RADIATION AND THE CELLULAR SYNTHESIS OF PROTEIN 



81 



YiG. 2. — The percentage uptake of L-arginine at 6 min as a function of dose. The plot of 



the percentage is on a logarithmic scale and it can be seen that if nlno is the ratio of 



uptake to original uptake, then the relation ln(w/«o) = constant X dose is obeyed. 



The results are shown in Fig. 4. In Fig. 4 we plot the nncorrected 

 cross-section found from the relation \n{nlno) = —SD against the 

 number of primary ionizations per cm generated by the bombarding 

 particle. We can include the data for cobalt bombardment by realizing 

 that if i is the number of primary ionizations per cm per heavy jDarticle 

 then for particles of very low linear energy transfer the equivalent 

 volume ionization density is the number of j^articles per cm'^ (D) times 

 the ionization per cm for each one (^) so that / = Di. Since equations 

 (1) and (2) are equivalent we can set VI = SD or VDi = SD or V =Sli. 

 Thus V appears as an initial slope, on the S versus i graph and has been 

 so represented. It can be seen from Fig. 4 that uracil and glucose be- 

 have quite differently toward heavy particle bombardment than the 

 other metabolites, with the possible exception of methionine. The 

 sensitivity to cobalt irradiation is quite high and yet the expected high 

 cross-section does not develop for heavy particle bombardment. The 

 sensitive region is roughly sj^herical of radius 160 A in the case of uracil 

 and 90 A for glucose. Since some radical migration must be occurring 

 it w^ould be expected that the actual organelle size is rather less. Both 



