High molecular weight kininogen potentiates the heparin-accelerated inhibition of plasma kallikrein by antithrombin: role for antithrombin in the regulation of kallikrein.

The effects of previously characterized interactions of high molecular weight kininogen (H-kininogen) with plasma kallikrein and with heparin on the regulation of kallikrein by the heparin-activated inhibitor, antithrombin, were investigated. H-kininogen, at levels sufficient to fully complex kallikrein, greatly potentiated the acceleration of antithrombin inhibition of kallikrein produced by heparin with high affinity for antithrombin. At I = 0.15, pH 7.4, 25 degrees C, kininogen thus maximally increased the heparin enhancement of the second-order rate constant for the antithrombin-kallikrein reaction from 13-fold (1.6 x 10(2) M-1 s-1 to 2.1 x 10(3) M-1 s-1) to 1200-fold (1.9 x 10(5) M-1 s-1). In contrast, H-kininogen had no effect on the antithrombin-kallikrein reaction in the absence of heparin, nor did the protein enhance the rate constants of 1.7 x 10(4) and 3.4 x 10(4) M-1 s-1 for kallikrein reactions with its primary plasma inhibitors C1-inhibitor and alpha 2-macroglobulin, respectively, in the absence or presence of heparin. Consistent with these results, SDS gel electrophoresis of the 125I-labeled kallikrein-inhibitor complexes formed in a mixture of these kallikrein inhibitors at their relative plasma concentrations indicated that antithrombin effectively competed with C1-inhibitor and alpha 2-macroglobulin for kallikrein, accounting for 54% of the total kallikrein complexes, only when both heparin and H-kininogen were present. Similarly, the presence of therapeutic levels of heparin (approximately 1 unit/mL) in normal, factor XII-deficient, and prekallikrein-deficient plasmas enhanced the rate of inactivation of added kallikrein by 2.3-fold and significantly altered the partitioning of radiolabeled kallikrein from predominantly C1-inhibitor and alpha 2-macroglobulin complexes (86-92%) to mostly antithrombin complexes (50-53%). Experiments in antithrombin-deficient and H-kininogen-deficient plasmas confirmed that the enhanced kallikrein inactivation rate and predominant formation of antithrombin-kallikrein complexes in heparinized plasma were dependent on antithrombin and H-kininogen. The contribution of antithrombin to kallikrein inhibition in plasma remained significant (approximately 40-70%) at optimal concentrations of unfractionated or size- and antithrombin affinity-fractionated heparin, in the presence of plasma levels of calcium and zinc ions, at 37 degrees C, and with minimal plasma dilution. These results suggest that antithrombin and H-kininogen may play important roles in the regulation of kallikrein activity in the presence of heparin or heparin-like glycosaminoglycans.