You are here:
Esterase metabolism of cholinesterase inhibitors using rat liver in vitro
Citation:
MOSER, V. C. AND S. J. PADILLA. Esterase metabolism of cholinesterase inhibitors using rat liver in vitro. TOXICOLOGY AND APPLIED PHARMACOLOGY. Academic Press Incorporated, Orlando, FL, 28(1-3):52-62, (2011).
Impact/Purpose:
The neurotoxicity of orgnanophosphate (OP) and carbamate pesticides, nerve agents, and industrial chemicals is due to inhibition acetylcholinesterase (AChE), leading to overstimulation of the cholinergic nervous system. The relative potencies of the chemicals across and within species depend in part on chemical-specific metabolic and detoxification processes. A-esterases and carboxylesterases are two enzymatic detoxification pathways that have been widely studied. We used an in vitro system to measure esterase-dependent detoxification of 15 AChE inhibitors. The target enzyme AChE served as a bioassay of inhibitor concentration following incubation with detoxifying tissue. Concentration-inhibition curves were determined for the inhibitor in the presence of buffer (no liver), rat liver plus calcium (to stimulate A-esterases and thereby measure both A-esterase and carboxylesterase), and rat liver plus EGTA (to inhibit Ca++-dependent A-esterases, measuring carboxylesterase activity. This screening method could be useful for informing human variability factors for these inhibitors. The detoxification patterns produced by the more well-studied OPs correlated well with previous literature, providing confidence in the system. None of the other inhibitors were detoxified to as great an extent, and for over half, there was no apparent esterase metabolism at all. For most of these inhibitors, these are the first data available for informing kinetic parameters and esterase-mediated detoxification. While we hypothesized that in vitro detoxification would predict age-related differences in sensitivity, this was accurate for most, but not all, chemicals for which such data exist. Our findings indicate that detoxification patterns are chemical-specific, and that the influence of known susceptibility factors such as PONI polymorphisms should not be generalized across these inhibitors.
Description:
A variety of chemicals, such as organophosphate (OP) and carbamate pesticides, nerve agents, and industrial chemicals, inhibit acetylcholinesterase (AChE) leading to overstimulation of the cholinergic nervous system. The resultant neurotoxicity is similar across mammalian species; however, the relative potencies of the chemicals across and within species depend in part on chemical-specific metabolic and detoxification processes. A-esterases and carboxylesterases are two enzymatic detoxification pathways that have been widely studied. We used an in vitro system to measure esterase-dependent detoxification of 15 AChE inhibitors. The target enzyme AChE served as a bioassay of inhibitor concentration following incubation with detoxifying tissue. Concentration-inhibition curves were determined for the inhibitor in the presence of buffer (no liver), rat liver plus calcium (to stimulate A-esterases and thereby measure both Aesterase and carboxylesterase), and rat liver plus EGTA (to inhibit Ca++-dependent A-esterases, measuring carboxyl esterase activity). Point estimates (concentrations calculated to produce 20, 50, and 80% inhibition) were compared across conditions and served as a measure of esterasemediated detoxification. Results with well-known inhibitors (chlorpyrifos oxon, paraoxon, methyl paraoxon, malaoxon) were in agreement with the literature, serving to support the use of this assay. Only a few other inhibitors showed slight or a trend towards detoxification via carboxylesterases or A-esterases (mevinphos, aldicarb, oxamyl). There was no apparent A-esterase- or carboxylesterase-mediated detoxification of the remaining inhibitors (carbofuran, chlorfenvinphos, dicrotophos, fenamiphos, methamidophos, methomyl, monocrotophos, phosphamidon), suggesting that the influence of esterases on these chemicals is minimal. Thus, generalizations regarding these metabolic pathways, and their influence on human variability, may not be appropriate. As with other aspects of AChE inhibitors, their metabolic patterns appear to be chemical-specific.
