55 



400- 



100 200 



TIME (MINUTES) 



their experiments, there is a continual 

 breakdown and repair, which results in 

 the maintenance of the enzyme level in 

 the unirradiated cell at some equilibrium 

 value. In the irradiated cell this process 

 of repair does not take place or it is in- 

 terfered with in one way or other. 



BARRON: Were those cells sus- 

 pended in a medium containing nitrogen? 



SHERMAN: The cells were sus- 

 pended in nitrogen-free medium. But, 

 even so, Billen's hypothesis was that 

 there is still some normal repair in 

 these cells and that this is inhibited. The 

 experiments of Billen and Lichstein (Z) 

 on the adaptive formation of hydrogen- 

 ase, which is interfered with markedly 

 by irradiation, were used as evidence for 

 this. However, this enzyme may be a 

 special case because it requires certain 

 amino acids, glutamate, among others, 

 for its formation. 



Figure 2. The influence of temperature 

 on X-ray damage to bacterial respiration. 

 The control cups contained 54x 10°and the 

 experimentals, 56 x IC* colony-forming 

 organisms. Filled circles represent the 

 controls; triangles, the irradiated cells. 



That protein synthesis can go on 

 after irradiation is well established. 

 Holweck and Lacassagne (4) long ago ob- 

 served that yeast cells would form giant 

 cells, and Brace (5) showed that the for- 

 mation of giant cells was not due to the uptake of water, but that protein synthe- 

 sis was taking place. Also we have some data that show that the nitrogen in the 

 medium is utilized as rapidly by irradiated as by nonirradiated cells. 



Another kind of evidence that protein synthesis can take place in X irra- 

 diated cells is furnished by the experiments of Spiegelman, Baron and Quastler 

 (6). They reported that galactozymase formation was not impaired by exposures 

 that resulted in inability of more than 99 percent of the cells to form new colo- 

 nies. 



These examples suggest that enzyme synthesis can and does go on in 

 irradiated cells. What may be happening in the experiments by Billen et al. , is 

 that cellular repair and protein synthesis continue, but that the repair is aber- 

 rant. This results in the gradual establishment of a set of abnormal enzymes. 

 Their data (Figure 1) is suggestive of a progressive deterioration. 



Another explanation proposed by Billen was that there may be a certain 

 proportion of enzymes in excess of the enzyme requirement, which comes into 

 play to replace those that have been damaged. However, after exposure of 5,000, 

 15, 000 and 60, 000 r, the period of normal respiration appears to be nearly the 

 same. Cells exposed to 5,000 r were inhibited least; at 15,000 r, an intermedi- 

 ate inhibition of respiration was observed, and at 60, 000 r the rate of O-^ con- 

 sumption fell quite rapidly to low values. 



The curve we have been discussing was from an experiment con- 

 ducted at 37"C (Figure 1). Cells treated in the same way but incubated 



