Pyrethroid-hydrolyzing esterases in southern armyworm larvae: Tissue distribution, kinetic properties, and selective inhibition

Abstract

Esterase activity hydrolyzing both [1 RS,trans]- and [1 RS,cis]-permethrin was detected in crude homogenates of the following southern armyworm ( Spodoptera eridania Cramer) larval tissues: cuticle, gut, fat body, head capsule, Malpighian tubules, and silk gland. Neither substrate was detectably hydrolyzed by hemolymph. The highest esterase activities per insect equivalent of tissue were found in cuticle, gut, and fat body for the trans isomer and in cuticle and gut for the cis isomer. Each preparation hydrolyzed the trans isomer more rapidly, but the degree of specificity varied greatly between tissues. Differences in apparent K m and V max values between the three most active tissues were threefold or less for trans isomer hydrolysis, but differences between tissues of up to 100-fold were found for K m and V max values for cis isomer hydrolysis. Hydrolysis of the trans isomer in cuticle, gut, and fat body homogenates was only partially inhibited by α-naphthyl N-propylcarbamate (NPC). Concentrations of NPC giving maximal inhibition of trans isomer hydrolysis had little effect on the hydrolysis of the cis isomer. These results demonstrate that pyrethroid-hydrolyzing activity is broadly distributed in insect tissues and results from the combined activity of several esterases with different properties. It is likely that the trans and cis isomers of permethrin are hydrolyzed by separate enzymes in this insect.