Pseudo-type acetylcholinesterase from insecticide-resistant Culex tritaeniorhynchus.

Acetylcholinesterases (AChEs) were extracted with Triton X-100 from the head and thorax of organophosphorus insecticide-resistant (R) and susceptible (S) strains of Culex tritaeniorhynchus, and purified by high performance liquid chromatography with a DEAE-cellulose column. Both AChE (R) and (S) hydrolyzed acetylcholine with similar specific activities of 5.34 and 6.84 μmol/min/mg protein, respectively. Hydrolysis of benzoyl-choline by both enzymes was one-seventh less than that of acetylcholine. When acetylthio-choline was used as substrate, the specific activity of AChE (R) (3.37 μmol/min/mg protein) was lower than that of AChE (S) (11.6 μmol/min/mg protein). This tendency in acetylthiocholine was observed in other thiocholinesters such as propionyl-and butyrylth-iocholine. The inhibitory power of neostigmine to AChE (R) was 1/4000 lower than that to AChE (S), and the inhibitory powers of fenitrooxon and malaoxon to AChE (R) were also 1/1000 and 1/80 lower than those to AChE (S). Moreover, the inhibitory power of BW 284 to AChE (R) was also lower than that to AChE (S). However, BPA was more inhibitory to AChE (R) than (S). These results suggest that AChE (R) is modified at the binding site of known inhibitors of true AChE and becomes a pseudo-type of AChE when mosquitoes acquire resistance to organophosphorus insecticides.