On the role of the major platelet membrane glycoproteins in platelet adhesion to collagen

The effect of modification of intact rabbit platelets with chymotrypsin, trypsin or thrombin on platelet adhesion to collagen fibers was determined. Experiments were performed in vitro with washed platelets suspended in a medium containing EGTA to prevent platelet aggregation. Platelet adhesion was unaffected by chymotrypsin or thrombin treatment but was reduced upon trypsin treatment. The latter effect could be attributed to the ADP released upon trypsin treatment. In the presence of high concentrations of AMP, adhesion was restored to control values. Release also occurred on thrombin treatment but this did not influence adhesion. The effect of the enzymes on platelet membrane glycoproteins was monitored by gel electrophoresis. Chymotrypsin caused the loss, at least, of the regions susceptible to lactoperoxidase-catalyzed iodination and to periodic acid-Schiff (PAS) staining from the major membrane glycoproteins I, II and III of rabbit platelets. Such degradation also occurred to glycoproteins I and II with trypsin, but thrombin was without detectable effect on the major membrane glycoproteins. The results indicate that degradation of the major membrane glycoproteins does not affect adhesion to collagen. Adhesion appears to be mediated either by protease-resistant parts of these molecules or by other membrane entities not readily attacked by proteases.