Ionizing Radiations and Enzymes 



31 



The diminishing efficiency of the protective substance 

 when its concentration is increased can be explained best by- 

 assuming that a protector molecule, after reaction with a 

 radical, may possibly be in a metastable state, or may have 

 formed another organic radical and thus be able to "hand on" 

 the effect of the first collision with a radical to the indicator 



CHANCING quotient' FOR CP 

 CURVES ARE THEORETICAL, POINTS EXPT 



A DIMETHYLUREA WITH CP 3<^7*/ 

 C B ° GLUCOSE WITH CR SO^V*^ 



C.R 90 Wm^ 



LOCO 



10 



»' 



CONC. IN 



lO-" 



c 



10 



lO" 



Fig. 3. Curves showing "Changing quotient" for carboxypeptidase. Curves 



are theoretical, points experimental (Dale, Davies and Meredith, 1949. 



Reproduced by permission of Fhil. Trans.). 



molecule. Protector molecules reacting in the described 

 manner would fail to fulfil their function as protectors. 



Barron and co-workers (Barron and Dickman, 1949; 

 Barron et al., 1949) assume that the principal point of attack 

 by oxidizing radicals is the SH group in enzymes in which 

 SH is essential for enzymic activity. Inactivation could be 

 prevented by blocking the SH groups with mercaptide- 

 forming reagents, and lost activity restored by adding gluta- 

 thione to reduce the disulphide, provided that the X-ray 

 doses were not so high as to lead to denaturation of the 



