TUENOVKll OF PHOSPHATIDES 363 



Fat Metabolism and Phosphatide Turnover 



A possible connection between fat metabolism and phosphatide turn- 

 over was repeatedly discussed. Bollmam and Flock (1946) compared 

 the amount of phosphatides turned over in the liver and plasma with 

 the amount of fat metabolized by the rat and arrived at the following 

 conclusion. 



Assuming, as found by these authors, 0.175 mgm of phosphatide P 

 to be renewed per hour in a rat liver weighing 5 gm, an equal amount 

 of phosphatide P must have been metabolized in the liver or have 

 left this organ during that time, the latter amounting to 0.048 mgrii. 

 These figures account for a sufficient phosphatide turnover in the liver 

 and in the plasma to metabolize or transfer fat equivalent to only 3 per 

 cent of the caloric needs of the rat and indicate that phosphatide forma- 

 tion apparently is not an obligatory step in fat oxidation or transfer. 



For the turnover rate of the 150 gm rat we found about twice the value 

 given by the above authors. Thus, according to our results, the amount 

 of phosphatides turned ovei- should be twice the above figure and, 

 if glycerophosphate is assumed to be the ultimate phosphatide pre- 

 cursor, even three times the last mentioned value, but still only 18 per 

 cent of the caloric needs of the rat. Consequently, our findings do not 

 contradict Bollman and Flock's conclusion. 



In the above consideration no account was taken of the possibility 

 of the existence of a minor phosphatide fraction which may be renewed 

 very rapidly, as discussed on p. 361, nor was the phosphatide turnover 

 in other organs than the liver accounted for. 



Summary 



(1) The extent of incorporation of intracellular ortliophosphate P of the liver 

 phosphatides of the rat decreases with increasing age. While it amounts to 12 per 

 cent per hour for the liver phosphatides of a 4-day-old rat, the corresponding 

 figure for a 1.5-year-old rat is 6, intermediate figures being obtained for rats of 

 intermediate age. 



(2) The calculation of the "rate of turnover" from the ratio of the end value 

 of the specific activities of phosphatide P and the mean value of the specific 

 activities of orthophosphate P during the experiment supplies, in experiments 

 lasting 4 hours or less, almost identical figures with those obtained when, accord- 

 ing to ZiLVERSMiT et al., the calculation is based on the change of the specific 

 activity of phosphatide P with time. 



(3) Replacement of the specific activities of liver inorganic P by corresponduig 

 values of ATP Pg^ of the liver leads to very similar turnover rate values. 



(4) The ratio of the turnover rate of the phosphatides of the total liver tissue, 

 the mitochondria, and the cell nuclei of the liver is found to be 1 : 0.67 : 0.42. 



(5) The percentage of labelled P administered present in I mgm liver phospha- 

 tide P of the rat, which is as high as 0.121 after the lapse of 1 day, declines to 

 0.00066 after the lapse of 84 days. 



