Cross-Linking of a2-Plasmin Inhibitor to Fibrin by Fibrin-stabilizing Factor

itor in blood plasma is higher than that in serum obtained from the blood clotted in the presence ofcalcium ions, but is the same as that in serum obtained in the absence of calcium ions. Radiolabeled a2-plasmin inhibitor was covalently bound to fibrin only when calcium ions were present at the time of clotting of plasma or fibrinogen. Whereas, when batroxobin, a snake venom enzyme that lacks the ability to activate fibrin-stabilizing factor, was used for clotting fibrinogen, the binding was not observed. When fibrin-stabilizing, factor-deficient plasma was clotted, the specific binding of a2-plasmin inhibitor to fibrin did not occur even in the presence of calcium ions and the concentration of a2-plasmin inhibitor in serum was the same as that in plasma. Monodansyl cadaverine, a fluorescent substrate of the fibrin-stabilizing factor, was incorporated into a2plasmin inhibitor by activated fibrin-stabilizing factor. All these findings indicate that a2-plasmin inhibitor is cross-linked to fibrin by activated fibrin-stabilizing factor when blood is clotted. Analysis of a2-plasmin inhibitor-incorporated fibrin by sodium dodecyl sulfate gel electrophoresis showed that the inhibitor was mainly cross-linked to polymerized a-chains of crosslinked fibrin. Cross-linking of a2-plasmin inhibitor to fibrin renders fibrin clot less susceptible to fibrinolysis by plasmin.