Thrombin Inhibition by Antithrombin III on the Subendothelium Is Explained by the Isoform ATβ

We have studied the interaction of 125I-antithrombin (125I-AT) with microvascular endothelial cells (RFPEC) to localize the cellular site of anticoagulantly active heparan sulfate proteoglycans (HSPG). The radiolabeled protease inhibitor bound specifically to the above HSPG with a Kd of approximately 50 nM. Confluent monolayer RFPEC cultures exhibited a linear increase in the amount of AT bound per cell for up to 16 d, whereas suspension RFPEC cultures possessed a constant number of protease inhibitor binding sites per cell for up to 5 d. These results suggest that monolayer RFPEC cultures secrete anticoagulantly active HSPG, which then accumulate in the extracellular matrix. This hypothesis was confirmed by quantitative light and EM level autoradiography which demonstrated that the AT binding sites are predominantly located in the extracellular matrix with only small quantities of protease inhibitor complexed to the cell surface. We have also pinpointed the in vivo position of anticoagulantly active HSPG within the blood vessel wall. Rat aortas were perfused, in situ, with 125I-AT, and bound labeled protease inhibitor was localized by light and EM autoradiography. The anticoagulantly active HSPG were concentrated immediately beneath the aortic and vasa vasorum endothelium with only a very small extent of labeling noted on the luminal surface of the endothelial cells. Based upon the above data, we propose a model whereby luminal and abluminal anticoagulantly active HSPG regulate coagulation mechanism activity.

Fibrin II monomer has a dramatic inhibitory effect on the rate of heparin-catalyzed inactivation of human alpha-thrombin by antithrombin III. At 6 microM fibrin II monomer, equivalent to the concentration of fibrinogen in plasma, the second-order rate constant was reduced by a factor of 308--from 2.05 x 10(8) M-1.s-1 to 6.65 x 10(5) M-1.s-1. Fibrin II monomer minimally affected the uncatalyzed rate of thrombin inactivation showing a reduction in the second-order rate constant by a factor of only 1.6. Fibrinogen and the product of plasmin degradation of fibrinogen, fragment E, at 6 microM concentrations also decreased the second-order rate constant for heparin-catalyzed thrombin inactivation, but by factors of only 2.7 and 1.9, respectively. On the basis of these observations it is proposed that protection of thrombin from inactivation by heparin-antithrombin III by fibrin II monomer can explain the limited efficacy of heparin in preventing coronary reocclusion in patients treated with tissue plasminogen activator and other fibrinolytic agents.

We have examined the role of heparinlike molecules in the regulation of coagulation by perfusing rat hindquarters with purified human thrombin and with its plasma inhibitor, antithrombin. Our data indicate that contact of the hemostatic components with the endothelium enhances the rate of thrombin-antithrombin complex formation by as much as 19-fold over the uncatalyzed rate of enzyme-inhibitor interaction. Heparinlike molecules are responsible for the antithrombin accelerating activity. The amount of thrombin-antithrombin complex generated within the hindlimb preparation after pretreatment of the vasculature with purified Flavobacterium heparinase or with addition of platelet Factor IV to the hemostatic components, was equal to the uncatalyzed levels. These heparinlike molecules appear to be tightly bound to the luminal surface of the endothelium, since they could not be detected within the physiologic buffer that was perfused through the animal. The above mucopolysaccharides function in a manner similar to commercial heparin, since modification of antithrombin at a site critical for heparin-dependent acceleration of the protease inhibitor resulted in a level of interaction product identical to the uncatalyzed amount. Finally, addition of diisofluorophosphate-thrombin to the enzyme perfusion stream reduced the amount of thrombin-antithrombin complex formed in the animal by 30-40%, which suggested that thrombin bound to the endothelium as well as enzyme free in solution are accessible to antithrombin that has interacted with heparinlike molecules present on the endothelium.