BIOSYNTHESIS OF PURINES AND PYRIMIDINES 281 



Dimroth et al.,"^^ who found that in Torula serine-/3-C'^ contributed more 

 of its isotope to position 6 than to positions 2 and 8 of the purine ring. 



Investigations on the carbon precursors of hypoxanthine were initiated 

 by Greenberg" with the observation that formate-C^* and bicarbonate-C" 

 were incorporated into hypoxanthine by a pigeon liver homogenate. The 

 same author later demonstrated the incorporation of glycine-C^^ into 

 hypoxanthine by the same system.^® Greenberg^^ also found that the syn- 

 thesis of hypoxanthine can proceed in dialyzed extracts of pigeon Uver. 

 Furthermore formaldehyde-C", as well as formate-C^"*, was incorporated 

 into hypoxanthine in the pigeon liver homogenate, and dilution studies 

 suggested that both substances were converted to a common intermediate 

 before incorporation. Localization of the isotope within the molecule, by 

 degradation of the hypoxanthine, was carried out in none of these cases. 



In the meantime Schulman and Buchanan-^ reported independently 

 that particulate-free extracts of pigeon liver can bring about hypoxanthine 

 synthesis. Schulman et al.,^^ continuing these experiments, made the very 

 important discovery that the synthesis of hypoxanthine in a nonfortified 

 particulate-free extract of pigeon liver takes place via the reaction of glycine 

 with CO2 and formate in the molar ratio of approximately 1:1:2. These 

 proportions are good additional evidence that glycine, CO2 , and formate 

 are fundamental carbon units from which hypoxanthine is synthesized. 



(2) Nitrogen Precursors. The starting point for all further work with 

 simple isotopic molecules was a paper by Barnes and Schoenheimer^ on 

 the synthesis of purines and pyrimidines from ammonia-N^^ Among other 

 things these authors showed that in pigeons an extensive synthesis of uric 

 acid took place from N^Mabeled ammonium citrate but they made no attempt 

 to determine the distribution of N^^ within the uric acid. 



Following the work of Sonne et al.,^^ Shemin and Rittenberg^' admin- 

 istered glycine-N^* to an adult human male, isolated uric acid from the 

 urine, and, by a partial degradation, determined the amount of N^^ in 

 positions 1 plus 3, in position 7, and in position 9. It was shown that the 

 greatest amount of N^* was located in position 7, and the authors concluded 

 that the N in this position, as well as in positions 4 and 5, was specifically 

 derived from glycine (Table II). 



Buchanan et al}^ separated nitrogen atoms 1 plus 3 from nitrogen atoms 

 7 plus 9 from uric acid obtained after administration of N^ ^-labeled am- 

 monium chloride to pigeons. Equal N^^ concentrations were observed in 



"^ K. Dimroth, E. Jaenicke, and E. W. Becker, Naturwissenschaften 39, 134 (1952). 



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



" G. R. Greenberg, Federation Proc. 9, 179 (1950). 



"G. R. Greenberg, Federalion Proc. 10, 192 (1951). 



28 M. P. Schulman and J. M. Buchanan, Federalion Proc. 10, 244 (1951). 



" D. Shemin and D. Rittenberg, /. Biol. Chem. 167, 875 (1947). 



