288 PETER REICHARD 



homogenates,** and into adenine and guanine from nucleic acids of yeast. ^^ 

 Williams and Buchanan^" furthermore isolated a soluble enzyme system 

 from yeast which incorporated the carboxamide-C" into inosinic acid. 



The importance of the carboxamide or a derivative of it for the biogene- 

 sis of the purines has thus been well established. The problem of ring- 

 closure to form the purine ring (Fig. 4) has received attention by Schul- 

 man and Buchanan. ^^ It was found that in pigeon liver extracts formate 

 and carboxamide reacted mole for mole to form hypoxan thine. Shive 

 et al.^^ had postulated that p-aminobenzoic acid or some compound formed 

 from it functions as a coenzjrme during ring-closure. Later Rogers and 

 Shive®^ obtained evidence that folic acid might be this coenzyme. The re- 

 sults of Buchanan and Schulman,^^ which will be discussed in more detail 

 below, demonstrated the participation of the citrovorum factor in the 

 enzymic exchange of formate and the carbon in position 2 of the purine 

 moiety of inosinic acid. It is thus probable that the citrovorum factor is 

 closely related to the transformation of the "1-C" derivative to position 2 

 of the purine ring. 



Bergmann et al.^* studied the effect of compounds containing "labile 

 methyl" groups on the accumulation of the carboxamide by sulfadiazine- 

 inhibited E. coli. Of all the methyl compounds tested only DL-methionine 

 in the presence of small amounts of p-aminobenzoic acid definitely sup- 

 pressed the accumulation of the carboxamide. Neither formate nor form- 

 aldehyde shared this ability. The identification by Cantoni^* of *S-adeno- 

 sylmethionine as an active transmethylating agent might fit these results. 

 Recently Peabody^^ has described the formation in pigeon liver extracts of 

 a nonvolatile formate derivative which was not further described. This 

 compound could be used for the ring-closure to form hypoxanthine. 



The problem of whether or not the carboxamide per se is an intermediate 

 in hypoxanthine synthesis has been investigated by Greenberg^^ and by 

 Schulman and Buchanan. ^^ Both investigators incubated a labeled pre- 

 cursor (formate-C^^ and glycine-1-C^* respectively) together with an excess 

 of nonlabeled carboxamide in a pigeon liver homogenate. In neither case 

 did the carboxamide contain any activity at the end of the experiment, 



68 M. P. Schulman, J. M. Buchanan, and C. S. Miller, Federation Proc. 9, 225 (1950). 



" W. J. Williams, Federation Proc. 10, 270 (1951). 



" W. J. Williams and J. M. Buchanan, J. Biol. Cheni. 202, 253 (1953). 



«' M. P. Schulman and J. M. Buchanan, /. Biol. Chem. 196, 513 (1952). 



«2 L. L. Rogers and W. Shive, /. Biol. Chem. 172, 751 (1948). 



" J. M. Buchanan and M. P. Schulman, J. Biol. Chem. 202, 241 (1953). 



" E. D. Bergmann, B. E. Volcani, and R. Ben-Ishai, J. Biol. Chem. 194, 521 (1952). 



« G. L. Cantoni, J. Am. Chem. Soc. 74, 2942 (1952). 



66 R. A. Peabody, Federation Proc. 12, 254 (1953). 



" G. R. Greenberg, J. Biol. Chem. 190, 611 (1951). 



