MECHANISMS INVOLVED IN THK IXITIATTON OF 

 RADIOBIOLOGICAL DAMAGE IN AEROBIC AND 



ANAEROBIC SYSTEMS 



L. H. GRAY 



Briti.'^h Empire Cancer Campaign Research Unit in Radiobiology, Mount 

 Vernon Hospital, Northivood, Middlesex, England. 



SUMMARY 



Loss of reproductive integrity in spores^ vegetative bacteria and cells of higher 

 plants and animals is in general a result of damage to a very small number of 

 macromolecules which are important foi- proliferation. In dry spores a good deal 

 is now known about the nature and lifetime of intermediates in the chemical 

 reaction chains leading up to the damage to these macromolecules. Water, when 

 present in small amounts, profoundly modifies the reaction chains and shortens 

 the lifetime of many intermediates. In cells of high water content a considerable 

 fraction of the damage may jjroceed from the radiolysis of the water. The conse- 

 quence of a particular kind of macromolecular damage, which has arisen along 

 a particular chemical pathway, may be greatly iafiuenced by nutrional factors, 

 both before and after irradiation. The influence of the chemical and nuti'itional 

 factors is interrelated. 



THE NATURE OF THE PROBLEM 



The aim of radiobiologists is to trace the course of events forward 

 as far as possible, from the initial interaction between the ionizing par- 

 ticles and the molecules present in the living cell, through the formation 

 of excited states, unstable intermediates of finite life, and on to chemical 

 changes which would be stable in a non-living system but which repre- 

 sent functional defects when they occur in certain organelles of the 

 living cell. These functional defects become in turn the starting point 

 for a further sequence of chemical changes, and may be regarded as 

 primary lesions at the biochemical level. 



The ionizing particles are moving too fast to excite molecular vibra- 

 tion levels directly. The energy is transferred first to the electronic 

 system of the recipient molecule and, almost always, in amounts which 

 are large compared with the activation energies for the ordinary 

 chemical reactions which the molecules are known to undergo. Thus 

 in the irradiated cell there arise, almost at random, centres of energy 

 absorption of such magnitude that many alternative pathways of 

 chemical reaction become possible simultaneously at each site. As 



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