Interactions of Heparin and a Covalently Linked Antithrombin Heparin Complex with the Components of the Fibrinolytic Pathway.

Unfractionated heparin (UFH) is used for the prophylaxis and treatment of thromboembolic diseases. UFH catalyzes inhibition by antithrombin (AT) of the serine proteases in the coagulation cascade. Additionally, UFH has been shown to interact with components of the fibrinolytic pathway in vitro. However UFH has several limitations which impact its utility as a therapeutic agent. Our lab has developed a novel covalent antithrombin-heparin complex (ATH) which inhibits most serine proteases of the coagulation pathway significantly faster when compared to non covalent mixtures of AT and UFH. However, the interactions of ATH with the components of the fibrinolytic pathway have not been studied before. Thus, the present study investigates possible serpin-heparin interactions of AT + UFH vs ATH within the fibrinolytic pathway.

Discontinuous second order rate constant assays under pseudo-first order conditions were carried out to obtain second order rate constant (k₂) values for the inhibition of plasmin by AT+UFH versus ATH. Briefly, at specific time intervals 20 nM plasmin was inhibited by 200 nM AT + 0–5000 nM UFH or by 200 nM ATH in the presence of 2.5 mM Ca²⁺. Reactions were neutralized by the simultaneous addition of a solution containing polybrene and plasmin substrate S-2366™ in buffer. Residual plasmin activity was measured and the final k₂ values calculated. For experiments involving tPA, wells containing 40nM tPA and increasing concentrations of AT, UFH or ATH, at mole ratios ranging from 0 to 20:1, were incubated for 15 min. Reactions with tPA were neutralized by simultaneous addition of a solution containing either polybrene and tPA substrate, S-2288™ in buffer, (ATH and UFH) or only the substrate S-2288™ in buffer (AT). Enzyme activity was then determined by measuring rate of substrate cleavage (Vmax).

When plasmin was inhibited by AT in the absence of UFH, k₂ values of 2.82×10⁵ +/− 4.46×10⁴ M⁻¹ min⁻¹ were observed. The k₂ values increased with addition of successively higher concentrations of UFH up to a plateau with maximal k₂ of 5.74×10⁶ +/− 2.78×10⁵ M⁻¹ min⁻¹ at a UFH concentration of 3000nM. For inhibition of plasmin by ATH, k₂ values of 6.39 × 10⁶ +/− 5.88 × 10⁵ M⁻¹ min⁻¹ were observed. Inhibition of plasmin by ATH was not significantly different when compared to the highest k₂ values obtained with UFH. (p=0.36) No statistically significant difference in tPA enzyme activity was observed when Vmax values for tPA alone were compared with those in the presence of AT, UFH or ATH. (p=0.932, p=0.085, p=0.31 respectively)

The characteristic shape of the curve obtained from the k₂vs. UFH plot suggests that the mechanism responsible for inhibition of plasmin by AT+UFH involves conformational activation of the serpin. The k₂ values in this study for inhibition of plasmin by both AT+UFH and ATH were three orders of magnitude lower than k₂ values for inhibition of thrombin or factor Xa. Furthermore these results suggest that tPA is not inhibited by AT + UFH or ATH, and is not influenced by the presence of UFH alone. Cumulatively, this indicates that the fibrinolytic pathway is minimally impacted by AT + UFH or ATH, allowing maximal antithrombotic potential to be achieved during anticoagulation. Overall, the favourable anticoagulant properties of ATH combined with the findings of this study strengthens the utility of the covalent conjugate over conventional UFH for the treatment of thromboembolic disorders.

No relevant conflicts of interest to declare.