Vitronectin (VN; = complement S-protein), a plasma glycoprotein that is also associated with extracellular sites, was identified in washed human platelets contaminated with less than 0.05% of plasma VN. A specific enzyme-linked immunosorbent assay (ELISA) for VN has been developed and was used to detect and to quantitate VN in detergent extracts of washed platelets with 8.1 +/- 4.6 micrograms/10(9) platelets (n = 10), representing about 0.8% of the plasma VN pool. Platelet and plasma VN were similar by immunochemical criteria using Western-blot analysis, although platelet VN was mainly found as partially proteolyzed polypeptide. Total release of platelet VN occurred at optimal doses of Ca-ionophore 23187 or thrombin, whereas no VN was released by platelet treatment with digitonin or Staphylococcus alpha-toxin. During stimulation of washed platelets with various concentrations of thrombin, the nearly concomitant release of VN and plasminogen activator inhibitor-1 (PAI-1) together with platelet factor 4 indicated the association of VN with inner-platelet storage granules. Furthermore, platelet VN and PAI-1 in Ca-ionophore releasates comigrated during ultracentrifugation in high mol wt fractions of sucrose density gradients, indicating a possible association of both components. Complex formation of platelet VN and PAI-1 was verified by a sensitive enzyme-linked immunosorbent assay (ELISA) and accounts at least in part for a high molecular form of platelet VN. The identification of platelet VN and its binding to platelet PAI-1 raises the possibility that VN, in contrast to other adhesive proteins, may participate in localized regulatory functions of blood coagulation and fibrinolysis in platelet-matrix interactions and the protection of the matrix against proteolysis.

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