Protein Disulfide Isomerase Catalyzes the Formation of Disulfide-Linked Complexes of Thrombospondin-1 with Thrombin–Antithrombin III

Abstract

The recent demonstration of a protein disulfide isomerase (PDI) on the surface of and secreted from blood platelets raises the possibility that proteins involved in hemostasis and wound healing are also substrates of this enzyme. In this study purified preparations of platelet PDI, thrombospondin-1 (TSP), α-thrombin, and antithrombin III (AT) were used to demonstrate that PDI catalyzes formation of a TSP–thrombin–AT complex consistent with previous results with supernatant platelet activation. Concentrations of 1.25 μg/ml of PDI were sufficient to convert almost 50% of thrombin to TSP–thrombin–AT complex. Complex formation requires low concentrations of a reduced thiol and the reaction can be prevented byN-ethymaleimide. The complex is dissociated by reducing agents such as mercaptoethanol. Absence of Ca2+and the addition of EDTA increased the rate of complex formation, indicating that TSP in the Ca2+-free form is most effective. In the absence of AT a small amount of TSP–thrombin complex formed which was only 0–13% of maximal complex formation in the presence of AT. This result, in combination with kinetic studies showing rapid formation of thrombin–AT complex followed by conversion to ternary complex, suggests that the thrombin–AT complex is an obligatory intermediate in the reaction. Under optimal conditions over 70% of the thrombin is incorporated into the complex in 60 min. Heparin accelerated the reaction largely by enhancing formation of thrombin–AT complexes and had little effect on TSP. PDI coprecipitated with TSP from the supernatant solution of activated platelets, suggesting an association between PDI and its substrate. In summary, these data are consistent with a role for PDI-catalyzed formation of disulfide-linked complexes of TSP with other proteins.