Activity cycles and the regulation of digestive proteases in the posterior midgut of Stomoxys calcitrans (L.) (Diptera: Muscidae)

Abstract Proteolytic activity, determined using synthetic protease substrates, in the posterior midgut of blood-fed stable flies Stomoxys calcitrans , follows a cyclic pattern after feeding. Maximal activity of trypsin, carboxypeptidase B and chymotrypsin-like proteinase, occurs approximately 18 h after feeding. Carboxypeptidase A activity peaks within the first 6 h after feeding. Aminopeptidase does not follow the same cycle and the peak of maximum activity is sustained longer before declining slowly. Protein content of the posterior midgut correlates significantly (P N- benzoyl- dl -arginine -p- nitroanilide , r = 0.36 ( N = 41), with tosyl- l -arginine methyl ester, r = 0.82 ( N = 41), and with chymotrypsin-like proteinase activity, r = 0.53 ( N = 41). Carboxypeptidases A and B activities also correlate with protein content having correlation coefficients of r = 0.73 ( N = 41) and r = 0.65 ( N = 41), respectively. Aminopeptidase activity does not correlate with posterior midgut protein, r = 0.27 ( N = 40), not significant. Protease activity of trypsin, chymotrypsin-like proteinase, and carboxypeptidases A and B, correlated significantly amongst themselves; however, aminopeptidase did not correlate with any of the other proteases. These results indicate that all four extracellular proteases, trypsin, chymotrypsin-like proteinase, carboxypeptidase A and carboxypeptidase B are controlled by a secretagogue mechanism. Aminopeptidase is not controlled by the same protein secretagogue mechanism.