Elevated Esterases Exhibiting Arylesterase-like Characteristics in an Organophosphate-Resistant Clone of the Greenbug, Schizaphis graminum (Homoptera: Aphididae)

Abstract

The profiles of esterase activity in an organophosphate (OP)-susceptible (OSS) clone and an OP-resistant (OR-2) clone of the greenbug (Schizaphis graminum) were compared using nondenaturing polyacrylamide gel electrophoresis (PAGE) coupled with esterase assays after gel fractionations. A distinct peak of esterase activity was found in the esterase profiles of the OR-2 clone but not in the OSS clone. The peak represents the elevated esterases identified previously and constitutes a major biochemical difference between the OSS and the OR-2 clones. The esterases within that peak hydrolyzed five substrates, including phenyl acetate (PA), β-naphthyl acetate (β-NA), α-naphthyl butyrate (α-NB), p-nitrophenyl acetate (p-NA), and α-naphthyl acetate (α-NA). The most preferred substrate was PA followed by β-NA, α-NB, p-NA, and α-NA. Nondenaturing PAGE revealed that the major esterase peak was contributed by three different esterase bands. These esterases showed a similar affinity to β-NA and were highly sensitive to inhibition by both paraoxon and p-hydroxymecuribenzoic acid. In addition, the enzyme activity was slightly to moderately activated by Ca2+, but significantly inhibited by Triton X-100. These characteristics suggested that the elevated esterases in the OR-2 clone were arylesterases or arylesterase-like esterases with certain biochemical properties resembling phosphoric triester hydrolase. However, these arylesterase-like esterases were not able to efficiently utilize paraoxon and chlorpyrifos-oxon as their substrates. Thus, the elevated esterases identified in the OR-2 clone appeared to contribute to OP resistance by sequestrating OP molecules.