A carboxylesterase (EC 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.

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