45 



sidered. It is very difficult to determine the system that is going to protect 

 and the one that is going to act as a source of free radicals to enhance radiation 

 action. What I should like to do is to extend Dale's concept and to say that you 

 not only have protection but you may also have enhancement of X-ray action by 

 the constituents present. 



PATT: Do you mean to imply that the sodium chloride protection is 

 due, in this instance, to the sharing of free radicals? 



BARRON: I think it is a combination of the chloride with the protein. 



PATT: Dale's original protective effect was, I believe, somewhat 

 different and was more a matter of competition or sharing than of combination. 

 He showed subsequently that there were different types of protective agents and 

 that the relationship between the concentration of the protective substance and 

 the degree of protection was not entirely simple. While most of the effects 

 could be thought of in terms of the sharing of radicals, the protection by certain 

 agents fell off with increasing concentration. 



JONES: 1 think Dr. Barron's point as to the enhancement effect is a 

 very important one. I, for one, have found it very difficult to see how you can 

 get any irradiation effects comparable to a simple water system at the high con- 

 centration of original substance existing in the cell. Everything would protect 

 everything else. 



PATT: Yes, but then you have to go back to the thought that you are 

 not dealing with a homogeneous system. As suggested by Dale, there may be 

 dilute areas alternating with more concentrated areas and surface effects of 

 various sorts. In other words, you simply cannot compare the cell with a con- 

 centrated solution. 



JONES: It seems that the balance of this dual system, on one hand 

 enhancing radiation effect, on the other minimizing the damaging reaction of 

 ionization, would be quite sensitive and dependent upon the density of ionization 

 and upon the type of protective substances used. 



POLLARD: There isn't a consistent story on density of ionization. 

 Chromosome breakage is greater with more dense ionization. I see no simple 

 explanation that is possible. 



PATT: Yet in solution there is generally a greater effect with the low 

 than with the high ion density radiations. 



POLLARD: And for other things too, but not for chromosome break- 

 age, let's say. 



PATT: For most types of biological effect, the effectiveness general- 

 ly increases with increasing ionization density; yet we find the reverse situa- 

 tion when we work with simple aqueous systems. 



MAGEE: We often think about the high energy of ionizing radiation as 

 supplying the energy to make reactions go and perhaps intuitively we think that 

 radiation-induced reactions have high free energies, but we can also have re- 

 actions in the cell that are exothermal and have a negative free energy change. 

 In such cases, we can have terrific enhancement. You just sort of trigger these 

 off and they go on, so that the enhancement in such cases, can be very high. In 

 pure chemical systems there are reactions which have G values of many, many 



