BIOLOGICAL ROLE OF PENTOSE NUCLEIC ACIDS 499 



nuclear PNA should have the same molar composition: we have already 



seen that this is not the case (Elson and Chargaff/^- Crosbie et a/.,'^^ Mar- 

 shak'33) 



Work on the same lines and leading to similar conclusions has been 

 done with precursors labeled with other isotopes; the differences between 

 nuclear PNA and cytoplasmic PNA are, however, usually smaller than 

 those found with P^'^ Using labeled glycine, Bergstrand et al}^^ found 

 much larger quantities of N^* incorporated into the purines of nuclear 

 PNA than into the cytoplasmic fractions. Similar results have been ob- 

 tained by Payne et al.,^^^ who used formate-C'^, by Anderson and Aquist'^^ 

 for orotic acid-N^^, and, finally, by Smellie et al.,^^'' who compared P^^ 

 glycine-N^^, and formate-C^^: in all these experiments, nuclear PNA showed 

 a high rate of incorporation which exceeded that of the PNA of all cyto- 

 plasmic fractions. 



Mention should also be made of a paper by Hurlbert and Potter,'" 

 who studied in the rat the fate of labeled orotic acid which behaved as a 

 highly specific precursor for PNA pyrimidines. They found that, at the 

 beginning of the experiment, the specific activity of nuclear PNA is higher 

 than that of cytoplasmic PNA, but that it levels off later on. Once again, 

 the experimental results are compatible with the view that nuclear PNA 

 is a precursor of cytoplasmic PNA; but the possibility that both are being 

 synthesized independently, at different rates, cannot be excluded. These 

 problems are also discussed in Chapter 26. 



Nothing is as yet known about the metabolic pathways of PNA synthe- 

 sis in the nucleus and in the cytoplasmic particles; but, it might be signifi- 

 cant that work done in Mirsky's laboratory (Stern et al^^^) has shown 

 that the only enzymes which are found in high proportions in most nuclei 

 are adenosine deaminase, nucleoside phosphorylase, and guanase, i.e., 

 enzymes related to nucleoside or purine metabolism. Many other hydro- 

 lyzing enzymes, as well as catalase, were also studied, but none of them 

 was found in such high concentrations in isolated nuclei. Such an enzyme 

 distribution suggests an important role of the cell nucleus in the synthesis 

 of nucleotides and possibly of the nucleic acids themselves: this condu- 

 it A. Marshak, J. Biol. Chem. 189, 607 (1951). 

 ^^* A. Bergstrand, N. A. Eliasson, B. Norberg, P. Reichard, and H. von Ubisch, Cold 



Spring Harbor Symposia Quant. Biol. 13, 22 (1948). 

 1" A. H. Payne, L. S. Kelly, G. Beach, and H. B. Jones, Cancer Research 12, 5426 



(1952). 

 1'* E. P. Anderson and S. E. G. Aquist. 2nd Intern. Congr. Biochem., Paris p. 197 



(1952). 

 1" R. B. Hurlbert and V. R. Potter, J. Biol. Chem. 195, 257 (1952). 

 "» H. Stern, V. G. Allfrey, A. E. Mirsky, and H. Saetren, J. Gen. Physiol. 35, 559 



(1952). 



