TURNOVER OF LECITHIN, CEPHALIN AND sriilNGOirVELIN 



343 



Table 32. — Extent of Renkwal of LKirrmN, Cephalin and Sphingomyklin 

 IN THE Organs of the Rabbit ix the Course of Experiments Lasting 4 Hours 



AND 12 HorRS, Respectively 



The results are computed from the Tigures in Taliles 17 — 21. 



Orpin 



rercentage of phosphatides reuewed 



.\(" 



Lecithin 



Oeplinlin 



Sphingo- 

 rayelin 



I!(=) 



Lecitiiin 



Ccpliiiiin 



Sphinpo- 

 niyelin 



<•) Turnover rate calculated on the assumption that the formation of phosphatides took place with 

 incorporation of cellular inorganic P. 



<*) Turnover rate calculated on the assumption that the formation of phosphatides took place with 

 incorporation of extracellular inorganic P. 



than in 4 hours, seems less plausible. While it is not probable that the 

 cephalin present in the liver should have been formed with incorporation 

 of extracellular inorganic P, we must envisage the possibility that a 

 part of the cephalin located in the cell membranes is renewed with 

 incorporation of inorganic P located inside the membrane. We discussed 

 above the two extreme cases, formation of phosphatides with incorpo- 

 ration of cellular and of extracellular P. While penetrating through 

 the cell wall, the inorganic P may experience a more or less continuous 

 drop in its activity and the renewal of phosphatide molecules located 

 in the cell membranes could take place by incorporation of "intermediary" 

 labelled phosphate radicals. 



It is of interest to remark that in a laying hen, where the liver has 

 to supply large amounts daily of both lecithin and cephalin, the "slow" 

 cephalin fraction is also renewed at a remarkable rate and the rejuven- 

 ation of the average lecithin and cephalin in the course of 5 hours hardly 

 differs. 



Difference between "fast" and "slow" cephalin 



That the organs contain a small cephalin fraction which is renewed 

 at a fast rate and a larger one which is slowly renewed may possibly be 

 due to a difference in the chemical composition of these fractions. Since 



