Abstract
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 μM) metabolism was mediated by high- and low-affinity enzyme systems;Km1 was 2.7 ± 2.6 andKm2 was 757 ± 156 μM (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.
Footnotes
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Send reprint requests to: Dr. R. F. Tyndale, Rm. 4336, Dept. of Pharmacology, Medical Sciences Building, 1 King’s College Circle, University of Toronto, Toronto, Ontario, Canada, M5S 1A8. E-mail: r.tyndale{at}utoronto.ca
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Supported in part by National Institute on Drug Abuse Grant DA06889, Medical Research Council of Canada Grant MT14173, and the Center for Addictions and Mental Health, Canada. Parts of this work were previously presented at the following meetings: XIth International Symposium on Microsomes and Drug Oxidations, 1996; Congress on the Problems of Drug Dependence, 1996; and Society for Neuroscience meeting 1995.
- Abbreviations used are::
- CYP
- cytochrome P-450
- DEX
- dextromethorphan
- DOR
- dextrorphan
- CNS
- central nervous system
- QN
- quinine
- QD
- quinidine
- SPZ
- sulfaphenazole
- TRL
- troleandomycin
- ACSF
- artificial cerebrospinal fluid
- Received December 31, 1998.
- Accepted April 29, 1999.
- The American Society for Pharmacology and Experimental Therapeutics
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