8 THE PHYSIOLOGY OF PROTEIN METABOLISM 



Intestinal Digestion. 



The mam digestion of protein takes place in the small intestine, 

 most actively at the upper end by means of the trypsin of the pan- 

 creatic juice and probably also by the erepsin of the intestinal wall. 

 Trypsin acts on all forms of protein, which have been passed on from 

 the stomach, and reduces them to simpler products. The question as 

 to the extent of this splitting has been much discussed. Formerly it 

 was believed that proteoses and peptone were the end products, but 

 it is now generally hejd that the digestion proceeds to the formation 

 of abiuret products in the form of polypeptides and the comparatively 

 simple monoamino and diamino acids. (Abderhalden, Baumann, and 

 London (42), Kutscher and Seemann (236), Cohnheim (94).) The 

 difficulty has been to prove that the digestion proceeds to this extent, 

 as (i) the disintegration of protein does not take place suddenly, in an 

 explosive fashion, but proceeds gradually more like erosion, and (2) 

 along with this slow decomposition there is a steady absorption of the 

 simple products as they are formed. By the utilization of the poly- 

 fistular method, which has been elaborated by London (261, 262, 263), 

 evidence of the thoroughness of the decomposition has been obtained. 

 Abderhalden, London and Oppler (45), for example, traced the appear- 

 ance of tyrosine and glutamic acid after feeding with gliadin. They 

 found in the duodenum 075 grm. of tyrosine and 2*5 grm. of glutamic 

 acid, in the jejunum ri grm. of tyrosine and 20-9 grm. of glutamic acid, 

 and in the ileum a mere trace of tyrosine and 33 grm. of glutamic acid. 

 Very similar results were obtained, by the same method, after feeding 

 with caseinogen and with meat. This work was repeated and confirmed 

 later by Abderhalden, London and Reemlin (47). Not only then has it 

 been proved that the digestion is gradual, but further, that the rate of 

 digestion is greater than that which takes place in vitro. Abderhalden 

 and Gigon (22) have demonstrated that the digestive ferments can 

 combine with the amino acids formed in the course of digestion and 

 in this way become inactivated. This inactivation is liable to occur 

 during in vitro experiments, where the amino acids accumulate in the 

 digestive fluid, but in the case of the intestine absorption is constantly 

 taking place, and thus there is but little time for combination to occur 

 with its resultant slowing of the rate of digestion. Abderhalden and 

 Rona (13) have further shown that there is another difference be- 

 tween the actions of artificial juices, both gastric and pancreatic, and 

 the real juices in in vitro experiments. They found that pepsin powder 



