Synthesis of Nucleosides and Nucleotides 169 



guanosine (Hammarsten and Reichard, 1950) is utilized by 

 the rat. Thus it would appear that guanylic acid can be 

 incorporated directly into the polynucleotide, since prior 

 degradation to either guanosine or guanine cannot be involved. 



The incorporation of adenylic acid, which leads to both the 

 adenine and the guanine of the polynucleotide, could not in 

 itself be of much significance since degradation to the purine, 

 and incorporation thereof, could explain it. However, by 

 analogy to the result with guanylic acid, it may be that both 

 adenylic and cytidylic acids may be incorporated directly 

 and it seems possible that nucleotides may be the immediate 

 precursors in the biosynthesis of polynucleotides. The fact 

 that guanylic acid is the only potential intermediate between 

 adenine and polynucleotide guanine also suggests that the 

 conversion of an adenine, or a 2,6-diamino-purine, into a 

 guanine derivative may occur at the nucleotide stage. 



It is worthy of note that the sum of the renewals of poly- 

 nucleotide guanine from the two sources, that is, directly from 

 guanylic acid and indirectly from adenylic acid, is approxi- 

 mately equal to the renewal of polynucleotide adenine from 

 the adenylic acid alone, and that these values are in close 

 agreement with the approximately equal renewal of the two 

 purines observed in the original experiment with a mixture 

 of nucleotides. 



The relatively low utilization of the adenylic or guanylic 

 acids when compared to that of free adenine, or to that of 

 cytidylic acid, may be attributed to the greater abundance 

 of tissue enzymes capable of degrading the purine nucleotides. 

 However, the possibility that only one of the isomers present 

 is utilized for anabolic purposes could also help to account 

 for these results and must be considered. 



These purine nucleotides led to the incorporation of the 

 isotope into the purines of the ribose nucleic acids but there 

 was not more than a trace of incorporation into the deoxyribo- 

 nucleic acids. In the case of a pyrimidine ribose derivative, 

 either cytidine or cytidylic acid, there was a considerable 

 conversion into the deoxyribose polynucleotide pyrimidines, 



