Characterization of multiple variants of carboxylesterases which are involved in insecticide resistance in the mosquito Culex quinquefasciatus

Overproduced carboxylesterases A2 and B2 (EC 3.1.1.1.), involved in insecticide resistance in the mosquito Culex quinquéfasciatus were purified to homogeneity using 4,h instar larvae from a Sri Lankan resistant strain, PelRR. Esterase B2 (Mr= 62,000 ) was characterized and compared with the esterase A2 characterized previously. The kinetic constants for interaction with several insecticides indicate, as for the esterase A2, that the role of B2 in insecticide resistance is mainly sequestration. The bimolecular rate constant, ka, is the most important constant which correlates directly with the insecticide resistance ratios of the strain. A concentration of approximately 7.67 pmol of both A2 and B2 esterases accounting for about 0.4% of the total protein could be estimated per 4,h instar larva. Using several different methods an A2 : B2 ratio of 1 : 3 could be obtained for this strain. A2, B2, Bj and susceptible non-amplified ‘A’ and ‘B’ type esterases were purified from one susceptible and six more resistant strains of Cx quinquefasciatus, which originated from different geographical areas. Significant differences in the ka’s for insecticide kinetics were obtained for the enzymes from the different strains. The susceptible enzymes were markedly less reactive with insecticides than the resistant enzymes and this was shown even at the crude homogenate level. The qualitative differences observed among the resistant populations indicate the presence of a greater number of amplified allelic forms for the esterase loci A2, B2 and Bj than previously has been suggested. The A2 and B2 esterases were partially purified from three sub-colonies selected with three different insecticides from a single parental colony. Significant intracolony differences were observed in enzyme-insecticide interactions demonstrating the existence of different alleles of A2 and B2 within a single population. Antiserum raised against PelRR A2 esterase cross-reacted with both enzymes from the other strains although the reactivity of the B2 enzyme was about 50-fold less than that of the A2 esterase. Also the A2 antiserum strongly cross-reacted with insect acetylcholinesterase, the target site of the insecticides. Esterase A2 did not show any significant immunological relationship with the other resistance associated esterases tested from other insects. Some of the commercially available vertebrate carboxylesterases and cholinesterases also cross-reacted with the A2 antiserum.