Purification and characterisation 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 Oryzaephilus surinamensis was found to be due to elevated esterase levels. There was no evidence of elevated glutathione S-transferase or cytochrome P-450 levels in this strain, or presence of an insensitive acetylcholinesterase (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 and N-terminal amino acid sequences were different. There was no difference in K-m between enzymes from resistant and susceptible insects using alpha-naphthyl acetate as substrate, but the V-max of the enzyme from the resistant strain was 7-fold higher than that from the susceptible strain. Both enzymes were competitively inhibited by OPs, but the enzyme from the resistant strain had a 3-fold lower affinity for malaoxon compared to that from the susceptible strain. We hypothesise that the resistance in the 7012/1malRR strain is due to the presence of elevated levels of an esterase enzyme which apparently has reduced affinity for OPs and greater activity than the corresponding enzyme in the susceptible strain. Crown copyright (C) 1998 Published by Elsevier Science Ltd. All rights reserved.