PROTEIN METABOLISM 79 



albumoses and peptones, as Cohnheim 1 has shown, can be 

 resolved into simpler bodies — the amino acids, by the action 

 of a ferment (erepsin) discovered by him in the intestinal 

 mucous membrane. This ferment, it may be stated, does not 

 attack native proteins with the exception of casein. 



The important question which is now awaiting solution is 

 whether it is necessary that the protein be so far decomposed 

 before it can be utilised by the body tissues for transformation 

 or resynthesis into the complex body proteins. Many observa- 

 tions on this point have been carried out by investigating 

 the contents of the intestinal canal of animals killed a few 

 hours after a full meat meal. This method, however, has the 

 serious drawback that both the decomposition of the protein 

 and the absorption of the products go constantly on together, 

 thus no time is allowed for any accumulation of the products 

 in the gut. Further, it is quite possible that some of the 

 products formed may be more readily absorbed than others, 

 and in this way the material which is eventually obtained 

 from the intestine may not give a true indication of the real 

 nature of the decomposition. Kutcher and Seemann 2 have 

 carried out such experiments, and have demonstrated in the 

 intestinal contents the presence of quite a number of the well- 

 known decomposition products. They were able to isolate 

 leucin, tyrosin, lysin, and arginin, thus both mono- and di- 

 amino acids. Abderhalden 3 has also investigated this question 

 along the same lines. He was able to isolate from the intestinal 

 contents by Fischer's ester method alanin, leucin, glutamic and 

 aspartic acids, and perhaps traces of a-proline and phenyl- 

 alanin. Tyrosin was also found. Some polypeptide was also 

 present, and yielded on hydrolysis with acid glycocoll, a-proline, 

 and phenylalanin. 



A very important point was made a few years ago when 

 O. Loewi 4 proved that animals which were fed on the decom- 

 position products of proteins, obtained by prolonged digestion 

 with trypsin until complete disappearance of the biuret reaction, 

 could be kept alive. Not only did he keep the animals alive 

 with these products as the sole source of their supply of 



1 Cohnheim, Zeit.f. physiol. Chem. 33, 1901, 451. 



2 Kutscher and Seemann, Zeit.f . physiol. Chem. 34, 1902, 528. 



3 Abderhalden, Zeit. f. physiol. Chem. 44, 1905, 617. 



4 Loewi, Schmiedeberg's Archiv, 48, 1902, 303. 



