title: Plasma factor V activation is prevented by activated protein C in the presence of phospholipid vesicles, not platelets.
abstract: Activated protein C (APC) is a vitamin K dependent anticoagulant which catalyzes the inactivation of factor V a and VIII a, in a reaction modulated by phospholipid membrane surface, or blood platelets. APC prevents thrombin generation at a much lower concentration when added to recalcified plasma and phospholipid vesicles, than recalcified plasma and platelets. This observation was attributed to a platelet associated APC inhibitor. We have performed serial thrombin, factor V one stage and two stage assays and Western blotting of dilute recalcified plasma containing either phospholipid vesicles or platelets and APC. More thrombin was formed at a given APC concentration with platelets than phospholipid. One stage factor V values increased to higher levels with platelets and APC than phospholipid and APC. Two stage factor V values decreased substantially with platelets and 5 nM APC but remained unchanged with phospholipid and 5 nM APC. Western blotting of plasma factor V confirmed factor V activation in the presence of platelets and APC, but lack of factor V activation with phospholipid and APC. Inclusion of platelets or platelet membrane with phospholipid enhanced rather than inhibited APC catalyzed plasma factor V inactivation. Platelet activation further enhanced factor V activation and inactivation at any given APC concentration. Plasma thrombin generation in the presence of platelets and APC is related to ongoing factor V activation. No inhibition of APC inactivation of FVa occurs in the presence of platelets.
authors.affiliation: St. Mary's Hospital Centre, McGill University, Montreal, Canada.
authors.forename: S
authors.initials: S
authors.lastname: Solymoss
authors.forename: K T
authors.initials: KT
authors.lastname: Nguyen
mesh_terms: D001777:Blood Coagulation / Q000187:drug effects / Q000502:physiology; D001792:Blood Platelets / Q000187:drug effects / Q000378:metabolism; D005165:Factor V / Q000378:metabolism*; D015943:Factor Va / Q000378:metabolism; D006801:Humans; D066298:In Vitro Techniques; D010743:Phospholipids / Q000097:blood; D011486:Protein C / Q000378:metabolism* / Q000494:pharmacology; D013917:Thrombin / Q000096:biosynthesis
chemical_list: D010743:Phospholipids; D011486:Protein C; D015943:Factor Va; D005165:Factor V; D013917:Thrombin