BIOCHEMICAL STUDIES ON A MODIFIED AND NORMAL CHOLINESTERASE FOUND IN THE LEVERKUSEN STRAINS OF THE TWO-SPOTTED SPIDER MITE TETRANYCHUS URTICAE

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Abstract
The properties of two different cholinesterases present in the organophosphorus-insecticide-resistant Leverkusen strain and its susceptible counterpart were investigated and compared. The cholinesterase of the Leverkusen resistant strain is characterized by a low sensitivity to organophosphate inhibitors, as is represented by an increase in the enzyme-inhibitor affinity constant (KI). This statement is in accordance with the finding that the ratio of the Michaelis constants for the two cholinesterases is 4, whereas the ratio of the respective maximum velocities (vmax) was only 1.2. Since those organophosphate inhibitors are known to attack only the esteratic site of the cholinesterase, the above finding can be interpreted to mean that the cholinesterase of the resistant strain possesses an abnormally weak esteratic site in terms of its affinity for the substrate as well as for the inhibitor. To investigate the extent of alteration of the esteratic site, a series of organophosphate poisons was tested against these cholinesterases. It was found that the interstrain difference was maximal with the shortest dialkyl side chain of the phosphorus atom, and that the difference decreased with the increase in the dialkyl carbon chain length. Similar findings were made when eholinesters with different acyl groups were tested as substrates. Propionylcholine was hydrolyzed at a faster rate than acetylcholine in both mite strains, and distinct interstrain differences in the cholinesterase activity towards these substrates were observed; with butyrylcholine, however, this interstrain difference was undetectable. Properties of mite cholinesterase were compared with those of insect and mammalian cholinesterases.