SURFACE-INDEPENDENT ACCELERATION OF FACTOR-XII ACTIVATION BY ZINC IONS .1. KINETIC CHARACTERIZATION OF THE METAL-ION RATE ENHANCEMENT

The effect of zinc ions (Zn(II)) on the activation of factor XII in th e absence of a procoagulant surface was investigated by initial veloci ty kinetic studies at I = 0.15, pH 7.4, and 25-degrees-C. Zinc ions at concentrations greater than 160 muM potentiated 99-fold the k(cat)/K( M) for the activation of factor XII by kallikrein and, at an optimum c oncentration of 110 muM, accelerated 140-fold the apparent k(cat)/K(M) for factor XII autoactivation. High molecular weight kininogen had no effect on either metal-potentiated reaction. Analysis of the factor X II concentration dependence of initial activation rates revealed that Zn(II), at levels that saturate the effect, accelerates kallikrein act ivation of factor XII by lowering Km (from 52 to 7.3 muM) and raising k(cat) (from 2.6 to 31 min-1). For the autocatalytic activation reacti on of factor XII in the presence of optimal Zn(II), apparent K(M) and k(cat) values of 2.4 muM and 0.041 min-1, respectively, were determine d, but these parameters were not resolvable in the absence of the meta l ion. Zinc ions minimally affected kallikrein enzymatic activity and inhibited factor XIIa enzymatic activity with K(I) values of 20-40 muM , suggesting that the rate-enhancing effects of the metal ion are due to interactions with the substrate (factor XII) rather than with the e nzyme. The Zn(II) inhibition of factor XIIa enzymatic activity account ed for a decreased Zn(II) enhancement of factor XII autoactivation at high metal ion concentrations (> 110 muM). The Zn(II) concentration de pendence of the acceleration of factor XII activation reactions were s igmoid and characterized by Hill coefficients of 3.3-4.3, suggesting t hat cooperative binding of at least four zinc ions to factor XII was r esponsible for the Zn(II) potentiating effect. The Zn(II) enhancement of the rates of factor XII activation decreased both above and below p H 7.4 with midpoint pH values of 6.5-7.0 and 8.0, consistent with hist idine and possibly water ligands mediating Zn(II) binding to the prote in. Despite an apparent weaker binding of Zn(II) to factor XII at pH 6 .5, indistinguishable maximum accelerating effects of the metal ion we re observed at saturation at this pH, indicating that the increased po sitive charge of factor XII resulting from protonation at the lower pH did not mimic the effect of Zn(II) binding. These results imply that zinc ions induce a conformational change in factor XII that makes it a better substrate for its enzyme activators.