ABSTRACT
The objective of this study was to characterize the heparin-binding properties of
a protein secreted by mouse myeloma cells. The characterization was performed using
clinical assays, such as heparin activity assays and heparin-induced thrombocytopenia
(HIT) platelet activation assays. The tests were performed in the presence of heparin,
low-molecular-weight heparins (LMWH), or heparinoids and either heparin-binding protein
(HBP) or saline to determine whether the HBP affects the activity of heparins. The
characterization of the HBP using heparin activity assays showed that the HBP shortened
the prolonged clotting times of the activated partial thromboplastin time (aPTT) and
thrombin clotting time induced by high concentrations of unfractionated heparin. The
chromogenic assays for antithrombin (AT), thrombin inhibition, and factor Xa inhibition
demonstrated that this effect is related to heparin concentrations below 0.5 IU/ml.
The Heptest assay did not detect these differences. The HBP did not modify the anticoagulant
effect of any LMWH or low- or high-sulfated glycosaminoglycans in the aPTT assay.
Activation of donor platelets in the presence of unfractionated heparin, platelet
factor 4 (PF4), and HIT-serum was not counteracted by the HBP in any of the assays.
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 similar to PF4 by binding to a PF4 antibody.
These data suggest that the HBP isolated from mouse myeloma cells has a low affinity
to heparin and interacts with the secondary binding site to AT and also perhaps to
PF4.
KEYWORD
Heparin-binding protein - heparin - low-molecular-weight heparin (LMWH) - heparin-induced
thrombocytopenia (HIT) - platelet factor 4
