title: Effect of CYP2B6*6 and CYP2C19*2 genotype on chlorpyrifos metabolism.
abstract: Chlorpyrifos (CPF) is a widely used organophosphorus (OP) pesticide. CPF is bioactivated by cytochrome P450s (CYPs) to the potent cholinesterase inhibitor chlorpyrifos oxon (CPF-O) or detoxified to 3,5,6-trichloro-2-pyridinol (TCPy). Human CYP2B6 has the highest reported Vmax)/Km (intrinsic clearance--CL(int)) for bioactivation while CYP2C19 has the highest reported CL(int) for detoxification of CPF. In this study, 22 human liver microsomes (HLMs) genotyped for common variants of these enzymes (CYP2B6*6 and CYP2C19*2) were incubated with 10 μM and 0.5 μM CPF and assayed for metabolite production. While no differences in metabolite production were observed in homozygous CYP2C19*2 HLMs, homozygous CYP2B6*6 specimens produced significantly less CPF-O than wild-type specimens at 10 μM (mean 144 and 446 pmol/min/mg, respectively). This correlated with reduced expression of CYP2B6 protein (mean 4.86 and 30.1 pmol/mg, for CYP2B6*6 and *1, respectively). Additionally, CYP2B6*1 and CYP2B6*6 were over-expressed in mammalian COS-1 cells to assess for the first time the impact of the CYP2B6*6 variant on the kinetic parameters of CPF bioactivation. The Vmax for CYP2B6*6 (1.05×10⁵ pmol/min/nmol CYP2B6) was significantly higher than that of CYP2B6*1 (4.13×10⁴ pmol/min/nmol CYP2B6) but the K(m) values did not differ (1.97 μM for CYP2B6*6 and 1.84 μM for CYP2B6*1) resulting in CL(int) rates of 53.5 and 22.5 nL/min/nmol CYP2B6 for *6 and *1, respectively. These data suggest that CYP2B6*6 has increased specific activity but reduced capacity to bioactivate CPF in HLMs compared to wild-type due to reduced hepatic protein expression, indicating that individuals with this genotype may be less susceptible to CPF toxicity.
authors.affiliation: Department of Pharmacology and Toxicology, State University of New York at Buffalo, Buffalo, NY 14214, USA. Electronic address: alcrane@buffalo.edu.
authors.forename: Alice L
authors.initials: AL
authors.lastname: Crane
authors.affiliation: Dr. Margarete Fischer-Bosch Institute of Clinical Pharmacology, Auerbachstr. 112, 70376 Stuttgart, Germany. Electronic address: kathrin.klein@ikp-stuttgart.de.
authors.forename: Kathrin
authors.initials: K
authors.lastname: Klein
authors.affiliation: Dr. Margarete Fischer-Bosch Institute of Clinical Pharmacology, Auerbachstr. 112, 70376 Stuttgart, Germany; Department of Clinical Pharmacology, University of Tuebingen, Tuebingen, Germany. Electronic address: uli.zanger@ikp-stuttgart.de.
authors.forename: Ulrich M
authors.initials: UM
authors.lastname: Zanger
authors.affiliation: Department of Pharmacology and Toxicology, State University of New York at Buffalo, Buffalo, NY 14214, USA. Electronic address: jolson@buffalo.edu.
authors.forename: James R
authors.initials: JR
authors.lastname: Olson
mesh_terms: D019943:Amino Acid Substitution; D000818:Animals; D001189:Aryl Hydrocarbon Hydroxylases / Q000235:genetics / Q000378:metabolism*; D001711:Biotransformation; D016642:Bupropion / Q000378:metabolism; D019556:COS Cells; D002522:Chlorocebus aethiops; D004390:Chlorpyrifos / Q000378:metabolism*; D002800:Cholinesterase Inhibitors / Q000378:metabolism*; D065702:Cytochrome P-450 CYP2B6; D065731:Cytochrome P-450 CYP2C19; D005260:Female; D006720:Homozygote; D006801:Humans; D006900:Hydroxylation; D007306:Insecticides / Q000378:metabolism*; D007700:Kinetics; D008297:Male; D008862:Microsomes, Liver / Q000201:enzymology* / Q000378:metabolism; D010089:Oxidoreductases, N-Demethylating / Q000235:genetics / Q000378:metabolism*; D020641:Polymorphism, Single Nucleotide; D011994:Recombinant Proteins / Q000378:metabolism
chemical_list: D002800:Cholinesterase Inhibitors; D007306:Insecticides; D011994:Recombinant Proteins; D016642:Bupropion; 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; D004390:Chlorpyrifos