Spatial organization of digestion, secretory mechanisms and digestive enzyme properties in Pheropsophus aequinoctialis (Coleoptera: Carabidae)

Abstract

Aminopeptidase (soluble form M r 110,000), carboxypeptidase A (soluble form M r 47,000), maltase (a dimer composed of two identical M r 60,000 subunits) and trypsin (two charge isomers with M r 34,000) are found in major amounts in the crop and midgut tissue, whereas amylase (a trimer of three identical M r 18,000 subunits) and cellobiase (a trimer of three identical M r 27,000 subunits) occur mainly in the crop and midgut contents. Subcellular fractions of midgut cells were obtained by conventional homogenization, followed by differential centrifugation or differential calcium precipitation. The results suggest that part of the aminopeptidase and carboxypeptidase A activity is bound to microvilli, that major amounts of trypsin and maltase are trapped in the cell glycocalyx and finally that soluble aminopeptidase, amylase and cellobiase occur in intracellular vesicles. The data support the hypothesis that most protein and carbohydrate digestion takes place in the crop under the action of enzymes passed forward from the midgut, after being secreted by exocytosis. Nevertheless, part of the intermediate and final digestion occurs at the surface of the midgut cells. The peculiar features of the digestion of P. aequinoctialis beetles, including their partly fluid peritrophic membranes, are thought to be derived from putative Coleoptera ancestors.