192 CELLULAR METABOLISM 



for the increase in oxygen consumption cannot, however, be concluded 

 from these observations. This is a point of interest, among others, in 

 view of a possible connection between desoxyribonucleic acid synthesis 

 and oxygen consumption. 



Effect on P^^ Incorporation into Nucleic Acid 



From the viewpoint of the chemist cell division is necessitated by 

 the spectacular accumulation of synthetic products in the nucleus. By 

 interfering with mitosis, ionizing radiation can be expected to interfere 

 with many of the synthetic and rearrangement processes involved in 

 cell division. When comparing the rate of incorporation of labeled phos- 

 phate into the desoxyribonucleic acid extracted from the growing Jen- 

 sen sarcoma of irradiated and of control rats (15, 16, 16a), it was found 

 that irradiation with an x-ray dose of a few hundred roentgens or more 

 reduced the rate of formation of desoxyribonucleic acid in the sarcoma. 

 Assuming that the intracellular inorganic P or organic phosphate, which 

 comes comparatively rapidly into exchange equilibrium with inorganic 

 phosphate (96, 97), was the significant precursor of the desoxyribonu- 

 cleic acid phosphorus of the tumor (should this assumption not be ten- 

 able, the renewal figures would be still higher than stated above), the 

 percentage renewal of the desoxyribonucleic acid P of the sarcoma was 

 calculated to be about 2.5 per cent in 2 hr in the controls, while for the 

 irradiated sarcoma half that value was found. In experiments taking 

 1 hr or less Holmes (17) observed irradiation with 2000 r to reduce the 

 rate of incorporation of P^^ into the desoxyribonucleic acid of the 

 sarcoma to one-half of that found in the controls. 



As irradiation may influence the permeability of phase boundaries, 

 one may be tempted to interpret the decreased rate of incorporation of 

 P^^ in the desoxyribonucleic acid molecule of irradiated tumors as 

 caused by a diminished rate of passage of labeled phosphate into the 

 tissue cells of the irradiated sarcoma. That this is not the case can be 

 shown by a comparison of the ratio of the specific activities of the 

 plasma inorganic P and tumor inorganic P in the controls and in the 

 animals with irradiated sarcomas. A difference in the permeability is 

 indicated by the results obtained. This is, however, insufficient to ex- 

 plain the depressed rate of incorporation of P^^ into the desoxyribonu- 

 cleic acid of the irradiated sarcoma. Furthermore, Holmes (17) found 

 irradiation to influence to only a minor extent the rate of incorporation 

 of P^^ into ribonucleic acid in contrast to its incorporation into desoxy- 

 ribonucleic acid of the tumor. This result is not compatible with the 

 assumption that the effect of irradiation on the rate of formation of 



