Diazinon is activated by CYP2C19 in human liver

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 muM; 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 muM diazinon as substrate, (11-648 pmol/min/mg protein, n = 15), whereas, with 500 muM, variation was less (164-978 pmol/min/mg protein). Among eight P450-catalyzed reactions, the putative high-affinity component (50 ELM 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 muM 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 ELM) 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 muM), 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. (C) 2001 Academic Press.