Characterization of the cytochrome P-450 gene family responsible for the N-dealkylation of the ergot alkaloid CQA 206-291 in humans.
The ergot alkaloid CQA 206-291 (CQA) was converted by human liver microsomes (n = 16) almost exclusively to the N-deethylated metabolite (I), as identified by the on-line coupling of liquid chromatography and mass spectroscopy. Metabolite I formation exhibited monophasic and linear enzyme kinetics (2.9-300 microM), and a 5.6-fold interindividual variability (7.2-40.2 nmol/mg/hr). Chemical inhibition experiments revealed that imidazole antimycotic agents (ketoconazole, miconazole, and clotrimazole) were potent inhibitors of this N-deethylation. Polymorphically metabolized substrates (sparteine and phenytoin), well-established cytochrome P-450 probe substrates (antipyrine and tolbutamide), and steroid hormones (estradiol and testosterone) were noninhibitory, indicating that their metabolism is catalyzed by forms of cytochrome P-450 that do not catalyze this route of CQA biotransformation. The ergot alkaloids--dihydroergotamine, bromocriptine, and SDZ 208-911--were competitive inhibitors of metabolite I formation, suggesting that these compounds are metabolized by similar enzymes. Cyclosporine A was a potent competitive inhibitor of CQA metabolism, providing initial evidence that formation of metabolite I was catalyzed by proteins of the CYP3 gene family. This was substantiated by the finding that CQA metabolism was completely inhibited by a polyclonal antibody directed against a pregnenolone 16 alpha-carbonitrile-inducible cytochrome P-450 of rat liver. The rate of CQA metabolism correlated significantly to the level of CYP3A4 expression, the rate of cyclosporine A metabolism to each of the primary metabolites (M-1, M-17, and M-21), and the rate of midazolam 4-hydroxylation. COS 1 cells transfected with human CYP3A4 and CYP3A5 provided direct evidence that these enzymes catalyze the metabolism of CQA.(ABSTRACT TRUNCATED AT 250 WORDS)
Ball
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Drug Safety Assessment, Sandoz Pharma Ltd., Basel, Switzerland.
Maurer
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Zollinger
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Ladona
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M
Vickers
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D001666:Binding Sites, Antibody; D001667:Binding, Competitive; D003577:Cytochrome P-450 Enzyme System / Q000235:genetics* / Q000502:physiology; D003640:Dealkylation / Q000187:drug effects; D015259:Dopamine Agents / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000378:metabolism*; D004873:Ergolines / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D008862:Microsomes, Liver / Q000187:drug effects / Q000201:enzymology* / Q000378:metabolism; D005810:Multigene Family
D015259:Dopamine Agents; D004873:Ergolines; D003577:Cytochrome P-450 Enzyme System; C059483:CQA 206-291