Thrombin interaction with platelet glycoprotein Ib: effect of glycocalicin
on thrombin specificity
M Jandrot-Perrus, KJ Clemetson, MG Huisse and MC Guillin
Laboratoire de Recherche sur l'Hemostase et la Thrombose, Faculte Xavier
Bichat, Paris, France.
We describe here the alteration of thrombin specificity induced by its
interaction with glycocalicin. Glycocalicin is the external part of
platelet glycoprotein Ib alpha (GPIb alpha) and contains binding sites for
von Willebrand factor and thrombin. Taking advantage of its solubility, we
have used glycocalicin in competition assays on various thrombin
activities. Glycocalicin did not inhibit chromogenic substrate hydrolysis
nor diisopropylfluorophosphate iPr2 (PF) incorporation, indicating that
thrombin binding to GPIb does not alter access to or the conformation of
the thrombin catalytic site. Glycocalicin competitively inhibited thrombin
binding to fibrin (Ki = 0.1 mumol/L) and blocked fibrinogen clotting
activity of thrombin. Glycocalicin also inhibited thrombin binding to
thrombomodulin in a competitive manner (Ki = 3 to 5 mumol/L), but failed to
prevent thrombin interaction with protein C in the absence of
thrombomodulin. Previous results have indicated that GPIb binds to thrombin
within the anion binding exosite masked by the carboxy-terminal hirudin
peptide 54-65. The present results confirm the implication of the anion
binding exosite in GPIb recognition, and further indicate that the thrombin
binding site for GPIb overlaps with the thrombin binding sites for fibrin
and thrombomodulin, whereas it is distinct from the thrombin binding site
for protein C. Some of the structural requirements for thrombin binding to
GPIb appear to be very similar to those reported for binding to its
platelet receptor. However, thrombin-GPIb interaction does not appear to
compete with receptor hydrolysis but rather increases the sensitivity and
the rate of platelet responses elicited by the receptor.
Volume 80,
Issue 11,
pp. 2781-2786,
12/01/1992
Copyright © 1992 by The American Society of Hematology
