238 DIGESTION AND ASSIMILATION OF THE FOODSTUFFS 



THE DIGESTION OF THE PROTEINS. 



Until a very few years ago it was generally held that the proteins 

 were absorbed from the stomach and intestine in the form of Peptones, 

 and, indeed, prior to the first years of the present century, it was be- 

 lieved in many quarters that not merely peptones, but even unaltered 

 protein or at least infraproteins resulting from the very earliest cleav- 

 age of the native protein molecule might be absorbed without further 

 hydrolysis. 



This view of protein-absorption stood in rather striking contrast 

 to our knowledge of the absorption of other foodstuffs which were at a 

 comparatively early date known to undergo complete or nearly com- 

 plete hydrolysis prior to their absorption. Moreover, the elaborate 

 machinery of enzymes for the splitting of proteins not only to peptones, 

 but to amino-acids which is provided by the digestive organs would 

 appear, if the older view were correct, to exist without any necessary 

 purpose or function. Considering these facts it may appear strange 

 that so exceptional a view of protein-absorption should ever have gained 

 general acceptance; but, as usual in the historical development of 

 science, a misinterpreted experiment furnished the foundation for an 

 extensive edifice of erroneous hypothesis. 



The observation which led us astray was the outcome of an experi- 

 ment by Voit, who, in 1869, showed that undigested proteins, unmixed 

 with gastric or pancreatic juice, rapidly disappear when they are 

 introduced into a ligated loop of small intestine, while a little later it 

 was further found by Hofmeister that proteoses and peptones similarly 

 disappear when introduced into an isolated loop of intestine. The 

 latter of these observations received its correct interpretation when 

 Cohnheim, in 1901, showed that the Succus Entericus which is secreted 

 by the mucous membrane of the small intestine, contains an enzyme, 

 Erepsin, which hydrolyzes proteoses and peptones to amino-acids, 

 leaving, however, native proteins with the exceptions of casein and the 

 protamines, unattacked. The disappearance of peptone from an 

 intestinal loop is therefore accounted for by its hydrolysis by erepsin 

 into amino-acids. The disappearance of native proteins such as egg- 

 albumin from isolated loops of intestine is, however, a more difficult 

 matter to interpret, and it cannot yet be said to have been completely 

 elucidated. It is, however, certain that under normal conditions 

 unaltered proteins never reach the circulation by absorption from the 

 intestine for the following reasons: 



In the first place, when native proteins are injected into the circu- 

 lation, a proportion of the protein thus introduced appears in the urine. 

 Evidently it is treated as a foreign constituent of the blood and dis- 

 charged, in so far as that is possible, by the kidneys. Another portion 

 of the protein is discharged by the kidneys in a non-coagulable form 

 which is still, nevertheless, a protein. At the same time there is a 

 marked increase of proteose-like substances in the blood and some 

 increase of the urea-output. 



