666 



ADVENTURES IN RADIOISOTOPE RESEARCH 



SPECIFIC ACTIVITY OF THE FREE P OF DIFFERENT ORGANS 



The fact that greatly divergent figures are obtained for the specific 

 activity of the nucleic acid fractions is due mainly to the highly dif- 

 ferent rate at which nucleic acid is formed in the different organs, 

 molecules are built up. To calculate the rate of formation of nucleic 

 acid, it is thus necessary to know the specific activity of the "free" 

 phosphate present in the tissue cells. It does not suffice to know the 

 specific activity of the free P at the end of the experiment. We have 

 to determine this magnitude at different times in order to arrive at 

 a value of the average specific activity of the free P during the experiment. 

 The results obtained are seen in Table 1 which contains data on the 

 specific activity of the "free" P extracted from the organs. They are 

 obtained on rats killed at different times. In the case of the muscles, 



Table 1. — Specific Activity of the Free Phosphate Extracted from the 



Organs(i) 



Plasma 



Liver 



Kidney 



Spleen 



Mucosa of the 

 small intestine 



Muscle 



Testes 



Brain 



0.18 

 0.25 

 0.24 

 0.21 



0.19 

 0.11 

 0.080 

 0.054 



''' Specific activity usually denotes the activity of 1 mgm P in arbitrary units (often, the specific activity 

 of the plasma P is taken = 100). The above specific activities denote the •percentage of the ^-P administered 

 present in 1 mgm P. 



the free P was extracted from tissue samples taken from the narco- 

 tized rat. The tissue was placed in liquid air and extracted at once by 

 grinding with 10 per cent trichloroacetic acid. This precaution has to 

 be taken in order to avoid the decomposition of creatine phosphoric acid 

 present in large amounts in the muscle. While, in experiments of long 

 duration, as discussed below, the specific activity of the creatine phospho- 

 ric acid P does not differ from the specific activity of the free P, in ex- 

 periments of shorter duration, however, large differences were found, 

 and in these experiments a decomposition of creatine phosphoric acid 

 prior to the removal of the free P would result in a lowering of the 

 specific activity of the free P. 



