350 ADVENTURES IN RADIOISOTOPE RESEARCH 



with the labelled phosphate, is incorporated into the labelled phos- 

 phatides. If this assumption, holds and 1 mgm inorganic intracellular 

 liver phosphate contains throughout the experiment 1000 ^ap atoms, 

 the presence of 10 ^ap atoms in 1 mgm of phosphatide P at the end of 

 the experiment, lasting for example 1 hour, indicates that 1 per cent of 

 the phosphatide molecules present in the liver is formed during the 

 experiment. This figure indicates the percentage labelled molecules. As 

 some of the molecules will be turned over twice or even several times 

 during the experiments, and the formation of one labelled phosphatide 

 molecule from another labelled phosphatide molecule is not registered 

 by this method, the number of phosphatide molecules turned over in 

 1 hour will be greater than 1 per cent. If the average concentration of 

 the labelled molecules was 0.5 per cent during the experiment, about 1 

 per cent of this 0.5 per cent would be ''invisibly" renewed a second 



time. 



The total percentage of phosphatide molecules turned over will 

 thus be 1.005 per cent. The effect of more than 2 renewals of the same 

 molecule in experiments of restricted duration can be disregarded 

 and the same applies, in experiments of short duration, even for the 

 second renewal of a molecule, as the percentage error of this type of 

 experiments is quite appreciable. In the recent work by Bollman and 

 Flock (1948), for example, in which remarkably uniform results were 

 obtained, the deviations of the specific activity values obtained for both 

 the inorganic P and the phosphatide P are still of the order of ± 10 



per cent. 



In view of this fact an exact evaluation of turnover rates is often 

 without interest. However, if such an evaluation is required, one is 

 tempted to take into account, beside the repeated renewal of the phos- 

 phatide molecules in the course of the experiment, the constant dilution 

 of the labelled molecules by non- (or sHghtly) labelled ones which pene- 

 trate from the plasma into the liver. In the dog, 2 per cent of the liver 

 phosphatides were found by Zilversmit et al. (1943 b) to be replaced 

 by plasma phosphatides in the course of 1 hour, a somewhat greater 

 replacement being found by Hevesy and Hahn (1940 b) in the liver 

 of the rabbit. Assuming the liver of the rat to show a similar behaviour 

 to that of the dog, about 0.1 per cent of the phosphatide content of the 

 liver will be replaced by plasma phosphatide molecules in the 1 hour- 

 experiment. 



In our considerations we assumed the specific activity (activity per 

 mgmP)oftheorthophosphate P of the liver to remain constant during 

 the experiment. This is not strictly correct. In former experiments 

 (Ahlstrom and assoc, 1944) the mean value of the specific activity 

 of the orthophosphate P of the rat liver during a 2 hour-experiment 

 was found to be about 5 per cent larger than the end value. In 



