AND A0KI, N.: Significance of cross-linking of a2-plasmin inhibitor to fibrin in inhibition of fibrinolysis and in haemostasis

itor in blood plasma is higher than that in serum ob-tained 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 a2-plasmin 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 cross-linked fibrin. Cross-linking of a2-plasmin inhibitor to fibrin renders fibrin clot less susceptible to fibrinolysis by plasmin