Action of Plasmin on Plasma Membranes of Human Platelets

The action of plasmin on plasma membrane polypeptides of human platelets was studied with the aid of polyacrylamide gel disc electrophoresis in sodium dodecyl sulfate. When isolated membranes were treated with plasmin, a minor polypeptide band of mol. wt. 120,000 disappeared completely and major polypeptide bands of mol. wt. 40,000 and 100,000 were reduced in concentration. Plasmin digestion of intact platelets coupled with subsequent membrane isolation and gel electrophoresis resulted in only a slight decrease in staining intensity in a glycopeptide band with a mol. wt. 100,000. Platelet membranes contain more than three plasmin substrates, most of which, however, are resistant to this enzyme in intact platelets because of their orientation in the membranes and some other mechanisms.