TURXOVEK OF LECITHIN, CEl'HALIX AND SPniXGOMVElJX 345 



permits us to calculate the extent of new formation (tuinoxer) of the phosphati- 

 des on the assumption that this process takes place inside the cells. In case 

 the phosphatide molecules are renewed with incorporation of extracellular inor- 

 ganic phosphate, the specific activity of the latter enters the calculation. 



The specific activity of cephalin P extracted from different organs was found 

 in experiments of short duration (4 hours) to be much higher (up to 10 times)' 

 than that of lecithin P. With increasing time of expeiiment this difference was 

 found to diminish. In the fractions obtained from the rabbits liver, after the 

 lapse of 12 hours, both fractions showed the same activity. In organs like muscle 

 and brain, in which a slow phosphatide turnover takes place, an equal activity 

 of lecithin and cephalin is only reached after the lapse of several daj-s. 



Sphingomyehn is renewed in the liver at a slower rate than the ether- soluble 

 phosphatides. In the muscles, in experiments taking not longer than 12 hours, 

 sphingomyehn was found to be appreciably more active than lecithin, but less 

 active than cephahn; after the lapse of 9 days, sphingomyelin was found to be the 

 most active fraction. 



Two alternative explanations are put forward to explain the difference in the 

 behaviour of cephalin and lecithin : (a) a part (about 1/4) of the cephalin present 

 inside the cells is renewed at an appreciably higher rate than the average cephalin 

 present, while the bulk of the cephalin showed a similar turnover rate as the ave- 

 rage lecithin; or (b) a part of the cephalin located in the cell walls is renewed in 

 situ with incorporation of inorganic phosphate which has a higher specific activity 

 than the inorganic P located inside the cells. 



In the course of 50 days, all phosphatide molecules present in the Uver and 

 the skeleton were found to be renewed. However, only 74 per cent of the lecithin 

 and 71 per cent of the cephahn extracted from the muscles were newly formed 

 in the course of the experiment. In the brain tissue, 1/4 or more of the lecithin 

 and 1/5 or more of the cephalin molecules remained tinchanged. 



The amount of active lecithin and cephalin present in the plasma and corpuscles 

 was determined. The active plasma phosphatide molecules are not formed in the 

 circulation but in the organs and are led into the circulation. Most of the phos- 

 phatide molecules present in the corpuscles were incorporated during the for- 

 mation of the erythrocytes, but some turnover takes place inside the corpuscles^ 



