title: Regulation of factor Xa in vitro in human and mouse plasma and in vivo in mouse. Role of the endothelium and plasma proteinase inhibitors.
abstract: The regulation of human Factor Xa was studied in vitro in human and mouse plasma, and in vivo in mouse. In human plasma, 125I-Factor Xa bound to alpha 1-proteinase inhibitor, antithrombin III, and alpha 2-macroglobulin in a ratio of 4.9:1.9:1 as determined by gel electrophoresis and by adsorption to IgG-(antiproteinase inhibitor)-Sepharose beads. The distribution of Factor Xa in mouse plasma was similar. The clearance of Factor Xa in mice was rapid (50% clearance in 3 min) and biphasic. alpha 1-Proteinase inhibitor-trypsin, even at a 2,000-fold molar excess, failed to inhibit the clearance of Factor Xa, while alpha 2-macroglobulin-trypsin inhibited only the later phase of clearance. The plasma clearance of diisopropylphosphoryl-Factor Xa was more rapid than native Factor Xa (50% clearance in 2.5 min), and the clearance was blocked by diisopropylphosphoryl-thrombin. Electrophoresis experiments confirmed that by 2 min after injection into the murine circulation, 90% of the bound Factor Xa was on alpha 2-macroglobulin, in marked contrast to the in vitro results. Organ distribution studies at 3 and 15 min with 125I-Factor Xa demonstrated that the majority of radioactivity was in the liver, with significant radioactivity also present in lung and kidney. Autopsies performed 30 s after injection of 125I-Factor Xa also demonstrated significant binding to the aorta and vena cava. These studies indicate that Factor Xa binds to specific thrombin-binding sites on endothelial cells, and that this binding alters its proteinase inhibitor specificity. Factor Xa binds to alpha 2-macroglobulin in vivo, whereas the predominant in vitro inhibitor of Factor Xa is alpha 1-proteinase inhibitor.
mesh_terms: D000818:Animals; D000990:Antithrombin III / Q000032:analysis; D001011:Aorta / Q000032:analysis; D004591:Electrophoresis, Polyacrylamide Gel; D004727:Endothelium / Q000032:analysis; D005170:Factor X / Q000032:analysis* / Q000378:metabolism; D015951:Factor Xa; D006801:Humans; D066298:In Vitro Techniques; D007700:Kinetics; D051379:Mice; D011480:Protease Inhibitors / Q000032:analysis*; D014018:Tissue Distribution; D014682:Vena Cava, Inferior / Q000032:analysis; D000511:alpha-Macroglobulins / Q000032:analysis*
chemical_list: D011480:Protease Inhibitors; D000511:alpha-Macroglobulins; D000990:Antithrombin III; D005170:Factor X; D015951:Factor Xa