Electron Spin Resonance in the Study of Radiation Damage 259 



in the reduced form, or other — SH agents may exert their protective effects by 

 forming an — S — S — link with an — SH of the protected molecule, as Eldjarn 

 ct al. proved for cysteamine. Electron sinks which collect the electrons knocked 

 out of the holes, and thus prevent them from causing damaging reactions, 

 would also be protective agents. The most desirable electron storage tank 

 would be a molecule which would accept the electron without itself becoming 

 dissociated, would hold it loosely, and would give it up easily when it was 

 needed elsewhere. 



I should like to add that the electron sources (traps for electron holes) 

 attached to side chains are not necessarily restricted to protective action against 

 direct hits : they may also protect from some of the indirect effects. Certain 

 free radicals produced in the medium around the protein might exert their 

 damage simply by steahng away an electron from some point in the protein. 

 This would of course be replaced by an electron borrowed from the protective 

 group, just as if the electron had been removed by irradiation. The effects of 

 ionized Oo or HoO — if there are such things — would be, I suppose, to ionize 

 the protein when they came near it. An OH radical might react with the 

 protein molecule, or it might simply ionize the protein and form 0H~. I do 

 not know which would happen in a particular case. I simply wish to illustrate a 

 possible unrecognized protective mechanism against indirect action of the 

 radiation. Some specialists on radiation effects evidently believe that the 

 damage to the protein of the cells is due mainly to the indirect effect of radicals 

 produced in the medium around the protein molecules and that the protective 

 action of such agents as cystine or glutathione is entirely the elimination of 

 these radicals before they get to the protein. I do not mean to imply that such 

 effects and the mechanism proposed to protect against them are not very im- 

 portant. What seems clear is that protection is also needed against direct hits 

 as well, and if possible against those radicals or charges produced in the medium 

 which survive long enough to reach the protein. Because of the ability of the 

 protein to transfer a charge, it now seems possible to provide this type of pro- 

 tection too. In fact it has already been achieved in some measure by Eldjarn e/ a/. 



The protective mechanism which I have proposed is strikingly related to 

 enzyme activity. Pollard's group at Yale, and perhaps others, have been 

 making experiments which show, I beheve, that a single hit in a large enzyme 

 molecule by an ionizing particle is enough to destroy the enzyme activity of 

 that molecule. This is not strange if the sensitive sites for the enzyme activity 

 are synonymous with the sinks or sources for electrons about which we have 

 been talking, and the ability of the enzyme molecule to conduct a hole or 

 excitation is required for enzyme action. 



I hke to think of the protective agents which are described here as enzymes 

 which prevent reactions. I know, of course, that the normal function of an 

 enzyme is to cause reactions. Some enzymes, so I understand, exert their 

 catalytic action by accepting spare electrons for a time and giving them up 

 again later. In the vivid language of Professor Henry Eyring, they take over 

 the unnecessary children (electrons) during the divorce proceedings and give 

 them back after the remarriages have taken place. One kind of 'protective 

 enzyme' supplies children to prevent divorces (broken bonds) and then later 

 recovers children indistinguishable from those given up (electrons all). Another 



