NUCLEIC ACID IX SARCOMA SLICES 775 



The determination of the amount of nucleic acid formed during the 

 experiment is made more difficult by the impact of quite considerable 

 obstacles in calculating the specific activity of intracellular phosphate 

 for in vitro experiments. 



The activity of free intracellular phosphorus for in vivo experiments 

 may be calculated from the specific activity of the total free P in the 

 tissue and from the specific activity of the free plasma P. We know the 

 magnitude of the extracellular volume and we also know that the phos- 

 phate concentration and the phosphate activity of the extracellular 

 fluid correspond approximately to those of the plasma when the time 

 of experiment amounts to a few hours or more. These quantities are 

 determined in every experiment. We also know the free P content of 

 the tissue and its activity. In liver, for instance, when the specific acti- 

 vity of the free tissue P (intracellular plus extracellular) is 1000 and that 

 of the plasma 1500, the liver and plasma contain 20 and 4 mgm of free P, 

 respectively, and when one-quarter of the liver weight consists of extra- 

 cellular fluid, we obtain a value of 974 for the specific activity of the 

 free intracellular phosphorus. The difference between the specific acti- 

 vities of the intracellular and of the total free P may be considerably 

 greater than in the above case, e. g. in muscle tissue where phosphate 

 slowly penetrates into the cells. In such cases, however, the difference 

 may be calculated likewise by means of a similar procedure. 



This procedure cannot be used for in vitro experiments. In this instance 

 the tissue is washed with a phosphate-free Ringer's solution in an at- 

 tempt to remove the extracellular phosphate. In this method it is hardly 

 possible to avoid washing out a part of the intracellular phosphate. 

 This does not nullify the determination of the correct value for the spe- 

 cific activity of the intracellular free phosphate, but decomposition of 

 a part of the existing organic phosphorus compounds possibly takes place 

 during the washing and phosphate is thus split off. This phosphate is 

 probably only slightly active and therefore "dilutes" the activity of the 

 original free phosphate. The result of this circumstance is underestima- 

 tion of the specific activity of the free intercellular phosphorus and a 

 corresponding overestimate of the calculated amount of nucleic acid 

 formed during experiment. 



FuKCHGOTT and Shorr^^\ when experimenting with sections of heart 

 muscle, found a 32 per cent cleavage of the creatine-phosphoric acid 

 present at the beginning of the experiment in a period of 90 min at 

 37.55° C, and Chaikoff and co-workers^^) found a decomposition of 



^i^R. F. FuRCHGOTT and E. Shorr, J. Biol. Chem. 151, 65 (1943). 

 ^2) B. A. Fries, H. Schachner and J. L. Chaikoff, J. Biol. Chem. 144,59 

 (1942). 



