102 A. I. RINGER 



no enzyme which has any effect on the protein molecule by causing it to 

 split into smaller compounds. 



In the stomach we find an enzyme, pepsin, which is secreted in an 

 inactive or zymogen state, and which is activated by the hydrochloric acid 

 of the gastric juice. The activation of this en^vme can be accomplished 

 also by organic acids, like oxalic, lactic and tartaric acids, or by inorganic 

 acids like nitric, phosphoric and sulphuric. 



The pepsin in acid solution has the power of splitting the protein mole- 

 cule into simpler or "derived proteins." The longer digestion proceeds the 

 smaller will be the size of the molecules of the "derived proteins" and the 

 further these molecules will get away from the colloidal state and ap- 

 proach the crystalloidal. By means of fractional precipitation with am- 

 monium sulphate or zinc sulphate, various fractions can be recognized, 

 representing different stages in the digestion. These fractions are not 

 definite chemical entities, but mixtures of what are known as meta-pro- 

 teins, coagulated proteins, proteoses and peptones. Under no circumstances 

 and no matter for how long pepsin is allowed to act on protein does its 

 digestion lead to amino acid formation. 



The hydrochloric acid plays an important part in the protein digestion. 

 It causes a swelling of the protein, and a breaking up of the larger 

 particles, converting it into a sort of gelatinous mass. The pepsin is thus 

 enabled to make its way into the interior of the particles with much greater 

 ease. 



The products of protein digestion are passed on into the intestines, 

 where they meet the secretions from the pancreas, liver and intestines. 

 These render the mixture alkaline and thus prepare it for the action of 

 trypsin, which acts only in alkaline mediums, and which is secreted by 

 the pancreas in an inactive state, trypsinogen, and which is activated by 

 the enterokinase of the succus entericus. 



The trypsin acts on the peptic digestive products and also on the native 

 proteins which have entered the intestines. The trypsin carries the di- 

 gestion of the proteins mostly to the peptid stage, i. e., small chain com- 

 pounds of amino acids, and to a considerable extent to the amino acid 

 stage. Ty rosin, leucin, tryptophan and cystin are the amino acids that 

 usually appear first in trypsin digestion. 



When a. protein is completely digested the products fail to give the 

 biuret reaction, and when trypsin acts on protein long enough it carries 

 the digestion to the stage where no biuret reaction is obtainable. E. Fischer 

 and Abderhalden have shown that certain peptids exist which are composed 

 of phenylalanin and prolin, which resist the action of trypsin and can 

 only be broken up by another enzyme which is secreted by the intestinal 

 glands and is known as erepsin. This enzyme has the power of breaking 

 up all peptones into amino acids, 



