Elevated esterases exhibiting arylesterase-like characteristics in an organophosphate-resistant clone of the greenbug, Schizaphis graminum (Homoptera: Aphididae)

The profiles of esterase activity in an organophosphate (OP)-susceptible (O SS) clone and an OP-resistant (OR-2) clone of the greenbug (Schizaphis gram inum) were compared using nondenaturing polyacrylamide gel electrophoresis (PAGE) coupled with esterase assays after gel fractionations. A distinct pe ak of esterase activity was found in the esterase profiles of the OR-2 clon e but not in the OSS clone. The peak represents the elevated esterases iden tified previously and constitutes a major biochemical difference between th e OSS and the OR-2 clones. The esterases within that peak hydrolyzed five s ubstrates, including phenyl acetate (PA), beta-naphthyl acetate (beta-NA), alpha-naphthyl butyrate (alpha-NB), p-nitrophenyl acetate (p-NA), and alpha -naphthyl acetate (alpha-NA). The most preferred substrate was PA followed by beta-NA, alpha-NB, p-NA, and alpha-NA. Nondenaturing PAGE revealed that the major esterase peak was contributed by three different esterase bands. These esterases showed a similar affinity to beta-NA and were highly sensit ive to inhibition by both paraoxon and p-hydroxymecuribenzoic acid. In addi tion, 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 arylest erase-like esterases with certain biochemical properties resembling phospho ric triester hydrolase. However, these arylesterase-like esterases were not able to efficiently utilize paraoxon and chlorpyrifos-oxon as their substr ates. Thus, the elevated esterases identified in the OR-2 clone appeared to contribute to OP resistance by sequestrating OP molecules. (C) 2000 Academ ic Press.