TUBXOVKIl OF PHOSPHATIDES 349 



cellular orthophosphate than the value obtained in the direct deter- 

 mination of tissue orthophosphate, even if the extraction was preceded 

 by viviperfusion. 



In the case of the rat liver, which is showing both a high capillary- 

 wall and a high cell membrane permeability, we found in all our experi- 

 ments lasting 2 hours to 84 days within ±10 per cent identical values 

 between the specific activities of tissue orthophosphate P and ATP Pg, 

 P3, indicating that in these cases it is not advantageous to consider the 

 specific activity of ATP P^, P3 instead of the corresponding value of 

 the tissue orthophosphate P. In experiments, however, in which the 

 formation of labelled phosphatides in the mouse liver was investigated, 

 only 10 min following intravenous injection of labelled phosphate, 

 a marked difference between the specific activity values of orthophos- 

 phate P and ATP Pg 3 was found. Taking the specific activity of the 

 plasma orthophosphate P to be 100, the corresponding values of liver 

 orthophosphate P and ATP P23 were found to be 31.9 and 21.5, respect- 

 ively. Assuming the extracellular volume (25%) of the liver to have 

 the same concentration (5 mgm%) and specific activity as the plasma 

 orthophosphate P, we find in 100 gm liver 1 .3 mgm extracellular orthophos- 

 phate P having an activity of 1.3x100 = 130, while the total tissue 

 orthophosphate P, the concentration of which is 32.3 mgm%, has an 

 activity of 1030; thus, in this case, the activity of the extracellular orlho- 

 ])hosphate P makes out 13 % of the activity of the total liver ortho- 

 phosphate. In experiments of such short duration it is advisable to con- 

 sider the specific activity of ATP Pg 3 or preferably that of ATP P3, 

 as a measure of the specific activity of the intracellular orthophosphate P. 



As another case in which the specific activity of ATP P, 3 has to be 

 regarded as a measure of the specific activity of the intracellular ortho- 

 phosphate P, that of the lung may be mentioned. One hour after admi- 

 nistration of labelled phosphate by subcutaneous injection to 170 gm 

 rats, the relative specific activity values (the plasma orthophosphate 

 value being taken = 100) for the tissue inorganic P and ATP Pg 3 were 

 found to be 52.7 and 35.6, respectively, the higher value of the specific 

 activity of the tissue inorganic P being possibly due to the presence 

 of highly active extracellular orthophosphate P in the tissue orthophos- 

 phate investigated. 



CALCULATION OF THE RATE OF RENEWAL 



In all previous calculations, except a quite recent one to be discussed 

 on p. 358, carried out hitherto it was assumed that labelled phosphate, 

 or another precursor which comes into rapid exchange equilibrium 

 (compared with the rate of formation of the lal)elled ])hosphalides) 



