107 



changes in which the static dielectric constant comes in and provides very ef- 

 ficient shielding. The electrostatic effect produced by ionizing radiation is 

 very quick and in a certain period of time operates with practically no dielec - 

 trie constant and is therefore much more effectual. 



The major thing about present developments is that there is beginning to ap- 

 pear a framework on which one can assess the facts that are available, which is 

 something that is urgently needed. In radiobiology we have an enormous wealth 

 of facts. Some of them are dubious and all of them have to be graded according 

 to the investigators. But they do exist. You have no way of picking between 

 them except when they diametrically oppose each other. It is very important, 

 really, that we have some feeling that there is a rationale in the background 

 somewhere. 



With regard to the reactions which occur in biological tissue, it is important 

 to remember how complex biological tissue is. We have heard it ad nauseam 

 but we might as well hear it again. I think that all of us who have come from 

 physics and chemistry into biology have come very optimistically and very en- 

 thusiastically and have been caught up short. 



POLLARD: It is a lot simpler than I had expected. 



BURTON: It should be made a matter of record that Dr. Pollard shook his 

 head in the negative. 



KAMEN: A major difficulty with the biological system is that a given sub- 

 stance inside the cell behaves differently from the same substance outside the 

 cell. Consider a piece of liver. It has lots of the enzyme catalase in it and it 

 is important in liver function. Now, irradiate it. Then take the liver and ex- 

 tract the enzyme and you have a nice protein which you can get out crystalline. 

 When you have done this and then measured its activity, nothing has happened 

 to it. On the other hand, if you take the same enzyme, as Forssberg (6) has 

 shown, and irradiate it in vitro, either in the dry state or in the wet state, with 

 the same dosage it is completely inactivated. 



Of course, there are explanations for this, but the trouble is that this kind 

 of phenomenon makes it dubious as to how to interpret what is going to happen 

 in a cell, because not only does it depend on whether the enzyme is adsorbed on 

 something or is in solution, it depends very much on what other things are 

 around to react with it, and this leaves a situation which is not one for opti- 

 mism but certainly has to be kept in mind. 



MAZIA: When you are working with the living cell, you have a situation in 

 which your system can replace the inactivated units rapidly, and so you may 

 have all kinds of radiation effects which are without any biological significance, 

 and are not detectable unless you are able to set up special experimental situa- 

 tions to detect them. Radiation chemists sometimes study radiation effects on 

 enzymes, but it is by no means certain that inactivation of enzymes is of any 

 great inportance in over-all radiobiological effects. 



KAMEN: Apparently the system has a certain capacity for repair which is 

 in addition to the protective effect of the other protein which is around the en- 

 zyme. So the radiation effect depends on whether it is metabolizing or not. 

 There are many things you can say about radiation, some of which are fairly 

 general. Thus, there always seems to be considerable delay before the effect 

 of radiation is noted. Bombard a system with an enormous dosage and you have 

 to wait a while before the effect is apparent -- the biological effect, that is. 



