THE Ml TABOLIS \l OP PROI 61 1 



acids. During digestion of protein the liver does not .- : ] » j » • - -■ 1 1 - to wait 

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

 ins to destroy the unnecessary & by conversion into urea; on 



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

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

 harmony with tin' well established fact that, when protein is given ' 

 starving animal, the greater pari of its nitrogen is soon excreted 

 urea, Leaving only a small fraction to be used fur rebuilding tl rted 



tissues i see page 6 13 , 



The amino acids thai are absorbed by the extrahepatic tissues become 

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



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

 during absorption of protein is not perceptibly greater than in tl 

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

 comes of tin 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 tiss i< - 

 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. Ii goes on, for example, during starvation; indeed, in 

 this condition the percentage of free amino acids in the museles is. if 

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

 imal. In starvation also the migration of amino acid Lg on am 

 the various o rga us of which those whose activity jsential 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 sei page 568 . Thes :periments further show that 

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



in the same v. ,-i\ as glycogen and fat. If amino acids were of valui 



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

 by starvation. As to how long a period of time elaps 

 incorporation of tl "bed amino acids into tissue protein and tl 



subsequent liberation again by autolysis, we arc entirely ignorant. 



Tl arches which we have just been considering do not throw any 



light on the relativi valu< of different proteins in 



They do not inform us as to which of the amino acids n 



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



with since the organism can manufacture them V\ mow ' 



the higher animals can synthe8l 'lie amino oil. 



but not others, such as tryptophane; but winch amino acids be' 



