FXIIIa-catalyzed cross-linking of fibrin. Cross-linking of fibrin by FXIIIa was examined by 8% SDS-PAGE under reducing conditions as described in “Materials and methods.” Fibrinogen (0.25 mg/mL) was mixed with FXIIIa (3.3 U/mL) and thrombin (0.07 U/mL) and the reaction was incubated for the specified time at 37°C in 20 mM HEPES, pH 7.4, 0.12 M NaCl, 0.67 mM CaCl₂ buffer. The reduced fibrin chains (α, β, γ, cross-linked γ-γ dimer, and cross-linked α-chain polymers) are indicated on the right side of the gels. The variant fibrinogens used were (A) γ387I, (B) γ387R, (C) γ387L, (D) γ387M, and (E) γ387A. Densitometric analyses were performed and γ-γ/β ratios were calculated and plotted in panel F: γ387I (♦), γ387R (▵), γ387L (▴), γ387M (▪), and γ387A (□).