Inhibition of Intrinsic Xase by Protein S

Abstract

Protein S is a vitamin K–dependent plasma protein that functions in the feedback regulation of thrombin generation. Our goal was to determine how protein S regulates the intrinsic pathway of blood coagulation. We used plasma, including platelet-rich plasma, and in vitro methods to determine how the intrinsic pathway of blood coagulation is regulated by protein S. We obtained the following results: (1) activated partial thromboplastin time assays with protein S–supplemented plasma confirmed that protein S prolongs clotting time; (2) a modified activated partial thromboplastin time assay with factor IX (fIX)–deficient plasma confirmed that protein S affects fIX-initiated clotting; (3) a fIXa/factor VIIIa (fVIIIa)–mediated thrombin generation assay with either platelet-rich plasma or factor-deficient plasma, initiated with a limiting amount of tissue factor, was regulated by protein S; (4) in the presence of phosphatidylserine vesicles, protein S inhibited fIXa in the absence and presence of fVIIIa; and (5) protein S altered only the K _M for factor X activation by fIXa in the absence of fVIIIa and both k _cat and K _M in the presence of fVIIIa. From our findings, it can be concluded that protein S inhibits fIXa in the presence or absence of fVIIIa in an activated protein C–independent way.