Heparin cofactor and plasma antithrombin in relation to the mechanism of inactivation of thrombin by heparin

Revisiting the Pharmacology of Unfractionated Heparin

Unfractionated heparin (UFH) is a commonly used anticoagulant therapy for the acute treatment and prevention of thrombosis. Its short duration of action, reversibility of effect by protamine sulfate, and extensive clinical experience are some of the advantages that support its use. However, the choice of dose and dosing regimen of UFH remains challenging for several reasons. First, UFH has a narrow therapeutic window and wide variability in the dose-response relationship. Second, its pharmacodynamic (PD) properties are difficult to characterise owing to the complex multidimensional mechanisms of interaction with the haemostatic system. Third, the complex heterogeneous chemical composition of UFH precludes precise characterisation of its pharmacokinetic (PK) properties. This review provides a comprehensive mechanistic approach to the interaction of UFH with the haemostatic system. The effect of chemical structure on its PK and PD properties is quantitatively described, and a framework for characterisation of the dose-response relationship of UFH for the purpose of dose optimisation is proposed.

Effect of protamine sulfate on antithrombin III activity

When protamine sulfate was added to heparinized plasma in vitro for neutralization of heparin, the activities on both thrombin and Xa known as heparin cofactor in antithrombin action were completely abolished. However, progressive activities on thrombin and Xa both recovered within 30 minutes after protamine sulfate addition. When equivalent heparin was again added, heparin cofactor activity was immediately restored. Based on the fact that protamine sulfate did not show any direct action on the antithrombin III molecule, the presence of AT III with progressive activity was considered to play an important role in the rebound phenomenon of heparin after heparin neutralization with protamine sulfate.

Antithrombin BM from human plasma: an antithrombin binding moderately to heparin

A human antithrombin was purified app. 60 fold from Cohn fraction IV, to give a single band of about 70.000 molecular weight in polyacrylamide gel electrophoresis. Compared to the similar antithrombin III, this glycoprotein binds only moderately to porcine heparin (hence its name Antithrombin BM), thus requiring higher heparin concentration for full thrombin inhibitor function, and lower ionic strength for elution from a heparin sepharose column. In these respects it resembles "heparin cofactor A", which is, however, characterized by a substantially larger molecular weight. From AT III, AT BM further differs in its absolute dependency on the presence of heparin(oids) for antithrombin activity, in its more pronounced inhibitory specificity largely restricted to thrombin, and in the absence of substantial immunological crossreaction with antibody to AT III. Based on comparative measurements of antithrombin activity in the presence of different amounts of heparin, up to 40% of the antithrombin activity present in human blood may be attributed to AT BM. The in vivo role of this new inhibitor remains to be elucidated.

A simple two-step isolation procedure for human and bovine antithrombin II/III (heparin cofactor): a comparison of two methods

Human and bovine antithrombin II/III have been isolated by a simple procedure essentially using only affinity chromatography on heparin-agarose and polyethylene glycol precipitation. An additional ammonium sulphate step removes polyethylene glycol and trace contaminants. The final preparation is homogeneous by SDS-, disc- and agar slide electrophoresis. End group analysis of human antithrombin II/III shows histidine as the N-terminal amino acid. The pure preparation showed both progressive antithrombin activity and heparin cofactor activity.

Action of heparin on thrombin-antithrombin reaction

Thrombin partially purified from bovine plasma can be inactivated at 60 degress C. In the presence of 10 units of heparin the extent of inactivation decreases. When thrombin and heparin are mixed and incubated for 5 min at 0 degrees C before gel filtration on Sephadex G-200, thrombin with heparin is eluted prior to either thrombin or heparin laone. These data suggest a complex formation between thrombin and heparin. Immobilized heparin binds thrombin. The enzyme can be eluted with 0.05 M Tris-HCl buffer, pH 7.3, containing an ion mixture of Na+, K+ and Ca2+ at 73, 3 and 11 mM, respectively, at 0 degrees C and with 0.05 M Tris-HCl buffer, pH 7.3, containing 0.5 M NaCl at 20 degrees C. During the same chromatographic procedure, antithrombin-III (heparin cofactor) partially purified from human plasma is eluted with 0.05 M Tris-HCl buffer, pH 7.3, at 0 degrees C as well as 20 degrees C. Although, as described in the literature, heparin binds to antithrombin, our findings suggest another possibility, i.e. that the binding of heparin to thrombin induces a conformational change in the enzyme facilitating a complex formation between thrombin and antithrombin-III.