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The magnesium ion-dependent hydrolysis of pyrophosphate catalyzed by yeast inorganic pyrophosphatase is markedly inhibited by calcium ion. The kinetics of inhibition have been measured and analyzed in terms of a model involving strong interactions between CaPPiand two metal-complexed forms of the enzyme, EMg and ECa. Inhibition constants for these interactions have been estimated by nonlinear regression and are compared with those obtained by equilibrium measurements. These results are discussed in terms of several theories for specificity in metal ion activation of catalysis.

Because of the requirement for divalent metal ion activation, inorganic pyrophosphatase activity is measured in a complex equilibrium mixture containing free metal ion, free PPi, and metal ion-PPicomplexes. Using crystalline yeast inorganic pyrophosphatase the binding to the enzyme of each of these individual components of the equilibrium has been examined. The dissociation constants of the binary enzyme-metal ion complexes are 2, 11, 16, and 800 µmfor Mn2+, Zn2+, Mg2+, Ca2+, respectively. In the absence of free metal ions neither PPinor CaPPibind to the enzyme; in the presence of excess Ca2+, CaPPiis bound quite strongly (Kdapparent = 7 x10-9m). The enzyme has two binding sites for CaPPiper mole of enzyme dimer. The results of these static binding studies are consistent with the kinetic analyses presented in the following papers.

Diffusion in polyethylene glycols has little relation to their molecular weight or viscosity. Diffusion in methyl-silicone elastomer SE-30 was anomalous, so that comparison with the light methyl-silicone oil SF-96-200 is unreliable. Diffusion of n-alkanes appeared much faster in the elastomer with an indication that this would be untrue of more strongly retained alkanes, but the oil gave neater more symmetrical peaks. This behavior is unexplained. Energy of diffusion is lower in the methyl-silicones, making them more suitable for programmed temperature chromatography. Equations are given relating diffusivities of n-alkanes in methyl-silicones and of n-alkanols in polyethylene glycols to carbon number and temperature.