606 VI. OCCURRENCE OF LIPIDS IN THE ANIMAL 



choline phospholipids (mostly phosphatidylethanolamine). Except for 

 the high phospholipid content, the composition of the lipid mixture ex- 

 tracted from the mitochondria was similar to that of the whole unfrac- 

 tionated liver. 



In addition to occurring in the nuclei and mitochondria, the lipids also 

 appear to be concentrated in the other particulate matter of the cytoplasm. 

 Kretchmer and Barnum 392 reported that, although the large granules and 

 microsomes contain only 38% of the solids of the cytoplasm of mouse liver 

 cells, they account for 62% of the total lipids, and 85% of the phospho- 

 lipids. Lecithin and cephalin are the phospholipids concentrated in the 

 particulate matter, while the supernatant also contains sphingomyelin. 

 This finding was based upon the demonstration of a N : P ratio of 1 : 1 in the 

 particulate phospholipids, as compared with a 1.3:1 ratio of N to P in the 

 phospholipids present in the supernatant fraction. A marked variation in 

 fatty acids between the particulate and the supernatant fractions also ob- 

 tains. In the former case, the acids are highly unsaturated, and contain 

 20% of tetraenoic acids. Although unsaturated acids are likewise present 

 in the supernatant fraction, a much larger proportion of monoethenoid 

 acids and a lower level of tetraenoic acids have been reported. Ada 393 

 found a difference in composition between the small granules (microsomes) 

 and the large granules in rabbit liver. The microsomes contained 43.4% of 

 lipid, while the large granules had 29.6% of lipid material. The total solids 

 in both of these fractions were accounted for as lipid, protein, and nucleic 

 acid. According to Ada, 26.4% of the liver cytoplasmic phospholipids 

 occurred in the large granules, 64.6% was in the microsomes, and 9% was 

 accounted for in the supernatant fraction. 393 



On the basis of studies with isotopic P, it was found that at least three 

 phospholipid fractions occur in the cytoplasm of the liver cells; each of 

 these fractions has specific activities. The rate of synthesis of phospho- 

 lipid is slower in the nuclei than in the cytoplasm. 393 ' 394 Apparently phos- 

 pholipids are synthesized separately and metabolized independently of 

 each other in different morphological structures of the liver cell. 



In an overall study of the comparative lipid composition in various struc- 

 tures of liver cells, Chauveau and associates 395 found that there was a pro- 

 gressive increase in the unsaturation of the fatty acids of phospholipids 

 from mitochondria, through the microsomes to the supernatant phase. 



392 N. Kretchmer and C. P. Barnum, Arch. Biochem. Biophys., 31, 141-147 (1951). 



393 G. L. Ada, Biochem. J., 45, 422-428 (1949). 



394 G. Hevesy, Nature, 158, 268 (1946). 



395 J. Chauveau, G. Clement, J. Clement, and E. Le Breton, Compt. rend., 232, 2261- 

 2263 (1951). 



