Selective inhibition of factor Xa in the prothrombinase complex by the carboxyl-terminal domain of antistasin

Inhibition mechanisms of hematophagous invertebrate compounds acting on the host blood coagulation and platelet aggregation pathways

To facilitate blood feeding, hematophagous invertebrates have evolved a sophisticated array of physiological compounds that counteract homeostatic systems and inflammatory reactions of the vertebrate host. For this reason, hematophagous invertebrates possess a variety of anticoagulation components that are inhibitors of coagulant factors or antagonists of the platelet receptor. The examination of kinetic data and the crystal structure analysis have exposed the inhibition mechanisms for many of these anticoagulant reagents. Here, we attempt to classify the antihemostatic molecules and to focus on the kinetic approaches that have been instrumental in defining these mechanisms.

Overview of anticoagulant drugs for the future

More efficacious, safer, and easier to use anticoagulants are under development. Multiple agents have been shown to be effective in ex vivo or animal thrombosis models and several have progressed to clinical studies. Investigators have not yet determined if pharmaceuticals that inhibit coagulation factor activity earlier in the cascade (for example, inhibitors of tissue factor/factor VIIa, factor IXa, or Xa) are superior to those that block the cascade at a later point. Orally bioavailable drugs for the long-term treatment of thrombotic disorders, particularly those that do not require monitoring, are needed and are under development. Local delivery of anticoagulants or genes modulating anticoagulant control at sites of increased thrombogenicity, such as in diseased arteries, is a promising treatment modality that may decrease systemic bleeding problems. Much about the initiating pathophysiologic events leading to venous thrombotic disease needs to be elucidated before such local therapy can be tested in the venous vasculature. While awaiting better anticoagulants to become routinely available, we need to improve patient management with existing drugs by instituting anticoagulation clinics, promoting patient self-monitoring, and improving efforts to educate patients and health care providers about the use of anticoagulant drugs.

Hypersulfated low molecular weight heparin with reduced affinity for antithrombin acts as an anticoagulant by inhibiting intrinsic tenase and prothrombinase

In buffer systems, heparin and low molecular weight heparin (LMWH) directly inhibit the intrinsic factor X-activating complex (intrinsic tenase) but have no effect on the prothrombin-activating complex (prothrombinase). Although chemical modification of LMWH, to lower its affinity for antithrombin (LA-LMWH) has no effect on its ability to inhibit intrinsic tenase, N-desulfation of LMWH reduces its activity 12-fold. To further explore the role of sulfation, hypersulfated LA-LMWH was synthesized (sLA-LMWH). sLA-LMWH is not only a 32-fold more potent inhibitor of intrinsic tenase than LA-LMWH; it also acquires prothrombinase inhibitory activity. A direct correlation between the extent of sulfation of LA-LMWH and its inhibitory activity against intrinsic tenase and prothrombinase is observed. In plasma-based assays of tenase and prothrombinase, sLA-LMWH produces similar prolongation of clotting times in plasma depleted of antithrombin and/or heparin cofactor II as it does in control plasma. In contrast, heparin has no effect in antithrombin-depleted plasma. When the effect of sLA-LMWH on various components of tenase and prothrombinase was examined, its inhibitory activity was found to be cofactor-dependent (factors Va and VIIIa) and phospholipid-independent. These studies reveal that sLA-LMWH acts as a potent antithrombin-independent inhibitor of coagulation by attenuating intrinsic tenase and prothrombinase.

Reversible modulation of human factor Xa activity with phosphonate esters: media effects

Enantiomers of 4-nitrophenyl 4-X-phenacyl methylphosphonate esters (X = H, PMN; CH3 and CH3O) inactivate human factor Xa with rate constants 8-86 M(-1)s(-1) at pH 6.75 in 0.025 M Hepes buffer, 0.15 M NaCl and 2 mM CaCl2 at 7.0+/-0.1 degrees C. The stereoselectivity of the inactivation of factor Xa is 2-10 and favors the levorotatory enantiomers. The pH-dependence of inactivation of factor Xa by (-)-PMN is sigmoidal and consistent with the participation of a catalytic residue with a pKa of 6.2+/-0.1. Factor Xa reactivates from its phosphonyl adducts through a self-catalyzed intramolecular reaction, which is much influenced by the presence of phospholipids. The rate of reactivation in the absence of phospholipids is not pH dependent at pH <9, but it increases very much at pH >9. In the presence of phospholipids, the pH dependence of the rate constant for reactivation is sigmoidal in the pH 6.5-10.3 range and levels off at pH >9 indicating that the enzyme catalyzes its reactivation. The kinetic pKa for the recovery of factor Xa from its adducts with the PMNs is in the range of 6.7-8.1 and is consistent with the participation of the catalytic His57 in the reactivation process.