The insecticidal properties and, consequently, the toxic properties of the phosphorothionate insecticides results from their ability to phosphorylate and inactivate certain esteratic enzymes in insects and animals. The parent phosphorothionate itself is not capable of inhibiting these enzymes. However, it is metabolized by a mixed function oxidase enzyme in insects and animals to the phosphate analog which is capable of bringing about the inactivation. Further studies have revealed there is an additional product, formed by the action of what appears to be a different form of the mixed function oxidase enzyme. This is a product in which one of the ester bonds is cleaved to give the diester of phosphorothioic acid. Studies have shown that the rates of these two reactions are sensitive to change in the electronic environment about the phosphorus.
Neal RA. Enzymic Mechanism of Metabolism of the Phosphorothionate Insecticides. Arch Intern Med. 1971;128(1):118-124. doi:10.1001/archinte.1971.00310190122015