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V. N. TARUSOV 



outside the scope of analytical methods. Moreover, secondary reactions 

 begin to develop very rapidly and it is very difficult to differentiate the 

 former from the latter. 



At the same time the kinetic peculiarities of primary radiochemical 

 reactions in various components of cells and tissues leave a natural 

 imprint on the development of secondary reactions and on the bio- 

 logical manifestation of damage from radiation. This temporal de- 

 pendence helps to determine the character of the basic primary processes. 



The action of protective chemical agents is clearly directed towards 

 these primary reactions. These agents inhibit them. Therefore the study 

 of kinetics can provide us with valuable information concerning the 

 nature of j^rimary processes. The most peculiar feature in radiation 

 damage is the so-called oxygen effect, a lowered radiosensitivity of 

 irradiated organisms at low oxygen pressures. 



This effect, described by many authors for various organisms, has 

 been interpreted as a result of the decrease in the products of water 

 radiolysis and especially of the radical HOo- In order to understand the 

 mechanism of the reactions that are inhibited at low oxygen jjressure, 

 one must pay attention to the kinetic peculiarities of this phenomenon. 

 The peculiar kinetic feature of the protective action of oxygen, when 

 its pressure is lowered, consists in the fact that the protective effect 

 does not increase gradually with the lowering of oxygen pressure, but 

 goes up rapidly after a certain pressure threshold. Therefore the lethal 

 curves (Sumarukov, 1958) depending on oxygen pressure have always 

 the shape of parametric parabolas. (Fig. 1). 



This kinetic peculiarity has been established by many investigators 

 ^^'ho have studied the ])henomenon quantitatively on mice, tissue 

 (sarcoma cells) bacteria, bean roots and others. 



