METABOLISM OF THE NUCLEIC ACIDS 431 



These experiments have confirmed the metaboHc activity of nuclear PNA 

 as compared with cytoplasmic PNA and once more illustrate the stability 

 of liver DNA. An extensive paper by Barnum et al.-'^^ deals with the in- 

 corporation of P^- into the phosphorus-containing fractions of mouse mam- 

 mary carcinoma nuclei and cytoplasmic constituents at different time 

 intervals. Their results again confirm the very rapid rate of renewal of 

 nuclear PNA and the close metabolic similarity between the PNA's of 

 the particulate components of the cytoplasm. In common with other 

 workers, Barnum et at. have observed a distinction between the PNA of the 

 cell sap and that of the particulate fractions. A mathematical analysis of 

 the experimental time curves has been carried out, and the results are 

 shown to be consistent with the conception of a common precursor of 

 nuclear PNA, DNA, and cell sap PNA, the last of which is in turn the 

 precursor of the PNA of the cytoplasmic particles. Moreover, their data 

 are not consistent with the view that nuclear PNA serves as the precursor 

 of either cell sap PNA or the PNA of the particulate components of the 

 cytoplasm. A further pointer that cytoplasmic PNA does not necessarily 

 arise from nuclear PNA comes from the work of Brachet and SzafarZj^^" 

 who have examined the assimilation of orotic acid-2-C" by the PNA of 

 nucleated and enucleated portions of Acetabularia mediterranea. It was 

 found over a period of months, when the two halves were incubated sepa. 

 rately with the precursor, that the ratio of activity in the PNA of the 

 nucleated half to that in the enculeated half remained almost constant at 

 about 1.4. 



The metabolism of orotic acid in rat liver has been examined by Hurlbert 

 and Potter-" '^2 and Hurlbert and Reichard.^'^ Orotic acid is rapidly utilized 

 in the synthesis of free uridine-5-phosphate and its various derivatives. The 

 C^* content of the soluble uridine phosphate pool has been related to that 

 of the uridylic acid of the tissue PNA at various time intervals (Fig. 7). 

 The activities of the uridine derivatives rise very rapidly to a high level by 

 about 3 hr. Nuclear PNA also exhibits a sharp increase in activity with a 

 peak at about 3 hr. which is about 30 to 50 % of that for the free uridine 

 phosphate. 



A survey of the uptake of P^^ at various time intervals (2 hr. to 7 days) 

 by the DNA's, nPNA's, and cPNA's of normal rabbit appendix, bone 

 marrow, intestinal mucosa, kidney, spleen, and thymus has been carried 



2»9 C. P. Barnum, R. A. Huseby, and H. Vermund, Cancer Research 13, 880 (1953). 

 ^i" J. Brachet and D. Szafarz, Biochem. et Biophys. Acta 12, 588 (1953). 



211 R. B. Hurlbert and V. R. Potter, /. Biol. Chem. 209, 1 (1954). 



212 R. B. Hurlbert and V. R. Potter, Federation Proc. 12, 222 (1953). 



213 R. B. Hurlbert and P. Reichard, Acta Chem. Scand. 8, 701 (1954). 



