Differences in esterases from insect species : Toxicity of organo‐phosphorus compounds and in vitro anti‐esterase activity

Abstract

AbstractTwo types of esterase, the one capable of hydrolysing phenyl acetate and not acetylcholine, the other able to hydrolyse acetylcholine, have been examined in adults of four species of insect—Tenebrio molitor L., Tribolium castaneum Hbst., Dysdercus fasciatus Sign. and Blatella germanica L.The in vitro inhibition of both types of enzyme from each species has been examined with five organo‐phosphorus compounds : tetraethyl pyrophosphate, paraoxon, parathion, OO‐diethyl S‐p‐nitrophenyl phosphorothiolate and OS‐diethyl O‐p‐nitrophenyl phosphorothiolate. Both types of esterase were inhibited by high dilutions of inhibitors. Susceptibility of both types of enzyme appeared to vary from species to species and may provide a basis for specific differences in resistance to insecticides.There is no consistent correlation between the toxicity of the compounds examined and the in vitro inhibition of either kind of esterase. Parathion did not inhibit any of the esterases. The other four substances were, in general, less potent inhibitors of the esterases that hydrolysed acetylcholine than those that did not. Therefore both types of esterase are worthy of further investigation since there are insufficient data to decide which type of esterase is involved in the insecticidal action of organo‐phosphorus compounds.