46 HIRST 



destruction of inhibitors by viruses and the isolation of a receptor-Hke 

 substance from red cells (also destroyed by viruses) have strengthened 

 this assumption. The principal fact is that exposure of cells to virus 

 leaves them devoid of receptors but the Adrus remains intact (Hirst, 

 1942). 



For many years attempts were made to remove receptors from cells 

 and then show that they reacted with viruses of this group. These re- 

 sults were unsuccessful because any attempt to combine receptors in 

 solution with virus resulted in immediate destruction of the former. 

 This difficulty was circumvented when it was discovered that the re- 

 ceptor-destroying capacity of a virus could be abolished without affect- 

 ing the ability of the virus to combine with receptors (Anderson, 1948, 

 Hirst 1948) . This can now be readily done in a number of ways: treat- 

 ing the virus with heat, periodates, trypsin, etc. Such mildly injured 

 virus combines in normal fashion with receptors, either on cells or in 

 solution, but does not spontaneously elute. When such treated virus 

 is exposed to an excess of receptor in solution, the virus becomes so 

 saturated with receptors that it can no longer combine with or aggluti- 

 nate red cells and hence soluble materials which have this effect are 

 called inhibitors and virus which does not elute is called indicator virus. 



One way to explain these facts is as follows: virus and red cells are 

 bound to each other when agglutination occurs by mutually attractive 

 forces of virus enzyme and receptor substrate. After contact has been 

 made the cell substrate is destroyed, the bonding forces no longer exist 

 and the virus diffuses away. When the virus is mildly injured by heat 

 or other agents, the virus enzyme is sufficiently deformed so that it 

 can no longer function enz5anatically but enough configuration re- 

 mains that "specific" adsorption between enzyme and substrate takes 

 place. In the test for inhibitor, inhibitor competes with red cells recep- 

 tors for the virus. 



By the use of indicator viruses, inhibitors have been found in a 

 wide variety of biological material: practically all mammalian organ 

 emulsions, allantoic fluid, egg white, plasma, red cell extracts, ovarian 

 cyst fluid, etc. There are obvious qualitative differences in the inhibi- 

 tor from different sources and the chemistry of the active principle is 

 in a primitive state. In one case, that of egg white, the inhibitor has 

 been purified to a considerable extent and possesses very high biological 

 activity (Gottschalk, and Lind, 1949). The active principle is at least 

 in part polysaccharide and does not diffuse through cellophane mem- 

 branes. In the presence of virus a substance in the fraction is broken 

 down to smaller units which are dialyzable, and contain reducing sub- 

 stance but which have not been identified as yet. Satisfactory evidence 



