title: Protein S is essential for the activated protein C-catalyzed inactivation of platelet-associated factor Va.
abstract: The prothrombin-converting activity of Factor Xa was enhanced by thrombin-stimulated Factor V-deficient platelets and supplementary extraneous Factor Va, and also by thrombin-stimulated normal human platelets. Both extraneous Factor Va and intra-platelet Factor Va were equally inactivated by a gamma-carboxyglutamic acid-containing plasma protease, activated protein C. However, a relatively larger amount of activated protein C was required for efficient inactivation of platelet-associated Factor Va as compared with the amount of activated protein C needed for inactivation of phospholipid vesicle-associated Factor Va. Protein S, another gamma-carboxyglutamic acid-containing plasma protein, increased the rate of the inactivation of platelet-associated Factor Va about 25-fold. This stimulating effect was observed only slightly with the thrombin-modified protein S. Thus, it was concluded that protein S is essential for the process of inactivation of platelet-associated Factor Va by activated protein C.
mesh_terms: D001792:Blood Platelets / Q000502:physiology*; D005165:Factor V / Q000037:antagonists & inhibitors*; D015943:Factor Va; D005170:Factor X / Q000378:metabolism; D015951:Factor Xa; D006023:Glycoproteins / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D011486:Protein C; D017293:Protein S; D013917:Thrombin / Q000502:physiology
chemical_list: D006023:Glycoproteins; D011486:Protein C; D017293:Protein S; D015943:Factor Va; D005165:Factor V; D005170:Factor X; D013917:Thrombin; D015951:Factor Xa