THE SPECIFICITY AND COLLABORATION OF DIGESTIVE ENZYMES IN METAZOA

Summary1 The identity of the digestive enzymes found in vertebrates and invertebrates has been discussed.For the proteases the conclusion is reached that in the vertebrates as well as in the invertebrates (as far as investigated) the protein molecule is attacked by at least four different enzymes, a proteinase, carboxypolypeptidase, aminopolypeptidase and dipeptidase, which can be separated by adsorption. (Recent researches have, moreover, demonstrated the presence of a separate protaminase in the former trypsin of vertebrates.) The proteinase of Maia squinado has the same specificity and properties as the vertebrate trypsin. In the midgut gland of Helix a cathepsin is present. Further investigations must verify the distribution of these enzymes for other groups. In the vertebrates the action of these enzymes is preceded by that of pepsin (stomach), working in a strongly acid medium. This pepsin is nowhere found in invertebrates. The nature and distribution of the proteases seems to be the same for all the classes of vertebrates. It is doubtful whether the enterokinases of the vertebrates are entirely identical. The amylases of vertebrates are, as far as is known, α‐amylases. They need activation by salts (chiefly sodium chloride). The amylases of invertebrates resemble those of vertebrates. An inulase is not found in vertebrates, but is present in some invertebrates. Cellulase and hemicellulase (it is doubtful whether they are identical or not) are also lacking in the vertebrates, but are present in some invertebrates. The properties of these enzymes have been fully treated. The disaccharases of vertebrates are saccharase, maltase and lactase. Of these, maltase is the most common in the invertebrates, occurring together with amylase (though its separation has not yet been carried out). Some invertebrates, especially Helix pomatia , are able to hydrolyse many more sugars (also tri‐ and tetrasaccharides) than the vertebrates can do. These disaccharases are still awaiting further investigation. The fat‐ and ester‐splitting digestive enzymes of invertebrates have more the character of esterases than of lipases (or an intermediate character), whereas in the vertebrates the reverse is the case.2 The differences in localization of the vertebrate and invertebrate digestive enzymes have been discussed.It has been pointed out that in the vertebrates the digestive enzymes occur in chains and attack the food successively, whereas in invertebrates almost all the enzymes meet in the place where the digestion is performed and attack the food simultaneously. Through this localization the action of the pepsin of the vertebrates becomes possible, for which a medium with strongly acid reaction is required, and moreover the organism is protected against inundation with cleavage products. Only in fishes (and probably amphibia) among the vertebrates is no pronounced chain of carbohydrases present, but amylase and maltase are chiefly found together in the pancreas.