4 



POLLARD: Well, you can answer it for direct action. 



It is interesting that Fluke, Drew and I (4) measured the radiation 

 sensitivity of the transformation of just rough to smooth. Fluke and Marmur(5) 

 have measured the streptomycin and, I believe, one other transformation from 

 Hotchkiss' laboratory with substantial agreement, although with a rather inter- 

 esting extension that there may be two classes of molecules present. 



I had a letter from Dr. Latarjet to the effect that he has found some- 

 what similar, though rather less, sensitivity. One can say that the apparent 

 radiation sensitive volume for the direct action of radiation on nucleic acid cor- 

 responds to a molecule in the order of between 3 and 7 million molecular 

 weight -- very sensitive. So that the temptation for me, as a direct action 

 man, is to see this enormous figure and to want to say, "Well, that is the key 

 to radiobiology. " It is a great temptation because it is a huge figure. Never- 

 theless, it will be most unwise to take this attitude until we have the other data 

 as to whether, by any chance, 1 ion pair can also inactivate such nucleic acid 

 molecules by the medium of water. If that is true, than we have just as good, 

 in fact, a better line of approach in terms of the action of water. So these are 

 important data. Moreover, I think it is so important that it would be quite all 

 right if we knew it only within a factor of 20. So if one had a purified trans- 

 forming principle with only 40 percent inactivity it still would be worth working 

 on. 



BENNETT: I don't know if this is the type of thing you are thinking 

 about, but Dr. Stent at the University of California is incorporating essentially 

 carrier-free p32 phosphate into phage and determining how many disintegra- 

 tions are required to inactivate them. It is a different type of phenomenon, I 

 think. 



POLLARD: There are two things there. In addition to the ionization, 

 there is actually a change of atomic species plus a violent recoil and actual 

 motion of a heavy atom, which is a very drastic thing indeed, in the case of a 

 big molecule. Dr. Kamen knows about this because he did pioneer work on the 

 subject. (6) The effect of incorporating P^^ is some 30 times as great as the 

 effect of radiation from the outside. So that there is clearly something else 

 taking place that is not normally present in ordinary radiobiology. Radio- 

 biology is not concerned with making nucleic acid radioactive. If that were so, 

 the cross-section would be very small, I believe. 



KAMEN: I wonder if you could tell us how the half-life was measured 

 for radicals in water. 



POLLARD: I cannot describe it in detail because it involves a lot of 

 plane diffusion constants. The procedure was essentially that of Mazia and 

 Blumenthal. Two separate monolayers, one of catalase and one of bovine 

 serum albumin, were deposited on a chromium-plated glass slide which was 

 placed in a clean water solution, with precautions for no oxygen, and then X- 

 irradiated. Loss of function was measured by the ellipsometer technique in 

 one case. In the case of the catalase, it was described in terms of the amount 

 of hydrogen peroxide converted. In the case of the bovine serum albumin, it 

 was measured for the amount of specific antibodies that would hook on to the 

 surface of the albumin. Both gave quite similar figures. But the significant 

 thing about the experiment is that in both cases, exposures of the order of 

 200, 000 r were needed to produce any effect. This is quite different from an 

 enzyme that is distributed throughout the material; for example, for catalase 

 in bulk dissolved in water. Catalase would undoubtedly be inactivated by a 



