PURIFICATION AND CHARACTERIZATION OF AN ESTERASE INVOLVED IN RESISTANCE TO ORGANOPHOSPHORUS INSECTICIDES IN THE SAW-TOOTHED GRAIN BEETLE, ORYZAEPHILUS-SURINAMENSIS (COLEOPTERA, SILVANIDAE)

Malathion and fenitrothion resistance in a multi-organophosphorus (OP) -resistant strain (7012/1malRR) of the saw-toothed grain beetle Oryzae philus surinamensis was found to be due to elevated esterase levels. T here was no evidence of elevated glutathione S-transferase or cytochro me P-450 levels in this strain, or presence of an insensitive acetylch olinesterase (AChE). The most active and abundant esterase isozyme was purified from the resistant strain and an OF-susceptible strain. The enzyme in both strains was monomeric but apparent molecular weights an d N-terminal amino acid sequences were different. There was no differe nce in K-m between enzymes from resistant and susceptible insects usin g alpha-naphthyl acetate as substrate, but the V-max of the enzyme fro m the resistant strain was 7-fold higher than that from the susceptibl e strain. Both enzymes were competitively inhibited by OPs, but the en zyme from the resistant strain had a 3-fold lower affinity for malaoxo n compared to that from the susceptible strain. We hypothesise that th e resistance in the 7012/1malRR strain is due to the presence of eleva ted levels of an esterase enzyme which apparently has reduced affinity for OPs and greater activity than the corresponding enzyme in the sus ceptible strain. Crown copyright (C) 1998 Published by Elsevier Scienc e Ltd. All rights reserved.