PROTEIN METABOLISM 79 
albumoses and peptones, as Cohnheim? 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? 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®* 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‘ 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 
? Cohnheim, Zezt. f. physiol. Chem. 33, 1901, 451. 
? Kutscher and Seemann, Zeit. f. physiol. Chem. 34, 1902, 528. 
3 Abderhalden, Zez¢. f physiol. Chem. 44, 1905, 617. 
* Loewi, Schmiedeberg’s Archiv, 48, 1902, 303. 
