33 THE AMINO ACIDS 



stomach digestion which is at most a matter of hours. 

 They may be explained in several ways, as for instance 

 in those cases where extracts of the stomach were 

 employed amino acids may arise from autolytic proc- 

 esses, or perhaps in all cases from the action of hydro- 

 chloric acid alone. Protein is a labile molecule which 

 apparently needs slight inducement to start on the 

 downward path to its demolition into amino acids. 

 The formation of amino acids in gastric digestion 

 under normal conditions seems hardly probable. 

 Against such an idea may be set the fact that pepsin- 

 hydrochloric acid is utterly incapable of breaking 

 down artificial polypeptides thus far tested. On the 

 other hand, they are readily split by pancreatic juice. 

 It would seem therefore that in peptic digestion 

 neither amino acids nor relatively simple polypeptides 

 are normally found in significant amounts. 



Gastric digestion, however, has the very important 

 function of preparing protein for the later action of 

 trypsin and the intestinal juices. Fischer and Abder- 

 halden have shown that tryptic digestion is much more 

 rapid and complete when protein has been previously 

 acted upon by pepsin-hydrochloric acid. If casein is 

 first digested with an artificial gastric juice and then 

 subjected to the influence of trypsin amino acids like 

 proline and phenylalanine could be isolated. Treated 

 with trypsin alone casein failed to yield the free amino 

 acids ; instead a corresponding polypeptide was present. 



One may conclude therefore that although gastric 

 digestion fails to yield amino acids directly it aids in 



