Molecular Pharmacology, Vol 7, 611-619, Copyright © 1971 by the American Society for Pharmacology and Experimental Therapeutics

Insensitivity of Acetylcholinesterases to Organophosphorus Compounds as Related to Size of Esteratic Site

¹ Israel Institute for Biological Research, Ness-Ziona, Israel

The activity patterns of sensitive and organophosphorus-insensitive acetylcholinesterases of mites were tested with a homologous series of three acylthiocholines. The activities of organophosphorus-sensitive enzymes from two mite strains were higher with propionylthiocholine than with acetylthiocholine, and decreased sharply with butyrylthiocholine. On the other hand, the organophosphorus-insensitive acetylcholinesterases from three resistant mite strains displayed decreasing activity in the following order: acetylthiocholine > propionylthiocholine > butyrylthiocholine. An essentially similar difference between sensitive and insensitive acetylcholinesterases was also observed when 1/K_m or maximal velocities were determined for the three acylthiocholine substrates. Consequently, with propionylthiocholine as substrate, the activities, 1/K_m values, and maximal velocities of organophosphorus insensitive enzymes were much lower than those of organophosphorus-sensitive acetylcholinesterases.

In view of these results and of the considerably lower extent of inhibition of mite acetylcholinesterase by the O,O-diethyl analogue of malaoxon, as compared to that by malaoxon (S-(1,2-dicarbethoxymethyl O,O-dimethyl)phosphorothioate) itself, it is suggested that the esteratic site of the organophosphorus-sensitive mite enzyme is wide enough to accommodate the O,O-dimethylphosphoryl residue of malaoxon but not an O,O-diethylphosphoryl residue. On the other hand, the esteratic site of organophosphorus-insensitive acetylcholinesterase is not wide enough to accommodate even an O,O-dimethylphosphoryl residue.

The pattern of activities of organophosphorus-sensitive acetylcholinesterases from aphids, roaches, and Mediterranean fruit flies, and of organophosphorus-insensitive acetylcholinesterase from toad brains, was also tested with the three acylthiocholines. With the sensitive enzymes only the pattern of 1/K_m values was similar to that of the sensitive enzyme of mites for the three acylthiocholines tested.

Note:
ACKNOWLEDGMENTS The assistance of Mr. I. Rogosick in rearing the mites and aphids and the gifts of the D- and L-stereoisomers of the O,O-diethyl analogue of malaoxon by Dr. W. C. Dauterman, and of purified malaoxon by American Cyanamid Company, are gratefully acknowledged. Figure 1 was photographed by Dr. A. Vincze.