Abstract
The 1′-hydroxylation of rac-bufuralol, which is catalyzed by polymorphic CYP2D6 in humans, was studied in brain microsomes from male and female Wistar rats and from the female Dark Agouti rat, a model of the CYP2D6 poor metabolizer phenotype. The kinetics of the 1′-hydroxylation of bufuralol (1–1500 μM) by brain microsomes were biphasic. The activity of the high-affinity site of metabolism was consistent with Michaelis-Menten kinetics (apparentKm1 = 0.61–1.42 μM,Vmax1 = 4.3–4.8 fmol/min/mg of protein), whereas the low-affinity activity was better described by a Hill function (K50%(2) = 253–258 μM, Vmax2 = 817–843 fmol/min/mg of protein, n = 1.2–1.3). Values for kinetic constants were similar in all rat strains. Quinine was only a weak inhibitor of both the high- (apparentKi = 90 μM) and low-affinity (210 μM) sites of metabolism. In contrast, the kinetics of 1′-hydroxylation of bufuralol by rat liver microsomes were best described by a two-site Michaelis-Menten function.Vmax values were 3 to 5 orders of magnitude greater compared with those for brain microsomes (male and female Wistar), and liver microsomes from female Dark Agouti rats were significantly less active than those from Wistar rats. These data, together with the known potent inhibitory effect of quinine on bufuralol 1′-hydroxylation by rat liver microsomes, indicate tissue-specific differences in the enzymology of this reaction. The role of brain CYP2D enzymes remains to be clarified.
Footnotes
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Send reprint requests to: Dr. M. S. Lennard, Section of Molecular Pharmacology and Pharmacogenetics, Division of Clinical Sciences, University of Sheffield, Royal Hallamshire Hospital, Sheffield S10 2JF, UK. E-mail: m.s.lennard{at}sheffield.ac.uk
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↵1 Present address: Department of Drug Metabolism and Pharmacokinetics, SmithKline Beecham Pharmaceuticals, The Frythe, Welwyn AL6 9AR, UK.
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This work was supported in part by a Ph.D. scholarship from the University of Sheffield and by a Biotechnology and Biological Sciences Research Council Industrial Case Studentship in collaboration with SmithKline Beecham Pharmaceuticals.
- Abbreviation used is::
- DA
- Dark Agouti
- Received October 14, 1999.
- Accepted May 23, 2000.
- The American Society for Pharmacology and Experimental Therapeutics
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