INACTIVATION OF VIRUSES 367 



long been known, however, that X-rays produce free radicals and other toxic 

 products in the suspending medium that are responsible for the so-called 

 indirect inactivation. 



Direct and indirect effects can usually be separated quite well because 

 neutral compounds, especially proteins and nucleic acids, protect against the 

 indirect effect of X-rays by competing for the inactivating radicals. This 

 phenomenon was studied in detail for various enzymes by Dale (1940); 

 Friedewald and Anderson (1940) have shown that rabbit papilloma virus is 

 about 20 times more resistant in crude, protein-rich preparations than in a 

 semipurified state. This general rule has been amply confirmed (e.g., by 

 Latarjet and Ephrati, 1948), and the analogy to protection of viruses against 

 surface inactivation is obvious (see Section A2). 



The addition of protein or peptone protects phage very satisfactorily 

 against indirect effects (Luria and Exner, 1941a,b; Watson, 1950, 1952). In 

 nutrient broth the rate of inactivation of phage T2 is the same as in media 

 containing as much as 5 % gelatin, which shows that inactivation is caused 

 chiefly by direct effects (Watson, 1950). This, however, is not necessarily true 

 of virus activities other than infectivity. Latarjet and Fredericq (1955) found 

 that the ability of phage T6 to kill bacteria was only partially protected in a 

 medium containing 9 % yeast extract and catalase; freezing further reduced 

 the rate of destruction by a factor 5. In the frozen state the bacterial killing 

 abihty of phage T6 and of the closely related "colicm K" were destroyed ex- 

 ponentially at exactly the same rate, suggesting that the element on the 

 phage particle, which is responsible for killing of the host cell upon adsorp- 

 tion onto it, is similar to, if not identical w^ith, the colicin molecule. Both of 

 these protein units are extremely susceptible to radiation-produced radicals, 

 presumably because they are in direct contact with the aqueous phase. The 

 same holds for free DNA units: DNA, in the form of transformation principle, 

 is as susceptible to and as difficult to protect against inactivation by radicals 

 as is colicm (Eplirussi-Taylor and Latarjet, 1955); whereas phage DNA, 

 which is separated from the suspending medium by a protein coat, is easily 

 protected. 



Bachofer and Pottmger (1953, 1954a) have demonstrated that the effect 

 of radicals on purified phage Tl varies with the salt concentration. The effect 

 depends on the type of cation as well as anion and on pH; the "protecting" 

 role of the salts is not weU understood. 



Freezmg and drying are usually assumed to elinmiate very efficiently the 

 indirect effect (cf. Wood, 1954). However, Bachofer (1953) found that lyo- 

 philized and vacuum-dried Tl preparations differed in X-ray sensitivity; the 

 former being about tw^ice as sensitive as the latter. Bachofer et al. (1953), 

 studying inactivation of phage Tl at temperatures ranging from — 196 to 

 37°C., further observed a distinct decrease in sensitivity of the phage at low 



VOL. I — 25 



