Regulation of factor IXa in vitro in human and mouse plasma and in vivo in the mouse. Role of the endothelium and the plasma proteinase inhibitors.

The regulation of human Factor IXa was studied in vitro in human and mouse plasma and in vivo in the mouse. In human plasma, approximately 60% of the 125I-Factor IXa was bound to antithrombin III (ATIII) by 2 h, with no binding to alpha 2-macroglobulin or alpha 1-proteinase inhibitor, as assessed by gel electrophoresis and IgG- antiproteinase inhibitor-Sepharose beads. In the presence of heparin, virtually 100% of the 125I-Factor IXa was bound to ATIII by 1 min. The distribution of 125I-Factor IXa in mouse plasma was similar. The clearance of 125I-Factor IXa was rapid (50% clearance in 2 min) and biphasic and was inhibited by large molar excesses of ATIII-thrombin and alpha 1-proteinase inhibitor-trypsin, but not alpha 2-macro-globulin-trypsin; it was also inhibited by large molar excesses of diisopropylphosphoryl - (DIP-) Factor Xa, DIP-thrombin, and Factor IX, but not by prothrombin or Factor X. The clearance of Factor IX was also rapid (50% clearance in 2.5 min) and was inhibited by a large molar excess of Factor IX, but not by large molar excesses of Factor X, prothrombin, DIP-Factor Xa, or DIP-thrombin. Electrophoresis and IgG- antiproteinase inhibitor-Sepharose bead studies confirmed that by 2 min after injection into the murine circulation, 60% of the 125I-Factor IXa was bound to ATIII. Organ distribution studies with 125I-Factor IXa demonstrated that most of the radioactivity was in the liver. These studies suggest that Factor IXa binds to at least two classes of binding sites on endothelial cells. One site apparently recognizes both Factors IX and IXa, but not Factor X, Factor Xa, prothrombin, or thrombin. The other site recognizes thrombin, Factor Xa, and Factor IXa, but not the zymogen forms of these clotting factors. After this binding, Factor IXa is bound to ATIII and the complex is cleared from the circulation by hepatocytes.