Increased activity associated with reduced sensitivity of acetylcholinesterase in organophosphate-resistant greenbug, Schizaphis graminum (Homoptera : Aphididae)

The activity and sensitivity of acetylcholinesterase (AChE, EC 1.1.1.7) fro m an organophosphate-susceptible (OSS) and three resistant (OR-0, OR-1 and OR-2) strains of the greenbug (Schizaphis graminum) were biochemically comp ared. All three resistant strains displayed higher frequencies of individua ls with respect to both increased activity of AChE toward the model substra te acetylthiocholine (ATC) and reduced sensitivity of AChE to inhibition by paraoxon as compared with the OSS strain. Kinetic study indicated that ACh E from the OR-0, OR-1 and OR-2 strains had 3.3-, 2.7- and 2.3-fold, respect ively, lower affinity, but 1.5-, 2.2- and 2.0-fold, respectively, higher ca talytic activity toward ATC than AChE from the OSS strain. Significantly in creased activity of AChE in the resistant strains was also confirmed by non -denaturing polyacrylamide gel electrophoresis, and appeared to be associat ed with the increase of general esterase activity. Inhibition kinetics reve aled that AChE from the OR-0, OR-1 and OR-2 strains was 2.1-, 2.2- and 2.7- fold less sensitive to inhibition by paraoxon than that from the OSS strain . The study suggested that both qualitative and quantitative modifications of AChE had evolved in the resistant strains and were likely to significant ly enhance the overall resistance level in greenbugs. (C) 1999 Society of C hemical Industry.