Abstract
The esterase properties of two highly chlorpyrifos-methyl-resistant strains of Oryzaephilus surinamensis differentially resistant to fenitrothion were investigated. Isoelectric focusing of whole insect homogenates revealed qualitative differences in the isozymes present, with differences in both the number of esterases with elevated activity in each strain and the ionic charge of these isozymes between strains. These esterases were concluded to be B-esterases, with no evidence of A-esterase activity in whole insect homogenates. Native polyacrylamide gel electrophoresis and isoelectric focusing also revealed qualitative differences between strains in the esterase with the greatest activity, purified using ion-exchange chromatography. These differences were thought to be configurational, with SDS-PAGE revealing no difference in the electrophoretic mobility of the two esterase bands, each having a molecular weight of 71 kDa. There was also no difference found in the sequence of the first 15 N-terminal amino acids. Inhibition assays of the purified enzyme samples using the specific insecticidal inhibitors of chlorpyrifos oxon and fenitrooxon revealed that the esterases from these strains were highly sensitive to inhibition by chlorpyrifos-methyl oxon and significantly less sensitive to inhibition by fenitrooxon. The enzyme sample from insects of MH130f was found to be 247 times less sensitive to fenitrooxon than chlorpyrifos-methyl oxon inhibition. The esterase sample from insects of strain HH012f, highly resistant to both insecticides, was 21 times more sensitive to fenitrooxon inhibition than the sample from MH130f, with intermediate fenitrothion resistance. Differences in the qualitative esterase properties of these strains were proposed as an explanation of the differences in fenitrothion sensitivity and resistance.
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