THE ACTION OF IONIZING RADIATION ON ENZYMES AND VIRUSES 



between one atom and another. It is felt that this process of very rapid 

 exchange can only take place within the molecular structure where the 

 atoms are bonded by electron exchange mechanisms. The transfer of 

 energy therefore takes place rapidly along the chains and continually occurs 

 untif such time as it is either lost by some method of de-excitation such as 

 the return of an electron to an atom to restore the original condition. Until 

 that time, however, there is a rapidly moving region of excitation, which 

 also corresponds to a region of weak bonding since the electron exchange 

 necessary for bonding is not present. If a molecule has an alternative 

 configuration which can result from such a weakness then this may take 

 place. This may correspond to the formation of an unwanted crosslinking 

 in which case the molecule is permanently distorted and will no longer 

 function in its biological way. Or it may be that due to thermal agitation 

 certain weak links are present in the molecule at various places, and that 

 the presence of an additional weak link induced by radiation near these 

 regions may cause again the formation of an unwanted and semipermanent 

 structure. In this way radiation induces the permanent distortion of the 

 molecule into another form and so causes its inactivation. 



It is clear that such a process, while largely confined to the molecule 

 itself, may well have energy requirements imposed on it for eflfectivc action. 

 Thus it may not be sufficient for one bond breakage to occur : it may be 

 necessary that three or four be produced inside the molecule. This has 

 been found to be the case in the studies made at Yale, particularly for 

 ionizing action on the solubility of the molecule. For this to be lost it is 

 generafly found that three or four ionizations must occur inside the molecular 



structure. 



In view of the effects which are covered by the explanation given above, 

 which may still not be the true explanation but which gives a means of 

 compact description of what is Hkely to be occurring, it is possible by radia- 

 tion action involving heavy particles of different velocities, and also fast 

 electrons, to determine the following three things regarding a molecule. 

 (/) Its maximum cross section, (2) its sensitivity in terms of a number of 

 ion pairs needed for inactivation, (3) its approximate thickness. These 

 three quantities can only be determined if irradiations under many different 

 conditions are used. These can be illustrated by considering the irradiation 

 of B. subtiUs cells by deuterons and electrons and subsequent assay of these 

 cells for the two enzymes cytochromoxidase, and succinic-dehydrogenase. 



Studies by Pow^ell and the author'^ have shown that these enzymatic 

 properties are lost under deuteron and electron bombardment and that the 

 loss varies with ion density of the bombarding radiation in a manner which 

 permits the deduction of molecular sizes and thicknesses for these two 

 enzymes as follows. 



Enzyme 



Cytochrome oxidase 

 Succinic dehydrogenase 



Molecular Weight 



160,000 

 310,000 



Length 



200 A 

 390 A 



Thickness 



36 A 

 36 A 



72 



