PROTECTION 201 



in the transfer of the phosphate groups into the desoxyribonucleic acid 

 molecules, its inactivation frustrating this process. 



In the study of the effect of the ionizing radiation on cellular metabo- 

 lism it may be a profitable line of attack to compare the activity of 

 enzymes involved in carbohydrate metabolism and also of other enzymes 

 extracted from irradiated growing tissues and from growing tissues of 

 controls. Phosphatase activity in Kato's rabbit sarcoma upon irradi- 

 ation with 600 r was found to decrease after the lapse of only 7 days (52) . 

 A decrease in the activity of alkaline phosphatase present in the plasma 

 of the rat occurs after x-irradiation (53). 



The anomalies observed in the distribution of desoxyribonucleic acid 

 were found to be those produced in the distribution of phosphatase (102). 

 Parallelism between the intensity of the nuclear alkaline phosphatase 

 reactions and the turnover rate of desoxyribonucleic acid was empha- 

 sized by Brachet (54). This observation enhances interest in the study 

 of the inactivation of alkaline phosphatase. 



A reason why the inactivation of enzymes may perhaps be considered 

 the primary seat of inactivation is the low concentration of enzymes in 

 the tissues. The decomposition products of water or other tissue con- 

 stituents, which on their way to the enzyme escape reaction with the 

 numerous "protecting" molecules present in the tissue, may reach and 

 inactivate enzymes. The number of such successful molecules may, 

 however, not suffice to inactivate more common tissue constituents pres- 

 ent in larger amounts. This line of thought is based on experiments on 

 enzyme inactivation in vitro. The probability of inactivation decreases 

 with increasing concentration of the product to be inactivated (32). 



From the fact that inactivation is produced in some cases by very 

 restricted doses we have to conclude not only that some enzymes are 

 easily inactivated, or their new formation is easily prevented, but also 

 that the cellular placing of these enzymes must be such that they are 

 easily reached by the inactivating radicals (38a). The possibility can- 

 not, however, be excluded that sulfhydryl groups, for example, present 

 in numerous compounds involved in mitosis and other processes, and 

 present in a much higher concentration than those of — SH enzymes, 

 are oxidized by ionizing radiation and thus set out of action (31, 31a, 

 50). 



Quite apart from the important problem as to which is or are the 

 sensitive spots in desoxyribonucleic acid synthesis, and possibly in that 

 of other nuclear constituents as well, the sensitivity of the mitotic proc- 

 ess to the effect of ionizing radiation can to some extent be ascribed to 

 the great speed with which the synthesis of highly complicated mole- 

 cules takes place. Ample evidence is available that the reduction of 



