Biochemical characterization of α-amylases from gut and hemolymph of Rhynchophorus ferrugineus Olivieri (Col.: Curculionidae) and their inhibition by extracts from the legumes Vigna radiata L. and Phaseolus vulgaris L.

Abstract

α-amylase inhibitors represent an important tool in engineering crop plants against insect pests. For achieving this goal, it is necessary to find the nature of α-amylases and their properties for possible use in a pest management procedure. Because Rhynchophorus ferrugineus Olivieri is a devastating pest of palm trees in the southeast of Iran, we attempted to characterize α-amylases from larval gut and hemolymph, and to study their interaction with inhibitors extracted from the common bean and the green mung bean. The optimal pHs for gut and hemolymph α-amylases were 4 - 5 and 5 - 6, respectively. Also, high gut amylolytic activity was found at temperatures of 40 – 50 °C. The highest and lowest specific α-amylase activities were detected in the guts of last instar and adult males, and in the hemolymph of last instar, respectively. As calculated from Lineweaver-Burk plots, the Km values for gut and hemolymph α-amylases of the last instar were 0.54 and 2.15 %, respectively, when glycogen was used as the substrate. Also, when starch was used as the substrate, the Km values for gut and hemolymph α-amylases were 1.37 and 0.15 %, respectively. Zymogram pattern in the native gel revealed that R. ferrugineus gut and hemolymph α-amylases had two isoforms. α-amylase inhibitors partially purified from Vigna radiata L. and Phaseolus vulgaris L. by ionic exchange DEAE cellulose column, inhibited the R. ferrugineus gut α-amylase activity by 19 ± 0.64 % and 11.56 ± 0.69 %, respectively.