128 



MICROSOMAL PARTICLES 



ence between these two, following Britten, Roberts, and French [11] and Cohen 

 and Rickenberg [12], is designated as the metabolic pool. In figure 2b, the 

 same process is employed on bacteria that have received 253,000 roentgens of 

 cobalt 60 irradiation. It can be seen that the total amount of radioactivity in- 

 corporated in the TCA-insoluble fraction is reduced; the rate of incorporation 

 is also less. The amount taken up by the whole cells is less, though not quite 

 to the same extent as the reduction in the TCA-insoluble fraction. In figure 2c, 

 still higher irradiation was employed, and the reductions in both fractions are 

 still more apparent. 



Similar data are shown in figure 3 for the uptake of proline. There is a 

 marked difference from the effects with methionine in that more irradiation is 

 necessary to reduce the amount incorporated into any fraction, about twice the 

 dose in the TCA-insoluble fraction and nearly 10 times in the whole-cell frac- 

 tion. Because of the disparity in these effects it can be seen that the pool actu- 

 ally rises after bombardment. 



For these two amino acids, with the exception of the proline pool, the amount 

 of activity remaining seems to be a diminishing exponential function of the 

 dose. The data scatter somewhat, but we have no real evidence in favor of a 

 multiple-hit type of process, where the activity remains nearly constant and 

 then rapidly falls. The per cent remaining activity for methionine incorpora- 

 tion is plotted against dose in figure 4. For proline incorporation the effect 

 of radiation is definitely less in the TCA-insoluble fraction than for methionine, 

 and very markedly less in the whole-cell case. Such dose effect curves can be 

 analyzed statistically in terms of an inactivation volume V, which is the sensi- 

 tive region that must escape an ionization in order to retain the effect being 

 measured. If the ionizations occur at a number / per unit volume, then the 

 average number of ionizations occurring in the sensitive region is IV, and by 

 the Poisson relation the probability that the region will escape is e~ Iv . Thus 

 the natural logarithm of the ratio remaining to that in the unirradiated con- 

 trol should be —IV. For 37 per cent remaining, the value of IV is unity. 



Table 1 summarizes the results of cobalt 60 studies. The first column gives 

 the 37 per cent dose found from the survival curves. The second column gives 

 the corresponding number of primary ionizations per cubic centimeter for such 

 a dose. Column three, the sensitive volume, is the reciprocal of the value in 



TABLE 1. Summary of Incorporation Studies in the TCA-insoluble Fraction of E. coli 



Irradiated with Cobalt 60 y Rays 



