Prothrombinase complex, coagulation

The appearance of anionic phospholipids, particularly phosphatidylserine, on the cell siuface activates prothrombinase complex culminating in the formation of thrombin (Bevers et al., 1982 Connor et al., 1989). The assay can be performed with pure coagulation proteins and specific chromogenic substtates to produce a very sensitive test to detect the appearance of phosphatidylserine on ceU siufaces. Nevertheless, it has been shown that changes in the disposition of phosphatidylethanolamine and sphingomyelin may interfere with the ability of phosphatidylserine-containing membranes to activate prothrombinase (Smeets et al., 1996). 

Schematic view of the role of coagulation factor Xa in arterial thrombosis. After endothelial injury, platelets adhere to the subendothelial matrix. The procoagulant activity of the arterial clot can be attributed to the formation of the prothrombinase complex on the platelet surface which cleaves prothrombin and produces thrombin. Thrombin subsequently acts as a strong agonist of further platelet aggregation.

An important step in the blood coagulation pathway is the formation of the prothrombinase complex. The latter is a mixture of factor V, factor Xa, Ca2+, and phospholipid. In this case, a phospholipid mixture with a net negative charge will allow the prothrombinase complex to form. This active enzyme is important in cleaving prothrombin to yield thrombin. The most active phospholipid mixture for in vitro studies has proven to be phosphatidylserine-phosphatidylcholine. Subsequently the hypothesis has developed that phosphatidylserine is key to the formation of prothrombinase. 

Factor Xa and its co-factor Va form the prothrombinase complex, which activates prothrombin to thrombin. Thrombin then activates other components of the coagulation cascade, including factor V and factor VIE (which activates factor XI, in turn activating factor K), and activates factor Vni and releases it from being bound to vWF. 

The first development of ctor Xa inhibitors was met with less interest than ffirombin inhibitors. These early inhibitors had low affinity to ctor Xa, low selectivity and low potency (55,56). Because of the increase in enzymatic activity of the coagulation cascade once the prothrombinase complex is formed, a potent ctor Xa inhibitor is required to have an extremely high affinity for the enzyme. The first fector Xa inhibitors did not fulfill this requirement. 

Secretion. Exocytotic liberation of granular constituents has been demonstrated in leukocytes, platelets and neurological cells. Platelets attacked by alpha-toxin secrete platelet factor 4 and factor 5 (Arvand et a/., 1990 Bhakdi ef al., 1988). Release of the latter leads to assembly of platelet-bound prothrombinase complexes that generate thrombin. Alpha-toxin can thus promote coagulation via its perme-abilizing action on platelets. Release of granule constituents from leukocytes has been observed after permeabilization of these cells by HlyA. For example, large amounts of elastase are secreted into the extracellular medium (Bhakdi eta/., 1989). 

---