116 H. Laser 



Laser, H. (1954). Nature, Lond., 174, 753. 

 Laser, H. (1955). Nature, Lond., 176, 361. 

 MiCHAELis, L., Schubert, M. P., and Smythe, C. V. (1936). J. biol. 



Chem., 116, 587. 

 Pirie, a., Heyningen, R. van, and Boag, H. W. (1953). Biochem. J., 



54, 682. 

 Stapleton, G. E., Billen, D., and Hollaender, A. (1952). J. 



BacterioL, 63, 805. 

 Sutton, H. C. (1952). Radiation Chemistry, Disc. Faraday Soc, no. 12, 



p. 281. 

 Tahmisian, T. N., and Devine, R. L. (1955). Radiation Res., 3, 182. 



DISCUSSION 



Alper: Mr. Paul Howard-Flanders and I have some results which 

 are relevant to what Dr. Laser has been discussing. We took Esch. coli B 

 in a starving condition and bubbled oxygen through them for a good 

 long time, so that they should have exhausted most or a good deal of 

 their endogenous substrate, in order to see what the radiosensitivity 

 under nitrogen would be. At first it seemed as if the nitrogen survival 

 curve was quite different if one had treated the bacteria in this way; 

 but a more rigorous examination of this phenomenon showed that it 

 was, in fact, due to the circumstance that the sensitivity of Esch. coli B 

 is affected by extremely low oxygen concentrations. This means, of 

 course, that in order to distinguish very clearly between the survival 

 curves you get under oxygen or under complete anoxia, you have to 

 have extremely rigorous conditions of oxygen exclusion. These survival 

 curves were all done with very dilute suspensions of bacteria, and with 

 suspensions in which the gas mixture was bubbling through con- 

 tinuously, so that one could feel fairly confident at least about the 

 oxygen tension in the fluid surrounding the cells. Now there are several 

 points relevant to Dr. Laser's argument about the necessity for the cells 

 to be metabolizing in order to show the oxygen effect. Longmuir (1954, 

 Biochem. J., 57, 81) gives a figure which is analogous to the Michaelis 

 constant, the oxygen concentration at which the bacteria are respiring 

 at half their normal rate. This concentration is 2-2 X 10"® m oxygen, 

 and on this basis they would be respiring at 95 per cent of their normal 

 rate at something like 4 x 10 ' m; the region within which they are 

 reaching full oxygen radiosensitivity, however, is something of the 

 order of 10-20 (jlm. We have also done survival curves with these 

 various gas mixtures in the absence and in the presence of substrate. 

 When we compared the curve which represents the points in the absence 

 of substrate, with the curve obtained when succinate was added, there 

 was no difference for the nitrogen point, for complete oxygenation, or 

 for • 02 per cent oxygen. There was, in fact, only one point where the 

 succinate apparently made a slight difference, and since it was only at 

 the one point I think this will certainly need verification. Apart from 

 this experiment the succinate made no difference at all to the survival 

 curves. 



