1050 ABSORPTION 



while, we must conclude that a portion of the nitrogen of the food 

 passes over almost immediately. Consequently, urea may well be 

 employed as an index of the amount of protein absorbed-. Even the 

 intravenous injection of solutions of the ammo-acids into normal 

 animals does not result in their retention, as much as 90 per cent, of 

 the original amount disappearing from the blood in the course of 5 

 minutes after the injection. These facts, as well as others that might 

 still be mentioned, show conclusively that the amino-acids do not tarry 

 in the tissues, but are rapidly excreted so that their destruction prac- 

 tically equals their absorption. This need not surprise us, because it 

 has been shown previously that the tissue proteins are equilibrated in a 

 more exact manner than the fat and carbohydrates, and that a definite 

 relationship must, therefore, be retained between the amino-acids of 

 the blood and those of the tissues. But while the power of protein- 

 storage of the tissues is extremely limited, it seems that the liver is 

 much more elastic in this regard and is capable of assimilating 125 

 to 150 mgr. per 100 gr. of the original amount. Its power of absorbing 

 this material is also evinced by the fact that the concentration of the 

 amino-acids is less in the blood leaving this organ than in that enter- 

 ing -it. 1 



The deduction to be made from these data is that the liver utilizes 

 the amino-acids in the formation of urea. But since this body may 

 also be produced after the removal of the liver, this organ cannot be 

 said to be the only place in which urea is formed, although it is safe to 

 conclude that it is its chief source. Moreover, since it is the endeavor 

 of the system to remain in nitrogen-equilibrium, it is the function of 

 the liver to prevent any flooding of the tissues with amino-acids. 

 Consequently, this organ begins its function of forming urea almost 

 immediately after these substances have begun to be absorbed. 

 Besides, the liver also takes care of the protein waste, discharged in 

 consequence of the catabolism of the different tissues. 



The End-products of Protein Metabolism. Obviously, the tissue- 

 proteins are first split up into the amino-acids from which they were first 

 synthetized, and supposedly no decisive chemical difference exists 

 between these catabolic products and the amino-acids absorbed. We 

 know that the tissues possess this power of reducing their protein 

 material, because they are in possession of proteolytic enzymes (pro- 

 teases) which may be isolated from them in different ways. Thus, it 

 is a matter of common experience that pieces of tissues, when kept 

 under proper condition of temperature and moisture, undergo autolytic 

 changes which yield ammonia, glycine, tyrosine, tryptolane and other 

 basic substances. A similar process of autolysis occurs in malignant 

 tumors, and such a condition as cystinuria merely indicates that the 

 cystin is not taken care of by the body, owing to a derangement of 

 the metabolism of the amino-acids. It enters the urine, frequently 



1 Mendel, Ergebn. der Physiol., 1911. 



