35. BIOSYNTHESIS OF PURINE NUCLEOTIDES 



305 



ASPARTIC ACIDH> HN 

 FORMATE ->HC, < 



FORMATE 



CH„OPOH„ 



6LUTAMINE 

 ( AMIDE N 



RIBOSE-5-PHOSPHATE 

 Fig. 1. Precursors of inosinic acid. 



sors indicated in Fig. 1. Glycine 13 ' w plays the central role in the synthesis, 

 the carboxyl and a carbon atoms and the nitrogen atom contributing to 

 carbon atoms 4 and 5 and nitrogen atom 7 of the purine ring. C0 2 is the 

 precursor of carbon atom 6, and formate of carbon atoms 2 and 8. It was 

 more difficult, however, to determine the precursors of the three remaining 

 nitrogen atoms by in vivo experiments since the rapid metabolism of some 

 of the common nitrogenous compounds made it difficult to distinguish 

 between their reactions and those of ammonium salts. With the discovery 

 by Greenberg 15 that synthesis of the purine compound, hypoxanthine, could 

 be accomplished from radioactive precursors in homogenates of pigeon liver 

 it was possible to continue studies on the remaining nitrogenous precursors. 

 When soluble extracts 16 - 17 were used as the source of enzyme, the side 

 reactions of the amino acids were substantially reduced and ammonium 

 salts were not a major source of purine nitrogen. 



In pigeon liver extracts 18 the labeled substrates glycine, formate, and C0 2 

 were utilized for purine synthesis in the ratio of 1:2:1. In double-labeling 

 experiments two moles of amide N 15 of glutamine and one mole of N 15 - 

 labeled nitrogen of aspartic (or glutamic) acid were utilized for every mole 

 of C 14 -labeled glycine. 19 Upon degrading the labeled purine formed it was 

 found that N-3 and N-9 were derived from the amide nitrogen of gluta- 

 mine and N-l was derived from aspartic acid. 20 Although glutamic acid 



13 J. L. Karlsson and H. A. Barker, J. Biol. Chem. 177, 597 (1949). 



14 D. Shemin and D. Rittenberg, J. Biol. Chem. 167, 875 (1947). 



15 G. R. Greenberg, Arch. Biochem. 19, 337 (1948). 



16 G. R. Greenberg, Federation Proc. 10, 192 (1951). 



17 M. P. Schulman and J. M. Buchanan, Federation Proc. 10, 244 (1951). 



18 M. P. Schulman, J. C. Sonne, and J. M. Buchanan, J. Biol. Chem. 196, 499 (1952). 



19 J. C. Sonne, I. Lin, and J. M. Buchanan, J. Biol. Chem. 220, 369 (1956). 



20 B. Levenberg, S. C. Hartman, and J. M. Buchanan, /. Biol. Chem. 220, 379 (1956). 



