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 120 kDa and consisted of two 60 kDa subunits. The purified carboxylesterase activity was totally inhibited by 10−1 mM chlorpyrifos-methyl and by 10−3 mM 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.