BASIC RADIATION BIOCHEMISTRY 277 



culties. The constitution of nucleic acids is still a matter of controversy. 

 Although the compounds contained in the molecule are known, it has not 

 yet been discovered how they are joined together and, furthermore, the 

 methods of isolating nucleic acids yield variable products. It seems as if 

 they are nearer to their natural condition the less drastic the methods 

 used for their isolation. Thus the somewhat ill-defined identity of 

 nucleic acids has led to many investigators having specimens of varying 

 properties at their disposal, and this may account for the discrepancies in 

 the results reported. 



All investigations (Taylor, Greenstein, and Hollaender, 1948; Sparrow 

 and Rosenfeld, 1946; Butler, Gilbert, and Smith, 1950), however, seem to 

 indicate that depolymerization occurs as a result of X radiation, but 

 divergence of opinion exists as to the extent of this and its cause. It 

 seems to be certain that the change of viscosity is initiated by X radiation 

 but continues for several hours after irradiation has stopped until a new 

 equilibrium is reached. This new equilibrium depends on the radiation 

 dose given. Loss of birefringence is observed by most investigators, and 

 sedimentation tests in the ultracentrifuge point to a breaking up of 

 nucleic acid fibers into smaller fragments. There is perhaps a possibility 

 that the viscosity change once initiated by radiation is a self-propagating 

 colloidal change. In support of this explanation is the loss of alcohol 

 precipitability without any demonstrable chemical change, since tests for 

 ammonia and organic and inorganic phosphate of a dialysate were com- 

 pletely negative. Moreover, viscosity changes resulting from irradiation 

 are observed for a number of substances which, chemically unrelated to 

 nucleic acids, share with them the property of forming gel-like structures. 

 Sparrow (1944) has collected the literature on the effects of various radia- 

 tions on physical properties of irradiated substances. This list shows 

 that on exposure to X rays a decrease of viscosity occurs in substances 

 other than nucleic acids, e.g., gelatin, agar, apple pectin, synovial fluid, 

 and nucleohistone, and an increase in the protoplasm of Arbacia eggs. 

 According to Taylor et al. (1948) hydrogen peroxide added in high con- 

 centration (10~^ M) is without effect on the viscosity, whereas Butler et al. 

 (1950) find a change of viscosity with a concentration of hydrogen per- 

 oxide of 0.001 M, and this change is produced in their experiments by an 

 X-ray dose of 22,000 r. The question of whether the effect is caused by 

 hydrogen peroxide therefore appears to be undecided. 



CONCLUDING REMARKS 



In summing up the development of the research on biochemical radia-. 

 tion effects during the last twenty years it may be said that the most 

 striking advance made lies in the fact that on the basis of experimental 

 evidence both the direct and indirect action of ionizing radiations can be 



