306 General Discussion 



and that is not what matters at all. If anything matters it is the 

 functional part and not the store, and the notoriously small amounts 

 of change by moderate radiation doses in any substance can, of 

 course, have nothing to do with the bulk from which they are 

 changed by radiation. 



My first question is similar to that of the separation of substrate 

 and enzyme and — to extend it — the enzyme can diffuse or the en- 

 zyme is phase-bound and the substrate is diffusing to it. Evidence 

 has been brought forward by some workers that substrate and 

 enzyme, at least in certain cases, are localized separately and in 

 order to get to each other they have to be in transit of some form. 



One more point that I should like to mention is the apparent and, 

 to a certain extent, neglected importance of chain reactions. Some, 

 of course, are known and we have a very interesting example of a 

 dose-rate dependent chain reaction with respect to the liberation of 

 sulphur from thiourea, which at a dose rate of • 39 has G values of 

 17,000-20,000 and there is virtually no limit if one goes down still 

 further with the dose-rate. It is also known that for instance, the 

 oxidation of cysteine to the disulphide is a chain reaction, and 

 furthermore (which makes it so difficult to reconcile any scheme 

 devised by physical chemists for any reaction with radicals) that the 

 action of some of the radicals very often must cause changes which 

 lead to new radicals which are probably very difficult or nearly im- 

 possible to put into a reaction scheme. That refers to oxidations, to 

 dehydrogenations which are one-step reactions, leaving a radical 

 which again may do something, and the phenomenon I have shown, 

 namely the "changing quotient" (i.e. that the protective power per 

 unit mass of the substance added declines on increasing the concen- 

 tration) can be explained also by the formation of a radical from 

 protector molecules which again hands on the energy. 



Holmes: Dr. Hug and co-worker (Hug, O., and Wolf, I. (1956), 

 Progress in Radiobiology, p. 23. Edinburgh: Oliver & Boyd) 

 made a very useful contribution to the knowledge of the effects of 

 irradiation of systems in a steady state. They irradiated with X-rays 

 luminescent bacteria which were emitting light of a steady intensity. 

 A diminution of light-intensity was shown as soon as the irradiation 

 was begun and the intensity fell continuously as long as the irradia- 

 tion continued. Directly irradiation ceased the light emission began 

 to recover and became steady at an intensity rather lower than that 

 originally found. The recoverable part of this system was un- 

 doubtedly an irradiation-sensitive unit of the light system which was 

 restored by the further activity of the bacteria. 



Dale: That is quite possible. There is an American worker too, 

 who works with fireflies. It is a very sensitive reaction. 



