332 



ADVENTURES IN RADIOISOTOPE RESEARCH 



all through the experiment to keep the specific activity of the plasma 

 inorganic P at a constant level. In the experiments of short duration tak- 

 ing only 4 hours, the cephalin extracted from all the organs investi- 

 gated was found to be much more active than the lecithin. While the 

 sphingomyelin extracted from the liver did not much differ in its speci- 

 fic activity from that of the lecithin of this organ, in the muscle the 

 sphingomyelin was found to be much more active than the lecithin but 

 less active than the cephalin. 



In experiments taking 12 hours, lecithin and cephalin were renewed 

 in the liver to about the same rate while sphingomyelin was found to 

 show a slower turnover rate. The relative activity of lecithin and cepha- 

 lin was, thus, very materially different in the experiment taking 12 

 hours from that found in experiments of only 4 hours duration. This 

 is not the case with the different phosphatide fractions secured from 



Table 17. — Renewal of Lecithin and Cephalin 



Rabbit III. — Weight: 2.3 kgm 

 Intravenous injection during 234 min 



<•> 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 incor- 

 poration of extracellular inorganic P. 



Table 18. — Renewal of Lecithin, Cephalin and Sphingomyelin 



Rabbit IV. — Weight: 2.5 kgm 

 Intravenous injection during 215 min 



<'^ Turnover rate calculated on the assumption that the formation of phosphatides took place with 

 incorporation of cellular inorganic P. 



^^1 Turnover rate calculated on the assumption that the formation of phosphatides took place with 

 incorporation of extracellular inorganic P. 



<'> Fraction extracted with cold ether (not protein-bound lecithin?). 



*') Fraction extracted, after removal of the ether-soluble lecithin, with hot alcohol (protein-bound?). 



