MECHANISMS OF ( "HEMIC AL RADIATION PROTECTION 307 



Table 1. Protection against the direct action oj ^ x li)^ rads of 'Z M eV 

 electrons 07i DNA in the heads of herring sperm. '\ 



Protective agent ^jreseut iii DNA present as gel 



suspension (per cent) (per cent) 



None 79 



01 cysteamine 11 



0-02 cysteamine 51 



0-1 thiourea 66 



0-02 thiourea 77 



0-5 tyramine 54 



On irradiation the DNA becomes cross-hnked and is no longer soluble, but is present 

 as a gel. 



t The sperm heads were irradiated as a 12 per cent suspension in water. The sperm 

 head nucleoprotein does not swell and is present as a hard sphere. Under these con- 

 ditions tlie action of the radiation is entirely direct and free radicals formed in the water 

 play no part. 



ESR signal or irradiated sperm heads is not reduced at — 195°C by the 

 addition of cysteamine. In this respect DNA behaves quite differently 

 from serum albumin which has just been discussed. But, on warming 

 to room temperature the radical concentration in the DNA containing 

 cysteamine falls and direct evidence has now been obtained to show that 

 this is due to "repair" of the DNA radicals by cysteamine (Ormerod 

 and Alexander, 1961). 



MECHANISMS OF PROTECTION OF LIVING SYSTEMS, MAINLY 



MAMMALS 



The preceding examples have shown that the chemical reactivity of 

 cysteamine is such that it is capable of protecting by several different 

 mechanisms. Since cysteamine reduces radiation damage in a wide 

 variety of biological systems it is possible that the same mechanisms 

 are not involved in every case. For example the protection of dry seeds 

 shown by Moutschen et al., (1956) is parallelled by a reduction in the 

 ESR signal of irradiated frozen yeast cells (Smaller and Avery, 1959) 

 and an energy transfer mechanism seems to be indicated especially 

 as very high doses are involved. 



We believe that the Eldjarn and Pihl disulphide exchange hypothesis 

 can be rejected as a mechanism of biological protection. At best it only 

 provides for a reduction in radiation damage of proteins and these are 

 unlikely to constitute an important primary lesion for reasons given 

 in our other contribution to this colloquium. But even if damage to 

 protein should prove to be important the data reported in this paper 

 show that proteins are protected against both the direct, and indirect, 



