FACTORS ALTERING CONCENTRATION OF BLOOD LIPIDS 395 



was prolonged to 33 to 160 hours in liverless animals. Friedlander and 

 associates 233 noted that the rate of turnover of plasma phospholipids in dogs 

 was increased by the administration of choline. Zilversmit, Entenman, 

 and Chaikoff 234 confirmed the earlier results with choline which proved that 

 the specific activities of choline-containing phospholipids of liver and plasma 

 were markedly increased after a single dose of choline. Nearly all of the 

 increase was found ia the case of lecithin; that of liver sphingomyelin was 

 slight, due to its low concentration. Although the animals which responded 

 to choline showed an increased calculated turnover of liver lecithins, 

 there was no rise of that fraction in either plasma lecithin or plasma sphingo- 

 myelin. The authors conclude that the increases in specific activities of 

 plasma phospholipid must be interpreted as a reflection of similar increases 

 of their precursors in the liver. In the case of man, Cornatzer and Cayer 235 

 found that the rate of phospholipid turnover in the plasma of normal per- 

 sons varies between individuals, but is fairly constant in the same individual. 

 However, neither choline nor methionine produced any stimulatory effect 

 on phospholipid turnover under normal conditions. 



In completely depancreatized dogs with diabetes, in which condition 

 considerable amounts of fat migrate from fat depots to liver, the rate of 

 turnover of plasma phospholipids is not increased. 236 These results also 

 fail to support the concept that the phospholipids serve in the transport of 

 fatty acids from one tissue to another. In the case of cholesterol, London 

 and Rittenberg 237 reported that the half-time of serum cholesterol is eight 

 days, while the turnover time is twelve days. 



7. Factors Altering the Concentration of Blood Lipids 



Although the average values for blood constituents listed earlier are 

 reasonably accurate for the prediction of the several blood lipids in normal 

 individuals under postabsorptive conditions, a number of physiological 

 factors may cause marked variations from the usual values. Such condi- 

 tions as age, race, sex, species, and environment may have considerable 

 importance, as do also the ingestion of fatty meals, or prolonged fasting. 

 The most pronounced alterations from the normal are to be observed in such 



233 H. D. Friedlander, I. L. Chaikoff, and C. Entenman, /. Biol. Chem., 158, 231-238 

 (1945). 



234 D. B. Zilversmit, C. Entenman, and I. L. Chaikoff, J. Biol. Chem., 176, 193-208 

 (1948). 



235 W. E. Cornatzer and D. Cayer, /. Clin. Invest., 29, 534-541 (1950). 



236 1. L. Chaikoff, D. B. Zilversmit, and C. Entenman, Proc. Soc. Exptl. Biol. Med., 68, 

 6-9 (1948). 



237 I. M. London and D. Rittenberg, J. Biol. Chem., 184, 687-691 (1950). 



