An 81-year-old woman, who presented with sudden episodes of spontaneous
bleeding, was found to have a specific inhibitor of
factor XIII. Her
fibrin clots had approximately 70% gamma-gamma and no alpha
polymer formation, under conditions where normal
fibrin was fully cross-linked; the patient's clots were soluble in
urea or
monochloroacetic acid.
Factor XIII activity in her plasma was 24%, measured by the
dansylcadaverine incorporation assay. When mixed with normal plasma, the patient's plasma inhibited
fibrin cross-linking; however, in mixtures of patient and normal plasma, there was no inhibition of
factor XIII activity when assayed by the incorporation of
dansylcadaverine into
casein. Thus, this inhibitor was active against
fibrin cross-linking but not against
ligation of small molecules to
casein. Consequently, gel electrophoresis of reduced,
sodium dodecyl sulfate-solubilized
fibrin clots was a simple, quantitative method that was used to measure inhibitor activity. This inhibitor is unique and has been designated inhibitor New Haven. It was neutralized by
anti-IgG and anti-kappa. It did not inhibit the activation of
factor XIII but did inhibit
fibrin cross-linking. There was complex formation between the inhibitor and
activated factor XIII (A', A*) but not between A2 or
fibrinogen. Only A', A* and the 56-Kd fragment bound to affinity columns made with this
IgG. The inhibitor significantly decreased the binding of A', A* to
fibrin clots. These data indicate that the
epitope for this inhibitor is in a
fibrin binding site. It is hidden in the
zymogen and expressed on A' and A*, indicating that the conformational change occurring with the cleavage of the activation
peptide is sufficient to expose the
fibrin binding site.