CHEMISTRY OF VIRUS RECEPTORS 53 



Indicator virus is defined as a virus pretreated by heat under specified 

 conditions so as to destroy infectivity and enzymatic activity without 

 impairing the hemagglutinin (Briody, 1948; Stone, 1949a). Among the 

 inhibitors were a mucopolysaccharide from erythrocytes (De Burgh et al., 1948; 

 McCrea, 1953a), ovarian cyst mucin (Burnet, 1948), serum mucoprotein 

 (McCrea, 1948), urine mucoprotein (Tamm and Horsfall, 1950), meconium 

 mucoprotein (Curtain et al., 1953), sputum mucoprotein (M&rmion etal., 1953), 

 all of human origin, ovomucin of hen egg white (Gottschalk and Lind, 

 1949a; Eckerte^a/., 1949), and sheep submaxillary gland mucoprotein (McCrea, 

 1953b). The inhibitory capacity of these mucoids was lost upon treatment 

 with living (infective) virus or with RDE (Burnet, 1951; Gottschalk, 1954a). 

 Two inferences were drawn from the data presented. First, the coincidence in 

 the effect of the influenza virus particle and the vibrio enzyme on red cell 

 receptors, host cell receptors, and hemagglutinin inhibitory mucoproteins 

 was taken as strong support for Hirst's suggestion of the presence of an 

 enzyme at the surface of the virus. Second, the cellular receptors and the 

 soluble inhibitory mucoproteins were regarded as chemical analogs, their 

 common structural feature competing for and being susceptible to the 

 isodynamic enzymes of influenza virus and V. cholerae (Anderson et al., 1948). 

 These results and their interpretation provided a solid basis for a bio- 

 chemical and, eventually, a chemical approach to the problem of virus 

 receptors. Using ovomucin as substrate, Gottschalk and Lind (1949b) 

 presented the first chemical evidence for the activity of the virus enzyme. 

 They isolated by dialysis from the digest of ovomucin with living influenza A 

 virus a low molecular weight compound. Concomitant with the release from 

 the mucoprotein of the split product, and of this product only, the ovomucin 

 irreversibly lost its hemagglutinin inhibitory capacity, whereas the virus 

 retained its enzymatic activity. Heat-inactivated virus did not promote these 

 changes. RDE imitated in detail the effects of living virus on ovomucin. 

 The split product resembled A-acetylglucosamine with regard to reducing 

 power, nitrogen content, and absorption spectrum of the purple-colored 

 compound formed in the Morgan-Elson reaction. However, contrary to the 

 behavior of A 7 -acetylglucosamine, the split product decomposed with discolor- 

 ation on heating with very dilute mineral acid, as 2-deoxyhexoses do. In 

 addition, it gave the direct Ehrlich reaction (without alkali pretreatment). 

 The same product was obtained when the electrophoretically homogeneous 

 urine mucoprotein was acted upon by highly purified influenza B virus 

 (Gottschalk, 1951). The weight of the product split off amounted to about 

 1 % of the substrate used. These results, taken together with the biological 

 effects of RDE, indicated that the binding power of the cellular receptors for 

 the influenza virus and the competitive hemagglutinin inhibitory capacity 

 of the soluble receptors (mucoproteins) were closely associated with the 



