Dimethylphosphorothioates.

Five dimethylphosphorothioates were tested for their toxicity to rats, potentiation of malathion toxicity in rats, inhibition of carboxylesterase in vitro, and reaction with malathion in vitro. The compounds were: potassium salts of (CHS)P(O)O(I), (CHO)(CHS)P(O)S(II), (CHO)P(O)S(III), (CHO)P(S)S(IV), and (CHO)(CHS)P(O)O(V). The dimethylphosphorothioates are not toxic to rats (up to 1 g/kg, orally), they do not potentiate malathion toxicity in rats, and do not inhibit carboxylesterase activity in vitro (up to 1 mM concentrations). However, when the S-acid diesters (II, III, IV) are incubated with malathion for several days at room temperature or for several hours at 50° C they become methylated forming the trimethylphosphorothioates OSS-trimethyl phosphorodithioate, OOS-trimethyl phosphorothioate and OOS-trimethyl phosphorodithioate respectively, which potentiate malathion toxicity. Furthermore, these same acid diesters increase the rate of isomerization of malathion into OS-dimethyl-S-(1,2-dicarbethoxyethyl) phosphorodithioate (isomalathion) particularly, diester IV. The formation of the trimethylphosphorothioates and isomalathion from the interaction of the S-acid diesters with malathion was determined by thin layer chromatography (TLC), gas chromatography and mass spectrometry and could be detected by in vitro inhibition of carboxylesterase. TLC methods can detect 1 mg of the trimethylphosphorothioates and isomalathion per gram malathion. [ABSTRACT FROM AUTHOR]