title: Diazinon is activated by CYP2C19 in human liver.
abstract: Phosphorothioate compounds are used throughout the world as agricultural and domestic pesticides. Here, the activation of the phosphorothioate diazinon to diazoxon in human liver is described. In an initial study using three human liver microsomal samples, K(m) for diazoxon formation varied markedly (31, 208, and 660 microM; V(max) 1125, 685, and 1028 pmol/min/mg protein, respectively), suggesting the involvement of more than one P450 enzyme. A wide variation in activity was found using 50 microM diazinon as substrate, (11-648 pmol/min/mg protein, n = 15), whereas, with 500 microM, variation was less (164-978 pmol/min/mg protein). Among eight P450-catalyzed reactions, the putative high-affinity component (50 microM diazinon) correlated with S-mephenytoin 4'-hydroxylase activity (r = 0.686, p < 0.01), suggesting the involvement of CYP2C19. The putative low-affinity component (500 microM diazinon) correlated with both S-mephenytoin 4'-hydroxylase (r = 0.714; p < 0.005) and high-affinity phenacetin O-deethylase activity (r = 0.625; p < 0.05). This activity was partially inhibited by furafylline, troleandomycin, and ketoconazole. These data suggest contributions from CYP2C19, CYP1A2, and CYP3A4. None of the inhibitors affected the high-affinity component. Of seven heterologously expressed human P450 enzymes, CYP2C19 activated diazinon (500 microM) at the fastest rate, followed by CYP3A4, CYP1A2, and CYP2C9. Both hepatic microsomal S-mephenytoin 4'-hydroxylase and high-affinity phenacetin O-deethylase activities were strongly inhibited by diazinon (IC50 < 2.5 microM), while no effect was seen on midazolam 1'-hydroxylase activity. These data indicate that CYP2C19 is the major enzyme involved in diazinon activation in human liver, while other enzymes including CYP1A2 may play a more minor role.
authors.affiliation: Toxicology Unit, Section on Clinical Pharmacology, Faculty of Medicine, Imperial College, Du Cane Road, London W12 0NN, United Kingdom. WeKa@novonordisk.com
authors.forename: W A
authors.initials: WA
authors.lastname: Kappers
authors.forename: R J
authors.initials: RJ
authors.lastname: Edwards
authors.forename: S
authors.initials: S
authors.lastname: Murray
authors.forename: A R
authors.initials: AR
authors.lastname: Boobis
mesh_terms: D001189:Aryl Hydrocarbon Hydroxylases; D001711:Biotransformation; D065702:Cytochrome P-450 CYP2B6; D065731:Cytochrome P-450 CYP2C19; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D003976:Diazinon / Q000493:pharmacokinetics*; D004305:Dose-Response Relationship, Drug; D006801:Humans; D007306:Insecticides / Q000493:pharmacokinetics*; D007654:Ketoconazole / Q000494:pharmacology; D008099:Liver / Q000187:drug effects* / Q000201:enzymology; D008862:Microsomes, Liver / Q000187:drug effects* / Q000201:enzymology; D006899:Mixed Function Oxygenases / Q000378:metabolism*; D010089:Oxidoreductases, N-Demethylating; D011994:Recombinant Proteins / Q000493:pharmacokinetics; D013806:Theophylline / Q000031:analogs & derivatives* / Q000494:pharmacology; D014217:Troleandomycin / Q000494:pharmacology
chemical_list: D007306:Insecticides; D011994:Recombinant Proteins; D003577:Cytochrome P-450 Enzyme System; D013806:Theophylline; C050131:furafylline; D014217:Troleandomycin; D006899:Mixed Function Oxygenases; D001189:Aryl Hydrocarbon Hydroxylases; C585599:CYP2B6 protein, human; C045793:CYP2C19 protein, human; D065702:Cytochrome P-450 CYP2B6; D065731:Cytochrome P-450 CYP2C19; D010089:Oxidoreductases, N-Demethylating; D007654:Ketoconazole; D003976:Diazinon