Kinetic studies on surface‐mediated activation of bovine factor XII and prekallikrein

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Abstract
The kaolin‐mediated reciprocal activation of bovine factor XII and prekallikrein was divided into the following two reactions: the activation of factor XII by plasma kallikrein (reaction 1) and the activation of prekallikrein by factor XIIa (reaction 2). The effects of high‐Mr kininogen and kalin surface on the kinetics of these activation reactions were studied. High‐Mr kininogen markedly enhanced the rate of reactions 1 and 2 in the presence of kaolin, and the enhancements were highly dependent on the concentrations of the protein cofactor and amount of kaolin surface. For the activation of factor XII by plasma kallikrein (reaction 1), high‐Mr kininogen was required when a low concentration of factor XII and kaolin was used. The molar ratio of the protein cofactor to factor XII for optimal activation was found to be approximately 1:1. The apparent Km value and the kcat/Km value for plasma kallikrein on factor XII were calculated to be 4 nM and 5.2 − 107 s−1· M−1, respectively. The activation of prekallikrein by factor XIIa, (reaction 2) proceeded even in the absence of high‐Mr kininogen and kaolin. The addition of the protein cofactor and surface to the reaction mixture remarkably accelerated the reaction, and the apparent Km value for factor XIIa on prekallikrein was reduced from 1 μM to 40 nM. Moreover, the kcat/Km value was altered from 7.3 − 104 to 1.1 − 106 s−1· M−1. These results suggest that high‐Mr kininogen accelerates the surface‐mediated activation of factor XII and prekallikrein by enhancing the susceptibility of factor XII to plasma kallikrein, on the one hand, and the affinity of factor XIIa for prekallikrein, on the other hand. Kaolin may play an important role in the concentration and organization of these components on the negatively charged surface.