PROTEINS AND NUCLEIC ACIDS IN SOLUTION 49 



same conditions of y-irradiation in vacuo. It appeared that up to 

 45 SH-groups were oxidized per radical on irradiation in vacuo. On 

 irradiation in air this discrepancy increases, since the number of 

 peroxidized radicals (after the addition of O2), according to Alexander, 

 is equal to that of primary radicals, while the number of oxidized SH- 

 groups under these conditions increases twofold. This quantitative dis- 

 crepancy is so considerable that the direct oxidation of SH-groups by 

 radiation can hardly be explained by the formation of free organic 

 radicals through the breakage of valency bonds, since in this case a 

 closer correspondence between them and the number of oxidized 

 SH-groups should be expected. The suggestion of Alexander (1957) 

 on the role of molecular O-i" ions in the oxidative effects observed seems 

 to be more likely. 



However, it may be expected that lethal or sublethal radiation doses 

 (500 to 1,000 r) will lead to the appearance in the cell of hundreds or 

 thousands of protein or nucleic acid molecules changed to a greater or 

 lesser degree in chemical or structural respects. 



Moreover, the primary inactivated volumes in the cell calculated 

 according to statistical theory of the action of radiation have in many 

 cases the size of only one or several molecules. These calculations are 

 usually made on the grounds of the so-called "target-theory" which, 

 in its physical basis, gives just the method of calculating the value of 

 some inactivated volume with respect to various conditions of radiation 

 (kinds of radiation, intensity of the dose, etc.) without defining the 

 nature of this volume. Within this range the application of this theory 

 does not have serious objections. In a paper by Pollard and his co- 

 workers (1955) it was shown that statistical theory enabled one to 

 determine directly by means of radiation the molecular weight of 

 enzymes, hormones, viruses, etc., in a good agreement with the results 

 of other experimental procedures. In our laboratory similar results have 

 been obtained for preparations of lipoxidase (Budnitzkaya et al., 1956) 

 and insulin (Volkova et al., 1957). 



Thus the fundamental question of radiobiology is why does the 

 damage to only a fcAv molecules or of some cell portion of only mole- 

 cular size lead to the death of the cell, while damage to hundreds of 

 similar molecules in the cell produces no lethal effect, or, from the 

 point of view of the theory of open systems, how can damage to a few 

 molecules bring about essential disturbances in the stationary state of 

 the cell? 



The investigation of these questions was mainly directed in the litera- 

 ture to the establishment of specifical structural changes arising after 

 irradiation of the molecules of such biochemically important substances 



