ISOLATION AND COMPOSITION OF NUCLEI AND NUCLEOLI 129 



tone solution will suffice if the histone solution is not excessively dilute. A second 

 histone fraction, soluble in ammoniacal solution, can be precipitated by adding five 

 volumes of alcohol. 



b. Amount of Total Nuclear Protein. The amount of protein in cell nuclei 

 varies with the type of nucleus. In rat liver cell nuclei isolated from rats 

 fed ad libitum on a fox chow diet, the amount of total protein is about 80 

 to 85 %, if the nuclei are isolated in aqueous media. In Behrens-type nuclei 

 isolated in the writer's laboratory, it can be calculated from the DNA 

 content that the amount of total protein would be close to 90% on a dry 

 weight basis. The ratios of protein to DNA in the Behrens-type nuclei iso- 

 lated from cow and horse liver are 5.7 and 7.3, respectively, from which it 

 can be calculated that the corresponding percentages of protein are 85 and 

 88%. If an allowance for lipid is made, the percentage of protein in both 

 cases would be lowered slightly. On the other hand, the protein content of 

 calf thymus cell nuclei is only 74% (without allowance for hpid), and the 

 protein content of the nuclei of spermatozoa may be considerably lower than 

 the latter figure, although this point is not well established since the nuclei 

 of sperm cells have not been isolated by the Behrens procedure. In any 

 case, it seems clear that the protein content of cell nuclei is far higher than 

 was thought by the early workers, and that protein is the principal chemical 

 constituent of cell nuclei, at least in terms of the amount present. 



c. Albumin Content. The percentage of albumin in cell nuclei is not 

 known, but it cannot be high. 



d. Globulin Content. According to Kirkham and Thomas,'*^ the globulin 

 content of calf thymus nuclei is about 26 %, and that of calf liver about 42 %. 

 In this case, the nuclei studied were isolated by a modification of the 

 Behrens technique. The difference in the globulin content of the two types 

 of nuclei in question is reflected in a difference in the DNA content. Mirsky 

 et al.,'^^ also using nuclei isolated by a modification of the Behrens technique, 

 found that calf thymus nuclei contained 26% DNA and calf liver nuclei 

 15%) DNA (see under Percentage of DNA in Cell Nuclei, p. 122). Since the 

 DNA per resting somatic cell nucleus is constant in the calf (neglecting 

 polyploidy), the high percentage of globulin in the liver nuclei apparently 

 causes the percentage of DNA to be low. It should be noticed that any 

 albumin present in the nuclei studied by Kirkham and Thomas'*^ would 

 have been estimated as globulin. 



e. Histone Content. The histone fraction of cell nuclei may not be a 

 homogeneous protein-^^'^^^'^^^ The amino acid composition of histone has 

 been studied by Davidson and Laurie, i"' Hamer,i'''*-^"« and Mirsky et al.^^'' 



'<"' A. E. Mirsky and H. Ris, J. Gen. Physiol. 31, 7 (1947). 



1" J. N. Davidson and R. A. Laurie, Biochem. J. 43, Proc. xxix (1948). 



'""D. Hamer, Nature 167, 40 (1951). 



10* D. Hamer, Brit. J. Cancer 5, 130 (1951). 



