ACCELERATED HEAVY IONS ON ENZYMES AND YEASTS 203 



magnitude as one obtains by dehydrating tlie cells at room temperature 

 by the use of glycerol. For X-rays it was also established that the yeast 

 cells exhibited an oxygen effect (Birge and Tobias, 1954). Thus the 

 current experiments performed by Manney et al. (lOBU) were directed at 

 combined effects of glycerol -induced dehydration and the effect of 

 gaseous environments. With X-rays 6 M glycerol in water caused a 

 dose reduction factor of about 2-6 in aerated suspensions of cells. The 

 cells were exposed to the various heavy particles on the surface of a 

 millipore filter ; survival scorhig was made on the basis of colony counts 

 from single cell isolates plated on full nutrient agar medium. The RBE 

 has a peak for irradiation in air, in buffer with nitrogen or in glycerol. 

 Glycerol protects the cells with respect to buffer with an approximately 

 constant dose increase factor of 1 -7 for all LET values compared to the 

 nitrogen-buffer system and a factor of about 2-5 for the oxygen-buffer 

 system, which decreases to about 2-0 for the highest LET tested. The 

 RBE values are plotted in Fig. 4. 



While haploid cells have exponential survival curves for all LET 

 values, the shape of the diploid survival curves is modified at high LET 

 values in a way somewhat similar to those described by Barendsen for 

 human kidney cells. Correspondingly the RBE for high LET is about 

 3-4 for diploid cells. There appears to be more dominant lethal damage 

 from heavy ionization. Whether this is due to chromosome breaks that 

 fail to recombine, or a higher probability for the chromosomes to suffer 

 multix)le breaks from passage of a single heavy particle, we do not 

 know at the present time. 



Preliminary data obtained in our laboratory on the yield of mutations 

 at specific loci as function of the LET indicates that some loci have a 

 similar RBE to that of the inhibition of colony formation, others have a 

 decreasing RBE at high values of LET (Mortimer and Brustad, 1960). 



The mean dose necessary to kill a haploid yeast cell is, at heavy 

 ionization, where the cross-section plateau corresponds to a cross- 

 section of the approximate size of the nucleus of the yeast cells. Thus 

 it appears, that, as in the case of lightly ionizing particles, radiation 

 injury of heavy particles is particularly hazardous if interaction with 

 the cell nucleus is involved. 



These results, make one speculate as to the nature of heavy ion 

 effects in the aqueous medium. Glycerol is not only an antidote for the 

 oxygen effect but it also diminishes the effect in nitrogen. We might 

 surmise that irradiated glycerol scavenges not only OH radicals, but 

 oxygen as well. Or it may in some other, as yet unknown way, interfere 

 with the mediating action of water molecules. 



