TUKNOVEH OF PHOSPUATIDES 347 



average phosphatide (or lecithine and so on) molecule in llu^ liver remains 

 unchanged. The occurrence of a change may involve certain moieties of 

 the molecule only or a reassembly of all the moieties or. finally. 1 he mole- 

 cule may leave the organ unchanged, as do some of the phosphatide 

 molecules of Ihe liver, which interchange with phosphatide molecules 

 of the plasma. In the last mentioned case, the determination of the 

 rate of renewal (replacement) of the phosphatides does not encounter 

 difficulties. When replacing part of the plasma of one animal by plasma 

 of another animal, containing ^ap labelled phosphatides, we can follow 

 the rate of disappearance, thus the rate of replacement of plasma phos- 

 phatides. In the first mentioned cases w(> ma} , however, encounter great 

 difficulties due to our ignorance of the steps involved in the biosynthe- 

 sis of most types of molecules. We can, however, determine the upper 

 limit of the time during which the average phosphatide molecules of the 

 liver, for example, remain unchanged by administering labelled phosphate 

 and comparing the specific activity of the phosphatide P at the end of 

 the experiment with the specific activity of the intracellular inorganic P 

 which prevailed in the average during the experiment. In experiments 

 of short duration, the repeated renewal of a phosphatide molecule may 

 usually be neglected. Is that not the case, a more complicated calcu- 

 lation has to be used, as will be shown below (cf. p. 360). 



The rate of incorporation of labelled intracellular orthophosphate 

 may indicate the rate of renewal of the phosphate group of the phos- 

 phatide molecule, it will, however, not necessarily do so. If first with 

 the participation of orthophosphate, or with a phosphorus compound 

 which comes into rapid exchange equilibrium with orthophosphate 

 at a comparatively slow rate, a formation of phosphorus containing pre- 

 cursor of the phosphatide molecule takes place, and this formation is 

 followed by a relatively rapid incorporation of the precursor into the 

 phosphatide molecule, the following up of the rate of incorporation of 

 the intracellular orthophosphate into Ihe phosphatide molecules fails 

 to reveal the rapid interchange taking place between \hv ullimate 

 precursor and the phosphatide molecule. 



It is quite possible that the labelled orthophosphate is incorporated 

 Into the phosphatide molecule during the asseml)ly oi ihe molecule 

 and that thus orthophosphate or a phosphorus compound which comes 

 into rapid exchange equilibrium with orlhophosphatc as ATP is not 

 only an early, but also the last precursor of phosphatide P. It is, how- 

 ever, equally possible that glycerophosphate or another organic P com- 

 pound is the last precursor of phosphatide phosphorus. 



Following a single administration of labelled phosphate the specific 

 activity of the liver intracellular orthophosphate first increases ; in the 

 later phase of the experiment, however, when the decrease in specific 

 activity of the plasma inorganic P results in a partial exodus of ^^p 



