Determination of the role of elevated B2 esterase in insecticide resistance in Culex quinquefasciatus (Diptera: Culicidae) from studies on the purified enzyme

A method for the purification of elevated esterase B2 from the Sri Lankan Pel RR strain of Culex quinquefasciatus (Say) has been developed, using sequential column chromatography. The pure enzyme is stable, with no decrease in specific activity after several months, if stored in the presence of 25 mM dithiothreitol and 50% glycerol at –20°C. The enzyme has a pl of 5 and an estimated molecular weight for the monomeric enzyme of 63 kD from SDS-PAGE. In all Culex strains with elevated B2, another esterase (A2) is always co-elevated. The strains with these esterases are resistant to a broad range of organophosphorus insecticides. The previous purification and characterization of A2 from Pel RR indicated that the role of this enzyme was primarily to sequester rather than metabolize organophosphates. Purified B2, rapidly binds malaoxon, the activated form of malathion, but not malathion itself. The turnover rate of bound malaoxon is slow, suggesting that the role of this esterase in malathion resistance is also sequestration. The similar bimolecular rate constants for both A2 and B2 with malaoxon suggest both esterases are important in resistance, which may explain the almost complete linkage disequilibrium that the elevated forms of these two esterases are maintained in when under pesticide selection pressure.