Peculiarities of digestive function of proteinases in invertebrates—Inhabitants of cold seas

Abstract

Digestive proteinases of various taxa of invertebrates of the boreal seas have been studied: crustaceans Paralithodes camtchaticus, Pandalus borealis; molluscs Chlamys islandicus, Buccinum undatum, Serripes groenlandicus, and echinoderms Strongylocentrotus droebachiensis, Cucumaria frondosa, Asterias rubens, and Crossaster papposus. The presence of two proteolytic activity peaks in the acidic (pH 2.5–3.5) and lower alkaline ranges (pH 7.5–8.5) and a similar proteinase spectrum have been revealed in digestive organs of the studied animals. The proteolytic activity in digestive organs of the Barents Sea invertebrates exceeds significantly that of terrestrial homoiothermal animals, which seems to be an extensive compensation for poor differentiation of the digestive system and for low substrate specificity of the enzymes as well as for cold conditions of the habitat. The principal qualitative difference between vertebrates and invertebrates consists in that the latter have no pepsin activity, but do have the cathepsin activity that is absent in vertebrate digestive organs. Contribution to the acid proteolysis is made by lysosomal cathepsins, rather than by pepsins. Activity in the alkaline and neutral pH ranges is provided by serine proteinases. In digestive cavities of invertebrates, hydrolysis of proteins and mechanical processing of food occur only in the lower alkaline pH range, whereas acid proteolysis has intracellular lysosomal localization.