Acceleration of surface-dependent autocatalytic activation of blood coagulation factor XII by divalent metal ions

Abstract

The effect of divalent metal ions on the rate of dextran sulfate dependent autocatalytic activation of human blood coagulation factor XII was studied at pH 7.4 and 25.degree. C. Zn2+ and Cu2+, but not Co2+, increased the rate of factor XII activation induced by dextran sulfate with optimum effects at approximately 5 and 1 .mu.M, respectively, while Ca2+ acceleration required much higher concentrations (millimolar). Further investigation of the effect of Zn2+ on factor XII activation demonstrated a complete dependence on the presence of dextran sulfate, lack of inhibition by soybean trypsin inhibitor, the appearance of .alpha.-XIIa as the primary reaction product, and reaction kinetics characteristic of an autocatalytic process. These results were consistent with Zn2+ affecting only the rate of surface-mediated factor XII autoactivation. The initial turnover velocity of dextran sulfate induced factor XII autoactivation increased linearly with factor XII concentration in the absence of Zn2+ up to 0.9 .mu.M factor XII but showed saturation behavior over this same concentration range in the presence of 5 .mu.M Zn2+, indicating that Zn2+ increased the reaction rate primarily by lowering the apparent Km. Comparison of the kinetics of autoactivation at .mu. = 0.15 and 0.24 revealed that the enhancement in the apparent kcat/Km brought about by Zn2+ increased from 19-fold to 520-fold, respectively, due to a differential dependence of the Zn2+-stimulated and unstimulated reactions on ionic strength. Evidence that enhanced binding of factors XII and .alpha.-XIIa to dextran sulfate contributed to the Zn2+ rate enhancement was provided by the observation that factors XII and .alpha.-XIIa were eluted at higher ionic strengths from a dextran sulfate-agarose column in the presence of 5 .mu.M Zn2+ than in its absence. .alpha.-Factor XIIa bound more tightly to dextran sulfate-agarose than factor XII in the presence and absence of Zn2+, while .beta.-factor XIIa, which lacks the surface binding domain, did not bind to the column under the same conditions. The rate of dextran sulfate dependent contact activation in normal human plasma, but not in factor XII, prekallikrein, or high molecular weight kininogen deficient plasmas, was stimulated by Zn2+, suggesting a possible role for metal ions in promoting surface-dependent contact activation reactions under physiological conditions as well as in model systems.