Characterization of cytochrome P-450 2D1 activity in rat brain: high-affinity kinetics for dextromethorphan.
We investigated the enzymatic function, stability, and regional distribution of rat brain cytochrome P-450 (CYP) 2D1 activity. CYP2D1 is the homolog of human CYP2D6, a genetically variable enzyme that activates or inactivates many clinical drugs acting on the central nervous system (e.g., antidepressants, monoamine oxidase inhibitors, serotonin uptake inhibitors, and neuroleptics), drugs of abuse (e.g., amphetamine and codeine), neurotoxins (e.g., 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, 1,2,3, 4-tetrahydroquinoline), and endogenous neurochemicals (e.g., tryptamine). The CYP2D family has been identified in rodent, canine, and primate brain. Conversion of dextromethorphan to dextrorphan by rat brain membranes was assayed by HPLC and was dependent on NADPH, protein concentration, and incubation time. Significant loss of activity was observed in some homogenizing buffers and after freezing of whole tissues or membrane preparations. Dextromethorphan (0.5-640 microM) metabolism was mediated by high- and low-affinity enzyme systems; K(m1) was 2.7 +/- 2.6 and K(m2) was 757 +/- 156 microM (n = 3 rats, mean +/- S.E.). The enzyme activity was significantly (p <.01) and stereoselectively inhibited by CYP2D1 inhibitors quinine and quinidine (not by CYP2C or CYP3A inhibitors), and by anti-CYP2D6 peptide antiserum (not by anti-CYP2C, -CYP2B, or -CYP3A antibodies). The enzymatic activity demonstrated significant brain regional variation (n = 10 regions, p <.001). These data characterize CYP2D1-mediated dextromethorphan metabolism in rat brain and suggest that localized metabolism of other CYP2D1 substrates (drugs, neurotoxins, and possibly endogenous compounds) within the brain will occur. In humans, CYP2D6 is genetically polymorphic; the variable expression of brain CYP2D6 may result in interindividual differences in central drug and neurotoxin metabolism, possibly contributing to interindividual differences in drug effects and neurotoxicity.
Centre for Addiction and Mental Health and Departments of Pharmacology, University of Toronto, Toronto, Ontario, Canada. r.tyndale@utoronto.ca
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Tyndale
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Li
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Li
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Messina
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Miksys
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Sellers
D000429:Alcohol Oxidoreductases; D000818:Animals; D019138:Antibodies, Blocking / Q000494:pharmacology; D000996:Antitussive Agents / Q000032:analysis / Q000493:pharmacokinetics*; D001189:Aryl Hydrocarbon Hydroxylases; D001921:Brain / Q000201:enzymology*; D002531:Cerebellum / Q000187:drug effects / Q000378:metabolism; D002851:Chromatography, High Pressure Liquid; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D000072467:Cytochrome P450 Family 2; D003915:Dextromethorphan / Q000032:analysis / Q000493:pharmacokinetics*; D003917:Dextrorphan / Q000032:analysis / Q000493:pharmacokinetics; D007700:Kinetics; D008099:Liver / Q000187:drug effects / Q000378:metabolism; D008297:Male; D008566:Membranes / Q000187:drug effects / Q000378:metabolism; D009249:NADP / Q000378:metabolism; D018696:Neuroprotective Agents / Q000032:analysis / Q000493:pharmacokinetics; D051381:Rats; D017208:Rats, Wistar
D019138:Antibodies, Blocking; D000996:Antitussive Agents; D065607:Cytochrome P-450 Enzyme Inhibitors; D018696:Neuroprotective Agents; D003917:Dextrorphan; D009249:NADP; D003915:Dextromethorphan; D003577:Cytochrome P-450 Enzyme System; D000429:Alcohol Oxidoreductases; D001189:Aryl Hydrocarbon Hydroxylases; C504568:Cyp2d1 protein, rat; D000072467:Cytochrome P450 Family 2