Inhibition of intrinsic Xase by protein S: a novel regulatory role of protein S independent of activated protein C.
OBJECTIVE
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.


RESULTS
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.


CONCLUSIONS
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.
Department of Biochemistry and Biophysics, University of North Carolina, Chapel Hill, NC, USA.
Rima
R
Chattopadhyay
Tanusree
T
Sengupta
Rinku
R
Majumder
D001777:Blood Coagulation / Q000502:physiology*; D015949:Factor IXa / Q000037:antagonists & inhibitors* / Q000502:physiology; D015944:Factor VIIIa / Q000037:antagonists & inhibitors* / Q000502:physiology; D025461:Feedback, Physiological / Q000502:physiology; D006801:Humans; D066298:In Vitro Techniques; D010314:Partial Thromboplastin Time; D011486:Protein C / Q000502:physiology*; D017293:Protein S / Q000502:physiology*; D015398:Signal Transduction / Q000502:physiology; D013917:Thrombin / Q000502:physiology
D011486:Protein C; D017293:Protein S; D015944:Factor VIIIa; D015949:Factor IXa; D013917:Thrombin