610 METABOLISM 



the two experiments taken together, indicate that the amino acids after 

 their absorption can not remain in the tissues in a free condition for a 

 long time. It means that the amino acids during natural digestion must 

 ~be disposed of at a rate which is practically the same as that at wh'ich ab- 

 sorption is proceeding. 



THE FATE OF THE AMINO ACIDS 



To follow the metabolism of the amino acids further we must deter- 

 mine the end product into which they are converted. This is urea, 

 whose estimation can nowadays be made with considerable accuracy on 

 account of the discovery, by Marshall, of the action of urease in con- 

 verting its nitrogen into ammonia, which can then be estimated by com- 

 paratively simple methods (Folin). 



When the viscera are compared before and at various periods after 

 the intravenous injection of amino acids, the immediate increase in 

 amino nitrogen remains undiminished in all of them except the liver, in 

 which a very rapid reduction is observed to occur. At the same time 

 the percentage of urea in the blood steadily rises. These facts are illus- 

 trated in Fig. 187. 



The simplest interpretation of these results is that the liver converts 

 the amino acids into urea and discharges this urea into the blood. This 

 conclusion, however, it must be observed, is not inevitable; for it is pos- 

 sible that the amino acids may be condensed into polypeptides in the 

 liver, just as sugar is condensed by this organ into glycogen, and that 

 the increase in urea is merely coincident (Fiske). 



It must not be imagined that the conversion of the amino acids into 

 urea is exclusively a function of the liver. On the contrary, it is well 

 known that this process may occur in animals from which the liver has 

 been entirely removed. It is probably safe to conclude, however, that 

 the liver is the most active center for amino-acid transformation and 

 urea formation. 



When urea is estimated in samples of blood removed at short inter- 

 vals of time after the ingestion of a large amount of protein, it is found 

 that the increase becomes very early established. In one experiment, 

 before the food was taken the concentration of urea nitrogen in the blood 

 was a little over 10 mg. per cent; one hour after taking 500 grams of 

 meat, it had risen to about 18, and in two hours to nearly 25. Evidently 

 the increase had occurred about the same time as the passage of food 

 from the stomach into the duodenum. These facts indicate that urea 

 formation in the liver becomes stimulated long before the other tissues, 

 such as the muscles, have had time to take up their full quota of amino 



