ERNEST POLLARD 



energy can be obtained in terms of the energy of the particle and the number 

 of particles per cm^ incident on the enzyme or the virus. 



If an enzyme, such as trypsin, is subjected to bombardment by deuterons 

 then it is inactivated semilogarithmically according to the relation 



In nlfio = —SD 



In this formula n and rig are the activities of the enzyme after and before 

 irradiation respectively, D is the number of deuterons per cm- incident on 

 the trypsin, and S is the cross section, which measures the statistical probability 

 of inactivation of a trypsin molecule. It is found that the quantity S depends 

 on the density of ionization of a bombarding particle. This fits quite well 

 with the idea that the slower the particle the more dense the ionization and 

 the closer the quantity S approaches to the expected physical cross section 

 of the molecule of trypsin. 



To illustrate the kinds of studies that have been carried out the inacti- 

 vation of invertase may be considered. If this is subjected to bombardment 

 by deuterons at varying energies and also by high speed electrons, which 

 produce ionization effects which are substantially random in volume, then 

 the loss of ability to digest sucrose is lost according to a semilogarithmic 

 manner just as described for trypsin. So is the loss of ability to combine 

 with specific antibody to invertase. The two do not, however, follow the 

 same line. The serological activity is lost less rapidly, with more bombard- 

 ment, than the enzymatic activity. 



If the temperature at which the invertase is irradiated is raised then the 

 sensitivity of the invertase begins to increase at a temperature just below 

 that at which enzymatic activity would be lost due to temperature alone. 

 If the temperature is reduced to liquid air temperatures the sensitivity of 

 the enzyme diminishes. There is a range of 50°, or so, over which the 

 sensitivity does not vary very much, and at which the observed cross section 

 corresponds rather closely to that expected for the physical cross section of 

 the molecule. If in place of commercial preparations, the enzymatic activ- 

 ity in yeast cells is studied, it is found that the same inactivation is observed. 

 In other words the sensitivity of invertase in intact yeast cells is the same as 

 that in commercial preparations. 



It is perhaps easiest to see the nature of these effects of ionizing radiation 

 on enzyme molecules in terms of a proposed explanation. According to 

 this explanation we consider the enzyme to be made up of protein chains, 

 crosslinked in some way, and that a part of this whole protein molecule is 

 essential for the enzymatic activity, and also for other specific biological 

 activities. The same two parts need not be involved in different functions. 

 The arrival of ionizing radiation causes the ejection of an electron from any 

 one of the atoms in the whole molecule without there being any special 

 tendency to select one class of molecule. This results in the ejection of an 

 electron and leaves behind an atom which is positively charged due to the 

 loss of an electron. The fact that the electron is missing from an atom 

 means that it will tend to restore its lack of balance. This it may do by 

 transfer of an electron from a neighbouring atom and such transfer may be 

 extremely rapid. In view of this it is probable that the localization of the 

 ionized atom changes throughout the molecule as electrons are interchanged 



71 



