348 ADVENTURES IN RADIOISOTOPE RESEARCH 



from the liver cells into the plasma, the specific activity of the liver 

 orthophosphate P decreases. While, in the first phase of the experiment, 

 with increasing time an increasing incorporation of ^ap takes place 

 into liver phosphatides, in the second phase of the experiment, the 

 already labelled phosphatide molecules are continually renewed and, 

 as this renewal takes place in a medium of decreasing activity, the spe- 

 cific activity of the phosphatide P also decreases. The slower the orga- 

 nic compound is labelled in the first phase of the experiment, the slower 

 it will lose its activity in the second phase, as was already found in 

 early experiments with phosphatides and will be discussed below. 

 A precursor will thus have a higher specific activity than the product 

 formed in the early phase of the experiment and a lower one in the 

 second phase. To these two criteria an important third was added by 

 ZiLVERSMiT and assoc. (1943), viz. that the maximum specific activity 

 of the precursor P must coincide with the beginning decrease in specific 

 activity of the phosphatide P. Often the practical application of this 

 "Zilversmit criterium" encounters difficulties. Based on cumulative 

 evidence including specific activity-time relations, Zilversmit and 

 assoc. (1948) arrive, however, to the result that it is likely that glyc- 

 erophosphate is the pertinent precursor of liver lecithin. 



We shall in the following mainly consider the rate of incorporation of 

 orthophosphate, which has the same specific activity as the labile 

 phosphate of ATP, into phosphatides. Even though the consideration 

 that orthophosphate P is the precursor of phosphatide P, leads to a 

 lower limit of the turnover rate only, it involves many advantages 

 which one appreciates when faced with the task to calculate the turn- 

 over rates in investigations in which labelled carbon, nitrogen, sulphur, 

 etc. are applied as indicators. 



EXTRACELLULAR AND INTRACELLULAR ORTHOPHOSPHATE 



It is probable (Sacks, 1949) that orthophosphate participates in 

 phosphorylation processes during its passage through the cell bound- 

 ary. It is likely that 32p reaches the cells as ATP P3 or ATP P23 

 and then gets into rapid exchange equilibrium with the cellular ortho- 

 phosphate. 



The importance of the intracellular orthophosphate P as a precursor 

 —even if it were not the ultimate one— is enhanced by the 

 fact that within a very short time intracellular orthophosphate P and 

 ATP P3 (and to a large extent even ATP Pg) show the same activity 

 level. This is of interest both in view of the fact that ATP is an im- 

 portant phosphate donor and also because the specific activity of ATP P3 

 is a more reliable measure of the specific activity of the genuine intra- 



