Distribution, properties, and functions of midgut carboxypeptidases and dipeptidases from Musca domestica larvae

Abstract Dipeptidase and carboxypeptidase A activities were determined in cells and luminal contents of the fore‐, mid‐, and hind‐midgut of Musca domestica larvae. Dipeptidase activity was found mainly in hind‐midgut cells, whereas carboxy‐peptidase activity was recovered in major amounts in both cells and in luminal contents of hind‐midguts. The subcellular distribution of dipeptidase and part of the carboxypeptidase A activities is similar to that of a plasma membrane enzyme marker (aminopeptidase), suggesting that these activities are bound to the microvillar membranes. Soluble carboxypeptidase A seems to occur both bound to secretory vesicles and trapped in the cell glycocalyx. Based on density‐gradient ultracentrifugation and thermal inactivation, there seems to be only one molecular species of each of the following enzymes (soluble in water or solubilized in Triton X‐100): membrane‐bound dipeptidase (pH optimum 8.0; Km 3.7 mM GlyLeu, Mr 111,000), soluble carboxypeptidase (pH optimum 8.0; Km 1.22 mM N‐carbobenzoxy‐glycyl‐L‐phenylalanine (ZGlyPhe), Mr45,000) and membrane‐bound carboxypeptidase (pH optimum 7.5, Km 2.3 mM ZGlyPhe, Mr58,000). The results suggest that protein digestion is accomplished sequentially by luminal trypsin and luminal carboxypeptidase, by membrane‐bound carboxypeptidase and aminopeptidase, and finally by membrane‐bound dipeptidase.