Inhibition of Factor Xa in Prothrombinase Is Enhanced by Covalent Linkage of Antithrombin to Heparin.

The rate of prothrombin to thrombin conversion by factor Xa (Xa) is enhanced when Xa is incorporated into the surface-bound prothrombinase complex. However, in comparison to the free state, Xa within the prothrombinase complex is afforded protection from antithrombin + heparin (AT+H) inactivation. We have shown that, unlike AT+H, a covalent conjugate of AT and H (ATH) can neutralize fibrin-bound thrombin. In this study, AT+H and ATH were compared in their reaction with Xa +/− prothrombinase complex. Mixtures of CaCl₂, phospholipid vesicles, factor Va (Va) and prothrombin in TSP buffer, were combined with Xa. Following addition of either AT+H or ATH, time samples were neutralized with Na₂EDTA + polybrene + substrate (S-2222) and residual Xa activity measured. Second order rate constants (k₂) were calculated from plots of activity versus time. Results were compared to corresponding experiments with Xa alone. AT+H inactivation of Xa in prothrombinase occurred at a k₂ (x 10⁷ M⁻¹min⁻¹) of 2.34 +/− 0.09. In contrast, neutralization of free Xa by AT+H was significantly faster (k₂ = 8.34 +/− 0.18, p = 0.03). Reaction with ATH showed no significant rate difference for Xa inhibition in either the complexed or free states (18.5 +/− 3.3 and 16.3 +/− 3.7, respectively). Intriguingly, the rates achieved for ATH inhibition of complexed and free Xa were significantly greater than that for AT+H with free Xa (p=0.03 and p=0.02, respectively). We conclude that covalent complexes of AT and H do not encounter resistance in the inhibition of Xa in prothrombinase, as seen for non-covalent AT+H mixtures. Thus, it is possible for ATH to effectively inhibit the propagation phase of thrombin generation and thus dampen thrombin production via neutralization of Xa in prothrombinase.