Quantitative determination of the binding of β2- glycoprotein I and prothrombin to phosphatidylserine-exposing blood platelets

Edouard M. Bevers, Marie P. Janssen, Paul Comfurius, Krishnakumar Balasubramanian, Alan J. Schroit, Robert F A Zwaal, George M. Willems

Research output: Contribution to journalArticlepeer-review

21 Scopus citations


The plasma protein β2GPI (β2-glycoprotein I) has been proposed to mediate phagocytosis of apoptotic cells and to play a role in the antiphospholipid syndrome. This suggestion is based mainly on the presumption that β2GPI has an appreciable interaction with PS (phosphatidylserine)-exposing cell membranes. However, quantitative data on the binding of β2GPI to PS-exposing cells under physiologically relevant conditions are scarce and conflicting. Therefore we evaluated the binding of β2GPI to PS-expressing blood platelets. Flow cytometry showed that binding of β2GPI is negligible at physiological ionic strength, in contrast with significant binding occurring at low ionic strength. Binding parameters of β2GPI and (for comparison) prothrombin were quantified by ellipsometric measurement of protein depletion from the supernatant following incubation with platelets. At low ionic strength (20 mM NaCl, no CaCl2), a dissociation constant (K d) of 0.2 μM was found for β2GPI, with 7.4 × 105 binding sites per platelet. Under physiologically relevant conditions (120 mM NaCl and 3 mM CaCl2), binding of β2GPI was not detectable (extrapolated Kd > 80 μM). Prothrombin binding (at 3 mM CaCl2) was much less affected by ionic strength: Kd values of 0.5 and 1.4 μM were observed at 20 and 120 mM NaCl respectively. The low affinity and the presence of many lipid-binding proteins in plasma that can compete with the binding of β2GPI suggest that only a small fraction (< 5%) of the binding sites on PS-exposing blood cells are likely to be occupied by β2GPI. These findings are discussed in relation to the alleged (patho-)physiological functions of β2GPI.

Original languageEnglish (US)
Pages (from-to)271-279
Number of pages9
JournalBiochemical Journal
Issue number2
StatePublished - Mar 1 2005


  • Binding site
  • Dissociation constant
  • Phagocytosis
  • Platelet
  • Prothrombin
  • β-glycoprotein I

ASJC Scopus subject areas

  • Biochemistry
  • Molecular Biology
  • Cell Biology


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