H. A. BARKER 



hypoxanthine derivative. If a similar but reverse process occurs 

 in the fermentation of xanthine, an initial attack on the bonds 

 between the 1,2 or 2,3 positions in the purine would be required. 

 Actually all available evidence is against an initial splitting of 

 these bonds. For example, 4-amino-5-carboxamidoimidazole 

 is not attacked nor does it accumulate under conditions favorable 



HN-CO HN-CO HgN COOH 



0^ C-^^-CO -^iU °9 ^^^CH H,0 OC C-N<. 



HN-C-NH^ HN-C-NH' *- HN-C-NH" 



URIC ACID XANTHINE 4-URE1D0-5-CARB0XY- 



IMIDAZOLE 



-NH3 

 -CO2 



H N-CH '^ COOH 



2NH3+ I +HCOOH^^ CH-N^ ;^ f'^^CH 



COOH H^N-C-NH' H2N-C-NH' 



_^ 



4-AMlNO-IMIDA- 4- AMI N0-5-CARB0XY- 

 ZOLE IMIDAZOLE 



r 



CH,0H CH, CH, ^^ 



I 2 -NH, I 3 .2H I 3 +C0- 

 CHNH2 ^ CO 1- COOH 2 



COOH COOH 



Fig. 1. Fermentation of uric acid. The solid arrows indicate known 

 reactions, the dotted arrows postulated reactions. 



for xanthine decomposition. Moreover, in the presence of suit- 

 able inhibitors the major product of xanthine decomposition by 

 extracts has been identified as 4-ureido-5-carboxyimidazole 

 (28) which is formed by a rupture of the bond between the 1 and 

 6 positions. The ureido group of this compound is next attacked 

 enzymatically with the formation of ammonia, carbon dioxide, 

 and 4-amino-5-carboxyimidazole (27). The latter is decarbox- 

 ylated and then the imidazole ring of the resulting 4-amino 

 derivative is cleaved to give glycine, ammonia, and formate in 

 the cell-free system. This is undoubtedly a multistep process, 

 the details of which have not yet been worked out. 



84 



