Subunit structure and function of porcine factor Xa-activated factor VIII

Area of Science:

• Biochemistry
• Hematology
• Molecular Biology

Background:

• Factor VIII (fVIII) is a crucial cofactor in the intrinsic pathway of blood coagulation.
• Activation of fVIII by thrombin (factor IIa) and factor Xa involves distinct proteolytic cleavage sites.
• Understanding these activation pathways is key to comprehending hemostasis and developing anticoagulant therapies.

Purpose of the Study:

• To characterize the structure and function of factor Xa-activated porcine factor VIII (fVIIIaXa).
• To compare the activation and cofactor activity of fVIIIaXa with thrombin-activated fVIII (fVIIIaIIa).
• To elucidate the molecular basis for differences in procoagulant activity between fVIIIaXa and fVIIIaIIa.

Main Methods:

• Purification of porcine fVIIIaXa using Mono S HPLC.
• NH2-terminal sequence analysis of purified fVIIIaXa subunits.
• Analytical ultracentrifugation to determine molecular weight and species.
• Fluorescence anisotropy to assess binding affinity to factor IXa.
• Kinetic analysis of cofactor activity within the intrinsic factor Xase complex.

Main Results:

• A stable, five-subunit porcine fVIIIaXa preparation was obtained, with factor Xa cleaving fVIII at multiple sites including Arg219 and Arg490.
• fVIIIaXa and fVIIIaIIa exhibited similar binding affinities and 1:1 stoichiometry with factor IXa.
• fVIIIaXa demonstrated 4-fold lower procoagulant activity compared to fVIIIaIIa.
• Kinetic analysis indicated reduced activity of fVIIIaXa within the intrinsic factor Xase complex.

Conclusions:

• Factor Xa and thrombin generate distinct activated factor VIII species with differing functional consequences.
• The lower procoagulant activity of fVIIIaXa is attributed to its reduced efficiency within the intrinsic factor Xase complex.
• These findings highlight the nuanced regulation of the coagulation cascade by different activating proteases.