Hofmeister specific-ion effects on enzyme activity and buffer pH: Horseradish peroxidase in citrate buffer

Salt addition to enzymes in buffer always induce the problem of the respective influences of electrostatic interactions and anion specificity on buffer pH and enzyme kinetics. In the present paper the influence of some sodium salts (Na₂SO₄, NaCl, NaBr and NaNO₃) on the pH of a citrate buffer (c = 0.025 M), and on the catalytic constants of horseradish peroxidase (HRP) is studied. First, the pH changes due to the presence of salts in the buffer are examined; second, catalytic constants, K_m^ABTS, V_max^ABTS and V _max^ABTS/K_m^ABTS, are studied as a function of pH in buffer with and without added salt, at various salt concentrations and different pH values due to the salt additions. With a simple electrostatic model, it is possible to show that the glass electrode yields reasonable pH values even in the presence of fairly high 1 : 1 salt concentrations. For the catalytic efficiency, V_max ^ABTS/K_m^ABTS, a Hofmeister series is found with opposite deviations from the pure pH effect for salting-in and salting-out ions over a large range of salt concentrations. This usual Hofmeister series is a consequence of three, for the moment inseparable salt concentration and specific ion-induced phenomena: global bulk effects, local active site effects and surface effects.