168 G. B. Brown 



did furnish a precursor of the polynucleotide pyrimidines, 

 since none of the individual pyrimidines occurring in the 

 nucleic acids has been found to be utilized by the rat. 



Almost simultaneously Hammarsten, Reichard and Saluste 

 (1949, 1950) studied cytidine and uridine, also prepared from 

 yeast grown with isotopic ammonia, and found that it was 

 chiefly the cytidine which led to all of the pyrimidines of both 

 the pentose and the deoxypentose polynucleotides. 



Dr. Roll's results with cytidylic acid are quantitatively 

 almost a perfect parallel to the earlier results with cytidine 

 as far as the pyrimidines of both types of nucleic acids are 

 concerned. There is one interesting and perhaps very signi- 

 ficant point of difference, in that Hammarsten and co-workers 

 found that cytidine nitrogen was also incorporated to a very 

 considerable extent in both the adenine and the guanine 

 of the deoxypentose nucleic acids, while with cytidylic acid 

 we have not obtained any evidence of such a transformation. 



Our first studies of the metabolism of individual purine 

 nucleosides and nucleotides were carried out with the [8-^*C] 

 derivatives obtained by biosynthesis in yeast. The fate of 

 these has so far been studied only in this same yeast. Rela- 

 tively little (3 to 10 per cent) incorporation of any of these 

 ribose derivatives was observed, and since these small incor- 

 porations could have been due to incorporation of degradation 

 products, the only conclusion possible was that attachment 

 of the ribose or ribose phosphate did not facilitate the incor- 

 poration of the purines into the polynucleotides. In the light 

 of subsequent findings in the rat the small utilization by 

 yeast could perhaps be assigned greater significance. 



Dr. Roll has now completed studies in the rat of each of the 

 ^^N-labelled purine nucleotides prepared from yeast nucleic 

 acid. These were administered intraperitoneally and it was 

 found that both the adenylic acid and the guanylic acid are 

 utilized as precursors of the pentose polynucleotides (Roll 

 and Weliky, 1951). The behaviour of guanylic acid, which 

 leads only to the guanine of the polynucleotides, makes it 

 unique among guanine derivatives since neither guanine nor 



