BIOSYNTHESIS OF PURINES AND PYRIMIDINES 285 



CO2 



Amino group of -^^ 0=C. 3 j,CvL 9 / 

 glutamic or / ^N'^ " ^N 



aspartic acid / H ^^ H 



"Formate" 



V 



Amide group of 

 glutamine 



Fig. 3. Precursors of the various atoms in the purine ring of uric acid 



labeled ammonium citrate to pigeons or rats the purines from the nucleic 

 acids of the internal organs contained a significant amount of N^^ It was 

 further shown by deamination that the amino group of guanine contained 

 more isotope than did that of adenine in relation to the respective ring 

 nitrogens. This metabolic activity of the amino group in position 2 of the 

 purine nucleus, which was later confirmed by Reichard^^ using glycine- 

 N^^ as precursor in the rat, might evoke speculations as to the possible in- 

 stability of the carbon in position 2 of the purine ring. 



Degradation studies of purines from the nucleic acids of the rat, of pi- 

 geons, and of yeast have shown the same biosynthetic pattern for poly- 

 nucleotide purines as for uric acid. Abrams ei alP showed that the nitrogen 

 of glycine-N'* was preferentially incorporated into position 7 of guanine from 

 the polynucleotides of growing yeast. The biosynthesis of guanine obtained 

 from the mixed nucleic acids of the rat was studied by Heinrich and Wil^ 

 son.^^ The results showed that positions 2 and 8 were derived from for- 

 mate-C^^, position 6 from C^*02 , position 4 from glycine- 1-C^^ and posi- 

 tions 4 and 5 from glycine-l,2-C" (Table VI). Marsh^^'"" has studied the 

 incorporation of formate-C^* into adenine and guanine of nucleic acids 

 from the pigeon. Formate was incorporated mto positions 2 and 8 of both 

 purines, and the ratio of the specific activities in the two positions was 

 very nearly 1. These experiments would thus indicate that the purines 

 derive their carbons 2 and 8 from the same common precursor from the 

 same pool. Edmonds et al^^ have found that in yeast formate-C^* donates 

 its isotope to positions 2 and 8 of guanine from nucleic acids and that 

 position 5 of this purine was derived from the methyl group of glycine. 



3« P. Reichard, Acta Chem. Scand. 3, 422 (1949). 



"R. Abrams, E. Hammarsten, and D. Shemin, /. Biol. Chem. 173, 429 (1948). 



38 M. R. Heinrich and D. W. Wilson, J. Biol. Chem. 186, 447 (1950). 



39 W. H. Marsh, Federation Proc. 8, 225 (1949). 

 " W. H. Marsh, /. Biol. Chem. 190, 633 (1951). 



« M. Edmonds, A. M. Delluva, and D. W. Wilson, J. Biol. Chem. 197, 251 (1952). 



