ACCELERATED HEAVY IONS ON ENZYMES AND YEASTS 259 



Results obtained on lysozyme, trypsin and deoxyribonnclease by 

 Brustad are briefly reported here. When a sample of Nq enzyme mole- 

 cules was irradiated by a dose of/ particles per cm^ N enzyme molecules 

 remained which shoA\'ed activity accordhig to the well known relation- 

 ship 



N = Noe-af 



where a is the interaction cross-section. Each of the radiated particles 

 gave this sort of relationship with each of the enzymes studied, but 

 with cross-sections different for each of the LET values at which irradi- 

 ation of the molecule was carried out. According to a treatment by 

 Pollard ( 1 953) inactivation of the enzymes usually depends on producing 

 a primary ionization within some definite part of the enzyme molecule ; 

 the cross-section is related to the number of primary ionizations per 

 unit length k and the mean thickness of the enzyme molecule t by 



CT = (TO (1 -e-'^O 



Here ctq is the geometric cross-section of the molecule or of its 

 sensitive part. It may be obtained by radiating the molecule with very 

 heavily ionizing rays : at high ion density the cross-section should be- 

 come independent of LET. The data obtained in our laboratory do not 

 agree with this model without further modification. Experimentally 

 one does not obtam a plateau of cross-section with increasing LET. At 

 the highest measured cross-section the values obtained are for lysozyme 

 about 2-5 times the expected value, for tiypsin 2-2 times and for de- 

 oxyribonuclease 1-27 times what one would deduce from molecular 

 weight and other data known from independent sources. Although 

 Pollard and Barrett (1959) and Fluke et al. (1961) attempted to work 

 out methods whereby one would take into effect the S-rays produced by 

 the primary particles and in effect increase the cross-section for enzyme 

 inactivation. We believe that the modifying effect of S-rays are too 

 small to explain completely the large cross-sections reported. While 

 there can be no doubt that S-rays and their effects exist, some experi- 

 ments were undertaken to see how neighbouring molecules and other 

 environmental conditions might affect inactivation cross -section. It is 

 possible to design experiments which may elucidate the role S-rays 

 play in the inactivation cross-section. The oxygen effect provides one 

 approach. It is known since the work of Alexander (1957) that oxygen 

 can modify radiation injury due to lightly ionizing radiations (X-rays 

 or electrons) in dry macromolecules. In fact trypsin exhibits an 



