Digestive proteolytic and amylolytic activities in Trogoderma granarium Everts (Dermestidae: Coleoptera)

The Khapra beetle, Trogoderma granarium is a serious post-harvest pest of wheat. Digestive proteinase and amylase activities of larvae and adult insects were investigated using general and specific substrates and inhibitors. Gut pH in larvae was determined to be slightly acidic in the midgut and more acidic in the hindgut. However, midgut extracts from larvae showed an optimum activity for proteolysis against azocasein and haemoglobin at alkaline pH (broad optimum in the range 8–11). Proteinase extracted from larval guts hydrolyzed the synthetic substrates BApNA(Nα–benzoyl–L–arginine–p–nitroanilide) and SAAPPpNA (N–succinyl–alanine–alanine–proline–phenylalanine–p–nitroanilide) for trypsin- and chymotrypsin-like activities, respectively. The specific substrate for elastase-like activity SAAApNA (N–succinyl–alanine–alanine–alanine–p–nitroanilide) was not hydrolyzed by midgut extract of larvae. The most effective inhibitors of azocaseinolytic hydrolysis were trypsin and chymotrypsin-like serine proteinase inhibitors. Casein zymogram analysis revealed at least six proteinase bands in the larval midgut extract and showed the presence of trypsin-like serine proteinases. No cysteine proteinases were detected in larval midgut extract using specific cysteine proteinase substrates, inhibitors and activators. Carboxypeptidase A activity also was detected in larval midgut extract using FAPP(N-(3-(2-furyl) acryloyl)-l-phenylalanyl-l-phenylalanine) and showed optimal activity at alkaline pH. Activity of α-amylase using starch as substrate, was determined to be maximal around neutral pH (pH 6–8), which is correlated with the pH prevailing in the midgut. No proteolytic and amylolytic activities were detected using conventional substrates in the digestive system of the adult suggesting no significant protein and carbohydrate digestion occurs in the adult beetle which does not need to feed.