Platelets have more than one binding site for von Willebrand factor.

TLDR

Binding of radiolabeled von Willebrand factor to platelets stimulated by thrombin, ADP, or ADP plus epinephrine is specific, saturable, and reversible. A monoclonal IgG against a B‑cell/monocyte epitope, absent on platelets, was used to exclude Fc receptor–mediated effects. The results show that platelets have at least two distinct vWF‑binding sites: a GPIIb/IIIa‑dependent site engaged by thrombin, ADP, and ADP+EPI but not by ristocetin, and a GPIb‑dependent site mediating ristocetin binding, with thrombin‑induced binding increased in Bernard‑Soulier patients lacking GPIb, indicating exposure of an alternative site.

Abstract

The binding of 125I-von Willebrand factor (125I-vWF) to platelets stimulated by thrombin, ADP, and a combination of ADP + epinephrine (EPI) is specific, saturable, and reversible. Active platelet metabolism and divalent cations are required for binding induced by these stimuli, but not by ristocetin, suggesting the existence of different mechanisms involved in the vWF-platelet interaction. A monoclonal antibody directed against an epitope of membrane glycoprotein (GP) Ib had no effect on the binding of 125I-vWF to normal platelets stimulated by thrombin or a combination of ADP + EPI, but completely blocked ristocetin-induced binding. Binding induced by thrombin to GPIb-blocked platelets was specific. Moreover, thrombin-induced binding of 125I-vWF was increased, rather than decreased, in two patients with the Bernard-Soulier syndrome whose platelets lacked GPIb. Conversely, monoclonal antibodies directed against the GPIIb/IIIa complex had no effect on ristocetin-induced binding of 125I-v-WF to normal platelets, but blocked thrombin- and ADP + EPI-induced binding. To exclude effects mediated by the platelet Fc receptor, a monoclonal IgG directed against an epitope present on human B cells and monocytes, but not expressed on resting or stimulated platelets, was used. It did not affect 125I-vWF binding induced by any of the stimuli. These studies show that platelets have more than one binding site for vWF, and that they may be exposed by different stimuli.