INACTIVATION OF VIRUSES 415 



transforming principle is depolymerizcd and inactivated byDNAase (Zamenhof 

 et al., 1953) indicate tliat the resistance of the native viruses is attributable 

 to the protected situation rather than to any intrinsic resistance of its 

 nucleic acid. It is hardly likely that the nucleic acid is more accessible to 

 other enzymes with less specificity; for that reason, the statements that 

 phosphatases inactivate TMV (Pfankuch and Kausche, 1939) and herpes 

 virus (Amos, 1953) seem to need confirmation; in neither case was "re- 

 activation" attempted. 



The action of proteolytic enzyme on viruses is a different matter. As 

 already mentioned, Harris and Knight (1952) found that carboxypeptidase 

 attacked TMV, splitting off about 7 % of the total threonine but no other 

 amino acid. The uifectivity of the virus remained unchanged, xipparently, 

 threonine is the only C-terminal amino acid of the TMV protein. Knight 

 (1950) showed, furthermore, that a number of TMV variants behaved simi- 

 larly, yielding only threonine, as distinct from several other plant viruses, 

 each of which seemed to possess a characteristic pattern of C-terminal resi- 

 dues. Fraenkel-Conrat and Singer (1954) attempted by various means to 

 spht off iV-terminal residues from native virus protein, but without success. 

 They concluded, therefore, that the protein consists of a closed polypeptide 

 ring with "fringes" of C-terminal amino acids. As neither the individual 

 amino acids nor the general composition of virus proteins seem to possess 

 any distinctive features, the possibihty of such a closed structure and perhaps 

 other configuration pecuHarities offers for the time being the only clue to the 

 riddle of the remarkable resistance of many viruses to proteolytic enzymes. 

 After denaturation the virus protein seems to lose this resistance 

 (Kleczkowski, 1944). 



Not all virus proteins are equally resistant to proteolytic enzymes. The 

 study of this question, however, requires a number of safety measures to 

 yield conclusive results. Since A. Pirie (1935) found virus-inhibiting impurities 

 in crude enzyme preparations, it is obvious that only enzymes of satisfactory 

 purity should be employed. The possibility of effects upon the host rather 

 than the virus and of reversible inhibition of the nature previously described 

 should be ruled out. Identification of split products and analysis of the residue 

 (the procedure appHed by Knight) is highly desirable. Finally, denaturation 

 must be avoided and evidence presented that the enzyme attacks the native 

 virus protein. So far, few studies come up to this standard. 



Stanley (1934a) found no demonstrable change in the diffusion rate of 

 trypsin in a mixture with TMV and concluded that virus and enzyme did not 

 combine in vitro. Inhibition of infectivity, which appears immediately upon 

 mixing, was consequently assumed to be the result of enzyme effects upon 

 the host cells. Later Hills and Vinson (1938), using lower trypsin concentra- 

 tions, did observe a retardation of diffusion of both the virus and the enzyme, 

 VOL. I — 28 



