title: Estimating constants for metabolism of atrazine in freshly isolated rat hepatocytes by kinetic modeling.
abstract: This study estimated the kinetic constants for oxidative metabolism of atrazine (ATRA) and its chlorotriazine (Cl-TRI) metabolites, 2-chloro-4-ethylamino-6-amino-1,3,5-triazine (ETHYL), 2-chloro-4-amino-6-isopropylamino-1,3,5-triazine (ISO), and diaminochlorotriazine (DACT), using freshly isolated rat hepatocytes. Hepatocytes were incubated with 1.74, 44, 98, and 266 microM ATRA. Disappearance of ATRA and formation of the Cl-TRI metabolites were quantified over 90 min. At all incubation concentrations, ATRA was preferentially metabolized to ETHYL, producing ETHYL concentrations approximately 6 times higher than those of ISO. DACT concentrations peaked at 44 microM ATRA and decreased with increasing incubation concentrations, indicating non-linear metabolic behavior of ATRA with respect to DACT formation. A series of kinetic models were developed from these data to describe the dose and time-dependent oxidative metabolism of ATRA and the Cl-TRI metabolites. An integrated model for all the chloro-triazines included multi-substrate competitive inhibition of metabolism to describe the non-linear behavior of DACT production in relation to ATRA while simultaneously simulating the time-course behavior of the Cl-TRIs at all four ATRA concentrations. The maximal metabolic rate (V(max)) of ATRA metabolism and the Michaelis-Menten constant (K(M)) for the reaction were 1.6 microM/min and 30 microM, respectively. V(max) and K(M) values for ETHYL and ISO metabolism to DACT were also estimated using this modeling approach.
authors.affiliation: Department of Environmental and Radiological Health Sciences, Campus Delivery, 132 Physiology Building, 1680, Colorado State University, Fort Collins, CO 80523, United States.
authors.forename: Tami S
authors.initials: TS
authors.lastname: McMullin
authors.forename: Melvin E
authors.initials: ME
authors.lastname: Andersen
authors.forename: John D
authors.initials: JD
authors.lastname: Tessari
authors.forename: Brian
authors.initials: B
authors.lastname: Cranmer
authors.forename: William H
authors.initials: WH
authors.lastname: Hanneman
mesh_terms: D000818:Animals; D001280:Atrazine / Q000378:metabolism / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D005260:Female; D022781:Hepatocytes / Q000187:drug effects* / Q000378:metabolism; D006540:Herbicides / Q000378:metabolism / Q000494:pharmacology*; D066298:In Vitro Techniques; D007700:Kinetics; D050216:Metabolic Detoxication, Phase I / Q000502:physiology*; D008954:Models, Biological; D051381:Rats; D017207:Rats, Sprague-Dawley; D014227:Triazines / Q000378:metabolism / Q000494:pharmacology
chemical_list: D006540:Herbicides; D014227:Triazines; C060537:chlortriazine; D001280:Atrazine