Characterization of diuron N-demethylation by mammalian hepatic microsomes and cDNA-expressed human cytochrome P450 enzymes.
Diuron, a widely used herbicide and antifouling biocide, has been shown to persist in the environment and contaminate drinking water. It has been characterized as a "known/likely" human carcinogen. Whereas its environmental transformation and toxicity have been extensively examined, its metabolic characteristics in mammalian livers have not been reported. This study was designed to investigate diuron biotransformation and disposition because metabolic routes, metabolizing enzymes, interactions, interspecies differences, and interindividual variability are important for risk assessment purposes. The only metabolic pathway detected by liquid chromatography/mass spectometry in human liver homogenates and seven types of mammalian liver microsomes including human was demethylation at the terminal nitrogen atom. No other phase I or phase II metabolites were observed. The rank order of N-demethyldiuron formation in liver microsomes based on intrinsic clearance (V(max)/K(m)) was dog > monkey > rabbit > mouse > human > minipig > rat. All tested recombinant human cytochrome P450s (P450s) catalyzed diuron N-demethylation and the highest activities were possessed by CYP1A1, CYP1A2, CYP2C19, and CYP2D6. Relative contributions of human CYP1A2, CYP2C19, and CYP3A4 to hepatic diuron N-demethylation, based on average abundances of P450 enzymes in human liver microsomes, were approximately 60, 14, and 13%, respectively. Diuron inhibited relatively potently only CYP1A1/2 (IC(50) 4 microM). With human-derived and quantitative chemical-specific data, the uncertainty factors for animal to human differences and for human variability in toxicokinetics were within the range of the toxicokinetics default uncertainty/safety factors for chemical risk assessment.
Abass
Khaled
K
Department of Pharmacology and Toxicology, P.O. Box 5000, FIN-90014 University of Oulu, Oulu, Finland. khaled.megahed@oulu.fi
Reponen
Petri
P
Turpeinen
Miia
M
Jalonen
Jorma
J
Pelkonen
Olavi
O
D000818:Animals; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000096:biosynthesis* / Q000235:genetics; D018076:DNA, Complementary / Q000096:biosynthesis*; D003640:Dealkylation; D004237:Diuron / Q000378:metabolism*; D004285:Dogs; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006801:Humans; D066298:In Vitro Techniques; D007700:Kinetics; D008252:Macaca fascicularis; D013058:Mass Spectrometry; D051379:Mice; D008811:Mice, Inbred DBA; D008862:Microsomes, Liver / Q000378:metabolism*; D011817:Rabbits; D051381:Rats; D017207:Rats, Sprague-Dawley; D013552:Swine; D013556:Swine, Miniature
D065607:Cytochrome P-450 Enzyme Inhibitors; D018076:DNA, Complementary; D004791:Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System; D004237:Diuron