Determinants of Substrate Specificity for Factor XIII

Abstract

Plasma factor XIIIa (A*₂) is a regulator in balancing the opposing coagulation and fibrinolytic processes. Its enzymatic activity is to catalyze ε-(γ-glutamyl)lysyl bonds between certain substrate molecules to link them by strong bonds. The primary physiological substrates are crosslinks between the γ and α chains of fibrin that produce γ-γ-dimer and α-polymer, between α₂-plasmin inhibitor (α₂-PI) and α chains of fibrin, and between fibronectin and fibrin. We have characterized a unique factor XIII antibody that is specific for the middle 54-kDa section of A*₂. It does not react with the zymogen (A₂) or the inactive intermediate (A′₂), and it does not inhibit the active center, as do most patient antibodies to factor XIII. This antibody inhibits the formation of A*₂-fibrin complexes. Because of this specificity, the antibody was used to study other substrate interactions. It inhibited formation of fibronectin-factor XIIIa complexes, similarly to fibrin, and there was very little crosslinking of fibronectin to a fibrin clot. However, the amount of α₂-PI crosslinked to a fibrin clot was normal. It was concluded that this antibody interferes with exosite binding of fibrin and fibronectin in a similar way, while at least one critical exosite binding domain for α₂-PI is different from those of the other two substrates. Furthermore, with this antibody, it was shown that both α₂-PI-α chain crosslinking and α-polymer formation are necessary to normalize the rate of fibrinolysis.

Key words:

Factor XIIIa - fibrin - fibronectin - α₂-antiplasma - factor XIIIa-substrate complex