82 NEUTRON EFFECTS ON ANIMALS 



cellular elements of the blood were greatly reduced and the animal died 

 from the treatmicnt. In the case of both animals, Nos. 5 and 9, a longer 

 time (5 days) had elapsed before the blood samples were taken than with 

 the other animals. 



In neither the bone marrow nor the blood does the reduction in the ribo- 

 nucleinase appear to V)e a primary effect on the enzyme but appears more 

 likely to be in consequence of the reduction in the number of blood cells 

 in the blood or at their site of formation in the marrow. 



Ionizing radiations exert their effect on enzymes (23) either by direct 

 ionization within the molecule, or indirectly by ionization within the 

 a(]ueous environment. Inactivation by the latter mechanism is pre- 

 sumably by the intervention of an intermediate agent formed by ionization 

 or excitation of the water. The direct effect can only be demonstrated 

 with concentrated solutions or dry enzymes (23). In the case of large 

 molecules like viruses direct inactivation was readily demonstrated with a 

 1 per cent \irus solution (23), and it was noted that an ionization in the 

 water has a rather small probability of causing inactivation of the virus, 

 presumably because the activated water can lose its energy on contact 

 with the virus particle without invariably leading to inactivation of the 

 virus particle. The desoxyribonucleic acid, which occurs in the nucleus 

 of cells, might be expected to respond to irradiation like a virus since it has 

 a high molecular weight and high density (15). 



It has been found that ionizing rays which hardly influence respiration 

 or glycolysis very strongly influence cellular division (growth) (24). Cells 

 that are dividing slowly are least vulnerable to ionizing rays, presumably 

 having tim.e to eliminate toxic products before the sensitive process of cell 

 division occurs. These considerations suggest that the nucleic acid in the 

 nucleus of the cell may be the point of vulnerability of living systems to 

 X-rays and neutrons. 



X-rays and neutrons might cause a depolymerization of the nucleic acid 

 to tetra- and mononucleotides. It has been pointed out (G) that, since 

 nucleic acids and their hydrolytic products are inhibitory to the cellular 

 dehydrogenases, any influence changing the ratio of nucleic acid to the 

 more diffusible (i.e., more widely reactive) hydrolytic products might pro- 

 foundly alter the economy of the cell. 



In attempt to demonstrate an effect of neutrons on nucleic acid a 10 

 per cent solution of ribonucleic acid was treated with 750 n. A slight de- 

 polymerization occurred, measured by precipitation with the uranium 

 reagent (25), but it was not sufficient to affect the ability of the nucleic 

 acid to serve as a substrate for ribonucleinase. The molecular weight of 

 ribonucleic acid is, however, much less (2G, 27) than that of desoxyribo- 

 nucleic acid (15). Irradiation may not have to cause so great a change 



