ISOLATION AND COMPOSITION OF NUCLEI AND NUCLEOLI 125 



b. Ribonucleic Acid (PNA) 



(1) Percentage in Cell Nuclei. Unequivocal information is not available 

 at present concerning the percentage of PNA in cell nuclei but it seems 

 likely that the amount per nucleus, like the amount of protein per nucleus, 

 will prove to vary according to the diet, tissue, species, etc. 



The PNA content of liver cell nuclei obtained in aqueous solution at 

 pH 4 or pH 6 is about 2.5 %, according to the most recent work from the 

 writer's laboratory. Previous studies indicated a content of about 5%.*^ 

 Nuclei obtained by Hogeboom and Schneider by their most recent pro- 

 cedure-^ contained approximately one-quarter as much PNA as DNA; if 

 the percentage of DNA is ten, this would correspond to rather less than 

 3 % PNA. Rat liver cell nuclei isolated in high concentrations of acid usu- 

 ally contain only about one-tenth to one-eighth as much PNA,^^'*^ ^j. 

 though considerably lower amounts have been reported for other nuclei.*^ 



The work of Mclndoe and Davidson*^ shows that the ratio of PNA to 

 DNA in cell nuclei can vary considerably depending on the organ and the 

 species studied. For rat liver nuclei this ratio is in the neighborhood of 1 

 to 3.5; for rabbit liver, 1 to 6; for liver of tumor-bearing fowl, 1 to 5; for 

 fowl erythrocytes, 1 to 10; and for calf thymus, 1 to 17, as calculated from 

 a table of values showing the amounts of PNA and DNA per nucleus. 

 Slightly different ratios are obtained from another table showing concen- 

 trations of DNA and PNA in various cell nuclei. The mammalian cell 

 nuclei were isolated in moderately strong citric acid and the erythrocyte 

 nuclei in buffered saline. An appreciable drop in the amount of PNA per 

 nucleus seemed to occur in rats fasted for 72 hours. 



Mauritzen el al.^- found a slight increase in the ratio of PNA to total 

 nucleic acid in nuclei of regenerating rat liver as compared with those of 

 normal rat liver, and Leuchtenberger et alP found a higher ratio of PNA 

 to total nucleic acid in nuclei of certain mouse tumors than in nuclei of 

 "normal cells." 



Mirsky et al.^^ claim that the content of PNA in mammalian liver cell 

 nuclei isolated by their modification of the Behrens procedure is one-tenth 

 that of the DNA content. However, in nuclei of plant cells, Stern and Mir- 

 sky'^ find a PNA content about equal to the content of DNA. 



Mirsky et al}^ cite the high PNA content of liver cell nuclei obtained by 

 Dounce et al.^-^^ by a modification of the Behrens technique as an indica- 

 tion that these nuclei were grossly contaminated with cytoplasm, but they 

 do not mention that with one exception the figures given for PNA were 

 derived by calculation from analyses for total phosphorus and direct analysis 



«2 C. M. Mauritzen, A. B. Roy, and E. Stedman, Proc. Roy. Soc. (London) B140, 



18 (1952). 

 *' C. Leuchtenberger, G. Klein, and E. Klein, Cancer Research 12, 480 (1952). 



