Cross-linking of α2-plasmin inhibitor and fibronectin to fibrin by fibrin-stabilizing factor

Two plasma proteins, α₂-plasmin inhibitor and plasma fibronectin, are cross-linked to fibrin by plasma transglutaminase (R-glutaminyl-peptide : amine γ-glutamyl-yltransferase, EC 2.3.2.13, fibrin stabilizing factor) when blood coagulation takes place. The cross-linking reactions of these proteins were analyzed by polyacrylamide gel electrophoresis in sodium dodecyl sulfate (SDS) using these radioactively labeled proteins. Both proteins were cross-linked exclusively to the α-chain of fibrin, and each of these cross-linking reactions proceeded independently without being influenced by the other cross-linking reaction. The cross-linking of fibronectin to the α-chain proceeded steadily at a rate similar to that of the cross-linked polymerization of the α-chain. In contrast, the cross-linking reaction of α₂-plasmin inhibitor to fibrin proceeded markedly faster than that of fibrin polymerization but did not proceed further after reaching a certain relatively low level of cross-linking. Most of the cross-linked α₂-plasmin inhibitor molecules at this stage of the fibrin cross-linking process were in the form of complex with the α-chain monomer. The complex with the α-chain monomer was gradually transformed to a complex with the α-chain polymer as the cross-linking polymerization of the α-chain proceeded. The rate of the transformation was the same as that for the disappearance of the α-chain monomer, indicating that whether the α-chain was cross-linked to α₂-plasmin inhibitor or not, the α-chain underwent cross-linking polymerization at the same rate.