304 



Z. M. BACQ AND P. ALEXANDER 



100 



90 



1_ 

 CD Cl. 



C 



J ^ 

 ■- E 



OJ : 



cn 



80 



70- 



<L) — 



^D 

 CD E 

 Q_ 1- 



o 



c 



60- 



50- 



40 



1 2 3 4 5 6 



Dose 



Fig. 1. — Protection of a 1 per cent solution of seruni albumin by thiourea and by cystea- 

 mine against *''^Co y-rays. Radiation damage of serum albumin was followed by changes 

 in the sedimentation constant. The protecti\e agents react very much more readily 

 with OH radicals formed in the water so that the protein is unaffected until most of the 

 protective agent has been destroyed. 

 Dose in r x 10^ 



A, No protecting agent; 



B, 10-3M thiourea; 



C, 2x 10^3]vi thiourea; 



D, 2 X 10^3]V[ cysteamine. 



(b) Energy transfer 



The solid polymer polymethylmethacrylate is degraded by irradi- 

 ation. The energy that has to be deposited in the polymer to produce a 

 break in the polymer chain is 61 eV but if the polymer is cast as a film 

 containing 10 jDer cent of cysteamine then 140 eV has to be supplied to 

 achieve the same result (Alexander et al., 1954). This protection results 

 from the fact that more than half of the energy deposited in the poly- 

 mer is transferred to the relatively small amount of cysteamine present. 

 The possibility that this protection is due to a 'repair process' and not 

 to energy transfer has been excluded (Alexander and Toms, 1958). 



Gordy and Miyagawa (1960) demonstrated with electron spin reso- 

 nance (ESR) protection by cysteamine against the direct effect of 

 radiation in a protein. The number of free radicals, as given by the 

 magnitude of the ESR signal, left after irradiating zein was much less 

 if a small quantity of cysteamine was present. While an energy transfer 

 mechanism seems the most probable explanation, the possibility that 

 protection occurred by 'repair' or by disulphide exchange could not be 

 excluded. 



Libby et al., (1961), made a similar observation with bovine serum 



