METABOLISM OF PROTEINS 59* 



excreted, it is said, is therefore due, not to inability on the part of 

 the remaining tissues to form uric acid, but to the absence of the 

 ammonia which they require for its formation. This criticism, if 

 it were admitted as against the current interpretation of such ob- 

 servations on the bird's liver, could scarcely be denied some validity 

 as against the current interpretation of similar observations on the 

 results of interference with the mammalian liver. It is therefore 

 important to point out that there is still the same deficiency of uric 

 acid when alkali is administered to neutralize the acids, although 

 ammonia ought now to be available. There can be no question, 

 then, that the liver in birds is the seat of an extensive synthesis of 

 uric acid, and there is little doubt that ammonia compounds are 

 essentially concerned in the process, whatever the role of the lactic 

 or other acids may be. A similar synthetic formation of uric acid 

 from ammonia and a derivative of lactic acid may take place in 

 mammals, and probably exclusively in the liver, but it is of much 

 less importance. Another way in which uric acid arises both in 

 mammals and in birds is by the splitting and oxidation of nucleins. 

 This is by far the most important mode of formation in mammals, 

 as synthesis is the chief mode of formation in birds. In both groups 

 of animals the oxidative production of uric acid takes place, not in 

 any particular organ, but in the tissues in general, including the liver. 

 It has been shown that when air is blown through a mixture of 

 splenic pulp and blood, uric acid is formed from purin bodies already 

 present in the spleen. When the quantity of these is increased by 

 the decomposition of nucleins induced by slight putrefaction, the 

 yield of uric acid is also increased. Uric acid is also formed by the 

 perfectly fresh surviving spleen, liver, and thymus in the presence 

 of oxygen, and the quantity is increased when purin bodies are 

 artificially added. 



Sources of the Uric Acid. It is well established that in the bird 

 it arises both from amino-acids derived from the hydrolysis of 

 protein and from nuclein compounds and their derivatives in the 

 food and tissues. The amino-acids constitute by far the greatest 

 source of uric acid in these animals and in the reptiles, and it is 

 practically certain that the course of the decomposition of the 

 amino-acids and the form in which nitrogen is liberated from them 

 in its transformation into this end-product are not essentially differ- 

 ent from what obtains in the formation of urea in the mammal and 

 the amphibian. This is sufficiently illustrated by the rdle played by 

 ammonia and ammonia compounds in the production of uric acid 

 in the birds and their congeners. In the mammal, the taking of 

 food rich in nucleated cells, and therefore in nucleo-proteins and 

 nucleins, the characteristic conjugated proteins of nuclei (thymus 

 gland, pig's pancreas, and herring roe), or of food rich in purin 

 bases (Liebig's meat extract), increases the quantity of uric acid in 



