306 Z. M. BACQ AND P. ALEXANDER 



protein is prevented from forming a radical in tlie protein by transfer 

 to the cysteamine in which it is dissipated without radical formation. 



(c) Repair 



Cysteamine was shown to prevent radiation induced cross-linking of 

 polyvinyl alcohol pyrrolidone in aqueous solutions (Alexander and 

 Charlesby, 1955a, b).t The mechanism of cross-hnking jDrobably occurs 

 as follows : 



RH OH° R° 



polymer + radical from ■ — -^ polymer radical 



water 



R° -f RH ^ R —R +H2 (cross-linked polymer) 



Cysteamine protects by transferring hydrogen to R° 



2R° -1-2 SH C2H4NH2 ^ > RH -1-NH2C2H4S.S.C2.H4.NH2 



Profecfion of DNA against direct action^ 



Inactivation of bacteriophage by the direct action of X-rays is re- 

 duced in the presence of cysteine (Watson, 1952) or cysteamine (Marco- 

 vich, 1958; Howard-Flanders, 1960), and this is presumably due to 

 protection of DNA. 



DNA becomes cross-linked by direct action if oxygen is excluded. 

 Cysteamine protects against this process and it seems possible that the 

 protection of phage is due to the prevention of cross-linking. Alexander 

 and Stacey (1959) studied the effect of cysteamine on the cross-linking 

 of DNA in the sperm or the isolated sperm heads from herring, salmon 

 or trout. The sperm heads consist of a nucleoprotamine complex (65 

 per cent DNA and 35 per cent protamine) which is not swollen and 

 contains less than 50 per cent water. On irradiation the DNA becomes 

 cross-linked. This effect can be ascribed entirely to the direct action of 

 radiation, and the free radicals formed in the water play no part. Table 

 I shows that cysteamine reduces the amount of cross-linking. The 

 mechanism of protection does not seem to be energy transfer since the 



t In this system oxygen appears to protect when tlie polymer concentration is low by 

 reacting to give a peroxide, 



R° + 02 R02'= 



which cannot cross-link. This oxygen effect would not apjiear as protection in a bio- 

 logical system, since the molecule RH would lose its biological activity by peroxidation 

 as well as hy cross-linking. 



X DNA can of course be protected against the indirect action of radiation by com- 

 petitive removal of the radicals but the cysteamine concentration has to be high, as 

 DNA reacts very readily with the radicals formed from water. Indeed DNA was shown 

 (Alexander, 1953; Alexander et al., 195.")) to be an excellent jirotective agent against the 

 indirect action of radiation on a polymer. 



