TURNOVER OF PHOSPHATIDES 



361 



phosphate specific activity was observed. The presence of a highh^ 

 active glycerophosphate in the liver did thus not markedly accelerate 

 the formation of labelled phosphatides. The above stated specific acti- 

 vity figures were found by Artom and Swanson 6 hours aflci' feeding 

 of the labelled compounds. 



In view of the fact that the results of only one experiment is stated, 

 the results of this investigation, which aimed at the elucidation of a 

 very different problem, can however not be considered to invalidate 

 ZiLVERSMiT et al. conclusions. A closer investigation of the phos- 

 phatide formation in the liver following feeding of labelled glycero- 

 phosphate would not be without interest. 



Search for the Existence of a Small Phosphatide Fraction of Rapid 



Turnover Rate 



It is conceivable that a small fraction of phosphatides is present in the liver, 

 which is renewed at a more rapid rate than the average phosphatide molecule 

 present. The existence of such a fraction could influence much the conclusions 

 drawn in the last paragraph which are based on the number of phosphatide mole- 

 cules turned over during a time unit. 



Table 10. — Specific Activities of P Fractions of Mice 10 



MlNT'TES FoLLOWaXG InTRAVENOT'S InJECTIOX OF ^"P 



' Activity of 1 mgm P in percentage of the activity administered. 



Let us assume that of 100 phosphatide molecules 99 are renewed to an extent 

 of 18% in 1 hour, while 1 phosphatide molecule is renewed 100 times during the 

 experiment. Our experiments would hardly reveal the presence of such a fraction 

 responsible for 118 renewal processes, while our turnover experiments would 

 reveal a 19% renewal of the average phosphatide molecules, only. One hundred 

 renewals pei- hour is a large figuie in view of the fact that the very i-apidly rejuv- 

 enated ATP Pg 3 in experiments in vitro is found to be renewed 72 times per 

 hour only (Meyerhof et al., 1938). But even a 1% phosphatide fraction renewed 

 10 times in the course of 1 hour would be responsible for 10 renewal processes 

 beside the 18 performed by the rest of 99 "^o phosphatide molecules, increasing 

 the number of molecules turned over to 28. 



The presence of a small rapidly renewed fraction should become noticeable^ 

 in experiments of very short duration. 



We carried out experiments in which about y^ microcurie ^zp was injected 



into the tail vein of each group of 10 mice; the (22 — 26 gm) animals were killed 

 by decapitation idler the lapse of 10 minutes, the organs pooled, and the specific 



