Platelet‐Associated Coagulation Factors: Immunological Detection and the Effect of Calcium

Abstract

Normal platelets were examined by conventional coagulation methods and specific immunological techniques for the presence of platelet-associated coagulation factors. Platelets washed in buffer containing calcium chloride gave different results form those washed in the absence of calcium. Factor XI was not detected in washed platelets by any technique. Factor II and factor X were removed from platelets by washing in calcium-free buffer but not by calcium-containing buffer. Procoagulant factor VIII was readily removed or inactivated by washing in the presence or absence of calcium, but factor VIII related antigen was consistently demonstrated after multiple washes in either buffer. Factor V activity and antigen was detected in platelet suspensions after multiple washes although the presence of calcium in the washing buffer gave higher factor V assays and stronger immunological reactions than when calcium-free buffer was used. The results are consistent with the presence of calcium dependent binding sites in platelets which may have a role in the enhancement of factor V and phospholipid reactions.

von Willebrand protein facilitates platelet incorporation in polymerizing fibrin.

“…Thrombin not only converts fibrinogen to fibrin, but also induces platelet activation and secretion. Platelets are also a source of Factor V (22), fibrinogen (23), and vWF (24, 25), the latter protein being important not only for platelet adhesion, but also as a carrier for Factor VIII,. Even in the process ofclot dissolution, plasmin proteolyzes both fibrin and glycoprotein Tb, a vWF receptor, from the platelet surface (26), thereby facilitating fibrin and platelet removal from the thrombus.…”

Immunological Evidence for Prothrombin in Human Platelets

“…As references, 8 pg purified protein alone (lane 7) or after incubation with thrombin (lane 8), and thrombin alone (lane 9) were electrophoresed directly with Az3l[7caused release of most of the antigen, suggestirlg that most was internal, probably in o-granules. Some of the antigen, however, may be tightly associated with the platelet surface, since chymotrypsin, which removes surface proteins without activating platelets (16), caused release of some of the platelet antigen, and since platelets washed in L0 mM EDTA to remove adsorbed coagulation proteins (6) were agglutinated by antiserum or immune IgG. Previous experiments, using procoagulant (enzymatic)'assays (6,17,18) and indirect immunofluorescence (6,19), have failed to identify prothrombin associated with platelets.…”

Mechanisms of Pathogenesis of Arterial Thrombosis: Potential Sites of Inhibition by Therapeutic Compounds

“…32 In addition, stimulated platelets secrete a number of clotting factors, including fibrinogen, Factors X, Va, IX, and XII, and vWF. [33][34][35] Finally, platelets provide a procoagulant phospholipid for blood coagulation. This phospholipid reduces the concentration of the various activated factors, such as Factor Xa required for subsequent activation of Factors VIII and IX.…”