364 george bosworth brown and paul m. roll 



2. Relative Incorporations into PNA and DNA of Liver 



a. Phosphate 



The initial studies^" with phosphate-P^^ involved only its incorporation 

 into DNA, but in 1944 Brues et a/."" investigated the incorporation into 

 both the PNA and the DNA of rat liver. Three of eight days after injection 

 they found a 5- to 6-fold greater incorporation of the P'^ into the PNA in 

 normal rat Hver (Table V, p. 368, lines 1 and 2). However, in liver which was 

 regenerating after partial hepatectomy, they found that after 3 days the 

 ratio of the renewals of the PNA and the DNA was only 1.3, in accord with 

 the evidence that there is an increased synthesis of DNA in the growing 

 tissue, and after 13 days the activity of the PNA had dropped below that 

 of the DNA to give a ratio of 0.6, which was the first evidence to suggest 

 that the phosphorus of the DNA synthesized during growth was retained 

 longer than was that of the PNA (Table V, lines 10 and 11). Hammersten 

 and Hevesy^^ found, 2 hours after P^^ administration, a liver PNA: DNA 

 ratio of about 33 (27 to 40) (Table V, line 4) (but only 3 for spleen and 2 

 for intestine, lines 16 and 17). At 2 hours Davidson"^ found a ratio of 7 

 (line 3) and also the decrease in the ratio in regenerating liver (line 12). 

 Other values for regenerating liver (lines 13-15), and for fetal liver (lines 

 34-37) also show the lower ratios. 



With improvements in the techniques (cf. Chapters 10 and 11) of sepa- 

 rating the PNA from the DNA and of freeing the DNA from other highly 

 active contaminants, the ratios obtained tend to be higher. On the other 

 hand, improvements in the separation of the PNA fractions from more 

 highly active phosphorus-containing compounds tend to result in lower 

 ratios. Subsequent experiments on the incorporation of phosphate into liver 

 nucleic acids in several species, covering periods of a few hours to a day or 

 more (Table V, lines 5-9, 18, 19, 21-33, 48, and 49), have yielded various 

 ratios which sometimes exceed the 33 of Hammersten and Hevesy, and 

 even higher ratios for the relative incorporations into nuclear PNA (nPNA) 

 and the DNA (most notably, lines 8, 9, and 25). Certain low PNA: DNA 

 ratios obtained with chromatographically separated nucleotides^^ (Table 

 V, lines 13 and 21) were based upon a high-molecular- weight PNA which 

 represented only a portion of the "soluble" PNA of the cytoplasm. ^^ 



"o A. M. Brues, M. M. Tracy, and W. E. Cohn, J. Biol. Chem. 155, 619 (1944). 

 '" J. N. Davidson, Cold Spring Harbor Symposia Quant. Biol. 12, 50 (1947). 



112 J. N. Davidson, S. C. Frazer, and W. C. Hutchison, Biochem. J. 49, 311 (1951). 



113 J. N. Davidson, R. Logan, E. R. M. Kay, and R. M. S. Smellie, unpublished re- 

 sults. 



11^ R. M. S. Smellie, W. M. Mclndoe, and J. N. Davidson, Biochim. et Biophys. Acta 



11, 559 (1953). 

 11* A. H. Payne, L. S. Kelly, G. Beach, and H. B. Jones, Cancer Research 12, 426 



(1952). 



