a>-AMINO-AClDS 37 



[1898] found it once more in dog's urine. In both cases the substance 

 was constantly present ; no other case of its occurrence in urine has 

 been observed and it would appear that the two dogs presented a rare 

 anomaly of metabolism. Recently Hunter [1912], although unable to 

 find a dog secreting urocanic acid, obtained the same substance by 

 prolonged tryptic digestion of caseinogen and was able to identify it 

 by comparison with a specimen of iminazolylacrylic acid which Barger 

 and Ewins [1911] had obtained as a degradation product of ergo- 

 thioneine and had also synthesised. 



Among closely related substances from human urine we may men- 

 tion histidine itself, a base yielding a picrolonate C 5 H 7 O 2 N 3 , C 10 H 8 O 5 N 4 

 melting at 244, and a base giving an aurichloride C 15 H 36 O 13 N 8 , HAuCl 4 

 very soluble in water and blackening at 100. These bases were ob- 

 tained by Engeland [1908, 3] who regards the second as amino-imin- 

 azolylacetic acid, a lower homologue of histidine, and the third as 

 probably & polypeptide of histidine. According to Engeland histidine 

 is broken down more readily by carnivora than by herbivora ; the urine 

 of rabbits and horses gives a stronger reaction with p-diazobenzene 

 sulphonic acid than that of the cat or dog. 



OH 



Kynurenic Acid, 



v 



N 



Long ago Liebig [1853] discovered an acid which occasionally 

 separated from dog's urine in minute quantity. The substance was 

 further investigated by Schmiedeberg and Schultzen [1872] and by 

 Kretschy [1881-84] who showed that the product formed by heating 

 the acid above its melting point, the so-called kynurine, C 9 H 7 ON, was an 

 oxyquinoline, and that kynurenic acid was therefore an oxyquinoline 

 carboxylic acid. Heated with zinc dust kynurine was reduced to 

 quinoline, and on oxidation of kynurenic Kretschy obtained oxalyl- 

 anthranilic acid, 



COOH 



\NH . CO . COOH. 



Hence, when Wenzel [1894] had shown by synthesis that kynurine 

 is 4-hydroxy-quinoline, kynurenic acid was found to be either 4-hydroxy- 

 3 -quinoline carboxylic acid, or 4-hydroxy-2-quinoline carboxylic acid. 



OH OH 



^\/\ COOH or ^\/\ 



I II I I I! I 



I /"*/-\/"\TT 



>x /\ A L/UUri. 



N N 



