Anber_1989_Pestic.Biochem.Physiol_33_283

In an Italian parathion-resistant strain of Amblyseius potentillae, two genes for resistance have been found. A first one had been isolated in strain A which was shown to have a highly paraoxon-insensitive acetylcholinesterase. The second one has now been isolated in another substrain, B, in which an enzyme hydrolysing several organophosphates was found. This enzyme has a reduced activity of hydrolysis of alpha- and beta-NA of about 50% that in the susceptible S strain. Hydrolysis of paraoxon is 266 nmol/g-1/hr-1 (Vmax) in strain B, whereas it is not measurable in strains S and A. The Km of paraoxon hydrolysis is 5.7 x 10-7 M. Both Vmax are higher than for paraoxon hydrolysis in resistant house flies and aphids (Myzus persicae). Two methyl OP compounds are hydrolyzed slower than paraoxon. DEF appeared hardly to inhibit the enzyme and does not act as a synergist. Reactivation after paraoxon inhibition was measured as an increase of alpha-NA hydrolysis, the rate of dephosphorylation (k3) being 0.137 hr-1. Electrophoresis shows one strong band and three faint ones in the same pattern in strain S and B with alpha-NA, but the bands in B are fainter. The strong band in B is much less sensitive to paraoxon inhibition than that in strain S. This is the fourth case in which there is evidence for a mutant esterase in resistant arthropods, with increased attack on OP insecticides and a reduced activity to other esters, as opposed to cases with increased activity to both OPs and other esters due to gene amplification. The relation between the increased hydrolysis of OPs and the degree of resistance is discussed.