Effects of oligosaccharides in plasma glycoproteins on platelet interactions

Thrombus formation at blood interfaces is a critical factor in the design of medical devices and artificial organs. The thrombo-genicity of polymers was correlated with protein adsorption data from plasma. Surfaces with preferred adsorption of glycoproteins, such as fibrinogen and gamma-globulin, over albumin were shown to be more thrombogenic. The mole ratio of adsorbed fibriogen or gamma-globulin to adsorbed albumin provided an index to the thrombogenicity of the polymer surface. The mechanisms of platelet interaction and consequent platelet adhesion onto polymer surfaces were indirectly verified by the established hypothesis that the glycoproteins with terminal sugar units such as galactose or N-acetylglucosamine of incomplete oligosaccharide chains may interact with platelets via an enzyme-substrate complex formation. The degree of complex formation or platelet interaction with glycoproteins, which subsequently affects platelet aggregation and release, was determined by varying the number of exposed glactose groups at the terminus of oligosaccharide chains. The number of exposed galactose termini was controlled by enzyme treatments of neuraminidase and galactose oxidase or periodate treatment. Galactose oxidase-treated glycoproteins showed decreased aggregation and releasing activity as well as a weaker interaction with platelets due to the decreased number of the inteact terminal galactose groups. Periodate treatment, which selectively oxidizes carbohydrate residues of the glycoproteins, also significantly reduced the platelet aggregation and the serotonin releasing activity of the glycoproteins. The neuraminidase treatment, by which the increased number of galactose groups was exposed at the terminal position of the oligosaccharide chains, caused a greater interaction of platelets with the proteins followed by an increased serotonin release from the platelets. However, the increased number of terminal galactose groups failed to induce the correspondingly increased aggregating activity. The neuraminidase treatment resulted in a conformational change arising from removal of sialic acid. This conformational change, confirmed by circular dichroism spectra of the protein, may be responsible for the nonincreasing aggregating activity of the glycoproteins in spite of the increased terminal galactose groups. Such a conformational change could be observed by UV irradiation of the glycoproteins which also reduced the aggregating activity of the proteins. Therefore, the involvement of glycoproteins in platelet aggregation may require conformational integrity of the protein for the maximal aggregation effect. The similar interaction of platelets with the glycoproteins was determined from platelet retention experiments using protein-immobilized beads packed in columns. The retention of platelets, when citrated whole blood was passed through the columns containing protein-immobilized Sepharose 2B, was increased with increasing numbers of terminal galactose residues. Immobilization of galactose to beads also significantly increased the platelet retention in the galactose-immobilized bead columns. The results of the platelet interactions with adsorbed proteins on polymer surfaces were directly observed by scanning electron micrography and light micrography. These micrography results showed a similar pattern to the results previously described. An increased number of adhered platelets was observed on the neuraminidase-treated glycoprotein coated polymer surface, while platelet adhesion was markedly decreased on the galactose oxidase-treated or the periodate-treated glycoprotein coated surface to a level equal to that of albumin coated surfaces. The use of sialyltransferase inhibitors, such as aspirin and UDP, depressed ADP-induced platelet aggregation. The results obtained are supportive of the involvement of this proposed enzyme mechanism for platelet-protein interactions.