718 VII. LIPID DISTRIBUTION IN SPECIFIC TISSUES 



Fat synthesis occurs in the liver before it can be demonstrated in any other 

 organ when animals are fed on fat-low diets in the presence of deuterium. 90 

 Under such conditions, no comparable reaction can be demonstrated in the 

 depot fats. However, Bernhard and Schoenheimer 94 were able to prove 

 that some fatty acids are synthesized in tissues other than the liver, al- 

 though the rate was found to be markedly higher in hepatic tissue. These 

 workers concluded that the half-life of average saturated fatty acids in the 

 liver is only about one day, as contrasted with an average figure of five to 

 nine days in other tissues. In later work from this same laboratory, 95 it 

 was indicated that the half-life of liver fats in mice is 2.6 to 2.8 days, while 

 that of depot fats is five to six days. 



Variations have been shown to obtain in the rate of turnover of liver and 

 carcass phospholipids similar to those previously noted for the entire lipid 

 content. Thus, Sinclair 96 showed that the iodine number of the liver phos- 

 pholipids increased within one day to 65% of that reached at equilibrium, 

 which occurred at five days when a cod-liver oil diet was employed. On the 

 other hand, the rise in carcass phospholipids was only 27% of the maximum 

 in one day, and the equilibrium value was not observed until thirty-five 

 days later. Cavanagh and Raper 97 reported that the D 2 content of liver 

 phospholipids observed after the feeding of deuterated linseed oil was such 

 as to indicate a replacement of 14% of the fatty acids in liver phospholipids 

 within 6 hours after the feeding of this compound. Tolbert and Okey 98 

 calculated the turnover time of the phosphate of the choline-containing 

 phospholipids in the livers of fasted rats as 10.9 hours; that for the total 

 phospholipids was estimated as 8.9 hours. The same order of turnover 

 prevails in carcass lipids; however, in the brain, the fatty acids exhibit 

 the most rapid interchange. The average values for the half-life of these 

 lipids are given in Table 2. 



Table 2 

 The Rate of Turnover of Lipids in Rat Tissues (half-life in days)" 2 



Lipid Liver Carcass Brain 



Fatty acids 1-3 6-9 10-15 



Phospholipids 1-2 5-7 >200 



Cholesterol 5-7 12-15 >100 



a E. S. West and W. R. Todd, Textbook of Biochemistry, Macmillan, New York, 1951. 



94 K. Bernhard and R. Schoenheimer, J. Biol. Chem., 133, 713-720 (1940). 

 96 De W. Stetten, Jr., and G. F. Grail, J. Biol. Chem., 148, 509-515 (1943). 



96 R. G. Sinclair, J. Biol Chem., 95, 393-408 (1932). 



97 B. Cavanagh and H. S. Raper, Biochem. J., S3, 17-21 (1939). 



98 M. E. Tolbert and R. Okey, J. Biol. Chem., 194, 755-767 (1952). 



