L. H. GRAY 



recently conducted by Dewey and Boag^^, which will be reported more fully 

 in Paper 29, very large doses of electron radiation have been given to bacteria 

 in a single 2 [xsec pulse. Bacteria which were aerobic ( 1 per cent oxygen in the 

 gas phase) and had the corresponding radio-sensitivity at the beginning of 

 the pulse, responded as if they had been nearly anaerobic, owing, as we be- 

 lieve, to the radio-chemical utilization of all the oxygen initially present in 

 the cell during the early part of the radiation pulse. These experiments open 

 the way to an experimental evaluation of the maximum interval which may 

 elapse between irradiation and the availability of the oxygen if the oxygen is 

 to influence the overall radiation response. The extreme rapidity with which 

 a cell can show a change from aerobic to anaerobic sensitivity, and con- 

 versely, would be consistent with the view that oxygen is interacting with 

 short-lived chemical species produced by irradiation. 



An event of decisive importance in the development of the theory that 

 oxygen influences radiation damage by participating in the chemical changes 

 initiated along the tracks of individual ionizing particles, was the observation 

 of Thoday and Read^*-^^ that while oxygen greatly influences the damage 

 to root meristem cells when this is initiated by X-rays, it has litde or no 

 influence when the damage is initiated by a-rays. It is now known that the 

 ratio of the aerobic to the anaerobic sensitivity of many cells is a function of 

 the LET of the ionizing particles, being greatest for the low LET radiations 

 such as y and X radiation, intermediate for neutrons, and least for high 

 LET radiations such as a-particles. This situation has a rather complete 

 counterpart in radiation chemistry, since a number of chemical reactions 

 are known for which the ratio of aerobic to anaerobic yield is similarly depen- 

 dent on LET. Among the earliest chemical reactions to be studied in detail 

 in this connection were the oxidation of ferrous sulphate and the formation 

 of hydrogen peroxide by the irradiation of pure water, and in each case it 

 was postulated that the role of oxygen was to interact with the hydrogen 

 atoms H . which result from the radiolysis of water to form HOg • , which is 

 a powerful oxidizing agent. Accordingly it was suggested that oxygen might 

 have a similar role in radiobiology^*^. It now appears doubtful whether 

 HOg* is an important intermediate in chemical reactions induced in aqueous 

 solution at neutral pH since the radical is believed to be largely dissociated 

 into H^ and O2 above ~ pH 2^\ but it is still generally accepted as an 

 intermediate in the oxidation of ferrous to ferric in aerobic solutions at very 

 acid pH. Alper^^ proposed that in radiobiology the oxygen molecule 

 enhances damage by combining with the organic radical rather than the 

 hydrogen atom. This still leaves open the question as to how far the primary 

 radicals formed by the radiolysis of water contribute to biological damage, 

 since the organic radical with which oxygen interacts may have been formed 

 either as a result of ionization of the molecule itself, or, secondarily, by 

 reaction with an H. or OH. radical. Hutchinson's data^''*''^ indicate that 

 as regards the inactivation of two enzymes and a co-enzyme in the living yeast 

 cell, the indirect process is the more important. The idea that the oxygen 

 molecule reacts with an organic radical accords better not only with what has 

 been said above about the dissociation of HO.,* at neutral pH, but also 

 with the low concentration (~ 5 (xM/l.) at which oxygen is effective radio- 

 biologically^^. 



12 165 



