TURXOVEll OF PHOSPHATIDES 



359 



Phosphatide Turnover in Cell Nuclei and Mitochondria 



The figures obtained by difi'erent investigators for ilic turnover 

 rate of phosphatides (or for the turnover rate of lecithin, cephalin) 

 are average figures, as the turnover rate in different types of cells and 

 even in different parts of the same cell (cf. Hevesy, 1947) differ. In 

 Tables 7 and 8 the specific activities of the various isolated fractions 

 resp. the percentage replacement of phosphatide P of the total liver 

 tissue of the cell nuclei and of the mitochondria of Ihe liver by tissues 

 inorganic P, glycerophosphate P and total liver, resp., P is stated. 

 Isolating glycerophosphate we made use of the method applied by 

 Entenman et al. (1948). 6 ^curie of ^sp were administered by subcuta- 

 neous injection to each of 6 rats weighing about 150 gm. The animals 

 were killed after the lapse of 2 hours. 



The phosphatide P turnover of mitochondria makes out 67% only 

 of the corresponding figure of the total phosphatide P while the corre- 

 sponding figure for the cell nuclei is 42 only. The last mentioned figure 

 was in a former investigation found to be 65. The discrepancy is clue 

 to the fact that in the method used formerly of separation of cell nuclei 

 (DouNCE, 1945) the nuclei containing fraction contained also mito- 

 chondria, in which as seen above, phosphatides are turned over at a 

 more rapid rate than in the cell nuclei. 



The activity of 1 mgm phosphatide P in percentage of the activity 

 of 1 mgm liver orthophosphate P listed in Table 8 indicates the lower 

 limit of the percentage replacement of phosphatide P in the course of 

 the experiment taking 2 hours. By a fortuitous coincidence, the end 

 value and the average value, of the specific activity of liver orthophos- 



Table 8. — Percent.4igk Replacement of the Phosphatide P of 



THE LrV'ER 



phate are almost identical in an experiment taking 2 hours. This is not 

 the case for the specific activity of gylcerophosphate P, as the labelled 

 glycerophosphate accumulates gradually in the course of the experiment 

 by incorporation of labelled orthophosphate P. We can assume the 

 average value of the specific activity of glycerophosphate P during the 



