TURNOVER OF LECITHIN, CEPHALIN AND Sl'lllNGOMYELIN 



319 



turnover as it would if the formation of the phosphatide molecules took 

 place with participation of the less active cellular P. It is obvious that 

 the sensitivity of our radioactive indicator will be very different in the 

 two cases mentioned above. Though it is much more probable that the 

 turnover of the phosphatide molecules takes place inside the cells we 

 have also recorded, in the fifth column of the above mentioned tables, 

 the turnover rates calculated on the assumption of an extracellular for- 

 mation of the phosphatide molecules. The values thus obtained give the 

 lower limit of the turnover rate, while those obtained in column 4 give 



Table 2. — Specific Activity of the Inorganic P and Phosphatide P Extracted 



FROM the Organs 



Rabbit I. — Weight: 2.4 kgm 

 Tiitraveiions injection during 4 hours 



Fraction 



Specific activity 

 in relative units 



Plasma inorganic P 



Liver tissue inorganic P at the end of the experiment 



Liver tissue inorganic P corrected for the change in plasma activity 



during the experiment 



Liver cellular inorganic P at the end of the experiment corrected as 



above 



Liver cellular inorganic P average value during the experiment . . 



Liver phosphatide P 



Kidney tissue inorganic P at the end of the experiment 



Kidney tissue inorganic P corrected for the change in plasma activity 



during the experiment 



Kidney cellular inorganic P at the end of the experiment corrected as 



above 



Kidney cellular inorganic P average value during the experiment 

 Kidney phosphatide P 



100 

 36.2 



44 



40.8 



20.4 



3.0 



67.7 



82.3 



82.0 



73.5 



5.5 



the upper limit. It is conceivable that some of the phosphatide molecules 

 are renewed inside the cell wall. In that case the inorganic P entering the 

 newly formed phosphatide molecules will have a specific activity being the 

 intermediary betw^een that of the extracellular and the cellular P. A conti- 

 nuous drop of the specific activity of the inorganic P in the cell wall 

 may therefore take place while the phosphate penetrates from the inter- 

 spaces into the cells. 



In the corpuscles the phosphatides are known to be practically con- 

 centrated in the stroma^^), andthe thickness^^) of the latter to correspond 



(1) B. N. Ebickson, H. H. Williams, S. S. Beknstein, J. Arvin, R. L. Jones 

 and J. G. Macy, J. Biol. Chem. 122, 515 (1938). 



(2) Danielli, J. Cell. Comp. Physiol. 7, 393 (1936). 



