360 



ADVENTURES IN RADIOISOTOPE RESEARCH 



experiment to be about half its end value. Correspondingly, we have 

 to multiply the figure 18.75 listed in Table 8 by 2 to arrive at the per- 

 centage renewal of the liver phosphatides, assuming glycerophosphate 

 to be the last phosphatide precursor; when, on the other hand, assuming 

 orthophosphate P to be a relevant precursor, a value of 12.8% is obtained. 

 The turnover rate calculated on glycerophosphate "basis" is thus about 

 3 times the value obtained, supposing that orthophosphate P is the 

 relevant precursor. 



In experiments on dogs, Zilversmit et al. (1948) found a similar 

 ratio (3.5) for the turnover rate of liver lecithin calculated on the assump- 

 tion that glycerophosphate P resp. orthophosphate P is the ultimate 

 phosphatide P precursor. Assuming glycerophosphate to be the pre- 

 cursor of phosphatides, the turnover time of the liver phosphatides 

 of the 150 gm rat works out to be 5 hours. 



As stated on p. 359, several arguments were put forward in support 

 of the view that glycerophosphate is the ultimate precursor of phospha- 

 tides and that the comparison of the specific activity of the liver phos- 

 phatide P with the corresponding value of the liver glycerophosphate 

 P supplies a 3 times as high, and correct, turnover rate, as does compari- 

 son with the liver orthophosphate P. This would indicate that it is the 

 formation of labelled glycerophosphate which takes comparatively 

 k>ng time, while the incorporation of labelled glycerophosphate into 



Table 9. — Effect of Feeding of Labelled Glycerophos- 

 phate RESP. Labelled Orthophosphate on the Formation of 

 Labelled Phosphatides in the Liver of the Rat 



(Artom and Swanson 1948) 



the phosphatide molecules is performed at a comparatively rapid rate. 

 This conclusion is not easy to reconcile with a recent finding by Artom 

 and Swanson (1948). These authors state that 6 hours following feeding 

 of labelled glycerophosphate, the specific activity of liver glycerophos- 

 phate P was much greater than the corresponding value of orthophos- 

 phate P of the liver and also much greater than the orthophosphate P 

 value of the liver following feeding of labelled sodium phosphate. The 

 specific activity of phosphatide P of the liver was, however, not signi- 

 ficantly greater in these experiments in which a high liver glycero- 



