Effects of a heparin-binding protein on blood coagulation and platelet function

The objective of this study was to characterize the heparin-binding propert ies of a protein secreted by mouse myeloma cells. The characterization was performed using clinical assays, such as heparin activity assays and hepari n-induced thrombocytopenia (HIT) platelet activation assays. The tests were performed in the presence of heparin, lowmolecular-weight heparins (LMWH), or heparinoids and either heparin-binding protein (HBP) or saline to deter mine whether the HBP affects the activity of heparins. The characterization of the IMP using heparin activity assays showed that the HBP shortened the prolonged clotting times of the activated partial thromboplastin time (aPT T) and thrombin clotting time induced by high concentrations of unfractiona ted heparin. The chromogenic assays for antithrombin (AT), thrombin inhibit ion, and factor Xa inhibition demonstrated that this effect is related to h eparin concentrations below 0.5 IU/ml. The Heptest assay did not detect the se differences. The HBP did not modi, the anticoagulant effect of any LMWH or low- or high-sulfated glycosaminoglycans in the aPTT assay. Activation o f donor platelets in the presence of unfractionated heparin, platelet facto r 4 (PF4), and HIT-serum was not counteracted by the HBP in any of the assa ys. The characterization of the HBP using a PF4-enzyme-linked immunosorbent assay (ELISA) confirmed the lack of structural identity with PF4. However, the optical density data indicated that the protein structure may be simil ar to PF4 by binding to a PF4 antibody. These data suggest that the HBP iso lated from mouse myeloma cells has a low affinity to heparin and interacts with the secondary binding site to AT and also perhaps to PF4.