414 S. GARD AND O. MAAL0E 



G. Enzymes 



Lojkin and Vinson (1931) reported that TMV was inactivated by trypsin, 

 presumably on account of the proteolytic action of the enzyme. Stanley 

 (1934a), while confirming the inhibitory effect of trypsin, found no indication 

 of a protein breakdown. Characteristically, the effect was established im- 

 mediately upon addition of trypsin and did not proceed further on incuba- 

 tion. Full infectivity could also be restored by separation of the components 

 in the mixture, e.g., by means of isoelectric precipitation. 



A similar reversible reduction in uifectivity by enzymes has later been 

 repeatedly described, e.g., of TMV by papain (Bawden and Pirie, 1937) and 

 by ribonuclease (Loring, 1942); of influenza virus by papain (Towarnizki, 

 1948); and of bacteriophage by ribonuclease (Jerne and Maalee, 1957). 

 Trypsin was supposed by Stanley (1934a) to reduce the infectivity of TMV, 

 not by interaction with the virus, but by affectmg the susceptibility of the 

 host plant. In the other cases, virus and enzyme seem to combine to form an 

 inert complex, displaying neither infectivity nor enzyme activity. The TMV- 

 ribonuclease complex is insoluble in the absence of electrolytes, under these 

 conditions formmg a fibrous precipitate; the complex dissociates on dilution, 

 releasing fully active virus (Loring, 1942). Reduction in infectivity of bacterio- 

 phage T4 is observed primarily at low ionic strengths and infectivity is re- 

 stored after digestion of the complex with trypsin (Jerne and Maalcje, 1957). 

 The mechanism is not yet fully understood. As enzyme treatment of the host 

 cell alone can have an adverse effect upon its viability and synthetic capacity 

 (Jerne and Maakje, 1957), the apparent inactivation might at least partly be 

 explained by a damage to the host cell and impairment of the substrate of the 

 virus. That this is probably not the whole explanation is evident, however, 

 from the fact that enzymatically inactive proteins with high isoelectric points, 

 such as clupeine or globin (Bawden and Pirie, 1937; Towarnizki and Karlina, 

 1950), likewise reduce the infectivity, indicating that a complex formation 

 of oppositely charged substances may be the decisive factor. Studies of virus/ 

 cell adsorption rates might serve to throw more fight upon this question. 



Under such conditions reports on inactivation by enzymes, presumably 

 caused by enzymatic breakdown of the virus, must be cautiously judged. In 

 addition, most of the early studies were carried out with very crude enzyme 

 preparations and equally impure virus material; it is not always evident that 

 effects observed were attributable to the action of enzymes. Since crystal- 

 lized enzymes have become available, it has been shown repeatedly that 

 neither RNAase nor DNAase attack the native viruses, a fact that has been 

 utilized for purification purposes (cf. Schwerdt and Schaffer, 1956; Hershey 

 et al., 1951). The observations that free virus UNA is broken down by RNA 

 asc (Hart, 1955b; Gierer and Schramm, 1956) and that the Hemophilus 



