Reversible cross-linking of α2-plasmin inhibitor to fibrinogen by fibrin-stabilizing factor

α2-Plasmin inhibitor, a primary inhibitor of fibrinolysis, is cross-linked to fibrin by plasma transglutaminase (glutaminyl-peptide:amine γ-glutamyltransferase, EC 2.3.2.13, activated fibrin-stabilizing factor) when blood coagulation takes place, α2-Plasmin inhibitor was found also to be cross-linked to fibrinogen by plasma transglutaminase. The inhibitor was cross-linked exclusively to the Aα-chain of fibrinogen, and the cross-linking reaction proceeded very rapidly. The reaction was almost completed before the formation of the γ-chain dimers of fibrinogen which precedes cross-linking polymerization of the Aα-chain of fibrinogen. The maximum level of inhibitor cross-linking achieved was approx. 30% of the inhibitor present at the start of the reaction. The level of cross-linking of the inhibitor was not changed when the cross-linking reaction was preceded by dimerization of fibrinogen. The cross-linking reaction was found to be a reversible one, since the cross-linked complex of the inhibitor and fibrinogen was partly dissociated to each of its components when the complex was incubated with plasma transglutaminase. These results suggest that the self-limiting nature of the cross-linking reaction between α2-plasmin inhibitor and fibrin(ogen) is due to the reaction equilibrium favoring dissociation of the complex, and not due to the development of structural hindrance in polymerizing fibrin(ogen).