PROTEIN AND CIRCULATION 77 



words of Van Slyke and Meyer: "The amino acids, 

 with perhaps some peptides, from the intestine enter 

 the circulation, from which they are immediately 

 absorbed by the tissues. The power to take them up 

 from the blood stream is common to all the tissues, 

 but is limited. The muscles of the dog, for example, 

 reach the saturation point when they contain about 

 75 mgm. of amino acid nitrogen per 100 grams. The 

 liver, however, continually desaturates itself by metab- 

 olizing the amino acids that it has absorbed, and con- 

 sequently maintains indefinitely its power to continue 

 removing them from the circulation so long as they do 

 not enter it faster than the liver can metabolize them. 

 When the entrance is unnaturally rapid, as in our 

 injection experiments, or when the liver is sufficiently 

 degenerated, as observed clinically in some pathological 

 conditions, the kidneys assist in removing the amino 

 acids by excreting them unchanged. Death may result 

 when the above agencies for preventing undue accu- 

 mulation of protein digestion products are over- taxed. 



"In regard to the synthesis of tissue proteins it 

 appears reasonable to believe that, since each tissue 

 has its own store of amino acids, which it can replenish 

 from the blood, it uses these to synthesize its own 

 proteins." 



Concerning the manner in which the free amino 

 acids are utilized by the tissues two possibilities may 

 be assumed, and according to Van Slyke and Meyer 

 these are : 1. The amino acids serve as a reserve energy 

 supply, like glycogen, or as a reserve of tissue-building 



