Question

Comparisons of kinetic properties of acetylcholinesterase purified from azinphosmethyl-susceptible and resistant strains of colorado potato beetle

Answer 1

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The affinity (Km) and hydrolyzing efficiency (Vmax) of acetylcholinesterase (AChE, EC 1.1.1.7) purified from a near-isogenic azinphosmethyl-resistant (AZ-R) strain of Colorado potato beetle (CPB) to selected substrates, including acethylthiocholine, acetyl-(β-methyl) thiocholine, and propionythiocholine, were lower than those of AChE purified from a susceptible (SS) strain. AChE from the SS strain was significantly inhibited by higher concentrations of acethylthiocholine and acetyl-(β-methyl) thiocholine, whereas AChE from the AZ-R strain was activated by higher concentrations of all four substrates examined. AChE from the AZ-R strain was 16- and 1.4-fold less sensitive to inhibition by azinphosmethyl-oxon and dichlorvos, respectively, but was 3.1- and 4.6-fold more sensitive to inhibition by paraoxon and diisopropyl fluorophosphate, respectively, than that from the SS strain. Kinetic studies with the selected AChE inhibitors, including eserine, tetramethylammonium, tetrapropylammonium, and propidium, indicated that there was no significant difference in inhibition by eserine, tetrapropylammonium, and propidium between the two strains. However, AChE from the AZ-R was 1.8-fold less sensitive to inhibition by tetramethylammonium than AChE from the SS strain. Propidium produced a linear mixed-type inhibition, whereas the other three inhibitors produced a competitive-type inhibition in both strains. AChE from the AZ-R strain was also 1.3-fold less sensitive to inhibition by α-chaconine, but was 1.7- and 1.8-fold more sensitive to α-solanine and tomatine, respectively, than AChE from the: SS strain. These findings support our original contention that this aspect of overall azinphosmethyl resistance in CPB was due to a modified form of AChE that functions as a site insensitivity resistance mechanism. Modifications of AChE obtained from the AZ-R strain may occur both in the catalytic center and at the peripheral anionic site or in a single region where it modifies the binding of these ligands to both sites..

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