Ionizing Radiations and Enzymes 29 



acid oxidase (Dale, 1942), that the two components making 

 up the enzyme, namely the flavineadenine dinucleotide and 

 the specific protein, when irradiated together lost in terms of 

 oxygen uptake about 60 per cent of their activity, this loss 

 being due to the inactivation of the protein only while the 

 dinucleotide was protected. When irradiated singly, and then 

 joined, the loss was about 90 per cent. 



It is important to be aware of possible differences in the 

 response to radiation by non-enzymic substances which may 

 be present as protecting co-solutes, e.g. as substrates for 

 enzymic action or as products of metabolism. The protection 

 effect offers a method by which the effect of radiation on 

 non-enzymic substances can be measured when they are 

 co-solutes in an enzyme solution. The enzymic activity is 

 then the reference against which the effect of radiation on the 

 co-solute is measured. Such measurements make it possible 

 to detect radiation effects when ordinary analytical methods 

 applied to the non-enzymic substance itself would sometimes 

 fail because of the smallness of the effect. 



The results of such experiments are as follows (Dale, 1947; 

 Dale, Davies and Meredith, 1949): for large molecules the 

 protective power is roughly proportional to the molecular 

 weight and no specificity is found. However, if one considers 

 small molecules, of which a special atomic group forms the 

 greater part, very marked specificities appear, which, even if 

 they do occur in big molecules, would get lost in the over- 

 whelming excess of other atomic groups of average reactivity 

 with radicals. The outstanding examples of such a specific 

 effect is given by a comparison of the protective effects of urea 

 and thiourea. Whereas urea is hardly protective at all, the 

 substitution of O by S in thiourea causes a 10,000-fold increase 

 in protective power. 



Without going into details of the specificity of the pro- 

 tection effect, I should like to stress that sulphur-containing 

 compounds, and sulphur itself, play a special role, and that 

 generally the remainder of the molecule has an effect on the 

 protective power of any particular atomic group. 



