The Surface‐Dependent Autoactivation Mechanism of Factor XII

The dependency of concentrations of Zn2+ and the negatively charged surfaces, phosphatidylinositol phosphate (PtdInsP), sulfatide and dextran sulfate, on the autoactivation of human factor XII, has been studied. While the autoactivation induced by sulfatide, and low concentrations of dextran sulfate, was unaffected by the presence of Zn2+, that induced by PtdInsP and higher concentrations of dextran sulfate was completely dependent on Zn2+: the excess of Zn2+ needed to induce maximal activity with PtdInsP was 12‐fold the concentration of factor XII, while with dextran sulfate it was 40‐fold. Determination of the Zn2+‐binding properties of factor XII revealed that a total of four zinc ions could bind to each factor XII molecule. The first bound zinc ions (Kd 0.1 μM) induced an increase in the intrinsic tryptophan fluorescence of factor XII, while further titration up to a 40‐fold surplus resulted in a quenching of the fluorescence. Binding of the zinc ions that caused the quenching had an average Kd of approximately 1 μM, independent of whether it was determined from the fluorescence changes or by equilibrium filtration. Low concentrations of both sulfatide and PtdInsP induced a flu