TURNOVER OF PHOSPHATIDES 351 



recent experiments Ave found this figun^ to be aboiil 10 ])rv cent. Tn 

 these experiments the lal)elled sodium ])h()sphate was a(hninis1ered l)v 

 subcutaneous injection. If, however, the labelled phos])hate is given 

 by intrav(>nous injection, as in some of the experiments of Bollman 

 et al. (1948), a greater difference, viz. 24 per cent, is found between 

 the mean and the end value of the specific activity of the orthophosphate 

 P. While subcutaneous injection is foUowc^l by an increase in the speci- 

 fic activity values of the plasma inorganic P during the first phase of the 

 experiment followed by a comparatively slow decrease, intravenous 

 injection is followed by a rapid decrease in the specific; activity of the 

 plasma inorganic P which is the precursor of the liv(>r inorganic P. 

 Intravenous injection is thus a less favourable procedure when we aim 

 at a constant activity level of the orthophosphate P of the liver in 2 

 hour-experiments. In experiments like the last mentioned one, and also 

 when maximum precision is wanted, even in the first mentioned type 

 of experiments we have to take into account the change of the specific 

 activity of the orthophosphate P during the experiment. 



The percentage ratio of the specific activities of the end value of 

 the phosphatide P and of the mean value of the orthophosphate P 

 during the experiment indicates the lower limit of the percentage of 

 new phosphatide molecules (new as to their P content) formed during 

 the experiment. As already mentioned above, if we want tf) know the 

 percentage of phosphatide molecules turned over during the experi- 

 ment, we must consider among others a repeated renewal of the same 

 molecule as well and the percentage ratio of the mean specific activities 

 of the phosphatide P and the inorganic P during the experiment. In 

 most experiments of restricted duration, for example in our experiments 

 taking 2 hours, the correction due to the repeated renewal lies well 

 within the errors of the experiment. 



In experiments of long duration in which a large percentage of the 

 phosphatides is renewed, the calculation carried out by Zilveesmit 

 et al. (1943 a) can be used with advantage. This calculation is based 

 on the change of the specific activity of the phosphatide P with time; 

 specific activity values at different times have thus to be known as well 

 as the mean value of the specific activity of the inorganic P during 

 the experiment. While the last mentioned method is generally appli- 

 cable, the first mentioned method can only be applied advantageously 

 when the percentage turned over is not over 20 — 30 per cent. Its advant- 

 age is that the knowledge of one phosphatide activity value, that deter- 

 mined at the end of the experiment, suffices to calculate the turnover 

 rate. If a correction of the repeated renewal of the same molecule is 

 wanted, it suffices to assume that the mean value of the specific 

 activity of the phosphatide P during the experiment is half of its 

 end value. 



