28 W. M. Dale 



and the indirect actions. Even at a concentration of 20 per 

 cent weight for volume the indirect action is predominant. 

 This presentation apphes to enzymes which have a concen- 

 tration-independent yield of the order of 0-18. If the yields 

 are lower, i.e. if bigger doses are required to achieve the same 

 effect, the chances of the direct action grow correspondingly. 



We have now to consider the situation when an enzyme 

 solution is irradiated whilst the solution contains a second 

 solute. Then the available radicals will be shared by both 

 solutes according to their respective concentrations and 

 reactivity, and as a result they will mutually reduce the radi- 

 ation effect. In other words, the presence of a second solute 

 "protects" the first against the effects of radiation. This 

 protection effect is responsible for an exponential curve when 

 the activity of an irradiated enzyme solution is plotted against 

 radiation dose. As soon as irradiation has started there are 

 two types of solute molecules present, active and inactive, 

 the inactive ones still reacting with radicals, i.e. sharing radi- 

 cals, though this reaction is not scored. The opposite effect, 

 namely that the presence of a second solute leads to an increase 

 of radiation effects, i.e. sensitization, has so far not been 

 observed with enzymes, though it does occur with other sub- 

 stances. This applies in particular to the presence of oxygen 

 as a second solute, which in many biological radiation re- 

 sponses causes an increase of radiation effects via the HO 2 

 radicals and HgOg. No such increase has been observed with 

 carboxypeptidase (Dale, Gray and Meredith, 1949), ribo- 

 nuclease (Colhnson, Dainton and Holmes, 1950) and trypsin 

 (McDonald, 19546, 1955). Carboxypeptidase (Dale, Gray and 

 Meredith, 1949) and ribonuclease (Colhnson, Dainton and 

 Homes, 1950) are stable towards HgOg and are inactivated 

 by the — OH radical. Trypsin is also inactivated by the — OH 

 radical but is reversibly inhibited by HoOg independent of the 

 time of contact, while irradiation has an irreversible effect 

 (McDonald and Moore, 19556). 



The protection effect has been shown to operate quite 

 generally. In particular it was shown in the case of D-amino 



