ACTION OF IONIZING RADIATIONS ON CELL CONSTITUENTS 



far radiobiological work has been of great importance to analyse the possible 

 mechanisms of action of radiations and one is just beginning to uncover the 

 possible mechanisms of primary steps. Unfortunately this has led to very 

 little information concerning the cellular constituents whose alterations 

 produce biological effects. 



(7) Effects on proteins irradiated in vivo 



Physical chemists may study the action of radiations on various chemical 

 or physical properties of proteins, if these are treated pure or in solution. 

 This is not possible when the same constituents are irradiated in vivo : 

 methods of purification are not yet adequate and biochemical properties of 

 proteins must be relied on : enzymes may fortunately be studied a very 

 short time after irradiation of whole organisms in rapidly prepared extracts 

 or homogenates : we believe that effects on enzymes found immediately 

 after the irradiation of a variety of diflferent cells or organisms would be of 

 fundamental importance. 



Barron and his co-workers studied the effects of X-rays on several enzymes 

 in solution and were able to distinguish between two classes of differently 

 susceptible enzymes : those whose activity depends on reduced — SH groups 

 and which are inactivated with high ionic yields (adenosine triphosphatase, 

 hexokinase, succinic dehydrogenase, triose phosphate dehydrogenase) and 

 those whose inactivation needs a higher expenditure of energy (such as 

 ^-amino-acid oxidase, carboxypeptidase, trypsin) and whose activity is not 

 dependent on — SH groups". This scheme is certainly not a definite one, 

 for ribonviclease, classified as radio-resistant by Barron, has recently been 

 shown to be dependent on — SH groups^^^; and the relative difficulties 

 to oxidize these groups by ionizing radiations remains to be explained. 



But — SH enzymes are within the cell in close contact with reducing 

 agents like glutathione, ascorbic acid or other types of protecting agents 

 and their oxidation in vivo by ionizing radiations, although highly probable, 

 has not been proved to be of direct importance for the cell. The importance 

 of Barron's contribution is to have found specific chemical changes respon- 

 sible for biochemical effects and to have shown that these eflfects may be of 

 great biological importances^. Enzymes have, however, other chemical 

 groups indispensable for their biochemical effectiveness and these may also 

 be susceptible to radiations : this chapter of enzymology is still in its infancy 

 and only a few active groups of a limited number of enzymes are known so 

 far. Free carboxyl groups are needed for the activity of insulin and lyzozyme, 

 some free phenol or indole groups as well as amide or guanidyl groups are 

 needed for both these enzymes and also for trypsin ; S — S groups are required 

 for insulin whilst free amino groups are needed for lyzozyme ^2. Most of 

 these groups are known to be susceptible to chemical effects of ionizing 

 radiations and it is probable that if radiation eflfects on specified protein 

 side groups were better understood one might be able to deduce, as Barron 

 did for — SH groups, which ones of these other groups are most important. 

 Research of this type might lead to the discovery of other protective 

 mechanisms than those already found. 



We have summarized in Table I what is known of the effect of ionizing 

 radiations on different individual enzymes. On examination of these data, 



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