THE ACTION OF IONIZING RADIATION ON 

 ENZYMES AND VIRUSES 



Ernest Pollard 

 Biophysics Division, Sloane Physics Laboratory, Yale University 



The purpose of the studies which have been conducted for the past five 

 years in the Biophysics Division at Yale University have been to develop 

 radiation methods as a tool for the study of fundamental cellular processes. 

 Three reviews of this work have appeared^- '^- ^. The work is, therefore, 

 not so much directed at the understanding of radiobiological action on 

 large organisms, as it is at developing a means of study. 



The following features about radiation can be exploited for the study of 

 cells. The high energy release produced by radiation, the fact that this is 

 localized and that the localization can be varied by choosing the conditions 

 of irradiation, and the fact that penetration into the cell without damage 

 of the cell wall can be achieved by ionizing radiation. 



The work to be described follows logically from the early work of Lea, 

 Smith, Holmes and Markham^. These workers studied the effect of high 

 doses of X-rays on two dry enzymes and concluded from the nature of the 

 inactivation that a moderately reliable estimate of the molecular size of the 

 two enzymes ribonuclease and myosin could be obtained. The principle 

 of this work is to assume that ionizing radiation produces randomly distri- 

 buted high energy releases which produce the great majority of their effect 

 inside any one molecular unit. The statistics of inactivation permit the 

 derivation of a parameter which can be called the inactivation volume in the 

 case of irradiation where the energy release is distributed randomly in 

 volume and a cross section in the case where energy release is confined to 

 dense swaths of ionization. Both of these methods of irradiation were 

 clearly understood by Lea. 



The techniques of modern nuclear physics permit irradiation in a very 

 much more precise and flexible way than was available prior to the second 

 world war. In particular the use of cyclotron-produced particles for 

 irradiation is very attractive. A group at Yale has exploited this by modify- 

 ing a relatively small cyclotron, with an external beam, which passes through 

 a defocusing system on to a shutter in front of a series of samples. The 

 shutter is operated when each sample is put in place and exposures of known 

 beam for known times are given and in this way inactivation of quite small 

 and relatively insensitive molecules can be obtained. 



The energy release by fast charged particles is well known from a com- 

 bination of theory and experiment. These have been described in the review 

 articles already quoted. The important feature is that the spacing between 

 energy releases can be changed by varying the speed of the bombarding 

 particle and that these energy releases do not spread very far from the 

 track of the particle itself. Therefore some idea of the localization of the 



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