69 



DUBOIS: In 1 hour? 



SHERMAN: Yes. 



BENNETT: I would say that such a change is less than 25 percent. 



KAPLAN: If you irradiate as much as half the liver in vitro how do you 

 make sure that the internal liver cells have access to oxygen? I think Vincent 

 Hall showed some time ago that on irradiation of tumor fragments in vitro, the 

 radiosensitivity of the tumor was very highly dependent on the size of the Trag- 

 ment used and even quite small fragments showed radioresistance of the cells 

 in the interior of the fragment simply because they didn't get the same oxygen 

 concentration. 



SHERMAN: That is an important point. We tried to minimize the 

 "oxygen" effect by cooling the liver immediately after removal and by irradiating 

 it in the cold within 5 minutes. The irradiation period was 1 minute, and the 

 tissues were kept at to l^C, until they were sliced and put into the incubation 

 medium . 



TOBIAS: Mr. Chairman, I am not a biochemist and probably my ques- 

 tion will seem naive, but I came here with the hope that the biochemists would 

 answer some very simple questions. For example, as you all know, about 1/10 

 of the dose necessary to kill will cause a very great delay in the division process 

 in a microorganism, and at the same time the cell itself will continue to grow, 

 perhaps to 20 times its normal volume and presumably it will continue synthe- 

 sizing proteins. I wonder if there are any clues as to what enzyme system would 

 be affected by this small dose of radiation. 



SHERMAN: Is this a simple question? 



TOBIAS: I assumed that it would be a simple question for a biochemist. 

 To answer it is beyond me, of course. 



CHARGAFF: A physicist can ask more questions than a hundred bio- 

 chemists can answer, I am afraid. 



I don't think that what produces cell division is so simple. I think it is 

 the most complex question in biochemistry. If you interfere with cell division 

 you probably interfere not with just one reaction but with many. 



COHN: I would suggest that the day we can equate growth or reproduc- 

 tion to a number of enzyme systems we will all take a long vacation. 



CARTER: I think you can paraphrase that by saying that the answer to 

 such a simple question would be given by a very simple biochemist. 



POTTER: I would say that that comment discourages further discus- 

 sion. I think Dr. Mazia has something worthwhile to say. 



MAZIA: Since it has come up several times, just for the sake of kick- 

 ing it around, let us suppose that radiation affects some DNA synthesizing mech- 

 anism; that in order for a cell that has been produced by a division to make the 

 next division, it has to double its DNA; that low-dose irradiation knocks out the 

 DNA forming mechanism; and the delay in division represents the time required 

 for its reformation. Let us suppose that this is the only important event when 

 you irradiate with the dose you have in mind. Would not the protein-synthetic 



