38 THE SIMPLER NATURAL BASES 



Camps [1901, I, 2] prepared both acids and wrongly concluded that 

 the former was identical with the acid from dog's urine, but Miss 

 Homer [1913] has shown, by the mixed melting point, that kynurenic 

 acid has the latter constitution. 



Liebig [1853], Kretschy [1881] and others had already found 

 that kynurenic acid only makes its appearance, or is most abundant, 

 in the urine of dogs fed on large quantities of meat. Many fruitless 

 investigations were undertaken to find the precursor of the acid, until 

 finally its formation was shown to depend on a product of tryptic 

 digestion of protein (Glaessner and Langstein [1902]). This Ellinger 

 [1904, I, 2] identified as tryptophane (see Plimmer's " Chemical 

 Constitution of the Proteins," Part I, p. 137). Abderhalden, London, 

 and Pincussohn [1909] have shown that the transformation of trypto- 

 phane into kynurenic acid does not take place in the liver. 



Kynurenic acid, taken by the mouth, is not excreted in the urine 

 in man and in the rabbit (Hauser [1895], Solonin [1897]); the 

 reason is probably that the acid is an intermediate product of metabol- 

 ism which is not destroyed so rapidly in the dog as in man. 



