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Ligation of fibrinogen by factor XIIIa with dithiothreitol: Mechanical properties of ligated fibrinogen gels

Human fibrinogen (concentration 8.4 mg/mL) was ligated (cross‐linked) with factor XIIIa and dithiothreitol (DTT) at pH 8.5, ionic strength 0.45. With 7.5 μg/mL of factor XIIIa alone, there was almost no γ‐γ ligation, but with 2 mM DTT added, oligomers appeared, and γ‐γ and Aα‐Aα ligation was nearly complete after 3 days. At 38 μg/mL of factor XIIIa, some γ‐γ and Aα‐Aα ligation occurred even without DTT. For fibrinogen concentrations of 4.0 and 8.4 mg/mL, 38 μ/mL factor XIIIa, 2.0 mM DTT, clot‐like gels formed and the shear modulus of elasticity increased slowly over several days to a constant value. The final modulus was similar in magnitude to those of ligated clots of α‐fibrin (clotted by thrombin) and α‐fibrin (clotted by batroxobin) under the same conditions. However, the opacity was somewhat higher; whereas in fine fibrin clots there is minimal lateral association of the protofibrils, in fibrinogen gels at the same pH and ionic strength the protofibrils (which are presumably single chains of fibrinogen monomers joined end to end at their D domains) are evidently associated in bundles (although not to the degree seen in coarse fibrin clots). Creep and creep recovery measurements showed almost perfect elastic behavior, with essentially no creep under stress and complete recovery after removal of stress. The modulus was scarcely affected by introduction of lithium bromide by diffusion to a concentration of 0.6M, which in unligated fibrin clots causes substantial softening. Whereas in fine fibrin clots (both αβ‐fibrin and α‐fibrin) factor XIIIa causes only γ‐γ ligation, addition of 2 mM DTT produced some α‐α ligation in these also.