Heparin binding to protein C inhibitor.

Area of Science:

• Biochemistry
• Molecular Biology
• Pharmacology

Background:

• Protein C inhibitor (PCI) is a plasma protein that regulates coagulation by inhibiting proteases like thrombin and activated protein C.
• Heparin is known to stimulate PCI's inhibitory activity, but the precise mechanisms of binding and acceleration are not fully understood.

Purpose of the Study:

• To elucidate the specific heparin-binding site on protein C inhibitor.
• To investigate how heparin binding accelerates proteinase inhibition by PCI.
• To determine the glycosaminoglycan specificity and optimal conditions for heparin-mediated acceleration.

Main Methods:

• Identification of the heparin-binding region within protein C inhibitor using biochemical assays.
• Assessing the effect of various glycosaminoglycans and polyanions on PCI's inhibitory activity.
• Kinetic analysis of proteinase inhibition rates in the presence of different heparin concentrations.

Main Results:

• The heparin-binding site on protein C inhibitor was localized to residues 264-283, distinct from related inhibitors.
• PCI demonstrated broad glycosaminoglycan specificity, including heparin and heparan sulfate, and was accelerated by non-sulfated polyanions.
• Heparin accelerated the inhibition of several proteases, including thrombin and activated protein C, dependent on ternary complex formation.

Conclusions:

• The study identified a unique heparin-binding site on protein C inhibitor and elucidated the mechanism of heparin-accelerated inhibition.
• The findings suggest that the significant rate enhancement of activated protein C inhibition by heparin contributes to PCI's regulatory role in the protein C system.