The insecticidal activity of O-ethyl O-2-isopropoxycarbonylphenyl N-alkylphoshoramidothioates was examined with reference to their activation in biologicalsystems. Toxicity to the adzuki bean weevil varied with different N-alkyl groups. N-isopropylphosphoramidothioate was the most toxic of the compounds tested, and N-unsubstituted, N-methyl-and N-ethylphosphoramidothioates were more toxic than fenitrothion. However, N-propyl and N-butyl homologs were less active than fenitrothion with the exception of the N-isopropyl homolog. LD50 values of the phosphoramidothioates for the insect were not correlated with in vitro anti-AChE activity of the phosphoramidates. 150 of N-isopropylphosphoramidate for acetylcholinesterases from adzuki bean weevil and bovine serum was higher than 10-3 M. When the phosphoramidothioates and phosphoramidate were incubated with rat liver microsomal system, AChE activity of bovine serum was strongly inhibited in the presence of NADPH. Inhibition of AChE activity was reduced by addition of SKF 525-A to the microsomal system. The compounds became also potent inhibitors for AChE by treatment with m-chloroperbenzoic acid. From these results, it was concluded that phosphoramidothioates and their oxons were activated oxidatively to inhibit AChE by the microsomal system as well as chemical treatment with peracid.
Isofenphos (0-ethyl 0-2-isopropoxycarbonylphenyl isopropylphosphoramidothioate) and isofenphos-oxon show different insecticidal activities to the housefly and the adzuki bean weevil between chiral isomers. The (+)-isomers of both compounds were more toxic than the (-)-isomers. Although 150 values of both chiral isomers of isofenphos-oxon for acetylcholinesterase (ACNE) were higher than 103 M, it was found that the insecticidal activity of isof enphos and its oxon was attributed to the inhibition of ACNE activity after the bioactivation of each chiral isomer with the rat liver microsomal NADPH system. ' It was reported that (+)-cyanofenphos (0-p-cyanophenyl 0-ethyl phenylphosphonothioate) showed more toxic to the housefly and the rice stem borer than the (-)-isomer.2) The (+)-cyanof enphos produced approximately 10-fold larger amounts of cyanof enphos-oxon as an activated metabolite in the rice stem borer than the (-)-isomer. 3) Moreover, it was found that stereoselectivity was dependent on the different metabolism of chiral isomers in organophosphorus compounds, such as S-2571 (0-ethyl 0-2-nitrophenyl isopropylphosphoramidothioate), EPN (0-ethyl 0-p-nitrophenyl phenylphosphonothioate) and fonofos (0-ethyl S-phenyl ethylphosphonodithioate). 4-6) This report is concerned with differential metabolism of the chiral isomers of isofenphos in the rat liver microsomal system. MATERIALS AND METHODS[0-Ethyl-1-14C]isofenphos with 3.
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