394 V. BLOOD LIPIDS 



also observed that the liver is the site of formation of the plasma phospholip- 

 ids in the case of the domestic fowl, although extrahepatic synthesis of 

 phospholipids was likewise demonstrated. 



In a further study of the role of the liver in phospholipid synthesis, Gold- 

 man et al. 230 found that 2% of injected palmitic acid-l-C 14 was present in the 

 phospholipid molecule in normal dogs after seven hours, while only 0.1% 

 was recovered in the liverless dogs. On the other hand, isotopic fatty 

 acids were found in the phospholipids of heart, skeletal muscle, kidney, 

 small intestine and its mucosa, and lung of hepatectomized dogs. The 

 authors concluded that the phospholipid synthesized in these tissues is not 

 normally concerned with the transport of fatty acids from one organ to 

 another. 



Not only is the liver concerned with the synthesis of plasma phospho- 

 lipids, but also this organ is the primary site of their removal from the 

 blood. 231 Labeled plasma phospholipids disappeared at a normal rate in 

 dogs after the removal of the gastrointestinal tract, but not after hepa- 

 tectomy. When labeled phospholipids were introduced into the blood, 40 

 to 50% were rapidly lost. 232 About one-third of this was deposited in the 

 liver, while considerable amounts were transferred to other tissues. Ap- 

 proximately 9 to 20% of the radiophospholipids which left the plasma re- 

 appeared in the thoracic duct lymph, indicating that some phospholipids 

 can leave the blood stream and undergo circulation through the lymph. 



6. The Turnover Rate of Plasma Lipids 



The rate of turnover registers the speed at which a tissue component is 

 replaced with new molecules of the same compound. Thus, it is related 

 both to the speed of synthesis and to the rate at which the substance is 

 excreted, decomposed, or converted to a new product. 



The chief interest in turnover rates in the plasma has been f ocussed on the 

 phospholipids. Although phospholipids can be synthesized in the small 

 intestine, kidney, muscle, and brain, the main organ in which this reaction 

 occurs is the liver. The turnover rate of phospholipids is greatest in this 

 organ. Entenman et a/. 231 found that the turnover rate for plasma phos- 

 pholipids was 6 to 10 hours in normal dogs weighing 7 to 18 kg., while it 



230 D. S. Goldman, I. L. Chaikoff, W. O. Reinhardt, C. Entenman, and W. G. Dauben, 

 /. Biol. Chem., 184, 727-733 (1950). 



231 C. Entenman, I. L. Chaikoff, and D. B. Zilversmit, /. Biol. Chem., 166, 15-23 

 (1946). 



232 W. O. Reinhardt, M. C. Fishier, and I. L. Chaikoff, /. Biol. Chem., 152, 79-82 

 (1944). 



