Sialic acid removal with neuraminidase

Another possible explanation (Simmons and Rios, 1972) is that loss pf sialic acid would lower the negative charge at the cell surface, thus increasing the deformability of the cell (Weiss, 1965) and enhancing its susceptibility to phagocytosis (Lee, 1968). Furthermore, no activity change could be detected, after removal of sialic acid residues with neuraminidase, in the in vitro determination of inhibition of isolated... 

Conflicting results were obtained in the study of the influence of sialic acid on the flow of ions through the cell membrane Click and Githens (1965) observed a sharp response of the K+ ions to the removal of sialic acid with neuraminidase in L1210 leukemia cells, whereas the transport of Na ions was only slightly inhibited the transport of ions was inhibited regardless of the direction of flow. In Ehrlich ascites cells, however, removal of sialic acid residues with neuraminidase did not alter the content of Na and ions in the cells. Only a very small reduction in the unidirectional fluxes of K ions was observed after neuraminidase treatment. These observations led Weiss and Levinson (1969) to conclude that anionic sites on the cell membrane were not of major importance in regulating the intracellular concentration of Na and K ions or the unidirectional, transmembrane flux of ions. 

On the basis of a concentration of 2.4 x 10 molecules of sialic acid per cell, a high sialyltransferase activity, and a measurable neuraminidase activity at the surface of the human blood platelet, Bosmann (1972) proposed that platelet aggregation could result from an interaction, at the surface of the cell, between the sialyltransferases and the galactosyl residues liberated by the action of the neuraminidase. This application of Roseman s (1970) theory to platelet aggregation would require a dynamic equilibrium of transfers and removals of sialic acid residues, with continuous formation, degradation, and reformation of the enzyme-substrate complexes. 

According to Rolla, ionic bonds are important in the associations between bacterial polysaccharides and protein-coated tooth surfaces (21). This was based on in vitro experiments on the afiinity of dextran for hydroxyapatite powder coated with salivary glycoprotein specifically, adsorption of dextran was inhibited by 0.5M. Prior treatment of the coated hydroxyapatite with neuraminidase also reduced adsorption of dextran. Neuraminidase would be expected to reduce the negative charge of the protein coat by removing ionized sialic acid moieties. Of course, reduced adsorption of dextran could result from conformational changes induced in the pellicle protein by the neuraminidase treatment, as was apparently effected by 4M or 8M urea, in other experiments. 

Many enzymes are glycoproteins, and variations in carbohydrate side chains are a common cause of nonhomogeneity of preparations of these enzymes. Some carbohydrate moieties, notably h/-acetylneuraminic acid (sialic acid), are strongly ionized and consequently have a profound effect on some properties of enzyme molecules. For example, removal of terminal sialic acid groups from human liver and/or bone alkaline phosphatase with neuraminidase greatly reduces the electrophoretic heterogeneity of the enzyme. 

Two approaches have been proposed to improve the electrophoretic separation between bone and liver ALPs. Both methods exploit differences in the carbohydrate portions of the two forms of ALP. In one, electrophoresis is carried out in the presence of wheat germ lectin, which retards bone ALP migration more than the liver enzyme migration. In the other, serum is treated briefly (i.e.> for 15 min at 37 °C), with neuraminidase to remove part of the terminal sialic acid residues. As the sialic acid residues of bone ALP are more readily attacked than those of liver ALP, the electrophoretic mobility of the bone form is reduced more than that of liver ALP. The improved separation allows quantitative estimates to be made by densitometric scanning (Figure 21-5). ... 

FSH, derived both from pituitary tissue and from urine, is inactive if all the sialic acid has been removed from the molecule (G6). The sialic acid in LH is more stable than that in FSH, but treatment of the hormone with neuraminidase removes 75% of the LH activity (S13). It appears that tryptophan (P4) and probably a sulfhydryl group (A7) are necessary for FSH activity. 

Plasmodia were found to neither contain sialic acid nor were they capable of its synthesis (Schauer et al., 1984). Hence, when declines in sialic acid were found in P. berghei, P. knowlesi and P. yoelii it was assumed they were due to changes either in the quality or quantity of the host sialic acids (Howard and Day, 1981 Howard et ah, 1980,1986). It was claimed that the decreased reactivity of surface sialic acid for neuraminidase in murine and simian malaria-infected red cells was due to O-acetylation. However, they found little or no decrease in P. falciparum-infected red cells (Howard et al., 1981) and concluded that extensive removal or modification of sialic acid did not occur with human malaria, in contrast to the murine malaria. I believe their results can be explained quite simply the samples contained too few parasitized erythrocytes (i.e. parasitemias were low and most of the parasites were small, immature trophozoites). Indeed, when we re-investigated the sialic acids in P. lophurae-infected red cells where the parasitemias were above 80% (Sherman and Jones, 1979) there was a significant reduction (from 79 nmol/mg protein to 36 nmol/mg protein) and this was also found in in vitro grown P. falciparum (Sherman, personal communication) and P. vivax (reference in Sherman et al., 2004). Although we still do not understand the mechanism for loss of sialic acid from malaria-infected red cells I am of the opinion that the lowered amount of surface sialic acid is both real and significant. 

T. cruzi-infected mice correlates with the degree of parasitemia. Further studies revealed that neuraminidase also removes sialic acid from the surfaee of myoeardial and endothelial cells, indicating that this molecule may play a critical role in the pathology of Chagas disease (15). 

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