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ToxSci Advance Access originally published online on January 10, 2006
Toxicological Sciences 2006 90(2):460-469; doi:10.1093/toxsci/kfj094
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© The Author 2006. Published by Oxford University Press on behalf of the Society of Toxicology. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Concentration-Dependent Kinetics of Acetylcholinesterase Inhibition by the Organophosphate Paraoxon

Clint A. Rosenfeld* and Lester G. Sultatos{dagger},1

* Drug Metabolism and Pharmacokinetics, Schering-Plough Research Institute, Lafayette, New Jersey 07843; and {dagger} Department of Pharmacology and Physiology, University of Medicine and Dentistry of New Jersey, New Jersey Medical School, Newark, New Jersey 07103

Received November 7, 2005; accepted January 3, 2006

For decades the interaction of the anticholinesterase organophosphorus compounds with acetylcholinesterase has been characterized as a straightforward phosphylation of the active site serine (Ser-203) which can be described kinetically by the inhibitory rate constant ki. However, more recently certain kinetic complexities in the inhibition of acetylcholinesterase by organophosphates such as paraoxon (O,O-diethyl O-(p-nitrophenyl) phosphate) and chlorpyrifos oxon (O,O-diethyl O-(3,5,6-trichloro-2-pyridyl) phosphate) have raised questions regarding the adequacy of the kinetic scheme on which ki is based. The present article documents conditions in which the inhibitory capacity of paraoxon towards human recombinant acetylcholinesterase appears to change as a function of oxon concentration (as evidenced by a changing ki), with the inhibitory capacity of individual oxon molecules increasing at lower oxon concentrations. Optimization of a computer model based on an Ordered Uni Bi kinetic mechanism for phosphylation of acetylcholinesterse determined k1 to be 0.5 nM–1h–1, and k–1 to be 169.5 h–1. These values were used in a comparison of the Ordered Uni Bi model versus a ki model in order to assess the capacity of ki to describe accurately the inhibition of acetylcholinesterase by paraoxon. Interestingly, the ki model was accurate only at equilibrium (or near equilibrium), and when the inhibitor concentration was well below its Kd (pseudo first order conditions). Comparisons of the Ordered Uni Bi and ki models demonstrate the changing ki as a function of inhibitor concentrations is not an artifact resulting from inappropriate inhibitor concentrations.

Key Words: acetylcholinesterase; organophosphate; paraoxon; kinetics.


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