262 



CORNELIUS A. TOBIAS, TOR BRUSTAD AND THOMAS MANNEY 



o 



> 





a 



D 

 U 



o 



a: 



4x|0" 



3 - 



2 - 



10' 



10^ 



10^ 



LET (MeV/g/cm^) 



2-5 



2-0 

 - 1-5 



1-0 

 0-5 



CD 



a: 



10' 



Fig. 4. — Relative biological effectiveness as function of linear enei-gy transfer for haploid 

 yeast cells in buffer and in glycerol, exposed to air or to nitrogen atmosphere. Note the 

 protective effect of glycerol over the entire range of LET values. 



in aqueous milieu is due to direct hit and how much is due to indirect 

 mechanisms mediated by water, particularly if one realizes that in a 

 living cell some of the enzyme may be in a specially protected inactive 

 state; e.g. trypsin may be present as trypsinogen. 



EXPERIMENTS WITH YEAST CELLS 



Interpretation of dose-effect relationships is becoming exceedingly 

 difficult when we realize that the effects can result either from direct 

 effects on macromolecules, from indirect effects on the aqueous cell 

 medium and also from macromolecular interactions following radiation 

 in the denser regions of the cells, e.g. in nucleus or mitochondria. Some 

 years ago Zirkle and Tobias (1953) proposed the "migration"' model for 

 explanation of radiation survival relationships obtained on cells of 

 different ])loidies. The model allowed reconciliation of indirect and 

 direct action mechanisms. The effects have also been studied with low 

 and high LET radiations. The cross-section of the resting wet haploid cell 

 in air was found to be constant for the heaviest particles, above LET of 

 109 eV cm2/g and its value is about lO^^cm^. Experimentally it still 

 has not been proved whether or not the radiation action has an inter- 

 mediate component at high LET. We have attempted to determine if 

 the aqueous medium of the nucleus is able to modify the heavy ion 

 effects. Our initial results with X-rays agree with data presented by 

 Gray at this conference. Wood has already shown that a change in 

 phase state modifies the radiosensitivity of the haploid yeast cell to 

 X-rays and that the increased radioresistance is about of the same 



