Study on the inhibition kinetics parameters of esterases enzymes from the red palm Weevil Larvae

Abstract

Aromatic esterases from the red palm weevil; Rynchophorus ferrugineus 5th instar larvae were partially fractionated using differential centrifugation technique to study some enzyme properties and possible use of some inorganic salts as enzyme inhibitors hoping for future use in the field of pest control. Cytosol soluble fraction contained most of esterases activity. Esterases catalysis was activated by organic solvents such as acetone. However, incubation of esterases with 100 μl of different molar concentrations of EDTA, CuSO 4 , NaF, CaCl 2 and NaCl for 10 min at 25 °C had variable effects. NaCl had no effect, but CaCl 2 showed some considerable activation. CuSO 4 was the strongest inhibitor for both α- and β-esterases catalysis, but β-esterases were more inhibited than α-esterases by the tested CuSO 4 concentrations. In vitro partial inhibition kinetics studies showed that IC 50 of CuSO 4 was 8.5 and 4.4mM for α- and β-esterases of the red palm weevil larvae, respectively. CuSO 4 was a competitive inhibitor for esterases. The Dixon plot analysis revealed that CuSO 4 had a dissociation constant (ki) value of 4 and 2 mM for α - and β-esterases from cytosol fractions, respectively. On the other hand, the Arrhenius plot showed that activation energy (Ea) was 14.92 and 24.89 kJ/mol for α- and β-esterases hydrolysis reactions. i.e. esterases from 5th instar larvae of R. ferrugineus had minimum activation energy to start naphthyl acetate hydrolysis. Studies must be extended to explore CuSo 4 activity in the field of pest control and the possibility of using it as insecticide synergist.