ANTI-ENZYME IMMUNITY 183 



preparation has yet been found that attacks purified virus prepara- 

 tions at an appreciable rate, or that has any permanent effect on their 

 infectivity. They tried the proteolytic effect of trypsin, pepsin, papain 

 and autolyzed preparations of kidney at a number of pH values 

 around that optimal for enzymic activity, but in no case were they 

 able to observe any proteolytic activity on the living virus. In con- 

 trast, all these enzymes were shown to be strongly active proteolyti- 

 cally when tested against the heat denatured virus which was rapidly 

 hydrolyzed. In the presence of a large amount of trypsin the infectivity 

 of the purified virus preparations was reversibly inactivated as a result 

 of a possible virus-trypsin complex formation. This inactivation of the 

 infectivity was observed to occur immediately after the virus and 

 enzyme were mixed, and no further loss followed incubation. By 

 precipitation with acid or dilute ammonium sulfate solution the 

 virus was recovered with its full activity from such non-infective 

 mixtures. Similarly, when various amounts of a solution of papain 

 were added to constant amounts of virus, a papain-virus precipitate 

 was obtained. From these precipitates the active enzyme was recovered 

 by extraction at pH 3.3. Virus was also recovered without undergoing 

 any change in chemical, infective or serological properties. 



According to Kleczowski (1944) pepsin combines with virus X 

 (without affecting infectivity, suggesting that different parts of the 

 virus particles are involved in combination) and casein, which are 

 substrates for its proteolytic activity, but not with tobacco mosaic virus, 

 which is not a substrate. Tobacco mosaic virus denatured by heat is 

 readily hydrolyzed by pepsin and combines with pepsin almost to the 

 same extent as potato virus X. On the other hand, more trypsin com- 

 bined with tobacco mosaic virus (with loss of infectivity), which is not 

 a substrate for its proteolytic activity, than with potato virus X, which is 

 a substrate. The combination of trypsin with tobacco mosaic virus could 

 account for the reversible inhibition of infectivity of the virus by 

 trypsin. This combination protects trypsin from spontaneous inactiva- 

 tion at pH 7.0. 



Invertase does not combine with potato virus X, with tobacco mosaic 

 virus, whether heat denatured or not, or with casein. 



d. The Inhibition of Enzymes by Enzymes and Viruses. Bayliss 

 emphasized also the presence of normal enzyme inhibitors in living 

 cells, particularly a proteolytic enzyme inhibitor present in the worm 



