Identification of the cytochrome P450 isoenzymes involved in the metabolism of diazinon in the rat liver

J Biochem Mol Toxicol. 1999;13(1):53-61. doi: 10.1002/(sici)1099-0461(1999)13:1<53::aid-jbt7>;2-2.


The metabolism of diazinon, an organo-phosphorothionate pesticide, to diazoxon and pyrimidinol has been studied in incubations with hepatic microsomes from control Sprague-Dawley (SD) rats or SD rats treated with different P450-specific inducers (phenobarbital, dexamethasone, beta-napthoflavone, and pyrazole). Results obtained indicate an involvement of CYP2C11, CYP3A2, and CYP2B1/2, whereas CYP2E1 and CYP1A1 do not contribute to the pesticide oxidative metabolism. Indeed, diazinon was metabolized by microsomes from control rats; among the inducers, phenobarbital and dexamethasone only increased the production of either metabolites, although to different extents. The production of the two metabolites is self-limiting, due to P450 inactivation; therefore, the inhibition of CYP-specific monooxygenase activities after diazinon preincubation has been used to selectively identify the competent CYPs in diazinon metabolism. Results indicate that, after diazinon preincubation, CYP3A2-catalyzed reactions (2beta- and 6beta-testosterone hydroxylation) are very efficiently inhibited; CYP2C11- and CYP2B1/2-catalyzed reactions (2alpha- and 16beta-testosterone hydroxylation, respectively) are weakly inhibited, while CYP2E1-, CYP2A1/2-, and CYP1A1/2-related activities were unaffected. Results obtained by using chemical inhibitors or antibodies selectively active against specific CYPs provide a direct evidence for the involvement of CYP2C11, CYP3A2, and CYP2B1/2, indicating that each of them contributed about 40-50% of the diazinon metabolism, in hepatic microsomes from untreated, phenobarbital-, and dexamethasone-treated rats, respectively. The higher diazoxon/pyrimidinol ratio observed after phenobarbital-treatment together with the significantly more effective inhibition toward diazoxon production exerted by metyrapone in microsomes from phenobarbital-treated rats supports the conclusion that CYP2B1/2 catalyze preferentially the production of diazoxon.

MeSH terms

  • Animals
  • Aryl Hydrocarbon Hydroxylases*
  • Cytochrome P-450 CYP1A1 / metabolism
  • Cytochrome P-450 CYP2B1 / metabolism
  • Cytochrome P-450 CYP2E1 / metabolism
  • Cytochrome P-450 CYP3A
  • Cytochrome P-450 Enzyme System / metabolism*
  • Cytochrome P450 Family 2
  • Dexamethasone / pharmacology
  • Diazinon / metabolism*
  • Diazinon / pharmacokinetics
  • Kinetics
  • Male
  • Membrane Proteins
  • Microsomes, Liver / drug effects
  • Microsomes, Liver / enzymology*
  • Organophosphorus Compounds / analysis
  • Phenobarbital / pharmacology
  • Rats
  • Rats, Sprague-Dawley
  • Steroid 16-alpha-Hydroxylase*
  • Steroid Hydroxylases / metabolism
  • Substrate Specificity


  • Membrane Proteins
  • Organophosphorus Compounds
  • Dexamethasone
  • Cytochrome P-450 Enzyme System
  • diazoxon
  • Steroid Hydroxylases
  • Cytochrome P-450 CYP2E1
  • Aryl Hydrocarbon Hydroxylases
  • CYP2C11 protein, rat
  • Cyp3a2 protein, rat
  • Cytochrome P-450 CYP1A1
  • Cytochrome P-450 CYP2B1
  • Cytochrome P-450 CYP3A
  • Cytochrome P450 Family 2
  • Steroid 16-alpha-Hydroxylase
  • steroid 16-beta-hydroxylase
  • steroid hormone 6-beta-hydroxylase
  • Phenobarbital
  • Diazinon