Solvent-, enzyme-, and structural-dependence of phenyl-substituted methyl carbamate inhibition of acetylcholinesterase

Abstract

This study explored how dilution protocols, enzymes, and solvents affected the performance of 2- and 3-phenyl substituted methylcarbamates with varying selectivity for Anopheles gambiae acetylcholinesterase (AgAChE). Protocol A was 100-fold suspension in buffer of a 0.1 M DMSO stock solution, followed by serial buffer dilutions to give declining DMSO in parallel with the inhibitor. Protocol B was identical to A except that the initial stock concentration was 0.01 M. Protocol C entailed DMSO serial dilutions of a 0.1 M DMSO stock solution, followed by suspension of each into buffer giving 0.1 % (v/v) DMSO in all incubates. An. gambiae enzymes from insect homogenates or a recombinant clone generally showed a progressive increase in Hill slope from 0.5 to 1 via protocols A-C, along with increased IC₅₀ values, with the 3-tert-butyl analog (1) epitomizing these effects. In contrast, propoxur displayed no consistent change in inhibition potency of any AChE, regardless of DMSO dilution procedure. DMSO at constant 35 μM or 0.000273 % had a midpoint effect on compound 1 inhibition and displayed competitive inhibition. Time course incubations (10–60 min) over a broad concentration range (10⁻¹² - 10⁻⁵ M) of 1 revealed saturable inhibition of high and low potency, with the high potency effect more sensitive to incubation time. In contrast, there was little change of inhibition potency or Hill slope for compound 1 with human AChE, or the AChEs of Drosophila melanogaster and Musca domestica under any DMSO dilution protocol. Moreover, when the dilution protocols were repeated using ethanol as a solvent, little change of inhibition potency or Hill slope was observed with any compound, enzyme, or dilution scheme. These results contradicted the expectation that higher solvent concentration would yield better solubility and more rapid and potent effects of these lipophilic insecticides. Molecular modeling suggests DMSO may be competing with carbamate binding to AgAChE or by stabilizing an allosteric subpocket within AgAChE.