rURNOVER OF LECITHIN, CEPHALIN AND SPHINfiOMYKLIN 313 



to be, for example, lu per cent of that of the inorganic F, \v(^ can conclude 

 that 10 per cent of the phosphatides were renewed during the experi- 

 ment. 



Due regard must, however, be given to the change of the specific 

 activity of the cellular inorganic P in the course of the experiment. 

 By administering the labelled phosphate in several portions of suitably 

 varying quantities in the course of the experiment, we can maintain 

 a constant specific activity of plasma and interspace phosphate- As to 

 the cellular concentration of ^^P, which is nought at the start of the 

 experiment and then gradually increases, we determine the change of 

 concentration with time experimentally and compare the specific acti- 

 vity of the phosphatide P extracted at the end of the experiment with 

 the average value of the specific activity of the inorganic P which pre- 

 vailed during the experiment. 



When determining the specific activity of the cellular inorganic P^ 

 due regard must be taken to the fact that a part of the tissue inorganic 

 32P is of extracellular origin. As the extracellular volume of the tissue 

 is known and the specific activity of the extracellular P does not differ 

 much from that of the plasma P, we can easily correct for the presence 

 of the extracellular P in the tissue inorganic P. Since the extracellular 

 phosphate in the case of the muscle tissue, for example, amounts to 

 only about 1/90 of the cellular inorganic P, the correction mentioned 

 above becomes only significant in experiments of short duration. If the 

 rate of penetration of the inorganic phosphate differs greatly in the cells 

 of different tissues, as it actually does, for example, in the case of the 

 liver and the muscle, we do not get proper information on the relative 

 rate of turnover of the phosphatides in these organs by comparing the 

 specific activity of the liver phosphatide P with that of the muscle 

 phosphatide P. Conclusions based on such a comparison will greatly 

 underestimate the relative rate of phosphatide turnover going on in the 

 muscle cells into which the inorganic P diffuses at a slow rate, in contrast 

 to its penetration into the liver cells. We will arrive, however, at correct 

 figures by comparing the ratio. 



specific activity muscle phosphatide P 

 specific activity muscle inorganic P 



with the corresponding ratio of liver products. 



If we wish to draw quantitative conclusions from experiments carried 

 out with elaidic acid as an indicator, we have to compare the elaidic 

 acid content of the organ phosphatides with thai of the elaidic acid 

 content of the fatty acid mixture present in the corresponding cells in 

 freely dispersed state. The latter magnitude is not known and the same 

 consideration applies 1o the work with douterated fa1 as an indicator. 



