Binding of Factor VIIa to Tissue Factor Permits Rapid Antithrombin III/Heparin Inhibition of Factor VIIa

• Published in: Blood 1993-05, Vol.81 (10), p.2600-2607
• Main Authors: Mohan Rao, L. Vijaya, Rapaport, Samuel I., Hoang, An D.
• Format: Article
• Language: English
• Subjects: Antithrombin III - pharmacology; Biological and medical sciences; Blood coagulation. Blood cells; Cell Membrane - metabolism; Coagulation factors; Electrophoresis, Polyacrylamide Gel; Factor VIIa - antagonists & inhibitors; Factor VIIa - isolation & purification; Factor VIIa - metabolism; Female; Fundamental and applied biological sciences. Psychology; Heparin - pharmacology; Humans; Kinetics; Molecular and cellular biology; Molecular Weight; Ovarian Neoplasms; Protein Binding; Thromboplastin - isolation & purification; Thromboplastin - metabolism; Time Factors; Tumor Cells, Cultured
• ISSN: 0006-4971; 1528-0020
• DOI: 10.1182/blood.V81.10.2600.2600

Because free factor VIIa is inactivated only very slowly by a plasma concentration of antithrombin III (AT III) even in the presence of heparin, it has been assumed that AT III plays no significant role in regulating the initiation of tissue factor-dependent blood coagulation. However, in the present study, we present evidence that factor VIIa bound to tissue factor, unlike free factor VIIa, is readily inactivated by AT III in the presence of heparin. In a reaction mixture containing calcium ions and approximately equimolar concentrations of relipidated tissue factor (8.9 nmol/L) and factor VIIa (10 nmol/L), AT III (100 µg/mL) plus heparin (1 U/mL) inhibited 50% of the factor VIla coagulant activity of the reaction mixture within 5 minutes. AT lll/heparin was also shown to inhibit the catalytic activity towards factor X of factor Vlla/tissue factor complexes formed on monolayers of an ovarian carcinoma cell line (OC-2008) that constitutively expresses surface membrane tissue factor. AT III, even in the absence of exogenously added heparin, substantially inhibited the functional activity of factor VIIa/cell surface tissue factor complexes on intact monolayers. AT III alone and AT lll/heparin, to a greater extent, also inhibited factor VIIa on “nonfunctional” factor Vlla/tissue factor complexes on intact monolayers, with resultant inhibition of their expression of factor VIIa/tissue factor catalytic activity toward factor X after cell lysis. The potential physiologic significance of these findings is discussed.