Abstract
2-, 3- and 4-Chloro-4′-phenylchalcones were weakly inhibitory against glutathione S-transferases (GST) from resistant (R) and susceptible (S) housefly abdomens. However, each of glutathione (GSH) conjugates of the above chalcones inhibited strongly both GSTs in an incompetitive fashion with respect to CDNB or DCNB. The inhibitory activity was in the order of 2-, 4- and 3-Chloro-compounds which was similar to the case of mouse liver GST. These facts indicated that the rate of GSH conjugation was slow in vitro insect system. On the other hand, the above chalcones increased the insecticidal activity of diazinon to R-fly in the order of 2-, 4- and 3-chloro-compounds. Diazinon is known to be more detoxified in R-fly than in S-fly by GST. Therefore, it is conceivable that such synergism is due to the conversion of the chalcones to the GSH conjugates by GST and the inhibition of GST by the conjugates, which decrease the detoxication of diazinon. There were differences between rat and housefly GSTs and between R- and S-fly GSTs. The affinity of CDNB was higher to R-fly GST than to S-fly GST, but both were far low as compared with the affinity to rat GST. On the other hand, the affinity of DCNB to GSTs was generally lower than that of CDNB and there was not so much difference between housefly and mouse or rat GSTs, although R-fly GST gave the lowest affinity. Comparison of subunits on sodium dodecylsulfate polyacrylamide gel electrophoresis suggested that fly GST was qualitatively different from rat GST. There was a minor difference between GSTs of R- and S-flies.
