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Protein S binds to and inhibits factor Xa
M J Heeb1, J Rosing, H M Bakker
1Scripps Research Institute, La Jolla, CA 92037.
Summary
Protein S directly inhibits factor Xa, a key enzyme in blood clotting. This interaction, independent of activated protein C, reveals new anticoagulant mechanisms involving protein S binding to factors Xa and Va.
Area of Science:
- Biochemistry
- Hematology
- Molecular Biology
Background:
- Human protein S is known to inhibit prothrombinase activity by binding to factor Va.
- However, this inhibition is not entirely dependent on factor Va, suggesting other interactions.
Purpose of the Study:
- To investigate the potential interaction between protein S and human factor Xa.
- To elucidate the direct anticoagulant mechanisms of protein S involving factor Xa.
Main Methods:
- Ligand blotting assays using factor Xa and its derivatives with protein S.
- Surface plasmon resonance to determine binding affinity (Kd) between protein S and factor Xa.
- Enzyme activity assays to measure the inhibition of factor Xa amidolytic activity and prothrombin conversion.
- Clotting time assays using protein S-depleted plasma.
Main Results:
- Protein S directly binds to factor Xa with a dissociation constant (Kd) of approximately 18 nM in fluid phase.
- Protein S inhibits factor Xa amidolytic activity and prothrombin conversion in a dose-dependent manner.
- This inhibition is phospholipid-independent but Ca2+-stimulated and is enhanced in the presence of factor Va.
- Protein S prolongs the factor Xa clotting time in a dose-dependent manner.
Conclusions:
- Protein S exhibits anticoagulant activity through direct binding to and inhibition of factor Xa.
- These mechanisms are independent of activated protein C, highlighting a novel role for protein S.
- Protein S interacts with both factor Xa and factor Va, contributing to its overall anticoagulant effect.