24 S. S. COHEN 



The structure of this compound, has recently been elucidated by a number 

 of workers, among whom Gottschalk has made important contributions. This 

 substance is a previously unsuspected condensation product of a substituted 

 hexosamine and pyruvic acid. Siahc acid resembles in many respects the 

 lactyl ether of hexosamine recently discovered in the cell walls of numerous 

 bacteria, e.g., B. megatherium (Strange and Powell, 1954; Park and Stro- 

 minger, 1957). 



The well-defined compound, neuramin-lactose, which has been isolated 

 from rat mammary gland, is cleaved by the viral enzyme and by EDE into 

 diacetyl-neuraminic acid and lactose, as presented in Fig. 2a. The name 

 '"neuraminidase" has been suggested for enzymes capable of catalyzing this 

 cleavage. 



In the liberation of siaHc acid from the mucoprotein of bovine submaxillary 

 gland by RDE, the 0-glycosyl linkage is cleaved between iV-acetyl-galactos- 

 amine and siahc acid, as presented in Fig. 2b (Gottschalk, 1957b). In the 

 urinary mucoprotein the neuraminyl derivative exists as the terminal end 

 of a small polysaccharide, which also contains hexosamine, galactose, 

 mannose, and fucose. 



It is not believed that the neuraminidase is important in viral penetration, 

 as is the hydrolase of the T-even phages. In the case of influenza virus, 

 penetration may be effected by a process more akin to pinocytosis. It has 

 been suggested that in this animal virus infection the enzyme may be 

 involved in the release of virus particles from the surface of the infected 

 ceU. 



In an important study by Adams and Park (1956) it was shown that a 

 specific strain of phage liberated from an encapsulated strain of Klebsiella 

 pneumoniae contains an enzyme for the hydrolysis of the polysaccharide 

 capsule. The synthesis of the enzyme is apparently mduced by infection by 

 tliis phage; on lysis the enzyme is both freely diffusible and attached to 

 virus. It is not clear whether particle-bound enzyme does not arise as a 

 fortuitous accident of adsorption. However, from the point of view of the 

 functional penetration of virus to the ceU wall, this question may not be 

 meaningful. 



The sole evidences of the metaboHc activity of the viruses, then, are 

 confined to the viral hydrolases noted above. These burglar tools are special 

 requirements of an infective cycle which involve the penetration of the 

 tough cell wall or other protective coat of a bacterium or an escape 

 through the less rigid but none the less tough plastic envelope of an animal 

 ceU. No evidence has yet been obtained for their existence in plant 

 viruses, although conceivably an enzyme of this character may be 

 found in those viruses which multiply in both plants and their insect 

 vectors. 



