The effect of secreted heparin-binding proteins on heparin binding to platelets.

Heparin is known to bind to activated platelets at two classes of sites. However, the true affinities of these sites for heparin have been uncertain because the measurements have been made in the presence of heparin-binding proteins secreted by the platelets. Of these proteins platelet factor-4 (PF4) has the greatest affinity for heparin and is present in relatively high concentrations. Furthermore, a portion of the secreted PF4 binds to the platelet surface. Nevertheless, by gel-filtering platelets in the presence of 5 micrograms/ml heparin, we prepared platelets that were virtually free of PF4. The high and low affinities of these cells for heparin were respectively ten- and two-fold greater than the apparent affinities measured in the presence of the secreted platelet proteins. In contrast, neither the high- nor the low-affinity heparin binding capacity of these cells was altered, indicating that PF4, even though it binds to both heparin and platelets, appears not to link heparin to platelets.