Digestive enzymes associated with the glycocalyx, microvillar membranes and secretory vesicles from midgut cells of Tenebrio molitor larvae

Abstract

Acetylglucosaminidase, amylase, cellobiase and maltase are more active in anterior midgut cells, whereas aminopeptidase, carboxypeptidase and trypsin are more active in posterior midgut cells of Tenebrio molitor larvae. Differential centrifugation of midgut homogenates prepared in saline (or mannitol) isotonic buffered solutions revealed that aminopeptidase is associated with membranes, which occur in subcellular fractions displaying many microvilli. Carboxypeptidase, trypsin and the carbohydrases are mostly found in the soluble fraction, although significant amounts sediment together with cell vesicles. Data on differential calcium precipitation of midgut homogenates and on partial ultrasound disruption of midgut tissue suggest that aminopeptidase is a microvillar enzyme and that the digestive enzymes recovered in the soluble fraction of cells are loosely bound to the cell glycocalyx. About 5% of the non-absorbable dye amaranth fed to T. molitor larvae remains in the midgut tissue after rinsing. Most dye was recovered in the soluble fraction of midgut cells. This provided further support for the hypothesis that the digestive enzymes found in the soluble fraction are actually extracellular and that the true intracellular enzymes are those associated with cell vesicles. The results suggest that the carbohydrases are secreted by exocytosis from the anterior midgut and carboxypeptidase and trypsin from the posterior midgut.