Estimating constants for metabolism of atrazine in freshly isolated rat hepatocytes by kinetic modeling

• Published in: Toxicology in vitro Vol. 21; no. 3; pp. 492 - 501
• Main Authors: McMullin, Tami S., Andersen, Melvin E., Tessari, John D., Cranmer, Brian, Hanneman, William H.
• Format: Journal Article
• Language: English
• Published: England Elsevier Ltd 01.04.2007
• Subjects: Animals; Atrazine; Atrazine - metabolism; Atrazine - pharmacology; Chlorotriazines; Competitive inhibition; Dose-Response Relationship, Drug; Female; Hepatocytes - drug effects; Hepatocytes - metabolism; Herbicides - metabolism; Herbicides - pharmacology; In Vitro Techniques; Kinetic modeling; Kinetics; Metabolic Detoxication, Phase I - physiology; Models, Biological; Oxidative metabolism; Rats; Rats, Sprague-Dawley; Triazines - metabolism; Triazines - pharmacology; ISO; Oxidative metabolism; PBPK; Competitive inhibition; DACT; Kinetic modeling; V max; K M; Cl-TRI; Chlorotriazines; ATRA; GSH; ETHYL; Atrazine
• ISSN: 0887-2333; 1879-3177
• DOI: 10.1016/j.tiv.2006.10.010

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 μM 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 μM 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 μM/min and 30 μM, respectively. V max and K M values for ETHYL and ISO metabolism to DACT were also estimated using this modeling approach.