Abstract
The insecticidal activities of chiral isomers of isofenphos (O-ethyl O-2-isopropoxycarbonylphenyl N-isopropylphosphoramidothioate) and isofenphos-oxon were examined with reference to their activation in rat liver microsomal-NADPH system. The chiral isomers of isofenphos and Isofenphos-oxon were completely separated by using HPLC with a mixture of n-hexane and 2-propanol on a CHIRALCEL OC column. To the housefly and adzuki bean weevil, the (+)-isomers of both the compounds were more toxic than the (-)-isomers. Especially, to the housefly the (+)-isomer of isofenphos-oxon was approximately 83.5-fold more toxic than the (-)-isomer. To both insects fenitrothion, the (+)-isomer and racemate of isofenphos were almost similar in insecticidal activity, although (-)-isofenphos was somewhat weaker. The I50 values of both chiral isomers of isofenphos-oxon for acetylcholinesterase (AChE) of the housefly head and adzuki bean weevil, however, were higher than 10-3M. After the chiral isomers of isofenphos-oxon were incubated with the rat liver microsomal system in the presence of NADPH, the (+)-isomer became to inhibit AChE of housefly head stronger than the (-)-isomer; the I50 values were 1.47×10-7M, 6.19×10-5M and 9.61×10-7M for the (+)-, (-)- and racemic isofenphos-oxons, respectively. When piperonyl butoxide (PB) was added to the microsomal-NADPH system, inhibition of AChE activity by the (+)- and racemic isofenphos-oxon was reduced. The data suggested that the (+)-isomer was easily bioactivated by the microsomal-NADPH system though the difference of affinity to AChE between bioactivated chiral isomers should be taken into consideration.
