Biochemical studies of A and B carboxylesterases from organophosphate resistant strains of an Italian Culex pipiens (Diptera: Culicidae)

Biochemical comparisons were made between esterases of elevated and nonelevated activity from an organophosphate resistant strain of Italian Culex pipiens. Isoelectric focusing demonstrated that nonelevated A4 and elevated A2 and B2 esterases had similar isoelectric points (pI) ranging from 5.6 to 5.9. Elevated A1 had a slightly higher pI of 6.1-6.5. A1, A2, and A4 esterases had molecular weights of approximately 110 kDa, whereas B2 had a MW of 65 kDa. Soluble A esterases were partially purified for kinetic studies, which revealed that there was no qualitative difference in their ability to hydrolyse the substrate α-naphthyl acetate. This similarity at the active site suggests that there may be no significant differences between alleles to make one more effective at hydrolysing or sequenstering insecticides. Quantitative differences in A esterase activity between strains were evident, supporting the hypothesis that A esterases are overproduced in insecticide resistant strains.