Purification and characterisation of a carboxylesterase from a chlorpyrifos-methyl-resistant strain of Oryzaephilus surinamensis (Coleoptera: Silvanidae)

Carboxylesterases from a chlorpyrifos-methyl-resistant strain (VOSCM) of the saw-toothed grain beetle, Oryzaephilus surinamensis (L.), are presumed to play a role in conferring resistance to chlorpyrifos-methyl. Colorimetric assays using substrates of p-nitrophenyl acetate, alpha-naphthyl acetate and beta-naphthyl acetate showed 4.8, 7.8 and 7.5 times higher carboxylesterase hydrolytic activities in VOSCM than those in VOS48, an organophosphorus insecticide-susceptible strain. Carboxylesterase zymograms showed different banding patterns between VOSCM and VOS48. A primary carboxylesterase in the VOSCM strain, not detected in VOS48, was purified and characterised by chromatographic and electrophoretic techniques. On the basis of native and SDS-polyacrylamide gel electrophoresis, the molecular mass of the purified carboxylesterase from VOSCM was 120kDa and consisted of two 60kDa subunits. The purified carboxylesterase activity was totally inhibited by 10^-1mM chlorpyrifos-methyl and by 10^-3mM chlorpyrifps-methyl oxon. The purified enzyme did not hydrolyse insecticide substrates. Therefore, these results indicate that the purified carboxylesterase may play an important role in chlorpyrifos-methyl detoxification by sequestration.