Formation of a ternary polyphosphate-thrombin-fibrin complex attenuates thrombin activity.

Area of Science:

• Biochemistry
• Hematology
• Molecular Biology

Background:

• Platelet-derived polyphosphate (polyP) acts as a procoagulant, enhancing thrombin generation and fibrin formation.
• PolyP, like heparin, interacts with thrombin and fibrin(ogen).

Purpose of the Study:

• To investigate if polyP, similar to heparin, facilitates thrombin binding to fibrin.
• To determine if this interaction forms a ternary polyP-thrombin-fibrin complex that modulates thrombin activity.

Main Methods:

• Assessed polyP and heparin affinity for fibrin using labeled polyanions.
• Quantified thrombin-fibrin affinity with and without polyanions via chromogenic assay.
• Measured thrombin inhibition rates by antithrombin-heparin in the presence of polyanions and fibrin.

Main Results:

• PolyP and heparin bind fibrin with high affinity (Kd ≈ 1.7 μM and 1.1 μM, respectively).
• In the presence of polyP or heparin, thrombin binds fibrin with significantly increased affinity (Kd ≈ 1.2 μM and 0.9 μM) compared to its absence (Kd ≈ 5.6 μM).
• The ternary complex protects thrombin from antithrombin-heparin inhibition but reduces its fibrin-inducing activity.

Conclusions:

• PolyP promotes ternary complex formation (polyP-thrombin-fibrin), mirroring heparin's action.
• This complex shields thrombin from inhibition but diminishes its procoagulant function in fibrin generation.
• Findings elucidate the regulatory role of polyP, thrombin, and fibrin in hemostasis.