Shi_2002_Pestic.Biochem.Physiol_72_72

A propoxur-selected resistant (R) strain of housefly (Musca domestica), from Shanghai, China, exhibited >185-, 47.2-, 41.3-, and 50.5-fold greater resistance to propoxur, methomyl, paraoxon, and dichlorvos, respectively, than a susceptible (S) strain. The turnover number (kcat) and catalytic efficiency (kcat/Kmm) of acetylcholinesterase (AChE), determined by 7-[(diethoxyphosphoryl)-oxy]-1-methylquinolinium iodide (DEPQ) as a titrant and acetylthiocholine as a substrate, were 180,805 min-1 and 846.9 x 10 6 M-1 min-1 for the R strain and 119,875 min-1 and 2378.5 x 10 6 M-1 min-1 for the S strain, respectively. The greater maximal velocity of AChE from the R strain appeared to be due to increased turnover number and higher concentration of the AChE active site. AChE from the R strain was 3.4-, 21.0-, and 21.4-fold less sensitive to three carbamates, eserine, methomyl, and propoxur, and 2.6-, 33.1-, and 19.3-fold less sensitive to three organophosphates, DEPQ, dichlorvos, and paraoxon, respectively, based on the bimolecular rate constant (ki). The differences in ki were attributed mainly to the decreased affinity of these compounds for AChE from the R strain as determined by the affinity constant (Ka). All these data suggested that decreased sensitivity of AChE was associated with decreased catalytic efficiency of the enzyme in the R strain.