MECHANISMS OF CHEMICAL RADIATION PROTECTION 305 



albumin which has been freeze-dried from a sohition containing cyste- 

 amine. Figure 2 shows that the intensity of the ESR spectruin was 

 much reduced by cysteamine (i.e. there were far fewer free radicals left 



PROT ECTION OF ALBUMIN BY CYSTEAMINE 



ALBUMIN ONLY ALBUMIN -f CYSTEAMINE lO* 



TREATMENT 5 X lo'r AT-I95°C 



TREATMENT WARMED TO 20 C 



TREATMENT OPENED TO AIR 



Fig. 2. — Protection of solid bovine serum albumin by cysteamine against the direct 



effect of ^^Co y-rays as shown by the electron spin resonance pattern which measures 



the number of radicals formed. The protein-cysteamine mixture was prepared by freeze- 



drying a solution containing 10 parts of protein and 1 part of cysteamine. 



in the protein). For this protein disulphide exchange can be excluded 

 as the mechanism of protection since, as already mentioned, cysteamine 

 does not react with the protein's disulphide groups under the conditions 

 used. 



Of particular interest is the fact that if the protein is irradiated at the 

 temperature of liquid nitrogen ( — 195°C) and its ESR spectrum 

 measured at this temperature as well, many more free radicals are seen 

 to be present than if the irradiation is done at room temperature. On 

 allowing the protein to warm up the intensity of the ESR signal im- 

 mediately decreases (i.e. the radicals disappear). Measurements at room 

 temperature only reveal those radicals that are inherently stable and 

 which are prevented from combining with one another by steric factors. 

 The measurement at — 195°C reveals all the radicals that are formed, as 

 at the low temperature subsequent chemical changes of the radicals are 

 prevented. Cysteamine also reduces the number of radicals formed at 

 — 195'C and this can only be due to energy transfer, and rejtair cannot 

 be mvolved. Cysteamine appears very radiation-resistant as far as the 

 formation of radicals is concerned, and when irradiated by itself gives 

 only a very low ESR signal even at - li)5°C. Thus, energy taken up by 



