ISOLATION AND COMPOSITION OF NUCLEI AND NUCLEOLI 147 



The third isolation of nucleoli was reported by Litt, Monty, and Bounce. ^^ 

 Here the nucleoli were obtained from rat liver cell nuclei previously isolated 

 in very dilute citric acid-gum arable solution at pH 6.0. The nuclei were 

 ruptured in distilled water at pH 6.2 by treatment in a 9,000-cycle mag- 

 netostriction sonic oscillator at 2 to 3° for 7 to 8 min. The nucleoli were 

 isolated by repeated gravity sedimentations and centrifugations in distilled 

 water and 1 % gum arable solutions. The use of already isolated nuclei as 

 starting material precludes confusion of the nucleoli with mitochondria or 

 other cytoplasmic particulates. The complete details of the isolation will 

 appear elsewhere. A photograph of these isolated nucleoh is given in Fig. 6. 



4. Chemical Analyses of Isolated Nucleoli 



Analyses have not been reported for the nucleolar preparations of Kra- 

 kauer. The starfish egg nucleoli of Vincent were chiefly protein, but con- 

 tained from 2.2 to 4.6 % PXA and small amounts of lipid. The PNA was 

 qualitatively different from that of the cytoplasm in regard to nucleotide 

 composition. No histone could be found (Caspersson considered that nu- 

 cleoli contained histone). Acid phosphatase was found present, but DPN- 

 cytochrome-c reductase, dipeptidase, and alkaline phosphatase were absent. 



It is possible that these nucleoli are of the Caspersson-Schultz variety, 

 but this point is not clear at the present time. 



The nucleoli isolated by Litt, Monty, and Bounce were analyzed for 

 BNA, PNA, and a few enzjTnes. The percentage of BNA was very high, 

 ranging from 12 to 18 %, while the percentage of PNA was very low — about 

 2 to 4 % at the most. The BNA content was determined both by colorimetric 

 and spectrographic procedures. The nucleolar concentration of BNA was 

 higher than that of the whole homogenate of nuclei after removal from the 

 sonic oscillator, and higher than that of either of the nonnucleolar fractions 

 obtained therefrom. ^^^'^ Some histone was present, as judged by the fact 

 that 0.2 A^ HCl extracted material which was precipitable by the addition 

 of ammonia. These nucleoli of rat liver cell nuclei thus resembled whole 

 chromosomes in their chemical composition, a fact which is not surprising 

 if we remember the finding of Warren Lewis that such nucleoli are indeed 

 special parts of chromosomes. 



The rat liver cell nucleoli were found to contain the enzymes aldolase, 

 arginase, and catalase. The specific activities of arginase and catalase were 

 considerably lower than those of the whole nuclear suspension directly 

 after its removal from the sonic oscillator, but the specific activity of aldo- 

 lase was approximately double that of the oscillated nuclear suspension, 

 and much higher than either of the nonnucleolar fractions. Aldolase may 

 thus be a real constituent of the nucleoli. 



193a These fractions were the fragmented chromosomes, spun down at 18,000 r.p.m., 

 and the opalescent supernatant from which material could not be sedimented un- 

 less the ultracentrifuge was used. 



