Thromb Haemost 1997; 77(04): 735-740
DOI: 10.1055/s-0038-1656043
Platelets
Schattauer GmbH Stuttgart

Thrombin Interaction with Platelet GPIB: Role of the Heparin Binding Domain

Erica De Candia
1  The Centro Ricerche Fisiopatologia dell’Emostasi, Istituto di Semeiotica Medica, Università Cattolica S. Cuore, Roma, Italia
,
Raimondo De Cristofaro
1  The Centro Ricerche Fisiopatologia dell’Emostasi, Istituto di Semeiotica Medica, Università Cattolica S. Cuore, Roma, Italia
,
Lulgi De Marco
Servizio Immunotrasfusionale e Analisi Cliniche, Centro di Riferimento Oncologico, Aviano, Pordenone, Italia
,
Moreno Mazzucato
Servizio Immunotrasfusionale e Analisi Cliniche, Centro di Riferimento Oncologico, Aviano, Pordenone, Italia
,
Matilde Picozzi
1  The Centro Ricerche Fisiopatologia dell’Emostasi, Istituto di Semeiotica Medica, Università Cattolica S. Cuore, Roma, Italia
,
Raffaele Landolfi
1  The Centro Ricerche Fisiopatologia dell’Emostasi, Istituto di Semeiotica Medica, Università Cattolica S. Cuore, Roma, Italia
› Author Affiliations
Further Information

Publication History

Received 25 June 1996

Accepted after revision 25 November 1996

Publication Date:
11 July 2018 (online)

Summary

The platelet membrane glycoprotein lb (Gplb) has a high affinity binding site for α-thrombin whose occupancy is thought to positively modulate the thrombin-induced platelet activation. In this study, aimed at further characterizing the thrombin-GpIb interaction, two thrombin anion exosites referred to as “heparin binding site” (HBS) and “fibrino#gen recognition site” (FRS) were investigated as the possible domains involved in Gplb binding. The role of thrombin HBS was explored by performing binding measurements of 125I-α-thrombin to purified glycocalicin (GC), the extracytoplasmic portion of Gplb, in the presence of heparin as well as after chemical modifications of the thrombin heparin binding site (thrombin-HBS phosphopyridoxylation). These studies showed that a) thrombin binding to GC could be competitively inhibited by heparin and b) the equilibrium association constant for thrombin-GC interaction was reduced up to ten-fold by chemical modifications at the HBS. On the other hand, the role of FRS in the thrombin-GC interaction could be excluded by other experiments showing that GC in solution could not influence the interaction of α-thrombin with two substrates which bind to both the catalytic site and the fibrinogen recognition site: 1) the thrombin receptor peptide 38-60 (TR, L38-E60) and 2) the A α-chain of fibrinogen. Altogether these results demonstrated that GC interaction with thrombin involves the enzyme heparin binding site, whereas the fibrinogen recognition site does not play a significant role.