Characterization and ecological implications of midgut proteolytic activity in larvalPieris rapae andTrichoplusia ni.

In their larval luminal midgut fluid,Trichoplusia ni (Lepidoptera: Noctuidae) andPieris rapae (Lepidoptera: Pieridae) contain endopeptidases as their primary proteases. Neither species has detectable exopeptidase activity. Studies using enzyme-specific substrates and inhibitors demonstrate that the endopeptidases are serine proteinases (both trypsinlike and chymotrypsinlike) with histidine at the active site. Optimal pH for the tryptic and chymotryptic activity is 8.5 and 8.0, respectively, forT. ni. and 8.0 and 9.0, respectively, forP. rapae. The efficiency of proteolytic digestion (as measured by the rate of in vitro digestion of a standard protein by the midgut luminal fluid) is positively correlated with the larval dietary protein requirement and is significantly influenced by the ratios of tryptic to chymotryptic activity present in the gut lumen of these two species of Lepidoptera.