Investigation Into Mechanisms of Prothrombinase Complex Inhibition by a Covalent Antithrombin-Heparin Complex.

Formation of the prothrombinase complex results in efficient generation of thrombin (IIa) by a vast enhancement of the factor Xa (Xa) reaction with prothrombin (II). However, the integration of Xa into the complex significantly decreases its availability for inhibition by antithrombin+heparin (AT+H). Our laboratory has developed a novel anticoagulant by covalently linking antithrombin to heparin (ATH). We have shown that ATH, compared to AT+H, significantly increased the rate of inhibition of clot-bound thrombin as well as several other coagulation factors in isolation. The present study was performed to extend understanding of the anticoagulant mechanisms of ATH by determining its inhibition of Xa within the critical prothrombinase system.

Pseudo first-order kinetic experiments for the inhibition of Xa in the prothrombinase complex were performed by reacting AT+H or ATH with CaCl₂, phospholipid vesicles (PC/PS), factor Va (Va), II, specific IIa inhibitor Pefabloc and Xa in buffer. At specific time intervals, the reactions were neutralized with Na₂EDTA, polybrene and Xa substrate (S-2222) in buffer, followed by measurement of residual enzyme activity. Second-order rate constants (k₂) were calculated from semi-logarithmic plots of residual Xa activity remaining versus time. To investigate the roles of individual components of the prothrombinase complex on the anticoagulant effects of ATH, additional experiments were performed where components of the complex (II, PC/PS or Va) were removed prior to reaction with AT+H or ATH.

The k₂value (× 10⁸ M⁻¹min⁻¹) for the inhibition of Xa alone by AT+H was 1.47 ± 0.08. Incorporation of Xa into the prothrombinase complex significantly decreased this value to 0.67 ± 0.05, p<0.001. In comparison, ATH inhibited free-Xa and prothrombinase-complexed Xa at significantly faster rates than AT+H (2.83 ± 0.15 and 2.11 ± 0.38 (p<0.01), respectively). Relative to intact prothrombinase, rates for inhibition by AT+H of Xa in complexes devoid of PC/PS, Va or II significantly increased to 1.49 ± 0.08, 1.44 ± 0.08 and 1.24 ± 0.04, p<0.001, respectively. In contrast, removal of II from prothrombinase significantly decreased the k₂ for Xa inhibition by ATH to 1.57 ± 0.04, p<0.05. Although removal of PC/PS from the system resulted in a higher k₂ (2.79 ± 0.14, p<0.01), removal of Va resulted in no significant change (2.09 ± 0.09, p=0.79) for ATH reactions.

Based on these results, we conclude that the prothrombinase complex hinders the inhibitory action of AT+H on Xa, whereas ATH is less affected. In fact, ATH inhibition of the prothrombinase complex was significantly higher than the inhibition rate for either free or prothrominase-bound Xa by AT+H. We speculate that, similar to inhibition of fibrin-bound thrombin, H in the non-covalent AT+H dissociate and bind to components within the prothrombinase that repel incoming AT+H and preventing Xa inhibition. These complexes cannot occur when AT is covalently linked to H, thus allowing for enhanced inhibition of prothrominase-bound Xa by ATH. Absence of II from the prothrombinase complex removes interference of AT+H reactions by any H complexes within prothrombinase, but may antagonize inhibition by the conjugate through unproductive interactions of ATH with uncovered sites close to Xa at the phospholipid surface. These findings reveal important features involved in the enhanced anticoagulant action of ATH against prothrombinase surface-bound enzymes.

No relevant conflicts of interest to declare.