RADIATION AND VIRUSES 343 



can still function in adsorption but not in penetration, reproduction will 

 1)6 blocked whenever receptor .1 is utilized, and the probability that 

 inactivation results from such a hit is the inverse of the number of areas 

 .1 , B, C, . . . present on the virus surface. Considerations of this type 

 underline the naivete of the target theory in its narrow form, as applied to 

 the analysis of virus inactivation. 



An interesting result has recently been obtained (Hershey et al., 1951) 

 from a study of the spontaneous inactivation of bacteriophage that con- 

 tains relatively large amounts of radioactive phosphorus. Bacteriophage 

 particles containing up to 100 or more P^^ atoms (out of a total of approxi- 

 mately 500,000 phosphorus atoms in nucleotides) show a definite insta- 

 bility, with a one-hit type of activity decay. On the average, one particle 

 is inactivated for every ten P^' disintegrations. The inactivation is 

 apparently not due to the emission of /3 rays, but to the nuclear event 

 itself. This result suggests either that only 10 per cent of the phosphorus 

 atoms of a phage particle are necessary for infectivity, the others being 

 dispensable, or that, when a phosphorus nucleus disintegrates, there is an 

 average probability 0.10 that this change will result in inactivation. 



Some authors (see Riehl et al., 1941) have discussed the problem of how 

 a hit in any one point of a large physical volume can produce inactivation 

 of a virus (or mutation of a gene) and have speculated on the po.ssible 

 need and mechanism for energy migration within a large biological mole- 

 cule to a specific site of action. Such an approach has not led very far, 

 however, since little is known about such energy-migration mechanisms. 

 The need to invoke their intervention in virus inactivation is not apparent. 



2-2. ULTRAVIOLET RADIATION 



Viruses have generally been exposed to ultraviolet radiation either in 

 stirred suspensions or in thin layers in order to avoid or equalize the 

 screening effect of impurities. A continuous-flow technique has also been 

 described (Levinson et al., 1944). The possibihty that in such experi- 

 ments some indirect effects of radiations may be observed has often been 

 neglected since the doses of ultraviolet radiation needed for inactivation of 

 viruses do not seem to produce appreciable amounts of toxic substances in 

 water. Toxic products might, however, originate from impurities. 



For most viruses the proportion of active virus has been reported to 

 decrease exponentially with the dose according to Eq. (9-1), the total dose 

 (intensity times time) being the relevant variable (Hollaender and 

 Duggar, 193G; Price and Gowen, 1937; Taylor et al., 1941; Latarjet and 

 Wahl, 1945; Oster and McLaren, 1950; Fluke and Pollard, 1949). One 

 ciuantum is apparently the effective hit. Recent data on bacteriophage, 

 for which the precise titration method makes it possible to obtain more 

 accurate inactivation curves, indicate deviations from the simple loga- 

 rithmic relation. Some phages (T2, T4, T6) exhibit a slow initial rate of 



