P32 INCORPORATION 193 



labeled desoxyribonueleic acid results from a change in the rate of in- 

 trusion of labeled phosphate into the tumor cells. 



In contrast to the incorporation of P^", the incorporation of C^'* into 

 desoxyribonueleic acid (88), and into ribosenucleic acid (124) (the rate 

 of incorporation into the purines of these compounds being investigated) 

 was found to be depressed, although the latter to a smaller extent than 

 the former. 



Holmes (98) recently investigated the effect of irradiation on the in- 

 corporation of S^^ and of N^^ into the protein moiety of nucleoproteins. 

 No interference was observed. 



When comparing the effect of irradiation on the incorporation of P^^ 

 or C^'* into desoxyribonueleic acid and into ribonucleic acid the following 

 consideration may be of interest. In the tumor a large part (one-half 

 or less) of the labeled desoxyribonueleic acid molecules represents ad- 

 ditional material due to growth (16). From the labeled ribonucleic acid 

 molecules a much smaller amount (one-sixth or less) (18, 19) represents 

 only additional material due to the growth, the rest being due to a re- 

 newal process which may take place without involving such fundamental 

 steps as does the formation of additional molecules. The blockage of 

 any of these steps due to irradiation will markedly influence the forma- 

 tion of labeled desoxyribonueleic acid molecules. As to the effect of ir- 

 radiation on the formation of ribosenucleic acid, an early observation 

 obtained by applying the ultraviolet technique indicated that formation 

 is promoted (61, 119). The results obtained in the study of P^^ incorpo- 

 ration into ribosenucleic acid of irradiated sarcoma did not, however, 

 indicate an increased formation, while C^* incorporation into purines 

 of both desoxyribo- and ribonucleic acid was found to be depressed by 

 irradiation. 



In a study Jones (20) compared the extent of incorporation of ad- 

 ministered P^^ into the tumor desoxyribonueleic acid of rats receiving 

 60 r whole-body irradiation with the incorporation in non-irradiated 

 controls. The difference was found to be 0.18 per cent depression of 

 P^^ incorporation per roentgen, which is the same order of magnitude 

 as the depression effect of radiation on red- and white-corpuscle for- 

 mation. This coincidence is interpreted by assuming the synthesis of 

 desoxyribonueleic acid to be a function of the mitotic activity of the 

 tissue, so that this also represents the effect of radiation on a prolifer- 

 ative process. 



Isotopic tracers can advantageously be applied in this type of investi- 

 gation. Not only is it difficult by the usual chemical methods to deter- 

 mine a change of only 1 per cent in the desoxyribonueleic acid content 

 of a tumor, but even if such a difference was ascertained — Stowell (21) 



