TURNOVER OF LECITHIN, CEPHALIX AND SPHINGOMYELIN 325 



tides do not, therefore, exclude the possibility that the labelled phospha- 

 tides present in the muscles were carried into them from other organs. 

 This possibility is, however, excluded by the result of experiments based 

 on the rate of entrance of labelled phosphatides into the muscles<i). 

 While, in the course of 4 hours, phosphatides showing a relative activity 

 of 0.54 units pass from the plasma into the muscles, phosphatides having 

 an activity of 160 units were found 1o be present in the muscles after the 

 lapse of the same time. 



In experiments of short duration Ihe creatine P of the muscles gets 

 only partly labelled and, therefore, a decomposition of creatinephos- 

 phoric acid prior to the extraction of the inorganic P will lead to a 

 ■"'dilution" of the activity of the inorganic P present as such in the muscle 

 tissue. The possibility that in our experiments, taking only a few 

 hours, too low values are obtained for the specific activity of the 

 muscle inorganic P cannot, therefore, be entirely discarded. As the 

 extent of the new formation of the muscle phosphatides is calculated by 

 comparing the specific activity of the phosphatide P with that of the 

 inorganic P, a too low value of the specific activity of the inorganic P will 

 manifestly lead to a too high value of the rate of new formation of the 

 phosphatides. 



Kidney phosphatides 



Kidney phosphatide P is found in experiments of short duration to be 

 more active than the phosphatide P extracted from all other organs. 

 From this fact we may, however, not follow that the kidney phospha- 

 tides are renewed at a faster rate than the phosphatides in the liver 

 or the intestinal mucosa. The labelled inorganic P of the plasma diffuses 

 with a remarkable speed into the kidney cells (see Table 1). This is 

 in no way surprising in view of the role of the kidney cells as to excretion 

 and re-absorption of phosphate. A result of this fast penetration of 

 active phosphate into the kidney cells will be a formation of active 

 phosphatide molecules already in the earliest stages of the experiment. 

 This is not the case in the cells of such organs into which the labelled 

 phosphate diffuses at a slower rate. 



Labelled phosphatides of the plasma 



The renewal of phosphatides in the plasma can only be determined in 

 experiments in vitro; in such experiments,^-) taking 4.5 hours, the specific 



(i^L. HAHN-and G. Hevesy, Nature 144, 204 (1939); Kgl. Dansle Vidensk. 

 Selskah, Biol. Medd. 15, G (1940). 



(2) L. Hahn and G. Hevesy, Mem. Carlsberg 22, 190 (1937). 



