4OHTz/1′OHTz) was markedly increased over the entire substrate concentration range (0.5–1000 μM). To explain these kinetic observations, a two-site binding model is proposed in which triazolam is hypothesized to bind within the CYP3A4 active site in spatially distinct orientations, which may lead to the formation of either the 1′-hydroxytriazolam or 4-hydroxytriazolam. Differential inhibition/activation is consistent with this two-site model and substrate inhibition is hypothesized to result from competition between the two sites for reactive oxygen.

Footnotes

  • Send reprint requests to: Larry C. Wienkers, Pharmacia Corporation, 301 Henrietta St., 7265-300-313, Kalamazoo, MI 49007-4940. E-mail: Larry.C.Wienkers{at}am.pnu.com

  • Abbreviations used are::
    P-450 or CYP
    cytochrome P-450
    b5
    cytochromeb5
    4OHTz
    4-hydroxytriazolam
    1′OHTz
    1′-hydroxytriazolam
    HPLC
    high-performance liquid chromatography
    ES
    enzyme bound with one substrate
    ESS
    enzyme bound with two substrates
    • Received May 23, 2000.
    • Accepted September 25, 2000.
  • The American Society for Pharmacology and Experimental Therapeutics
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Drug Metabolism and Disposition: 29 (1)
Drug Metabolism and Disposition
Vol. 29, Issue 1
1 Jan 2001
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Research ArticleArticle

Triazolam Substrate Inhibition: Evidence of Competition for Heme-Bound Reactive Oxygen Within the CYP3A4 Active Site

Michael L. Schrag and Larry C. Wienkers
Drug Metabolism and Disposition January 1, 2001, 29 (1) 70-75;

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Research ArticleArticle

Triazolam Substrate Inhibition: Evidence of Competition for Heme-Bound Reactive Oxygen Within the CYP3A4 Active Site

Michael L. Schrag and Larry C. Wienkers
Drug Metabolism and Disposition January 1, 2001, 29 (1) 70-75;
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