Acetylcholinesterase in insecticide resistant Culex tritaeniorhynchus: Characteristics accompanying insensitivity to inhibitors

The acetylcholinesterase (AChE) of the Toyama strain (Toyama) of Culex tritaeniorhynchus showed extremely low sensitivity to various kinds of organophosphorus insecticides (OPs) and carbamates. Several characteristics of the AChE of Toyama have been tested and compared with those of the OP susceptible strain (Taiwan). The AChE of Toyama is different from that of Taiwan in many respects. It showed no clear optimum pH and substrate concentration (AChE of Taiwan had both the optimum pH and substrate concentration). Affinity for ACh was high (K-m value for ACh is 0.65 times the value of Taiwan), but affinity for the substrate declined significantly for acetylthiocholine and propionylthiocholine. I-50 values for OPs were extremely high (66.1-7,079 times the value for Taiwan), and those for carbamates were also higher than those for Taiwan (20.4-112 times). These results suggest that the structure of the AChE of Toyama has changed to an ACh specific form which has a remarkably lower affinity to the OP compounds.