STUDIES IN THE MODE OF ACTION OF INSECTICIDES

Abstract

Homogenates of the thoracic nervous system of Locusta migratoria migratoriodes are able to hydrolyse acetylcholine (ACh) and o‐nitrophenylacetate (NPA), and this hydrolysis can be inhibited by tetraethylpyrophosphate (TEPP) at approximately the same molar concentration for both substrates. It is possible that one acetyl‐esterase is responsible for the breakdown of the two substances, and there is no reason to assume the existence of a specific acetylcholinesterase. In normal horse serum, on the other hand, the pseudocholinesterase is quite distinct from the enzyme responsible for the breakdown of NPA.In an attempt to correlate the inhibition of the locust nerve cord acetylesterase with toxic activity to insects and mice, four chlorinated diethyl‐phenylphosphates were tested as contact poisons against a number of insects and by injection against locusts and mice, and also as in vitro inhibitors of locust nerve cord acetylesterase and horse‐serum pseudocholinesterase. The chemicals were the 2‐chloro‐, 4‐chloro, 2:4‐dichloro‐ and 2:4:5‐trichloro‐ analogues of diethyl‐phenylphosphate.Good correlation exists between their in vitro activity against the nerve‐cord acetylesterase and their contact activity to aphids, but not between the former and injection toxicity to locusts. No correlation could be established between the inhibition of horse‐serum cholinesterase and injection toxicity to mice. It is thought likely that the inhibition of nerve‐cord acetylesterase is of greater importance in aphids than in other insects, where the toxic action of the phosphoric esters is at least partly concerned with other vital processes, and that a detoxication mechanism in the mammal breaks down some of the phosphoric esters, but not others.