Sialic acids cell surface presentation

As with ER, estimation of the fraction of sialic acid of the cell present in mitochondria is made difficult because of problems of purity. While sialic-acid-rich surface membrane material can be eliminated from preparations of mitochondria, lysosomes which also are rich in sialic acids (Tables I, V) often contaminate mitochondrial preparations. The data of Table V clearly show that the concentration of sialic acid in mitochondria from a variety of cells is low. In L cells only 4% of the total sialic acid is present in mitochondria (Table I), while in Ehrlich ascites carcinoma 17)% and in rat liver 11% are in the mitochondria (Wallach and Eylar, 1961 Patterson and Touster, 1962). It is possible that the latter two values are high because of contamination due to inadequate fractionation techniques. 

As in the case of the gangliosides in vertebrates, the sialoglycolipids in sea urchins are cell-surface antigens. A study of antigenic specificity for two disialoglycolipids (from the eggs of S. intermedius) whose structures differ in only one respect, that a sulfate group is present on the sialic acid... 

In order to prevent infection, for example, with an influenza virus, it is necessary to block at least one out of two of the substances present on its surface One of them is a neuraminidase, an enzyme that cleaves off sialic acid, and the other is a hemagglutinin, a sialic acid-specific lectin which aids the influenza viruses in docking onto oligosaccharides exposing sialic acid on the surface of host cells. Both compounds are prerequisite for the infection process. 

Examples of glyco-coated dendrimers are the sialylated dendrimers79 that have been developed to prevent infection by the influenza virus. This infection is mediated by the interaction of viral glycoprotein hemagglutinin and the sialic acid residues present on cell-surface glycoproteins and glycolipids. 

The data presented here demonstrate that lectins may be used to explore structural aspects of the carbohydrate moieties of specific cell surface bound antigens. Furthermore, it is suggested that the H-2D antigen possesses differences in the structure of the carbohydrate moieties which may reflect microheterogeneity due to the biosynthesis of only partial structures, differences in the number of sialic acid residues, the relationship of the carbohydrate chains to the specific antigenic determinants and/or major structural differences of the carbohydrate chains on different H—2D antigens. 

It is only the ability of modern chemistry to detect very small quantities of materials that made the following discovery possible. In some recent research it was reported that one subtle way in which cancer tumours cells differ from normal cells is how they metabolize carbohydrates present on their surfaces. Cancer cells have far more of the carbohydrate sialic acid, which can be detected with MRI (magnetic resonance imaging) analytical techniques. It was found that the sialic acid normally appears on the surface of the cells only in foetal development, but it appears abnormally in patients with gastric, colon, pancreatic, liver, lung, prostate and breast cancers, as well as in leukaemia. Research is continuing.1... 

The ability of viruses to infect specific cell types is dictated in part by the ability of these viruses to bind to particular structures or receptors on the surfaces of cells. In some cases, these receptors are carbohydrates. For example, influenza virus recognizes sialic acid residues present on cell-surface glycoproteins. The viral protein that binds to these sugars is called hemasslutinin(Fisure 11.31). 

The influenza virus is one of the most extensively studied vimses at present. The influenza virus recognizes sialic acid on the host cell surface, followed by infection and transmission [109]. Two spike-formed proteins (hemagglutinin and neuraminidase) are found on the surface of the influenza virus [110,111]. Hemagglutinin binds to the host cells through recognizing the ligand containing sialic acid on the surface of infected cells, while neuraminidase eliminates sialic acid from the cell surface to avoid inhibition of the transmission of newly formed virus particles. 

Figure 11.29 Viral receptors. Influenza virus targets cells by binding to sialic acid residues (purple diamonds) located at the termini of oligosaccharides present on cell-surface glycoproteins and glycolipids. These carbohydrates are bound by hemagglutinin (interaction circles), one of the major proteins expressed on the surface of the virus. The other major viral surface protein, neuraminidase, iis an enzyme that cleaves oligosaccharide chains to release the viral particle at a later stage of the viral life cycle.

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