Different forms of insensitive acetylcholinesterase in insecticide-resistant house flies (Musca domestica)

A new form of acetylcholinesterase insensitive to organophosphorus inhibitors was identified in house flies collected from animal farms in the U.K. Its degree of insensitivity to a number of inhibitors ranged from 4-fold (omethoate) to >40-fold (tetrachlorvinphos and dichlorvos) compared with our standard susceptible enzyme, and its spectrum of response to inhibitors differed widely from that of a previously studied insensitive form. In addition, the form of acetylcholinesterase reported here is unusual in having a 3.4-fold greater affinity (lower K_m) for acetylthiocholine than the normal enzyme. The possible toxicological significance of this is discussed.