ISOLATION AND COMPOSITION OF NUCLEI AND NUCLEOLI 121 



liver cell nuclei obtained at pH 6.0 by means of very dilute citric acid but, 

 since the values reported for total lipid were derived by adding the values 

 obtained for the various lipid fractions, it is likely that the values given for 

 total lipid, which ranged from 14 to 18 %, were too high. An overlapping of 

 the various fractions is to be expected, owing to the difficulty of obtaining 

 very pure fractions. 



Values of about 1 1 % for the total lipid of rat liver nuclei obtained by 

 means of 5 % citric acid have been reported by Levine and Chargaff .^^ The 

 total lipid was determined by weighing and hence is presumably a true 

 value. The fact that the values are somewhat higher than those reported 

 by the writer is no doubt attributable to the use of strong citric acid, which 

 extracts considerable protein from the nuclei.^ 



b. Individual Lipid Fractions 



Barnum et al}^ found a total phospholipid content of 3.4 % in mouse Uver 

 nuclei isolated in 2% citric acid. The fractionation of nuclear lipids was 

 first undertaken by Stoneburg^- and subsequently by Haven and Levy^^ 

 and Williams et al.^^ The results of these investigators show that nuclear 

 lipid tends to be rich in phospholipid but is somewhat low in neutral fat. 

 Haven and Levy found that the phospholipid of nuclei of Walker carcinoma 

 256 contained 60 per cent lecithin and 40 per cent cephalin, but only traces 

 of sphingomyelin.^^ Williams et al.^° on the other hand found a higher 

 lecithin-to-cephalin ratio for dog and rat liver nuclei. Cerebroside seems to 

 be very low or lacking in nuclear lipid. ®° Levine and Chargaff^' found 

 approximately a one-to-one ratio of choline to ethanolamine in hpid from 

 rat liver nuclei, ^^^th a small amount of serine also present. 



Stoneburg*- and Levine and Chargaff*^ found that their nuclear lipid was 

 insoluble in petroleum ether. This may have been due to the use of strong 

 citric acid in isolating the nuclei, since the writer has found that the lipid 

 extracted from nuclei isolated at pH 4 or pH 6 is entirely soluble in pe- 

 troleum ether. 



Stoneburg^- calculated that sufficient cholesterol was present in the nu- 

 clei of muscle cells to esterify most of the fatty acids of the neutral fat frac- 

 tion. This neutral fat amounted to about 40% of the total phospholipid. 

 Williams et a/.*" obtained similar results. The latter investigators also found 

 by direct experiment that most of the cholesterol of dog and rat liver 

 nuclei was esterified, but their total cholesterol was only about 11 % of the 

 phospholipid, which is a lower percentage than can be derived from Stone- 

 burg's data. 



It is likely that the work on the lipid of isolated cell nuclei is for the most 



«' C. Levine and E. Chargaff, Exptl. Cell Research 3, 154 (1952). 

 "C. A. Stoneburg, J. Biol. Chem. 129, 189 (1939). 



