title: In vitro rat hepatic and intestinal metabolism of the organophosphate pesticides chlorpyrifos and diazinon.
abstract: Chlorpyrifos (CPF) and diazinon (DZN) are thionophosphorus organophosphate (OP) insecticides; their toxicity is mediated through CYP metabolism to CPF-oxon and DZN-oxon, respectively. Conversely, CYPs also detoxify these OPs to trichloropyridinol (TCP) and 2-isopropyl-4-methyl-6-hydroxypyrimidine (IMHP), respectively. In addition, A-esterase (PON1) metabolism of CPF- and DZN-oxon also forms TCP and IMHP. This study evaluated the role intestinal and hepatic metabolism may play in both the activation and detoxification of CPF and DZN in Sprague-Dawley rats. Similar CYP- and PON1-mediated metabolic profiles were demonstrated in microsomes from liver or isolated intestinal enterocytes. The metabolic efficiency was estimated by calculating the psuedo-1st order rate constant from the metabolic constants by dividing Vmax/Km. In enterocyte microsomes, the CYP metabolic efficiency for metabolism to the oxon metabolites was approximately 28-fold greater for CPF than DZN. Compared on a per nmol P450 basis, the Vmax for CPF in enterocytes was approximately 2-3 times higher than in liver microsomes for the production of CPF-oxon and TCP. The Michaelis-Menten rate constant (Km) for the metabolism of CPF to CPF-oxon was comparable in liver and enterocyte microsomes; however, the enterocyte Km for TCP production was higher (indicating a lower affinity). The smaller volume of intestine, lower amount of CYP, and higher Km for TCP in the enterocyte microsomes, resulted in a lower catalytic efficiency (2 and 62 times) than in liver for oxon and TCP. PON1-mediated metabolism of CPF- and DZN-oxon was also demonstrated in liver and enterocyte microsomes. Although PON1 affinity for the substrates was comparable in hepatic and enterocytic microsomes, the Vmax were 48- to 275-fold higher, in the liver. These results suggest that intestinal metabolism may impact the metabolism of CPF and DZN, especially following low-dose oral exposures.
authors.affiliation: Battelle, Pacific Northwest Division, Chemical Dosimetry, P.O. Box 999, Richland, Washington 99352, USA. torka.poet@pnl.gov
authors.forename: T S
authors.initials: TS
authors.lastname: Poet
authors.forename: H
authors.initials: H
authors.lastname: Wu
authors.forename: A A
authors.initials: AA
authors.lastname: Kousba
authors.forename: C
authors.initials: C
authors.lastname: Timchalk
mesh_terms: D000818:Animals; D002478:Cells, Cultured; D004390:Chlorpyrifos / Q000378:metabolism*; D003976:Diazinon / Q000378:metabolism*; D020895:Enterocytes / Q000378:metabolism*; D022781:Hepatocytes / Q000378:metabolism*; D007306:Insecticides / Q000378:metabolism*; D007421:Intestine, Small / Q000166:cytology / Q000378:metabolism; D008297:Male; D008862:Microsomes, Liver / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley
chemical_list: D007306:Insecticides; D004390:Chlorpyrifos; D003976:Diazinon