Bioactivation of N-substituted N'-(4-imidazole-ethyl)thioureas by human FMO1 and FMO3.
Enzyme kinetic parameters of the bioactivation of thiourea-containing compounds by human flavin-containing monooxygenase enzymes (FMOs) FMO1 and FMO3 were investigated. A microtitre-based adaptation of methodology described for the thiourea-dependent oxidation of thiocholine was used to determine the turnover of thiourea-containing compounds by human FMO1 and FMO3. The results show that major differences in enzyme kinetic parameters for N-substituted N'-(4-imidazole-ethyl)thiourea exist between human FMO3 and human FMO1. Whereas Km values of N-substituted N'-(4-imidazole-ethyl)thioureas for human FMO3 are all in the millimolar range, the Km values for human FMO1 range from the low micromolar to the low millimolar range. Furthermore, among a series of N-p-phenyl-substituted N'-(4-imidazole-ethyl)thioureas an interesting structure-activity relationship is evident with both FMO1 and FMO3. Where the Km decreases with increasing electron-withdrawing capacity of the p-substituent in the case of FMO1, the opposite phenomenon may be the case with FMO3. The kcat values of the compounds were all comparable for FMO1, averaging 3.03 +/- 0.56 min-1, whereas more variation was found for FMO3 (3.71 +/- 2.01 min-1). Enzyme kinetic parameters Km and kcat/Km of human FMO1 for N-substituted N'-(4-imidazole-ethyl)thioureas show a high degree of correlation with the results obtained in rat liver microsomes, in which rat FMO1 is the most abundant form, whereas those of human FMO3 do not.
Onderwater
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RC
Leiden/Amsterdam Center for Drug Research, Division of Molecular Toxicology, Vrije Universiteit, Amsterdam, the Netherlands.
Rettie
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Commandeur
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Vermeulen
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D000818:Animals; D001711:Biotransformation; D006801:Humans; D007093:Imidazoles / Q000493:pharmacokinetics*; D010105:Oxygenases / Q000235:genetics / Q000494:pharmacology*; D051381:Rats; D011994:Recombinant Proteins / Q000235:genetics; D013329:Structure-Activity Relationship; D013890:Thiourea / Q000031:analogs & derivatives* / Q000493:pharmacokinetics*
D007093:Imidazoles; D011994:Recombinant Proteins; D010105:Oxygenases; C020066:dimethylaniline monooxygenase (N-oxide forming); D013890:Thiourea