Organophosphorus insecticides are one of the biggest groups of pesticides and most of them have sulfur directly connected with phosphorus atom. Thionophosphorus compounds exert their insecticidal activity by inhibiting acetylcholinesterase (AChE) after conversion to their oxon forms. This conversion is mediated in vitro by the mixed function oxidase system (mfo), which also gives apparent hydrolysis products of thiono compounds. Studies on the origin of oxygens incorporated into the products as a result of mfo and peracid oxidation gave the evidence for phosphorus oxythionate intermediate as the initial oxidation product of thiono compounds. The involvement of unstable phosphorus oxythionate in biological and abiotic reactions can be indicated by the presence of its conversion product, phosphinyl disulfide.
Thiono compounds often isomerize to thiolo compounds not only under abiotic conditions such as heat and light, but also in the presence of mfo. Certain thiolophosphorus compounds inhibit AChE in vivo, but not in vitro, indicating that an activation mechanism is involved, which is found to be the oxidation on thiolo sulfur. Activated form is unstable and is suspected to be the S-oxide.
Transformation of thiophosphorus compounds is understandable in terms of the involvement of such unstable and reactive intermediates, and such information will be useful in the designing of a new phosphorus insecticide.
