Summary
Activated protein C (APC) is a vitamin K dependent anticoagulant which catalyzes the
inactivation of factor Va and VIIIa, 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.
