Effects of the soybean (Kunitz) trypsin inhibitor on growth and digestive proteases of larvae of Spodoptera litura

Abstract

Extracts from the digestive tract of final instar larvae of Spodoptera litura, reared on artificial diet, contained trypsin-like (BApNA-hydrolysing) activity with a pH optimum of 10.5, an elastase-like (SA 2PLpNA-hydrolysing) activity with a pH optimum of 10.5 and leucine aminopeptidase activity with a pH optimum of 8.5. No chymotrypsin amidase-like (BTpNA-hydrolysing) activity could be detected, but chymotrypsin esterase-like (BTEE-hydrolysing) activity was measured with a pH optimum of 8.5. Titration, in vitro of the soybean (Kunitz) trypsin inhibitor (SBTI) against each enzyme at its optimal pH revealed that the inhibitor was most effective at retarding the trypsin-like activity, only slightly effective against the elastase-like and chymotrypsin esterase-like activity, but completely ineffective against leucine aminopeptidase. Incorporation of SBTI into the diet of neonate larvae at 0.2% (w/v) and 0.5% (w/v) retarded growth rate when compared with larvae fed artifical diet only. The observed differences in growth rate occurred within the first three days, resulted in weight differences that were sustained over the whole period of the trial (12 days) and correlated with inhibitor concentration; the slowest growth rate and the lower weights were observed for larvae fed with the higher SBTI concentration (0.5%) in their diet. Assay of the digestive proteases in these larvae demonstrated that only the trypsin-like activity was stimulated significantly in larvae fed on diet containing SBTI.