INSECTICIDE RESISTANCE AND MALATHION CARBOXYLESTERASE IN THE SHEEP BLOWFLY, LUCILIA-CUPRINA

Resistance to the organophosphorus insecticide malathion in geneticall y related strains of the Australian sheep blowfly Lucilia cuprina was examined. Separate lines of blowflies were established by homozygosis of the fourth chromosome of the parental RM strain. Both the RM and th e derived resistant (der-R) strains are approximately 100 times more r esistant to malathion than the related susceptible der-S strain, resis tance being correlated with a 45- to 50-fold increase in a malathion c arboxylesterase (MCE) activity. MCE has a pH optimum ranging between 6 .6 and 8.0 and is strongly inhibited by the carboxylesterase inhibitor s triphenyl phosphate, paraoxon, and diisopropylfluorophosphate. Subce llular fractionation revealed that MCE was localized predominantly to the cytosol and mitochondria in both resistant and susceptible blowfli es. A single MCE was purified to homogeneity from RM blowflies. It has a pI of 5.5, is a monomer of 60.5 kDa, and hydrolyzes malathion with a V-max of 755 nmol/min/mg protein and a K-m of 11.0 mu M. L. cuprina have thus evolved a remarkable MCE which is faster and more efficient at hydrolyzing a specific insecticide than any other insect esterase y et described