10 NEUTRON EFFECTS ON ANIMALS 



small to absorb radiation directly to any appreciable extent but the mole- 

 cules take a share in the radiation product and thus the number of molecules 

 in the main solute inactivated is reduced. So that the evidence of the 

 effects of radiation upon pure enzyme solutes cannot now be taken at face 

 value. 



Recent and very interesting work by Mitchell upon X-rays (5) much 

 along the same lines as our own, in regard to the effects of radiation upon 

 the nucleic acid of the cell nucleus, comes to the conclusion "Evaluation 

 of the ionic efficiencies shows that significant disturbances of both carbo- 

 hydrate and nucleic acid metabolism are produced by therapeutic doses of 

 X and gamma radiations, probably by means of enzyme inactivation". 

 After radiation in dividing cells he believes there is, due to mitotic inhibi- 

 tion, no significant increase in the thymonucleic acid synthesized by the 

 nucleus. This histochemical evidence suggests that the small increase in 

 nuclear absorption observed in some cases after radiation is due to an in- 

 crease in pentose nucleotides either formed within the nucleus or diffused 

 into it from the protoplasm. In our work upon neutrons, it has been con- 

 sidered that nucleic acid which is of large molecular weight, when broken 

 into smaller molecular weight, pentose nucleotides, may diffuse when the 

 larger molecular weight substance could not and that these nucleotides 

 will inhibit the action of the enzyme ribonuclease to inhibit the synthesis 

 of nucleic acid so necessary to mitosis and cell division. This work is 

 continuing and gives an interesting area of research into neutron mech- 

 anism. 



Chemical processes, and life is a chemical phenomenon, are involved in 

 the immediate effect of ionizing radiation. This is apparently true in 

 regard to the turnover of desoxyribonucleic acid and Hevesy (6) has shown 

 that after radiation with 1000 r of X-rays the turnover of phosphatide is also 

 diminished from the nuclei and from the cytoplasm less so. This is of 

 interest because of the importance of desoxyribonucleic acid phosphorus 

 and phosphatide phosphorus in the nuclei of dividing cells. 



One theory in regard to radiation effects is that there exists in the cell 

 a special sensitive volume within which ionizations are biologically effective 

 and that these account for subsequent changes. More than one ionization 

 may be required to produce a biological effect but any ionization which 

 occurs within the cell, but outside the sensitive volume, is ineffective. 

 This view of the mode of action of radiation has become known as the 

 "quantum hit" or target theory. Differences in sensitivity to radiation 

 are explained by the chance distribution of ionization in the vital volume 

 of the cell system. 



An alternative hypothesis is that chemical or metabolic changes are 

 produced in the cell (or in the environment ?) by irradiation and that the 



