Comparative study of antithrombin III.protease complex metabolism by fibroblasts and vascular endothelial cells

Antithrombin III-beta associates more readily than antithrombin III-alpha with uninjured and de-endothelialized aortic wall in vitro and in vivo

The properties of two isoforms, alpha and beta, of rabbit antithrombin III (ATIII) were compared in the presence of undamaged or de-endothelialized rabbit aortic wall. Similar quantities of ATIII-alpha and ATIII-beta bound to and rapidly saturated the endothelium in vitro, but the rate of transendothelial passage of ATIII-beta exceeded that of ATIII-alpha by 22%. Furthermore, ATIII-beta was adsorbed approximately twice as rapidly as ATIII-alpha by the subendothelium of the de-endothelialized aorta. Binding of both isoforms was decreased (ATIII-beta more than ATIII-alpha) by pretreating the subendothelial surface with heparitinase. Also, subendothelium-bound ATIII-beta was desorbed more readily than bound ATIII-alpha by thrombin. In vivo, the rate of uptake of iodine-131-labeled ATIII-beta from the circulation by the aortic wall and the major organs was 30-50% faster than that of iodine-125-labeled ATIII-alpha. In contrast, the uptake of 131I-ATIII-beta by the de-endothelialized aorta in vivo was three times faster than that of 125I-ATIII-alpha. By these criteria, ATIII-beta is the more active of the two isoforms. We surmise that plasma and, consequently, vessel wall levels of ATIII-beta may be vital for controlling thrombogenic events caused by injury to the vascular wall.

Characterization of the cellular binding domain and the effects of monoclonal antibodies and thrombin inhibitors on the binding and internalization of the antithrombin-III--thrombin complex by cultured cells

Antithrombin III (AT) binds to cultured cells mainly as a complex with thrombin or other serine proteases rather than in its free form. This implies that, upon complex formation, a new determinant appears on the AT molecule which is recognized by the cells. Fragmentation of AT by cyanogen bromide exposes this determinant and an 8-kDa fragment is recognized by cultured cells. The binding of this fragment to cultured cells is inhibited by antithrombin-III-thrombin (AT-T) complex, but not by free AT. The putative cellular binding domain of AT-T is located over amino acid residues 253-314 of AT, in the large loop close to the carboxy-terminus of the molecule. The cell-associated AT-T is internalized and degraded, forming the thrombin-modified AT (ATM). This process is inhibited by lysosomal degradation inhibitors, such as chloroquine or benzamidine. Thus, the appearance of ATM in cells is not a result of its binding to the cell surface, but probably a result of lysosomal degradation of cell-associated AT-T. Another degradation product, with a molecular mass of 43 kDa, appears in cells along with the appearance of ATM. Hirudin, a specific inhibitor of thrombin, inhibits the cellular internalization of AT-T complexes, indicating a possible role for thrombin activity in the internalization process of complexes. Monoclonal antibodies (mAb) against AT, whose epitopes on the AT molecule have been located, affect the binding of AT-T to cultured cells. Two mAb, A10 whose epitope is found outside of the cellular binding site, and B108 whose epitope may overlap this sequence, totally inhibit the binding of the complex to cells. Inhibition of binding results either from steric hindrance or induced changes in the binding site.

Monoclonal antibodies against antithrombin III. Identification of their epitopes and effects on antithrombin III activities

Four monoclonal antibodies with distinct epitopes were prepared against antithrombin III. None of them is directed against the heparin-binding region nor the active site, yet two mAb namely A36 and B108, interfere with antithrombin III inhibition of thrombin. The epitope of monoclonal antibody A36 is located within amino acid residues 1-393, at a site different from the active site since it recognizes antithrombin III and antithrombin-III-thrombin complexes with the same affinity. A36 partially prevents the intrinsic antithrombin III activity and has no effect on the heparin-enhanced antithrombin III activity when added to the antithrombin-III--heparin complex. If A36 is first reacted with antithrombin III and then heparin is added to the reaction mixture, A36 fixes the conformation of antithrombin III so that heparin binds to antithrombin III, but is not able to induce the conformational change in the antithrombin III molecule required for the enhanced activity. The epitope for monoclonal antibody B108 is located within residues 282-393, close to the active site. It does not recognize antithrombin-III-thrombin complexes by solid-phase radioimmunoassay. Its binding to antithrombin III induces a conformational change that enhances antithrombin III activity in a manner that resembles the heparin effect, but its effect is additive to the heparin effect, since when it was added to a reaction mixture which contained a saturating amount of heparin, inhibition of thrombin was enhanced. The epitope for monoclonal antibody A5 is located within residues 1-393, and its recognition of antithrombin III or antithrombin-III-thrombin is strongly dependent on the integrity of the disulfide bonds. A5 has no effect on antithrombin III activities. The epitope for monoclonal antibody A10 is well defined within a narrow range of 55 amino acid residues, 339-393, on the antithrombin III molecule, close to the active site, yet it has no effect on antithrombin III inhibitory activity. These monoclonal antibodies may be developed for various diagnostic or clinical purposes and offer a powerful tool for studying the conformational changes and structure/activity relationships in the antithrombin III molecule.