High affinity interaction between C4b-binding protein and vitamin K-dependent protein S in the presence of calcium. Suggestion of a third component in blood regulating the interaction.
The anticoagulant vitamin K-dependent protein S interacts with the complement regulatory protein C4b-binding protein (C4BP), both in purified systems and in plasma. The concentrations of these proteins in plasma are approximately equimolar (0.3 microM) and 30-40% of protein S in plasma is found in the noncomplexed state. Only the uncomplexed form of protein S displays anticoagulant activity and studies have shown that patients with a selective deficiency of free protein S have a high incidence of thrombosis. In this study, we report that the protein S-C4BP interaction is at least 100-fold tighter in the presence of Ca2+ than in EDTA. The KD in the presence of Ca2+ was estimated with a gel filtration technique to be less than 5 x 10(-10) M, whereas in the presence of EDTA, it was approximately 100-fold higher. Ca2+ titration experiments suggested that the Ca2+ sites which function in the protein S-C4BP interaction are of high affinity which, in turn, suggests that they may be independent of the gamma-carboxyglutamic acid region and may be present in the epidermal growth factor-like domains of protein S. The high affinity of the protein S-C4BP interaction in the presence of Ca2+ suggested that virtually all of the protein S in whole blood should be complexed with C4BP. However, even though the protein S-C4BP interaction in Ca2(+)-containing serum was shown to have the same high affinity as in purified systems, approximately 30-40% of the protein S in serum was free. These results appear best explained by the presence of a third component in whole blood which regulates the protein S-C4BP interaction, keeping approximately 30-40% of circulating protein S in its free, functionally anticoagulant form. It is speculated that persons with little free protein S may be deficient in this hypothetical third component.
Dahlbäck
B
B
Department of Clinical Chemistry, University of Lund, Malmö General Hospital, Sweden.
Frohm
B
B
Nelsestuen
G
G
D002118:Calcium / Q000494:pharmacology*; D002352:Carrier Proteins / Q000378:metabolism*; D002850:Chromatography, Gel; D015935:Complement C4b / Q000378:metabolism*; D003169:Complement Inactivator Proteins; D004492:Edetic Acid / Q000494:pharmacology; D006023:Glycoproteins / Q000172:deficiency / Q000302:isolation & purification / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D011092:Polyethylene Glycols; D011485:Protein Binding; D017293:Protein S; D013927:Thrombosis / Q000097:blood / Q000209:etiology
D002352:Carrier Proteins; D003169:Complement Inactivator Proteins; D006023:Glycoproteins; D017293:Protein S; D011092:Polyethylene Glycols; D015935:Complement C4b; D004492:Edetic Acid; D002118:Calcium