Serine and cysteine proteases of Rhynchophorus ferrugineus (Coleoptera: Curculionidae) larvae raised on date palms (Phoenix dactylifera)

Abstract

The red palm weevil Rhynchophorus ferrugineus (Coleoptera: Curculionidae) is a serious pest of date palms in many Middle East countries. The aim of the current study was to obtain a good understanding of the gut proteolytic activity of R. ferrugineus. Thus, the enzymes in the alimentary tract were isolated and their properties were investigated using a general protease substrates (azocasein and hemoglobin) as well as specific substrates of trypsin, chymotrypsin, elastase, cathepsin L, and cathepsin B. Results showed that in the gut of the RPW both serine and cysteine proteases are present although with different activities. Among the active proteases, trypsin was the most active in the gut followed by chymotrypsin and elastase. The activities of these three proteases in the larval gut were 1.32, 1.12 and 0.32U, respectively. Among cysteine proteases, cathepsin L and cathepsin B were active in the larval gut. The activity of cathepsin L was 0.82U while that of cathepsin B was 0.31U. The order of the activity of the proteases in the larval gut was trypsin>chymotrypsin>cathepsin L>elastase>cathepsin B. The effect of pH on general protease substrates hydrolysis showed two distinct peaks: one in the acidic area (pH 6.0) and another in the alkaline area (pH 10.0); these correspond to cysteine and serine protease activities, respectively. Zymogram analysis along with inhibition studies revealed that four proteases are present in the gut.