Plasma sialoglycoproteins

Table I lists nonhormonal plasma sialoglycoproteins, and when known, their molecular weight, number of sialic acid residues per mole, and carbohydrate content. Sialic acid content was estimated by either the Warren procedure (1959) or by treatment with microbial sialidase and measurement of sialic acid release and concomitant change in electrophoretic mobility or isoelectric point. Some of the physical and biological changes observed in the plasma sialoglycoproteins after removal of sialic acid residues are described below.

Cruz, T R, and Gurd, J. W., 1978, Reaction of synaptic plasma membrane sialoglycoproteins with intrinsic sialidase and wheat germ agglutinin, J. Biol. Chem. 253 7314-7318. 

Most cells in culture have a substantial amount of sialic acid, the bulk of which is associated with glycoproteins (Weinstein et al., 1970) and is primarily localized on the cell surface (Click et aL, 1971), cf. Chapter 3. Because the plasma membrane of cells is of prime interest in the transformation process, considerable research has been directed towards investigating changes in sialic acid and sialoglycoproteins after transformation. 

Several sialoglycoprotein hormones are present in relatively minor but essential quantities in the plasma. As early as 1948, it was reported that the biological activity of cmde chorionic and pituitary gonadotro-... 

A wide variety of diseases occur which are accompanied by changes in sialic acid levels in plasma. Various hypotheses have been put forward to explain the observed increases (Winzler, 1955 1971). It has been suggested that increased serum sialoglycoprotein levels are associated with tissue proliferation rather than tissue destruction (Shet-lar et al., 1949). Several questions remain unanswered (1) why are more sialoglycoproteins synthesized in many diseases, or in those where there are reduced levels, what are the controls exerted (2) do these compounds have a direct role in the disease process, or are they artifacts, and what changes are manifested by the enzymes involved in their synthesis or breakdown (3) if it were possible to restore the metabolism of these compounds to normal, would this combat the disease process and (4) are sialic-acid-containing compounds active in viral infections ... 

The aforementioned considerations suggest a function for the sialidase found associated with the plasma membranes of liver cells (Schengrund et al.y 1972 Visser and Emmelot, 1973) to desialylate circulating sialoglycoproteins for their subsequent uptake by the hepatocytes. The sialidase of erythrocyte membranes (Bosmann, 1974) may function in determining their half-life in the circulation. 

The activity of several lytic enzymes, including sialidase, is necessary for fertilization. While sperm are in the seminal plasma, inhibitors of these enzymes are present in the decapacitation factor (DF). Sialidase was found to be a component of sperm acrosomes by Srivastava et aL (1970). They found sialidase activity and a sialidase-like factor which rendered bound sialic acid reactive in the thiobarbituric acid assay (Warren, 1959) but did not release it from the molecule and, upon purification, appeared to be converted to sialidase. An inhibitor of the sialidase-like factor was found in a partially purified DF preparation. The DF is lost during capacitation, which can be broadly defined to include the changes sperm undergo to enable them to penetrate the vitelline membrane of the egg. Gould et aL (1971) have proposed that the role of sialidase in fertilization is to alter the tertiary and quaternary structure of sialoglycoprotein present in the zona pellucida to cause the zona pellucida to be less penetrable by spermatozoa and serve to reduce or block polyspermy. 

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