Inhibition of tissue factor:factor VIIa-catalyzed factor IX and factor X activation by TFPI and TFPI constructs

Area of Science:

• Biochemistry
• Hematology
• Molecular Biology

Background:

• Tissue factor pathway inhibitor (TFPI) is a Kunitz-type protease inhibitor.
• TFPI downregulates the extrinsic coagulation pathway by inhibiting factor Xa (FXa) and FVIIa.
• Optimal inhibition of FXa and FVIIa requires all three Kunitz domains (KD1, KD2, KD3) and protein S.

Purpose of the Study:

• To investigate the role of TFPI's Kunitz domains and protein S.
• To determine their function in the inhibition of FX and FIX activation.

Main Methods:

• Quantified inhibition of TF:FVIIa-catalyzed FX and FIX activation.
• Utilized full-length TFPI (TFPIFL) and TFPI constructs.
• Measured FXa and FIXa generation using chromogenic substrates.

Main Results:

• TFPIFL inhibited TF:FVIIa-catalyzed FIX activation (Ki = 16.7 nmol/L), enhanced by protein S (Ki = 1.0 nmol/L).
• TFPI1-150 and KD1-KD2 showed 10-fold higher Ki values and lacked protein S stimulation.
• Single Kunitz domains were poor inhibitors; TFPIFL was more active than TFPI1-150 or KD1-KD2 at low phospholipid concentrations.

Conclusions:

• Protein S significantly enhances TFPI's inhibition of FIX activation.
• Kunitz domain composition and phospholipid concentration affect TFPI activity.
• Binary TFPI:FXa complex formation is the rate-limiting step in TF:FVIIa inhibition.