336 SPECULATIONS ON CELLULAR ACTIONS 



to use the findings of radiation chemistry in our attempts to visuahze 

 possible mechanisms of radiobiological actions. 



Bearing in mind the foregoing facts of radiation physics and chem- 

 istry, I shall now present a model of an imaginary mechanism of radio- 

 biological action. For the sake of concreteness I shall first present a 

 special case and then attempt to generalize. 



Mechanism of Radiobiological Action 



As our special case (Fig. 1), let us consider a known radiobiological 

 action, namely, inhibition of division of a unicellular organism, for ex- 

 ample a yeast. We start (at top of Fig. 1) with the normal state (a) of 

 the cell ; this state includes the ability of the cell to divide if it is not ir- 

 radiated. We then imagine various states 0, y, d n, ir) through which 

 the cell passes to reach the end effect (co), which in this case is the failure 

 of the cell to divide. Intervening between these imaginary states are 

 the imaginary processes {A, B, C, M, P, Z) by which the cell passes from 

 state to state. The first process {A) is energy transfer from ionizing 

 particles to water molecules, the cell thus entering a state ((S) in which 

 ions derived from water are present and diffusing. (There is, of course, 

 good evidence that this state is not imaginary.) We now assume that 

 OH radicals are formed from the H2O+ ions (process B and state 7). 

 Next we imagine a slight complication of mechanism, namely the forma- 

 tion of some sort of peroxide by the reaction of OH with a suitable molec- 

 ular species (process C and state 5). The peroxide reacts with and in- 

 activates a species of gene (process M and state m) which normally is 

 essential for the synthesis of an enzyme necessary for accomplishment 

 of cell division. With the genie species inactivated, insufficient enzyme 

 is present (process P, state tt), and cell division is inhibited (process Z, 

 state co). 



The processes and states so far mentioned are assumed to be integral 

 parts of the mechanism of the specific radiobiological action under dis- 

 cussion. They are accordingly considered relevant to that particular 

 action. In addition, it appears extremely probable that there are nu- 

 merous states and processes (as /3', y', 5', fx', A', B', C, M') which are 

 irrelevant to the action leading to the effect oj (but may be relevant to 

 other actions in the same cell). For instance, process B' (formation of 

 H atoms) very probably occurs if OH radicals are formed (process B). 

 However, if the mechanism of the action under consideration is promoted 

 only through the OH radicals, as we have assumed, then the H atoms 

 are irrelevant to this action, although they may be reasonably expected 

 to promote some other action, which may or may not be observed. In- 



