326 



ADVENTURES IN RADIOSIOTOPE RESEARCH 



activity of the plasma phosphatide P was found to be smaller than 

 1/1000 of that of the inorganic P. 



In experiments in vivo, an exchange between plasma phosphatides 

 and organ phosphatides takes place and, as in some of the organs labelled 

 phosphatides are formed at a fast rate, we will soon after the administra- 

 tion of labelled phosphate find labelled phosphatide molecules in the 

 plasma, which were released from the organs. In fact, almost all phospha- 

 tide molecules found in the plasma were synthesized in the organs. 

 The labelled phosphatide content of the plasma, at different times, is 

 seen in Table 10. In this experiment, the labelled inorganic P content 

 of the plasma was kept constant during 9 days. 



Table 10. — Specific Activity of Phosphatide P and 

 Inorganic P of the Plasma 



Time 



KeUitive specific activity 



Inorganic P Phosphatide P 



4 hours 

 16 hours 

 25 hours 

 37 hours 

 45 hours 

 55 hours 



9 days 



100 

 100 

 100 

 100 

 100 

 100 

 100 



0.53 

 3.8 

 8.1 

 15.0 

 22.0 

 27.5 

 81.6 



Three consecutive processes have to precede the appearance of label- 

 led phosphatides in the plasma. Labelled inorganic P has to diffuse into 

 the cells of the liver and other organs in which the plasma phosphatides 

 are formed. The building up of the labelled phosphatide molecules 

 represents the second process, their release into the plasma the third. 

 In view of the time taken by these processes, it is easy to understand 

 that in the early stages of the experiment the change of the labelled 

 phosphatide content of the plasma has a more rapid than linear depend- 

 ence with time. 



Since a large part of the phosphatide molecules found in the plasma 

 originated from the liver, it is of interest to compare the amount of the 

 active phosphatides found in the plasma with that present in the liver 

 at the end of the experiment. 



As seen in column 3 of Table 11, after the lapse of 12 hours, the acti- 

 vity of the plasma phosphatides reached 3/4 of that of the liver phospha- 

 tides. A large part of the liver phosphatides is, however, not yet renewed 

 and a further substantial increase of the activity of the plasma phospha- 

 tides can only be expected by a corresponchng increase in the active 

 phosphatide content of the liver and other organs. 



