THE METABOLISM OF PROTEIN 611 



acids. During digestion of protein the liver does not appear to wait 

 until the other tissues have become saturated with amino acids before it 

 begins to destroy the unnecessary excess by conversion into urea; on 

 the contrary, this process sets in with the very first installment of amino 

 acid that reaches the liver by the portal blood. This conclusion is in 

 harmony with the well-established fact that, when protein is given to a 

 starving animal, the greater part of its nitrogen is soon excreted as 

 urea, leaving only a small fraction to be used for rebuilding the wasted 

 tissues (see page 643). 



The amino acids that are absorbed by the extrahepatic tissues become 

 very quickly converted into formed protein, as is evident from the fact 

 that the concentration of free amino acids in the tissues of an animal 

 during absorption of protein is not perceptibly greater than in those of 

 a fasting animal, and the question remains to be considered, What be- 

 comes of the protein thus formed? The answer is, that it is gradually 

 used up in the metabolic processes, so as to liberate again the amino 

 acids, which add themselves to those absorbed from the intestine and be- 

 come used again or excreted, according to the demands of the tissues at 

 the time for amino acid. 



This process of liberation of amino acid from the breakdown of body 

 protein goes on of course irrespective of absorption of amino acid from 

 the intestine. It goes on, for example, during starvation; indeed, in 

 this condition the percentage of free amino acids in the muscles is, if 

 anything, somewhat higher than that observed in an ordinarily fed an- 

 imal. In starvation also the migration of amino acid is going on among 

 the various organs, of which those whose activity is essential to the 

 maintenance of life, such as the heart and the respiratory muscles, are 

 supplied with amino acids from tissues that are less vital, such as the 

 skeletal muscles (see page 568). These experiments further show that 

 free amino acids can not serve to any significant extent as food reserves 

 in the same way as glycogen and fat. If amino acids were of value as 

 food reserves, we should expect the store of them to be depleted 

 by starvation. As to how long a period of time elapses between the 

 incorporation of the absorbed amino acids into tissue protein and their 

 subsequent liberation again by autolysis, we are entirely ignorant. 



The researches which we have just been considering do not throw any 

 light on the relative value of different proteins in tissue metabolism. 

 They do not inform us as to which of the amino acids must be absorbed 

 ready-made from the digested food, and which of them may be dispensed 

 with since the organism can manufacture them for itself. We know that 

 the higher animals can synthesize some amino acids, such as glycocoll, 

 but not others, such as tryptophane; but which amino acids belong to 



