PRIMARY EFFECTS (ON CHEMICAL COMPOSITION) 



243 



beings without prior knowledge of the genetic result. Butler et al (1 959) have 

 partially clarified a rather confused picture, made not the least bit simple 

 by the fact that the molecule is huge: as obtained from leucocytes it has a 

 molecular weight of about five million. Two standard methods of determin- 

 ing molecular weight (also outlined in Chapter 6) were used. One, by meas- 

 urement of the viscosity of DNA solutions and measurement of the speed 

 with which the molecules settle out in a high-speed centrifuge, showed that 

 the molecular weight falls during irradiation, as though the big molecule 

 were being split into pieces. The other, however, by light-scattering tech- 

 niques, gave a constant molecular weight during irradiation. The implica- 

 tion is that the molecule is broken all right, but the pieces do not completely 

 uncoil. With such a loosened structure, easier degradation by heat should 

 result, and that is just what has been found, not only for DNA but also for 

 several enzymes as well. 



loss of NH3 and loss of H2 

 rupture of hydrogen bonds 



rupture of sugar-base linkage and oxidation of the sugar 

 iberation of purine bases 

 breakdown of pyrimidine* bases 

 rupture of nucleotide chain 

 liberation of organic phosphate 



Nucleic 

 acid 



Figure 9-6. Things That Can Happen if a Macromolecule Such as DNA is Irradiated by 



Ionizing Radiation. 

 * Order of radioresistance: adenine > guanine » cytosine > uracil > thymine (on iso- 

 lated components). 



Enzymes are known to have various sensitivities to radiations, at least in 

 dilute solutions. The data on these are somewhat suspect because of the 

 marked effect of impurities. However, one of the most sensitive enzymes 

 seems to be carboxypeptidase; ribonuclease is ten times more resistant, and 

 catalase ten times more resistant yet. Some enzymes are inactivated even 

 when in the dry, crystalline state (this supports the target theorists). All 

 enzymes studied are inactivated in aqueous solutions by ionizing radia- 

 tions — this can mean direct target action or attack by radiation-produced 

 free radicals, probably both. 



Of the small molecules present in tissues, the most interesting from the 

 medical point of view are cysteine and certain other molecules containing the 

 sulfhydryl ( — SH) group. These molecules are particularly sensitive to oxi- 

 dation by radiation, and therefore are among the most effective protectors 



