66 S. S. COHEN 



Treatment of the microsomal fraction from liver with sodium desoxy- 

 cholate, as in Fig. 9, solubilizes protein, hpid, hemochromogen, and DPN- 

 cytochrome c reductase, components which appear to have been centered in 

 the reticular membranes. Dense ENA-protein particles, which are similar in 

 size and shape to the membrane-embedded particles, may now be isolated 

 by sedimentation (Palade, 1956; Palade and Siekevitz, 1956). 



As observed earHer, the amount of the entire microsomal fraction in rat 

 liver decreases under conditions of protein depletion (Muntwyler et at., 1950). 

 The synthesis of this fraction and of protein in general in the adrenal gland 

 has been shown to be stimulated by ACTH (Fia]a et at., 1956). The fractiona- 

 tion of this material into membrane and particles makes possible a more 

 precise dissection of the changes occurruig under various physiological 

 conditions. 



Hultin (1957) has begun in this way to analyze the composition of micro- 

 somal subfractions as a function of the nutritive state of the animal. After 

 removal of RNA with bicarbonate buffer at pH 8.2, he has made deoxycholate 

 extracts. The precipitability of these by ethanol is affected by the lipid 

 content and it has been possible to observe apparent changes in lipid content 

 in these extracts after starvation. One might ask whether, in the synthesis of 

 an RNA virus, e.g., poliomyeUtis, the synthesis of the components of the 

 microsomal fraction has been affected and, if so, if this has been particularly 

 pronounced in the hpoprotein membranes or in the dense RNA particles. 



7. Cell Sap or Soluble Supernatant Fraction 



This fraction includes the material failing to sediment in a gravitational 

 field which deposits the microsomal fraction. It also contains substances of 

 low density, such as Hpid droplets, which would migrate centripetally in a 

 gravitational field. About 30 to 50 % of the nitrogen of fiver, kidney, and 

 tumor is present ui this soluble fraction. In the earfiest analysis of leukemic 

 ceUs by Claude (1944), it appeared that aU of the RNA was associated with 

 the sedimentable fraction. However, in rat fiver the ceU sap contains 43 % 

 of the total nitrogen of the homogenate, 10 % of the RNA, and 66 % of the 

 acid soluble nucleotides (Anderson, 1956c). In general, large amounts of 

 nonsedimentable ribonucleoprotein are present in most rapidly growing ceUs. 

 The systematic analysis of this fraction has barely begun, several electio- 

 phoretic studies reveafing numerous fipid- and RNA-containing protein 

 components (Gjessing et al., 1951; Anderson, 1957). 



As noted earfier, metachromaticaUy stainiug mast ceUs have been frac- 

 tionated in. order to determine the distribution of heparin. This substance is 

 not present on mitochondria and the soluble fraction contained 82 % of the 

 heparin bound to very smaU particles, less than 10 m/z in diameter (Julen 

 et al., 1950). Of course, the distinction between particulate and soluble 



