[Comparative properties of metals activating inorganic pyrophosphatase]

Using an ion-selective electrode, the complex formation in a Zn2+-PPi system at pH 6,52 and ionic strength of 0,1 M, t=25 degrees, was studied. The concentrations of the ZnPPi complex (true substrate) and free cation Zn2+ (activator) were calculated from the values of the dissociation constants for the ZnPPi and Zn2PPi complexes. The activating effects of these complexes on inorganic pyrophosphatase were similar to those exerted by Mn2+ and Zn2+. The kinetic patterns of both complexes revealed three main sites for cation binding to the enzyme -- substrate complex. The excess of substrate and activator inhibited the enzymatic reaction. The Km and V values for ZnPPi are 4.1 and 1.2 times lower than those for MgPPi. Thus, Zn2+ appear to be a more efficient activator than Mg2+ at non-saturating concentrations. The data obtained suggest that the occurrence of the limiting step is practically the same for both metals.