74 



Discussion 



through it or put a vacuum on so that you get the gas out, you break 

 the structure down. 



Gray : If you remove the oxygen chemically then they are protected ? 



Hollaender: Yes. In the curve shown in Fig. 1 the change in viscosity 

 with increasing irradiation was seen. Now if AET is added (Fig. 2), no 

 effect on viscosity is observed without irradiation (upper curve). With 

 250 r a very large decrease is observed without the AET (middle curve), 

 but with small concentrations a large protective effect is seen. With 

 900 r (lower curve) more AET is needed for maximal protection. Much 

 more complete protection is observed with lower doses of radiation. 



CONCN. OF AET (mmoles/liter) 



Fig. 2. (Hollaender). Protective effect of *S'-j3-amino- 



ethyh'sothiouronium bromide HBr on crude rat 



thymus preparation (W. Fisher, unpublished). 



There is, however, a viscosity change which AET will not protect 

 against. 



de Hevesy: Dr. Lajtha mentioned the effect of the irradiation of the 

 spleen. Is this effect not due to an interference with the formation of a 

 humoral substance and is the much greater effectiveness of the whole- 

 body radiation not partly due to the supression of the formation of this 

 substance in the whole organism? 



Alexander: With regard to the irradiation of DNA in vitro, the resist- 

 ance of DNA to radiation, which has been stressed by Butler, is more 

 apparent than real. Dr. K. A. Stacey (1955, Int. Conf. TtadiohioL, 

 Cambridge, 1954, B.E.C.C. report, 32, 29) at our Institute irradiated 

 DNA from herring sperm in the dry state and then measured its mole- 

 cular weight by light scattering. From the change in molecular weight 

 the energy needed to break one polynucleotide bond can be determined. 



