Cholinesterase and Ali-Esterase Activity in Organophosphorus-Susceptible and -Resistant House Flies

Abstract

Cholinesterase and aliphatic esterase activity were measured colorimetrically in organophosphorus-susceptible and -resistant strains of house flies ( Musca domestica L.), with acetylcholine and methyl- n -butyrate as substrates. The effects of time and enzyme concentration on the hydrolysis of the substrates were determined. The specificity of head and body breis in hydrolyzing both substrates was measured, and distinct differences were shown in levels of esterase activity between the two body regions. Levels of cholinesterase activity were found to be nearly identical in susceptible and resistant flies, but aliphatic esterase activity was much lower in two resistant strains than in the susceptible colony, indicating a possible relationship between this esterase and resistance. Ali-esterase was four times as susceptible as cholinesterase to inhibition by parathion and five times as susceptible to inhibition by para-oxon. With malathion, cholinesterase was twice as susceptible to inhibition as ali-esterase.