Gastric Digestion—A Survey *

In the evolution of the digestive system of the mammal, the gastric mechanism appears to be the final and most recent addition to the alimentary armamentarium. It is found in all mammals and nearly all vertebrates. Its reported occurrence in lower animals is rare and scattered, perhaps even open to serious question. This statement is not in accord with much of the older literature. Pepsin has been described by students of general physiology in connection with a good many forms of lower animal and even plant life. Exact control of the H ion was not possible at the time many of these observations were made, and the autolytic mechanism was not always recognized. Extracts of tissues and digestive juices of a number of lower animal forms have been shown to digest fibrin in acid solution. Without further characterization, however, we should hesitate to assume that these enzymes were pepsin. In several instances such enzymes were shown by their discoverers to differ from mammalian pepsin in their resistance to acid, and in other ways. For a review of this early work see Otto von Fiirthe's Vergleichende Chemische Physiologie der niederen Tiere, 1903. More recently Dernby1" 14 describes the enzymes concerned in autolysis as a mixture of pepsin, trypsin, and erepsin. As we have pointed out8, however, such use of the terms trypsin and pepsin can only lead to confusion since the evidence in Dernby's data, and in our own, conclusively shows that mammalian tissue contains neither trypsin nor pepsin of the mammalian type. No autolysis of liver or kidney goes on at the optimum pH for trypsin. None goes on at the optimum pH for pepsin. And the primary protease of the tissue is destroyed at one-tenth the acidity which is optimum for the action of mammalian pepsin. The primary protease of liver or kidney does carry on the primary cleavages of native protein in acid solution, but at an optimum pH of approximately 4.5. It is destroyed at a pH of 2.5. Pepsin from the same animal acts most effectively at a pH of about 1.5 and is resistant to much higher acidities. This means that the optimum for pepsin is about a thousand times more acid, and its acid resistance about a hundred times that of the autolytic protease. An example of the sort of peptic enzyme found in lower forms is reported by Yonge6' in the hepatopancreas of the lamellibranch Mya. "A strong protease is present which acts in an acid medium almost exclusively, being destroyed by alkali even in the presence of fibrin. It might be described as a pepsin were it not for the fact that it reduces globulin to amino acids instead of merely to peptones as is the case with vertebrate pepsin. It also coagulates milk in the presence of CaC12." Yonge does not attempt to characterize more exactly the pH relations of this enzyme, and it is apparently intracellular