TUIliS'OVEK OF LECITHIK, CKi'JlALIlS' AND srillNCiOM VKLIN 317 



CALCULATION OF THE TURNOVER RATE 



In most of our experiments only a minor pari of the phospliatide 

 molecules present in the organ became labelled; we can, therefore, con- 

 sider the reaction leading to the formation of labelled phosphatides to 

 be a one-sided one and disregard the decomposition of lal)elled phospha- 

 tides during the experiment. As already mentioned on page 312, to arrive 

 at the value of the rate of the phosphatide turnover, we have to compare 

 the specific activity of the phosphatide P extracted from the organ at 

 the end of the experiment with the average specific activity of the cellu- 

 lar inorganic P found in the course of the experiment. The value of the 

 activity of the cellular inorganic P is obtained from that of the tissue 

 inorganic P after subtraction of the share due to the extracellular fluid. 

 The correction to be applied for the presence of extracellular P in the 

 tissue inorganic P is, in most cases, a small one. In the liver of the rabbit, 

 for example, out of 30 mgm. inorganic P only about 0.6 mgm is located 

 in the interspaces. We arrive at this figure by assuming that the inter- 

 spaces make out^^^ 22 per cent of the weight of the liver and the inorganic 

 P content of the interspaces is 3 mgm per cent. The specific activity 

 of the liver extracellular P is, after 4 hours, 2.5 times higher than the 

 specific activity of the tissue inorganic P; correspondingly, 5 per cent 

 of the total inorganic P activity of the liver is due to extracellular P. 



In the case of the muscle, we arrive by an analogous consideration 

 at the result that 25 per cent of the activity of the tissue inorganic P is 

 of extracellular origin. The extent of the correction to be applied increases 

 with decreasing length of the experiment, since in experiments of short 

 fluration only a small amount of labelled P penetrates into the cells. 



With regard to the considerations stated above, one must recognise the 

 possibility that some of the phosphorus which one identifies, even after 

 the most careful experimental procedure, as inorganic P, was in fact 

 present in the tissue in the form of very labile, not yet known, organic 

 phosphorus compounds. Very labile P compounds of that kind, if pre- 

 sent, would probably be in fast exchange equilibrium with the inorganic 

 P present, and their presence would therefore not influence much the 

 calculation given above. The labile P of adenyltriphosphoric acid comes, 

 for example, very quickly into exchange equilibrium with the inorganic 

 P of the tissues or the corpuscles; it is often permissible to replace the 

 specific activity of the inorganic P by that of the above mentioned labile 

 P. The behaviour of creatinephosphoric acid is discussed on page 325. 



When calculating the turnover rate of phosphatides, we must consider 

 the average specific activity of the cellular inorganic P prevailing during 

 the experiment. This value is obtained by determining the specific 



(1) J. F. Manery and B. Hastings, J. Biol. Chem. 127, 657 (1939). 



