title: Characterization of the in vitro kinetic interaction of chlorpyrifos-oxon with rat salivary cholinesterase: a potential biomonitoring matrix.
abstract: The primary mechanism of action for organophosphorus (OP) insecticides such as chlorpyrifos (CPF) involves the inhibition of acetylcholinesterase (AChE) by their active oxon metabolites resulting in a wide range of neurotoxic effects. These oxons also inhibit other cholinesterases (ChE) such as butyrylcholinesterase (BuChE), which represents a detoxification mechanism and a potential biomarker for OP insecticide exposure/response. Salivary biomonitoring has recently been explored as a practical method for examination of chemical exposure, however, there are few studies exploring the use of saliva for OP insecticides. To evaluate the use of salivary ChE as a biological monitor for OP insecticide exposure, a modified Ellman assay in conjunction with a pharmacodynamic model was used to characterize salivary ChE in adult male Sprague-Dawley rats. Comparison of rat saliva, brain, and plasma ChE activity in the presence of selective inhibitors of AChE and BuChE (BW284C51 and iso-OMPA, respectively) with different ChE substrates indicated that rat salivary ChE activity is primarily associated with BuChE (>95%). Further characterization of rat salivary BuChE kinetics yielded an average total BuChE active site concentration of 1.20+/-0.13 fmol ml(-1) saliva, an average reactivation rate constant (Kr) of 0.070+/-0.008 h(-1), and an inhibitory rate constant (Ki) of approximately 9 nM(-1) h(-1). The pharmacodynamic model successfully described the in vitro BuChE activity profile as well as the kinetic parameters. These results support the potential utility of saliva as a biomonitoring matrix for evaluating occupational and environmental exposure to CPF and other OP insecticides.
authors.affiliation: Battelle, Pacific Northwest Division, 902 Battelle Blvd., PO Box 999, Richland, WA 99352, USA.
authors.forename: A A
authors.initials: AA
authors.lastname: Kousba
authors.forename: T S
authors.initials: TS
authors.lastname: Poet
authors.forename: Charles
authors.initials: C
authors.lastname: Timchalk
mesh_terms: D000110:Acetylcholinesterase / Q000378:metabolism*; D000818:Animals; D002093:Benzenaminium, 4,4'-(3-oxo-1,5-pentanediyl)bis(N,N-dimethyl-N-2-propenyl-), Dibromide / Q000494:pharmacology; D001921:Brain / Q000201:enzymology; D002091:Butyrylcholinesterase / Q000378:metabolism*; D004390:Chlorpyrifos / Q000031:analogs & derivatives* / Q000493:pharmacokinetics*; D002800:Cholinesterase Inhibitors / Q000493:pharmacokinetics*; D003198:Computer Simulation; D004784:Environmental Monitoring / Q000379:methods*; D007306:Insecticides / Q000493:pharmacokinetics*; D008297:Male; D008954:Models, Biological; D051381:Rats; D017207:Rats, Sprague-Dawley; D012463:Saliva / Q000201:enzymology*; D013770:Tetraisopropylpyrophosphamide / Q000494:pharmacology
chemical_list: D002800:Cholinesterase Inhibitors; D007306:Insecticides; D002093:Benzenaminium, 4,4'-(3-oxo-1,5-pentanediyl)bis(N,N-dimethyl-N-2-propenyl-), Dibromide; D013770:Tetraisopropylpyrophosphamide; C009618:O,O-diethyl O-3,5,6-trichloro-2-pyridyl phosphate; D000110:Acetylcholinesterase; D002091:Butyrylcholinesterase; D004390:Chlorpyrifos