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) [12]. The present manuscript shows that this platelet form of PN (PN p ) does not complex [125 I ]‐thrombin that has been blocked at its active site, consistent with the conclusion that it is a thrombin inhibitor. When platelets are incubated with [125 I ]‐thrombin, PN p ‐[125M I ]‐thrombin complexers accumulate both in the medium and on the platelet surface. In the case of fibroblasts, PNI‐[ 125 I]‐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 PN p ‐[ 125 I]‐thrombin complexes that accumulate in platelet‐binding incubation medium do not bind to platelets. Thus, the platelet‐associated complexes must form by [ 125 I]‐thrombin binding to PN p that is associated with the platelet surface. Pretreatment of platelets with heparin markedly increases the number of PN p ‐[ 125 I]‐thrombin complexes that form on platelets. The basis for this increase in unclear. This effect seems incompatible with a heparinlike factor acting as the surface binding site for PN p .