138 NITROGEN METABOLISM 



B12, but not PAB or folic acid, enhanced the utiHzation of 

 AIC by mutants of Esch. coli exacting towards purines [6]. 

 These resuhs are in accord with previous observations that 

 the methyl groups of methionine serve as a source of one 

 carbon units in intermediary metabolism and that they are 

 related to formate, a substance known to be a precursor of 

 purine carbon in position 2. Furthermore, Bj^g bas already 

 been implicated in the metabolism of labile methyl groups 

 in mammals, and it is to be noted that this growth factor 

 had no effect on the utilization of the formyl derivative of 

 AIC, 4-formamino-imidazole-5-carboxamide, by the Esch. 

 coli mutants. Incubation of AIC with Esch. coli resulted in 

 the formation of a substance, possibly a pentoside, which 

 was five times more effective than AIC itself in supporting 

 the growth of the purine auxotrophs [5]. Though the routes 

 by which the various purine bases are synthesized may be 

 quite distinct, it is most likely that they have several steps 

 in common, and it is possible that one is formed directly 

 from another. The interconvertibility of the purines shows 

 species variation; thus whilst guanine and adenine are freely 

 interconvertible in Esch. coli [27] and Lh. casei, yeast is only 

 able to convert adenine to guanine. On the other hand, 

 Tetrahymena gelei, a protozoon exacting towards guanine, 

 can change guanine into adenine [26]. 



CO NH COOH CO 



NH2 C \ CH NH CH 



II CH II I II 



C y C.COOH CO C.COOH 



/ \ / / \ / 



NH2 N NH2 NH 



Amino-imidazolecarboxamide Anmiofumaric acid Orotic acid 



Synthesis of pyrimidines 



Although the pyrimidine ring system is also part of that 

 present in purines, it is apparent that the synthesis of these 

 two groups of nitrogenous bases proceeds by entirely inde- 

 pendent routes. For example, in experiments with yeast, 

 carbon from formate or glycine was incorporated into 



