PROTEASE NEXIN II IN HUMAN PLATELETS.

 

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Normal human fibroblasts secrete a protein into the culture medium named protease nexin II (PN II), which forms sodium do-decyl sulfate (SDS)-stable complexes with epidermal growth factor binding protein (EGF BP). These complexes then bind back to the same cells and are rapidly internalized and degraded. We recently purified PN II to apparent homogeneity and showed that it is a single chain polypeptide with an estimated molecular weight of 106 KDa. Other interesting properties of this protein include its ability to bind heparin and its stability to treatment with SDS or pH 1.5. In addition to EGF BP, PN II will also complex the gamma subunit of 7S nerve growth factor (NGF-gamma) and trypsin. Here we show that human platelets contain a protein which possesses the same properties as does PN II from cultured human fibroblasts. This platelet PN II (PL-PN II) is also a 106 KDa single chain polypeptide which can complex EGF BP, NGF-gamma and trypsin. PL-PN II also possesses the unusual stability to treatment with SDS or pH 1.5. In addition, both PN II and PL-PN II exhibit the same affinity for binding to heparin-Sepharose. Furthermore, rabbit polyclonal antiserum raised against PN II recognized PL-PN II on Western blot analysis demonstrating that both proteins are immunologically related. Treatment of fresh unactivated platelets with epinephrine or thrombin resulted in a release of PL-PN II into the supernatant suggesting that PL-PN II is stored in the platelet alpha granules.