Formation of protease nexin-thrombin complexes on the platelet surface

We have recently described a platelet factor that is similar to the fibroblast thrombin inhibitor protease nexin I (PNI). The present manuscript shows that this platelet form of PN (PNp) does not complex [125I]-thrombin that has been blocked at its active site, consistent with the conclusion that it is a thrombin inhibitor. When platelets are incubated with [125I]-thrombin, PNp-[125I]-thrombin complexers accumulate both in the medium and on the platelet surface. In the case of fibroblasts, PNI-[125I]-thrombin complexes that form in solution bind to the cells as a consequence of a receptor-mediated clearance process [Low et al, Proc Natl Acad Sci USA 78:2340, 1981]. We show here that the PNp-[125I]-thrombin complexes that accumulate in platelet-binding incubation medium do not bind to platelets. Thus, the platelet-associated complexes must form by [125I]-thrombin binding to PNp that is associated with the platelet surface. Pretreatment of platelets with heparin markedly increases the number of PNp-[125I]-thrombin complexes that form on platelets. The basis for this increase is unclear. This effect seems incompatible with a heparinlike factor acting as the surface binding site for PNp.