Selective inhibitors of methyl parathion-resistant acetylcholinesterase from Heliothis virescens

Acetylcholinesterase activity from the tobacco budworm, Heliothis virescens, was 22-fold less sensitive to inhibition by methyl paraoxon in larvae or adults of a methyl parathion-resistant Woodrow strain when compared to a susceptible, Florence strain. Resistant acetylcholinesterase was also less sensitive to the closely related structural analogues fenitrooxon and ethyl paraoxon, and to the N-methyl carbamates eserine, propoxur, and methomyl. Methyl paraoxon-resistant acetylcholinesterase was selectively inhibited by the nonanalogous organophosphate inhibitors monocrotophos and dicrotophos. This enzyme was also sensitive to 4-nitrophenyl di-2-thienylphosphinate, 4-nitrophenyl diphenylphosphinate, and N-n-propyl 1-naphthylcarbamate. Individual adult tobacco budworms were diagnosed as SS, RS, or RR genotypes by clusters formed in scatterplots contrasting pairs of inhibitors. Methyl paraoxon-resistant preparations were more active toward both acetylthiocholine and butyrylthiocholine substrates and were not inhibited by high substate concentrations. Inhibition results suggest that there has been a qualitative change in the resistant acetylcholinesterase producing greater sensitivity to certain inhibitors. The resistant strain may also possess a greater quantity of enzyme; however, this is not clear due to substrate inhibition in the susceptible strain and apparent substrate activation in the resistant strain.