1Factor XII-Independent Activation of the Complex of Prekallikrein with High Molecular Weight Kininogen: Implications for Hereditary Angioedema

Abstract

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 (C1INH) or corn trypsin inhibitor (CTI). CTI does not inhibit kallikrein. Addition of heat shock protein 90 (Hsp90) leads to conversion of prekallikrein to kallikrein in a zinc-dependent reaction. Our goal is to determine whether these reactions occur in plasma. C1INH was removed from factor XII deficient plasma by immunoadsorption. Plasma was incubated in polystyrene plates and assayed for kallikrein formation. We now demonstrate that prekallikrein-HK will activate to kallikrein in phosphate-containing buffers and the rate is further accelerated upon addition of Hsp90. In 1983, Fields et al (J Allergy Clin Immunol 72: 54-60, 1983) reported that incubation of HAE plasma at 37°C leads to progressive formation of bradykinin, presumably by activation of factor XII and formation of kallikrein when C1INH is absent. Activation of the prekallikrein-HK complex is an alternative possibility. Prolonged incubation of plasma deficient in both factor XII and C1INH led to conversion of prekallikrein to kallikrein and cleavage of HK as was seen with HAE plasma but not normal plasma. These results indicate that C1INH stabilizes the prekallikrein-HK complex to prevent HK cleavage either by prekallikrein and/or prekallikrein-HK autoactivation. In HAE, 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.