Partial characterization of phosphotriesterases from organophosphate-susceptible and -resistant populations of Rhyzopertha dominica (Coleoptera: Bostrichidae)

Phosphotriesterases from lesser grain borer, Rhyzopertha dominica (Coleoptera: Bostrichidae), were partially characterized and their roles in conferring organophosphate resistance were assessed. The optimal temperature for determination of phosphotriesterase activity was 37 degrees C using paraoxon as a substrate. The pH profile suggests the presence of more than one phosphotriesterase with different pH optima. The kinetic parameters. K-m and V-max, determined at 37 degrees C were 5.19 x 10(-3) M and 80.22 nmol/hr/mg protein, respectively, for the susceptible population and 5.82 x 10(-3) M and 189.47 nmol/hr/mg protein, respectively, for the resistant population. Frequency of individuals with high phosphotriesterase activity within the resistant population was significantly higher than that of the susceptible population. Phosphotriesterase activity in each of 15 resistant populations was also significantly higher than that of the susceptible population. Significantly higher hydrolysing efficiency to paraoxon, as judged by the V-max value and higher phosphotriesterase activity in resistant populations, suggests the involvement of phosphotriesterases as an organophosphate resistance mechanism in lesser grain borer. (C) 1997 Academic Press.