BIOSYNTHESIS OF NUCLEIC ACIDS 353 



guanylic acid/"'' only the fate of the purine or pyrimidine moiety is spe- 

 cifically known. 



4. Comparative Biochemistry of Purine Utilizations 



The utiHzations of various purines by a number of species have been 

 studied. The examples, from among the mammals, bacteria, protozoa, and 

 yeasts, show marked differences which cannot yet be correlated with any 

 other characteristics of the organisms. 



Among the purines studied, only two have been found to be extensively 

 utilized by the rat: adenine^^'^' as a precursor of both adenine and guanine 

 of the polynucleotides, and 2,6-diaminopurine^'''^ as a precursor of poly- 

 nucleotide guanine only. However, in the C57 mouse guanine was incor- 

 porated*^'^^ to a considerably greater extent, while adenine- 1, 3 -N 2^^ was 

 utilized^^ to a smaller extent than in the rat. In the mouse each purine is 

 converted into the polynucleotide derivative of the other. ^^-^^ 



In rabbit hyperplastic bone marrow slices, each purine is incorporated 

 and each is converted into the polynucleotide derivative of the other to a 

 small extent. In addition, free adenine, guanine, and hypoxanthine contain- 

 ing the label could be isolated after the administration of labeled adenine, 

 which suggested'* that interconversion of the free purines may occur, al- 

 though it has been pointed out that the possibility of mediation of ribosides 

 in the conversion is not excluded."* 



The utilization of purines by a number of microorganisms has been 

 studied, and different species present a whole spectrum of variations in 

 their patterns of purine incorporations and interconversions. The range of 

 variations in the extent to which the adenine and guanine are intercon- 

 verted by several species is depicted graphically in Fig. 2. 



In microorganisms with an obligate purine requirement, growth on a single purine 

 has been taken as presumptive evidence that all of the polynucleotide purines arise 

 from that single purine. However, organisms which do not require preformed purines 

 for growth may readily utilize conveniently available purines in competition with 

 the usual pathways of synthesis de novo; the use of isotopically labeled purines makes 

 possible a direct demonstration of their incorporation, and is the only possible direct 

 measure of the extent to which they are interconverted. In some instances the sparing 

 effect of added purines on the synthesis de novo from labeled precursors (other than 

 purines) has furnished'^ ■"'■'^•*' a reliable but less direct measure of the utilization 



">" P. M. Roll, H. Weinfeld, and G. B. Brown, Biochim. et Biophrjs. Acta, 13, 141 



(1954). 

 " G. B. Brown, J. Cellular Comp. Physiol. 38, Suppl. 1, 121 (1951). 

 '2 H. G. Mandel and P. E. Carlo, J. Biol. Chem. 201, 335 (1953). 

 " R. Abrams and J. M. Goldinger, Arch. Biochem. and Biophijs. 30, 261 (1951). 

 ^^ H. M. Kalckar, Fortschr. Chem. org. Naturstoffe 9, 363 (1952). 



