Examination of esterases from insecticide resistant and susceptible strains of the German cockroach, Blattella germanica (L.)

Esterases from insecticide resistant and susceptible Blattella germanica (L.) were examined biochemically. Two strains were utilized: Johnson Wax (JWax; susceptible), and Munsyana (MA; chlorpyrifos LD50 and LD95 resistance ratio 5.2 and 10.0). On native polyacrylamide gel electrophoresis (PAGE), MA had four visible esterase electromorphs (E1, E2, E3 and E4), whereas JWax had three (E1, E2 and E4). Esterases E1 and E4 were more intense in the MA strain, and none of these esterase electromorphs were acetylcholinesterases. Insecticide inhibition of native esterases within polyacrylamide gels showed an interaction of all electromorphs with the carbamate insecticide propoxur and complete inhibition of all electromorphs by the carbamate bendiocarb and the organophosphate insecticides chlorpyrifos oxon, malaoxon and paraoxon. The pyrethroid insecticides permethrin and cypermethrin had no inhibitory effects. Sequential Q-Sepharose and hydroxyapatite column chromatography was used to fractionate esterases from each strain into two groups (I and II). Following hydroxyapatite fractionation of these esterase groups, inhibition kinetic constants (k(a) and k3), and molecular weights were estimated. Results for ka (the rate of enzyme inhibition) indicated a greater affinity for organophosphate insecticides by MA esterases. Results for k3 (the rate of enzyme recovery) indicated lengthened times of MA esterase-inhibition by organophosphate insecticides. Therefore the role, if any, in organophosphate resistance played by MA esterases must be by sequestration. Molecular weight estimates were within the range (55-65 kDa) previously observed for esterases from both B. germanica and Culex quinquefasciatus.