io THE PHYSIOLOGY OF PROTEIN METABOLISM 



The presence of these amino acids, however, did not of necessity exclude 

 the possibility of a subsequent synthesis the condition might be ana- 

 logous to the splitting of fats into fatty acids and glycerol which pre- 

 cedes the fat synthesis in the intestinal mucous membrane. Cohnheim 

 (95) attempted to isolate the protein which he believed was synthetized 

 in the intestinal wall. He, however, found that an increase of protein, 

 i.e. a regeneration, never took place, but that the peptone was invari- 

 ably broken down to simpler decomposition products ; in other words 

 that the characteristic peptone reaction disappeared not because pro- 

 tein had been synthetized but because crystalline decomposition pro- 

 ducts were formed. He further found and isolated the ferment which 

 brought about this decomposition, and to it he gave the name erepsin. 

 This erepsin, on further investigation, was found to be capable of 

 breaking down the proteoses and peptones to their constituent amino 

 acids he was able to isolate leucine, tyrosine, lysine, histidine and 

 arginine but it could not attack native proteins, with the exception 

 of caseinogen and fibrin. Kutscher and Seemann (236) on investigat- 

 ing the fate of protein in a dog with a fistula in the middle of its small 

 intestine found that the protein flesh was reduced to amino acids 

 of which leucine, tyrosine, lysine and arginine could be isolated, but 

 neither proteose nor peptone was detected. This observation, that 

 leucine and tyrosine could be isolated from the normal intestinal con- 

 tents, was by no means new, as Kolliker and Miiller as long ago as 

 1856 had discovered leucine and tyrosine in the intestinal contents, 

 although of course they were unable to assign an explanation to their 

 presence. They concluded that they were either absorbed or broken 

 down further as they were unable to find them in the faeces. 



Kiihne (234) also detected leucine and tyrosine in the material 

 collected from an isolated loop of intestine, and he rightly de- 

 scribed them as being products arising from the breakdown of pro- 

 tein, but thought that they were rather by-products than normal 

 digestion products on the way to absorption. Schmidt-Mulheim (357), 

 who repeated Kiihne' s work, came, however, to the conclusion that 

 although such a breakdown took place it was quite unimportant 

 Sheridan Lea (371) came to similar conclusions as Kolliker and Miiller. 

 Macfadyen, Nencki and Sieber (272), who investigated the case of a 

 woman with a fistula at the lower end of the small intestine, found 

 that the intestinal contents contained soluble proteins and peptones, 

 but no leucine or tyrosine. 



Salkowski and Leube (352), on the other hand, put forward the 

 suggestion that the leucine might be considered as a product which, 



