312 ADVENTURES IN RADIOISOTOPE RESEARCH 



e) Incorporation of analogues of choline, in which arsenic replaces 

 nitrogen, into the phosphatide molecule 



Arsenic can be detected in the lecithin fraction isolated from the 

 liver and the brain of rats kept for 21 days on a diet containing arseno- 

 choline chloride^^). 



f) Incorporation of labelled phosphate into the phosphatide molecule 



This method will be discussed in detail. 



Most of the methods outlined above were successfully applied to show 

 that a marked turnover takes place in some of the organs, and the appli- 

 cation of the methods a), c), and f) lead to the result that the rate of the 

 phosphatide turnover is much faster in the intestinal mucosa and in 

 the liver than in the other organs. None but the "phosphate method" 

 was applied, however, to arrive at quantitative data as to the rate of 

 rejuvenation of the phosphatide molecules present in the different 

 organs. 



QUANTITATIVE DETERMINATION OF THE TURNOVER RATE BY 



USING LABELLED PHOSPHATE 



The formation of phosphatide molecules containing ^^p inside the 

 tissue cell can only take place when the process of phosphatide formation 

 was preceded by a penetration of ^^p into the cell, and the same applies 

 to all indicators used in turnover experiments. This point was hitherto 

 not considered. Its great importance is best seen by the following. 



To arrive at a proper figure for the turnover rate we have to compare 

 the percentage of ^^p in the total inorganic P of the cells with the per- 

 centage of ^2p in the total phosphatide P extracted from them. If these 

 ratios, which correspond to those of the specific activities of the inorganic 

 P and the phosphatide P, are found to be equal, we can conclude that 

 all phosphatide molecules were renewed during the experiment. In this 

 case, a proportional partition of ^^p between the inorganic P and the 

 phosphatide P present in the cells took place. This is only possible if 

 the phosphate radical of all the phosphatide molecules was split off in 

 the course of the experiment, a process which was then followed by a 

 synthesis of phosphatide molecules with incorporation of other phosphate 

 radicals in which ^^pQ^ was represented proportionally to its total 

 number present. If the specific activity of the phosphatide P is found, 



(i^A. Welch, Proc. Soc. Exp. Biol. Med. 35, 107 (1937). 



