Factor XII-independent activation of the bradykinin-forming cascade: Implications for the pathogenesis of hereditary angioedema types I and II.

Background: We have previously reported that prekallikrein expresses an active site when it is bound to high-molecular-weight kininogen (HK) and can digest HK to produce bradykinin. The reaction is stoichiometric and inhibited by C1 inhibitor (C1-INH) or corn trypsin inhibitor. Addition of heat shock protein 90 leads to conversion of prekallikrein to kallikrein in a zinc-dependent reaction.

Objective: Our goal was to determine whether these reactions are demonstrable in plasma and distinguish them from activation through factor XII.

Methods: Plasma was incubated in polystyrene plates and assayed for kallikrein formation. C1-INH was removed from factor XII-deficient plasma by means of immunoadsorption.

Results: We demonstrate that prekallikrein-HK will activate to kallikrein in phosphate-containing buffers and that the rate is further accelerated on addition of heat shock protein 90. Prolonged incubation of plasma deficient in both factor XII and C1-INH led to conversion of prekallikrein to kallikrein and cleavage of HK, as was seen in plasma from patients with hereditary angioedema but not plasma from healthy subjects.

Conclusions: These results indicate that C1-INH stabilizes the prekallikrein-HK complex to prevent HK cleavage either by prekallikrein or by prekallikrein-HK autoactivation to generate kallikrein. In patients with hereditary angioedema, kallikrein and bradykinin formation can occur without invoking factor XII activation, although the kallikrein formed can rapidly activate factor XII if it is surface bound.