The enzyme, esterase B2, involved in insecticide resistance has been purified and characterized from the mosquito Culex quinquefasciatus. The monomeric enzyme has an M(r) of 62000 and a pI of 5.0. This enzyme is compared with the esterase A2 previously characterized [Ketterman, Jayawardena and Hemingway (1992) Biochem. J. 287, 355-360]. The kinetic constants for interaction with several insecticides indicate, as for the esterase A2, that the B2 enzyme has a role in resistance. The rates and affinities of binding observed support the hypothesis that the role mainly is sequestration followed by the slow turnover of the insecticide. Although the B2 esterase appears to have a slightly higher rate of interaction with insecticides, the A2 has a much greater Vmax. with the xenobiotic substrates studied. The B2 esterase also appears to be present in the larvae to a lesser extent than the esterase A2.

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A carboxylesterase (EC 3.1.1.1) involved in organophosphate insecticide resistance has been purified and characterized from the mosquito Culex quinquefasciatus. The monomeric enzyme has M(r) of 67,000 and a pI of 5.2. It hydrolysed medium-chain-length mono- and di-acylglycerols in addition to xenobiotic esters. Kinetic constants determined for four insecticides, temephos, chlorpyrifos, fenitrothion and propoxur indicate the rates of acylation and the affinities of binding of the insecticides to this carboxylesterase are important. This supports the major role of the A2 carboxylesterase is the sequestration of the insecticide with a minor role in the slow turnover of the insecticide.