972 ADVENTURES IN RADIOISOTOPE RESEARCH 



In view of the fact that the specific activity of the phosphatides of 

 the ovary is lower than that of the yolk phosphatides, as seen in Table 3, 

 the former cannot be the source of the latter^^^\ 



Table 3. — Specific Activity of Phosphatide P 

 IN Organs or the Hen 



Organ 



Relative 

 specific activity 



Liver 



Plasma 



Ovary 



Yolk 



Intestinal mucosa 



100 



79 

 7.2 

 9.2 



18 



The specific activity of the precursor of a product obviously cannot 

 be lower than that of the product itself, at least at that phase of the 

 experiment in which the specific activity of the precursor increases with 

 time. Thus we have to conclude that the yolk phosphatides originate 

 from the plasma phosphatides. The source of the plasma phosphatides, 

 or at least their main source, must be the liver phosphatides as alone 

 the liver phosphatides have a higher specific activity than the plasma 

 phosphatides. Even the specific activity of the phosphatides of the 

 intestinal mucosa in which phosphatides are turned over at a remarkable 

 rate as seen in Table 3 is lower than that of the plasma phosphatides. 



While the plasma phosphatides contain mainly lecithin, appreciable 

 amounts of cephalin are also present in the liver. In view of the small 

 differences in the turnover rate of lecithin and cephalin in the liver, 

 the difference in the composition of liver and plasma phosphatides does 

 not influence the above conclusion. 



The most direct approach to the solution of the problem of the origin 

 of plasma phosphatides is a comparison of the specific activity of plasma 

 phosphatides in the intact and in the hepatectomized animal. In their 

 experiments Fishler et al S^"^^ iound that, after the administration of 

 labelled phosphate in contrast to the phosphatides of the plasma of the 

 intact dog, the plasma phosphatides of the liver-less dog contained only 

 a negligible amount of ^^P. As in both cases the same amount of labelled 

 phosphate was administered to the dog, this result fully confirms the 

 above conclusion that the liver is almost the sole place of formation of 

 plasma phosphatides^^s) 



(27) Fishler, Entenmann, Montgomery and Chaikoff, J. Biol. Chem. 150, 

 47 (1943); Entenmann, Chaikoff and Zilversmit, Ibid. 166, 15 (1946). 



(28) cf. also PopjAk and Beeckmans, Biochem. J. 46, 99 (1950). 



