MECHANISMS OF CHEMICAL RADIATION 



PROTECTION 



Z. M. BACQ 



Universite de Liege et Labomtoire de Reche.rches pour la Protection des 

 Populations Civiles, Liege, Belgium. 



AND P. ALEXANDER 



Chester Beatty Research Institute, Royal Cancer Hospital, London, England. 



SUMMARY 



The mechanisms by which added chemical substances — in particular cystea- 

 mine — ak-er the chemical changes j^roduced by ionizing radiations are discussed. 

 Examples are given of protection of proteins and nucleic acids by radical scaveng- 

 ing, energy transfer and repair. In mammals the protection given by — SH 

 compounds is not due to the production of anoxia and is believed to be brought 

 about by one or more of the chemical mechanisms working at the level of the 

 primary lesion. Protection of mammals by histamine is due to the production of 

 anoxia by pharmacological means because localized protection confined to the 

 site of injection is not seen. When considering the mechanisms of protection 

 every tissue or organ must be studied separately as several mechanisms may be 

 active simultaneously or synergistically. 



INTRODUCTION 



The term 'chemical protection' must be confined to those situations 

 (discovered in 1949 by Bacq and Herve and Patt et al., 1949) where the 

 administration of a chemical before irradiation reduces the biological 

 effects of a subsequent exposure to ionizing radiation. Post-irradiation 

 treatments are not protection; they are therapy. 



Chemical protection has been observed at all levels of radiobiology 

 ranging from chemical changes of individual substances to death from 

 radiation sickness in mammals. In every case, protection is much less 

 marked with densely ionizing than with sparsely ionizing radiations. 

 The uniform pattern of radiation protection and the fact that chemical 

 protectors are inactive in wet systems (e.g. vegetative cells) when ad- 

 ministered after (even very soon after) irradiation, indicate that these 

 substances act by reducing the extent of the initial or ])rimary chemical 

 lesions. 



Many reviews are available that give lists of the substances that 

 protect, of the organisms that have been used and of various cells, 

 tissues and functions that have been carefully studied in protected 



301 



