318 . BIOLOGICAL EFFECTS OF RADIATION 



that the loss in sohibiUty after radiation is due to an internal tautomeric 

 change, possibly a rearrangement of peptide linkages into closed-ring 

 systems, which would diminish the affinity for water, but one must confess 

 that we are as yet ignorant of the nature of the change we speak of as 

 " denaturation " and the mechanism by means of which it is brought 

 about. 



RELATION OF PROTEIN DENATURATION TO GENERAL PHYSIOLOGICAL 



LIGHT REACTIONS 



The best known result of radiation is the lethal action produced on 

 the hving cell. This is probably due to a combination of several causes, 

 one of which is the denaturation and coagulation of the cell proteins. 

 Changes in the colloids in living cells are reversible and an irreversible 

 change, such as that produced by radiation denaturation, would bring 

 about the death of the cell. The sensitivity curve of microorganisms to 

 ultra-violet radiation has been determined (43, 46) and the similarity 

 of the curve to the absorption curve of proteins is good evidence that the 

 absorption of radiation by cell proteins produces irreversible changes 

 resulting in death. As proteins are denatured at any pH, the primary 

 radiation change always takes place. This may be followed by gel 

 formation or coagulation if the protein is at its isoelectric point or if 

 salts are present in sufficient quantity. Photographs of microorganisms 

 killed by ultra-violet radiation show that coagulation has actually taken 

 place. 



In the eye media the cornea and lens are subject to protein denatura- 

 tion in the presence of ultra-violet radiation. The lens protein is not 

 at its isoelectric point so that it does not coagulate after radiation unless 

 there is an abnormal salt content, but it is very possible that denaturation 

 of the lens protein, even without subsequent flocculation, may affect 

 the normal nutrition of the lens. 



The skin cells are killed by radiation (ultra-violet, X-rays or radium) 

 and on disintegration liberate a substance which produces capillary 

 dilatation and there are probably extensive denaturation and coagulation 

 of tissue proteins leading to changes in the permeability of cells and a 

 shift in the water balance. 



The rapidly elTective ultra-violet rays are so strongly absorbed by 

 the skin that little, if any, radiation reaches the blood stream, but X-rays 

 and gamma rays penetrate all the tissues and the changes they produce 

 are consequently widespread. Isolated serum proteins are. rapidly 

 denatured by ultra-violet radiation and slowly denatured by X-rays 

 and gamma rays. When radiated as they occur in combination in the 

 serum, however, they show no flocculation and serum is known to exercise 

 a protective action in many radiation effects. Red cells hemolyze when 

 radiated in isotonic salt solutions but not when radiated in serum. 



