EFFECT OF X-RAYS ON NUCLEIC ACID IN JENSEN-SARCOMA 695 



activity, the weight of the magnesium ammonium phosphate sample 

 and the chemically determined phosphorus content of the nucleic acid 

 as shown in the following example: 



The activity of 1 mgm of nucleic acid phosphorus is equal to 8.60 X 

 X80x5/2.44x71x4 = 4.98 counting-tube pulses/min. The figure 8.60 

 denotes the activity of the magnesium ammonium phosphate containing 

 the nucleic acid phosphorus, 2.44 is the phosphorus content of the total 

 nucleic acid in the sarcoma (mgm); 80 is the weight of the whole of the 

 precipitated magnesium ammonium phosphate sample (mgm), and 71 is 

 the weight of sample placed under the counting tube; 5/4 is the correction 

 to be applied because only four-fifths of the solution obtained by ashing 

 the nucleic acid was used for the radioactive investigation. 



After this it is necessary to compare the radioactivity of 1 mgm of 

 nucleic acid phosphorus with the radioactivity of 1 mgm of free-phosphate 

 phosphorus in the sarcoma. The postulate mentioned on p. 690, that the 

 phosphate phosphorus isolated from the sarcoma has the same activity 

 which would have been found at any time during the experiment, does 

 not hold good. The absorption of the injected phosphate requires several 

 minutes and although the membrane of sarcoma cells is, as discussed 

 elsewhere^^\ highly permeable to the penetration of phosphate, the 

 infiltration of phosphate from the plasma (or from the extracellular 

 fluid) into the sarcoma cells likewise requires time. Furthermore, the 

 activity of the plasma phosphate changes during the experiment. At 

 first it increases and then decreases, as a result of the inflow of labelled 

 phosphate into the cells of the organ and vice versa. Strictly speaking, 

 therefore, we should measure the activity of 1 mgm of sarcoma phosphate 

 at different times and in this way calculate the mean value of the phos- 

 phate activity during the experiment. This average activity of 1 mgm of 

 phosphate should then be compared with the activity of 1 mgm of nucleic 

 acid phosphorus determined at the conclusion of the experiment. We 

 are, however, not so much interested in the accurate values of nucleic 

 acid metabolism of this acid in the sarcoma as in the effect of X-rays 

 on the nucleic acid metabolism. We have, therefore, replaced the above 

 somewhat tedious procedure by the following one. 



The activity of 1 mgm of nucleic acid phosphorus was compared with 

 the activity of 1 mgm of free plasma phosphorus determined at the end 

 of the experiment. The activity of the plasma phosphorus at first increases 

 during the experiment and later decreases; the activity value measured 

 two hours after injection is not very different from the mean value in 

 the course of the experiment. We have also compared the activity of 

 1 mgm of nucleic acid of the sarcoma with the activily of 1 mgm of free 

 phophorus in the liver, and have thus attained a second scale for com- 



iQ. Hevesy and H. Euler, Ark. Kern ISA, 15 (1940). 



