STUDIES ON THE KINETICS OF ACETYLCHOLINESTERASE IN THE RESISTANT AND SUSCEPTIBLE STRAINS OF HOUSEFLY (MUSCA DOMESTICA)

Acetylcholinesterase (AChE) in the susceptible (S) and the resistant (R) strains of housefly (Musca domestica) was investigated using kinetic analysis. The V max values of AChE for hydrolyzing acetylthiocholine (ATCh) and butyrylthiocholine (BTCh) were 4578.50 and 1716.08nmol/min/mg·protein in the R strain, and were 1884.75 and 864.72 nmol/min/mg.protein in the S strain, respectively. The V max ratios of R to S enzyme were 2.43 for ATCh and 1.98 for BTCh. The K m values of AChE for ATCh and BTCh were 0.069 and 0.034 mmol/L in the S strain, and 0.156, 0.059 mmol/L in the R strain, respectively. The K m ratios of R to S enzyme were 2.26 for ATCh and 1.74 for BTCh. The k i ratios of S to R enzyme for three insecticides propoxur, methomyl and paraoxon were 46.04, 4.17 and 2.86, respectively. In addition, k cat and k cat / K m for measuring turnover and catalytic efficiency of AChE were determined using eserine as titrant. The k cat values of AChE from the R strain for both ATCh and BTCh were higher than those values from the S strain. But the values of k cat / K m were in contrary to the k cat values with R enzyme compared to S enzyme. The AChE catalytic properties and sensitivity to the inhibition by three insecticides in the R and S strains of housefly were discussed based on contribution of V max , K m, k i, k cat and k cat / K m. All these data implied that AChE from the R strain might be qualitatively altered. We also observed an intriguing phenomenon that inhibitors could enhance the activity of AChE from the resistant strain. This “flight reaction” of the powerful enzyme might be correlated with the developing resistance of housefly to organophosphate or carbamate insecticides.