AbstractWe 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 (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‐125MI‐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 PNpthat 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 in unclear. This effect seems incompatible with a heparinlike factor acting as the s
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