INITIAL RADIATION DAMAGE AT SUB-CELLULAR LEVEL 11 



ref. Bacq and Alexander, 1961). We shall come back to this ])oint later. 



Another and perhaps more serious objection to an enzyme theory is 

 that almost all the biological effects and, in particular, cell killing are 

 brought about more effectively by densely ionizing radiations such as 

 a -particles and fast neutrons than by sparsely ionizing radiations such 

 as hard X-rays or y-rays. For a radiochemical reaction, shown to occur 

 in the cell, to be a candidate for the role of primary lesion, it must, at 

 least qualitatively, show the same relative efficiency for radiations of 

 different ionizing densities. This requirement effectively eliminates 

 most radiochemical reactions studied so far since in almost every case 

 densely ionizing radiations were shown to be less efficient. This is the 

 case for the inactivation of enzymes where a-particles are many times 

 less effective than sparsely ionizing radiations. 



Gordy and Shields (1958) have claimed that when proteins are 

 irradiated in the dry state, the energy is funnelled into the disulphide 

 bonds and Ehrenberg and Zimmer (1959) have ascribed great bio- 

 logical significance to this reaction. The experimental evidence for this 

 hypothesis was the observation that the electron spin resonance 

 patterns after irradiation of protein and of the sulphur-containing 

 amino acid cystine were similar. We have investigated this claim 

 (Libby et al., 1961) and Fig. 4 shows that the pattern of protein and 

 cystine are quite different and that there can be no question that the 

 same radicals are produced. The patterns only resemble one another 



COMPARISON OF IRRADIATED ALBUMIN WITH CYSTINE 



TREATMENT 5«IOrAT-l95C 



TREATMENT S.IOr AT 20°C 



TREATMENT OPENED TO AIR 



Fig. 4. — Comparison of the electron spin resonance pattern obtained by irradiating 

 bovine serum albumin and cystine with ^^Co y-rays in vacuo. The irradiations and 

 measui-ements were carried out at — 195°C and at 20°C. After measurement, the samples 

 were opened to the air and their ESR remeasured (for experimental details see Libbj' 



et al., 1961). 



