Heme binds to factor VIII and inhibits its interaction with activated factor IX

Area of Science:

• Biochemistry
• Hematology
• Molecular Biology

Background:

• Heme, a redox-active molecule released during tissue damage, can overwhelm scavenging proteins.
• Extracellular heme's potential impact on coagulation and its association with the Factor VIII (FVIII)-von Willebrand factor (VWF) complex are under investigation.
• Heme release sites overlap with FVIII's hemostatic function sites.

Purpose of the Study:

• To investigate the interaction between heme and FVIII.
• To determine the consequences of heme binding on FVIII's procoagulant activity in vitro.

Main Methods:

• ELISA assays to assess FVIII interaction with activated FIX.
• Surface plasmon resonance to quantify binding kinetics.
• Fluorescence quenching to study conformational changes.
• In vitro assays to measure FVIII procoagulant activity.

Main Results:

• Heme binds to multiple high-affinity sites on FVIII.
• Heme binding inhibits FVIII procoagulant activity in a dose-dependent manner.
• Heme-induced FVIII inactivation is linked to reduced interaction with activated FIX.
• Von Willebrand factor (VWF) prevents heme-mediated FVIII inactivation, unlike human serum albumin.

Conclusions:

• Factor VIII (FVIII) is identified as a novel heme-binding protein.
• Low heme concentrations do not inactivate FVIII, but high concentrations do.
• Heme-mediated inhibition of FVIII occurs under conditions mimicking extensive heme release.
• The role of heme in regulating coagulation via FVIII activity requires further investigation.