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CORNELIUS A. TOBIAS, TOR BRUSTAD AND THOMAS MANNEY 



increased radiosensitivity to oxygen by a factor close to two when radi- 

 ated by 50 kV X-rays. One would expect a similar oxygen effect due 

 to the more energetic S-rays in the track of a heavy ionizing particle 

 whereas the densely ionizing core of the track may not exliibit an oxygen 

 effect. For trypsin, carbon irradiation and exposure to even heavier 

 ions does not show an oxygen effect at one atmosphere pressure within 

 experimental error and we can safely say that the oxygen effect is less 

 than 20 per cent of the oxygen effect with X-rays. Thus it appears that 

 heavy ions exert most of their effect in the densely ionizing central 

 portion of their track. Another test is to compare the effect of two 

 different accelerated particles which have different atomic number but 

 both of which have the same linear energy transfer. Since the two kinds 

 of particles achieve the same LET by travelling at different velocities, 

 the 8-ray distributions will be somewhat different: the maximum 

 velocity of S-ray electrons is approximately the same as the velocity of 

 the primary particles. Comparison of data taken with neon and oxygen 

 or carbon and boron particles has so far failed to disclose a definite 

 S-ray effect. 



The remaining explanation for the large cross-section is the assump- 

 tion that the inactivated enzyme molecules do not all receive an ionizing 

 particle passing directly through them, but sometimes they can become 

 inactivated by interaction with neighl)Ouring radiated molecules, be 

 this chemical interaction, some sort of physical effect or excitation 

 energy transfer. Norman and Ginoza (1958) indicated the existence of 

 radiation protection of catalase by glutathione in the dry state, 

 Hutchinson showed some years ago that the nature of aggregation of 

 dry bovine serum albumin can alter its radiation sensitivity. Braams 



Fig. 2. — Modification of the radiation sensitivity of trypsin in the dry state by dextran, 

 ribose, cysteine and pinacyanole. Tlie data were obtained for accelerated carbon ions. 



