Digestive proteinase activity in corn earworm (Helicoverpa zea) after molting and in response to lowered redox potential

Insect digestive proteinases are often strongly influenced by ambient physicochemical conditions, such as pH, ionic strength, and oxidation-reduction potential. Although the effects of the former two parameters are well documented, the influence of redox potential on catalytic rates of digestive enzymes is not well understood. In this study, we manipulated the midgut redox potential of a generalist caterpillar (the corn earworm, Helicoverpa zea) by augmenting artificial diet with dithiothreitol, a powerful thiol reducing agent that lowers the redox potential in the lumen by 40-45 mV. Effects on total proteolytic activity, as well as on elastase, chymotrypsin, trypsin, leucine aminopeptidase, and carboxypeptidase A and B activities were measured using azocasein and nitroanilide model substrates. The profiles of proteinase activities in the epithelium and lumen were also monitored on days 1, 2, and 3 after the molt in penultimate instar larvae. Although the reducing agent strongly inhibited the activity of some proteinases in vitro, ingestion of the reducing diet failed to affect in vivo proteinase activities. There was also no effect on larval relative growth, consumption, or digestive efficiencies. We conclude that dietary reducing agents must lower midgut redox potential to below -40 mV to significantly impact digestive efficiency. Arch.