58 A. GOTTSCHALK 



reason to believe that the cellular receptors are built up in a similar way. Both 

 the terminal neuraminic acid of the prosthetic group and the polypeptide 

 core providing the framework for an orderly spatial arrangement of the 

 prosthetic groups are essential for attracting and binding the influenza virus 

 particle. Enzymatic removal from the prosthetic group of the terminal 

 acetylated neuraminic acid deprives the receptor of its binding power. 

 Breakdown of the protein framework has the same effect. Thus, it has long 

 been known that trypsin inactivates inhibitory mucoproteins (Burnet et at., 

 1947; Hirst, 1948; Gottschalk and Lind, 1949a). The oligosaccharide on its own 

 does not inhibit hemagglutination by indicator virus, as was shown with the 

 disaccheride (Fig. 2) and with sialyl-lactose. This trisaccharide, which is present 

 in milk, consists of diacetylneuraminic acid and lactose (Kuhn and Brossmer, 

 1956a). In this case, neuraminic acid is linked ketosidically to C 3 of galactose, 

 the ketosidic bond being of the a-type (Kuhn and Brossmer, 1958; 

 Gottschalk, 1957b). The trisaccharide is susceptible to virus and V. cholerae 

 neuraminidase. 



Just as different mucoproteins offer different structural situations to one 

 and the same virus, different strains of influenza virus present different sur- 

 face structures to one and the same receptor. Differences in reactivity of the 

 various influenza virus strains with one receptor, the red cell receptor, were 

 first observed by Burnet et al. (1946). They found that red cells from which 

 one strain of virus had become eluted were still agglutinable by certain other 

 strains. Graded decrease by RDE of the number of intact receptors available 

 at the red cell surface rendered in a definite order one virus strain after the 

 other unable to agglutinate the increasingly impaired red cell (receptor 

 gradient; for details see page 29). The physicochemical expression of this 

 stepwise decrease in intact receptors available or, as we know now, of the 

 stepwise loss of sialic acid residues is the gradual decrease in the net negative 

 charge of the red blood cells. Thus, the electrophoretic mobility of human 

 erythrocytes is reduced from the normal value of — 1.30 X 10~ 6 cm. 2 /sec. -1 / 

 volt -1 to a value characteristic for each virus strain of the gradient, the 

 lowest value of — 0.37 X 10~ 6 cm. 2 /sec. _1 / vo lt _1 being attained with swine 

 influenza virus (Ada and Stone, 1950). RDE action on human erythrocytes 

 reduces their electrophoretic mobility to — 0.17 X 10~ 6 cm^/sec.^/voltr 1 , 

 i.e., by 87 %, indicating that the net negative surface charge of human 

 erythrocytes is due nearly exclusively to the dissociated carboxyl group of 

 sialic acid (pK' = 2.60 ± 0.05 at 20°C. and 0.052V). The receptor gradient 

 probably reflects the degree of steric hindrance to the close fit between the 

 complementary combining groups of virus and receptor, exerted by the 

 surface structure of the individual virus strains. In a similar fashion Burnet 

 (1955) has invoked differences in "accessibility" of receptors to account for 

 the gradient. Since the surface structures of the virus receptors, whether 



