We have analyzed the binding of thrombin, a serine protease with central roles in hemostasis, to the subendothelial extracellular matrix (ECM) produced by cultured endothelial cells. This substrate provides a thrombogenic surface where hemostasis is initiated. Binding was saturable and equilibrium was achieved after 3 h incubation with 125I-alpha-thrombin. Scatchard analysis of thrombin binding revealed the presence of 5.1 X 10(9) binding sites per squared millimeter ECM, with an apparent Kd of 13 nM. The catalytically blocked enzyme, diisofluorophosphate (DIP)-alpha-thrombin competed efficiently with 125I-alpha-thrombin, indicating that the binding was independent of its catalytic site. Moreover, high concentrations of the synthetic tetradecapeptide, representing residues 367-380 of thrombin B chain (the macrophage mitogenic domain of thrombin), competed with thrombin binding to ECM, indicating that the binding site may reside in the vicinity of "loop B" region. Thrombin binds to dermatan sulfate in the ECM, as demonstrated by the inhibition of 125I-alpha-thrombin binding to ECM pretreated with chondroitinase ABC, but not with heparitinase or chondroitinase AC. This stands in contrast to 125I-FGF (fibroblast growth factor) binding to ECM, which was inhibited by heparitinase but not by chondroitinase ABC, ECM-bound thrombin exhibits an exposed proteolytic site as monitored by the Chromozyme TH assay and by its ability to convert fibrinogen to a fibrin clot and to induce platelet activation as indicated by 14C-serotonin release. ECM-bound thrombin failed to form a complex with its major circulating inhibitor-antithrombin III (AT III), compared with rapid complex formation with soluble thrombin. We propose that thrombin binds to subendothelial ECM where it remains functionally active, localized, and protected from inactivation by circulating inhibitors.
R Bar-Shavit, A Eldor, I Vlodavsky