Malathion toxicity and carboxylesterase activity in Drosophila melanogaster

Adults from nine strains of Drosophila melanogaster were exposed to a residual film of malathion. The LC₅₀ values indicated that there was a fourfold difference in susceptibility to malathion among the strains. 3-Nonylthio-1,1,1-trifluoropropan-2-one (NTFP), a carboxylesterase inhibitor, synergized the toxicity of the insecticide when four fly strains were exposed to a mixture of the inhibitor and malathion. Whole-body homogenates of adults were analyzed by wide-range isoelectric focusing (IEF). Carboxylesterase activities on the gels were stained using common substrates such as α- and β-naphthyl acetate and p-nitrophenyl acetate. Less conventional substrates, ethyl and isobutyl carbonates of α-naphthol and p-nitrophenol, were synthesized and used for staining gels and kinetics in solution. Two major bands of activity (pI 4.4 and 4.0) were detected with naphthyl substrates. A major peak of malathion hydrolytic activity (pI 6.3) was detected by running spectrophotometric assays for carboxylesterase activity on malathion using IEF gel slices and a rapid, semiautomated assay method. The peak region of activity on malathion did not coincide with bands obtained when naphthyl substrates were used to stain the gels. NTFP inhibited malathion hydrolysis in a solution from the IEF peak of activity and also inhibited esterase activity on α-naphthyl acetate on the gels. Recovered carboxylesterase activities on malathion and O-ethyl carbonate of p-nitrophenol were higher in the cytosolic than in the microsomal or mitochondrial cell fractions.