FERMENTATIONS OF NITROGENOUS COMPOUNDS 



83 



cleavage of the pyrimidine ring between positions 1 and 6 

 and results in the formation of 4-ureido-5-imidazolecar- 

 boxylic acid 63 (see Fig. 6) . The accumulation of this com- 



NH— CO 



CO C— N 



NH 2 COOH 



H 2 



\ 



CH 



NH— C— NH 



Xanthine 



COOH 



H 2 

 -NH,, -C0 2 



C— N 



\ 



CO C— N 



)CH 

 NH— C— NH 



4-Ureido-S -imidazolecarboxylic acid 



CH— N 



Sd H 2 Q 

 -NH 3 



CH 

 / 

 H 2 N— C— NH 



4-Amino-5 -imidazolecarboxylic acid 



[X] 



H 2 



H 2 N— C NH 



4-Aminoimidazole 



CH 2 — NH 



V CH 



C HN 



/\ 



O OH 



Formiminoglycine 

 Fig. 6. Conversion of Xanthine to Formiminoglycine. 



pound in crude cell-free extracts is made possible by the 

 addition of sequestering agents such as ethylenediamine- 

 tetraacetic acid which remove the Fe++ or Mn++ ions 

 required for its further decomposition. The next enzymatic 

 step is the cleavage of the ureido group to yield carbon 

 dioxide, ammonia, and 4-amino-5-imidazolecarboxyl acid. 64 

 The latter compound accumulates in an alkaline reaction 

 mixture. The mechanism of this reaction has not been ex- 

 amined in detail, but since orthophosphate is not required, 

 the reaction appears to be hydrolytic. The aminocarboxy- 



