310 ADVENTURES IN RADIOISOTOPE RESEARCH 



fatty acid, choline (or another organic base), and phosphate. We will 

 denote, in what follows, as turnover rate the rate of synthesis of phos- 

 phatide molecules from inorganic phosphate and other components 

 independent of the actual mechanisms involved, and we shall measure 

 this rate by determining the extent to which labelled phosphate present 

 in the cells of an organ is incorporated into these newly formed phos- 

 phatide molecules. As the phosphatide content of an organ is usually 

 constant, we can follow that with the formation of new phosphatide 

 molecules the decomposition of an equal or similar number of old mole- 

 cules goes hand in hand. The possibility must also be envisaged that new- 

 formation and decomposition of phosphatides do not take place in the 

 same organ, but that the newly formed molecules are synthesised in 

 one organ and carried into the other by the circulation. 



The turnover rate can also be measured by following the rate of incor- 

 poration of fatty acids or of choline, for example, into the phosphatide 

 molecule. The turnover rates measured by using different indicators 

 need not necessarily be identical. It would be conceivable, for example, 

 that the incorporation of the phosphate radical into the phosphatide 

 molecules w^ould be preceded by the formation of glycerophosphate and 

 that this process would be a comparatively slow one in contrast to all 

 other steps involved in the synthesis of the phosphatide molecule. 

 In this case, the turnover rate measured, using labelled P as an indi- 

 cator, would be slower than that found when using labelled fatty acids 

 or labelled choline. The opposite would be the case if the reorganisation 

 of the phosphate bond were to take place at a faster rate than the corre- 

 sponding release and incorporation of fatty acids or choline into the 

 phosphatide molecules. 



The question if and to what extent the rate of phosphate incorporation 

 into the phosphatide molecule differs, for example, from that of the fatty 

 acid incorporation into the latter cannot be answered at the time being. 



Feeding cats with mixed glyceride, the acids of wdiich were composed 

 to 85 per cent of elaidic acid, Sinclair^i^ found 12 hours later the plasma 

 phosphatide fatty acids to contain 19 per cent of elaidic acid. In our 

 experiments we found (comp. p. 326) that, after the lapse of 16 hours, 

 about 4 per cent of the phosphatides extracted from the plasma of rabbits 

 contained labelled phosphate. 



(i)R. G. Sinclair, J. Biol. Chem. 115, 215 (1937). 



