56 A. GOTTSCHALK 



the compound. This contribution proved beyond doubt the previously postul- 

 ated chemical analogy between cellular receptors and soluble inhibitory muco- 

 proteins, an analogy also reflected in the reduction of the net negative surface 

 charge of both human erythrocytes and inhibitory mucoproteins upon RDE 

 action (Hanig, 1948; Ada and Stone, 1950; Pye, 1955). It is of interest to note 

 that the sialic acid of all potent hemagglutinin inhibitors is NANA. Thus, lipid- 

 free extracts of equine erythrocytes (stromata), known to contain iV-glycolyl- 

 neuraminic acid, are devoid of inhibitory power, whereas the corresponding 

 extracts of human erythrocytes, containing NANA, inhibit virus hemagglu- 

 tination (Yamakawa, 1956). This observation may have some bearing on the 

 fact that equine red cells, in contrast to human red cells, are not readily agglu- 

 tinated by influenza virus (Clark and Nagler, 1943). Another example is BSM. 

 This mucoprotein has the highest sialic acid content known so far (17 %), of 

 which at least 80 % is terminal (Gottschalk, 1956b). Yet, its virus hemagglut- 

 inin inhibitory power is very limited. BSM efficiently inhibits only influenza 

 PR8 indicator virus; LEE indicator virus is inhibited very slightly, and eight 

 other influenza virus strains are not inhibited at all. When living PR8 or LEE 

 virus was allowed to act on BSM, no significant change in the net negative 

 charge of the muco-protein was observed (Curtain and Pye, 1955). According 

 to Blix (1958) the sialic acids of BSM are a mixture of ON-diacetylneur- 

 aminic acid, N-acetyl-O-diacetylneuraminic acid and N-glycolylneuraminic 

 acid. 



With regard to the position and linkage of sialic acid in inhibitory 

 mucoproteins, it was shown by Gottschalk (1956b) that in BSM sialic acid 

 occupies a terminal position and is joined through the potential keto group 

 glycosidically to the adjacent unit. RDE released 64 % of the total sialic acid 

 present in BSM (Gottschalk, 1955b, 1956b). Heimer and Meyer (1956) and 

 Faillard (1957) arrived at the same results. With UM as substrate, both 

 the influenza virus enzyme and RDE split off 50-60 % of the total sialic acid 

 present, and sialic acid only (Klenk et ah, 1955; Faillard, 1957). 



The over-all structure of the soluble receptors was early recognized 

 (Gottschalk, 1952) as that of conjugated proteins, containing as prosthetic 

 groups relatively small sized polysaccharides or oligosaccharides. In the case 

 of UM, the prosthetic group, detachable by alkali, consists of galactose, 

 mannose, fucose, glucosamine, galactosamine, and NANA in the molar ratios 

 6:3:1:6:2:3. The molecular weight of the prosthetic group is of the order 

 of 12,300 (assuming acetylation of the amino sugars). Since the molecular 

 weight of UM is 7 X 10 6 and its carbohydrate content about 21.5 % 

 (calculated as anhydro sugar), approximately 120 individual prosthetic 

 groups are assumed to be attached to the protein core (Gottschalk, 1958). 

 BSM contains 11.4 % ^-acetylgalactosamine and 17 % OiV-diacetylneura- 

 minic acid, i.e., about equimolar quantities of the two components; in addition, 



