Protein S is required for bovine platelets to support activated protein C binding and activity.
Gel-filtered platelets accelerate activated protein C inactivation of factor Va in a reaction that requires the presence of protein S. With protein S present, specific activated protein C binding to the platelet surface is observed (Kd = 11 +/- 3 nM, 203 +/- 20 sites/platelet). The concentration dependence of the activated protein C-mediated factor Va inactivation is in close agreement with the binding. The observed binding is specific since protein C does not compete with activated protein C. Platelet-bound activated protein C is approximately 8000 times more active than the solution-phase enzyme. Platelet activation with thrombin results in formation of a site capable of accelerating factor Va inactivation by activated protein C in the absence of added protein S. This cell surface site is blocked by the addition of affinity purified antibodies to protein S. We conclude that protein S is required for activated protein C binding to the platelet surface and subsequent rapid factor Va inactivation. Platelet activation leads to the expression of either protein S or an antigenically related protein which can substitute for exogenously added protein S.
Harris
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Esmon
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D000818:Animals; D001665:Binding Sites; D001792:Blood Platelets / Q000187:drug effects / Q000378:metabolism*; D002417:Cattle; D005165:Factor V / Q000037:antagonists & inhibitors; D015943:Factor Va; D006023:Glycoproteins / Q000097:blood* / Q000494:pharmacology / Q000502:physiology*; D007700:Kinetics; D011486:Protein C; D017293:Protein S; D012701:Serotonin / Q000097:blood; D013917:Thrombin / Q000494:pharmacology
D006023:Glycoproteins; D011486:Protein C; D017293:Protein S; D012701:Serotonin; D015943:Factor Va; D005165:Factor V; D013917:Thrombin