The Effect of Magnesium Ion Concentration on the pH Optimum of the Spinach Leaf Alkaline Fructose Diphosphatase

The pH optimum of the spinach leaf alkaline fructose diphosphatase depends on the concentration of magnesium ion. At 5 mm MgC12 the pH optimum for enzyme activity is 8.5 and there is very little activity at 7.0. Increasing concentration of magnesium ion up to 40 mm, however, causes a shift of the pH optimum to 7.5. Appreciable activity is observed at pH 7.0 at the higher magnesium ion concentrations (20 to 40 mm). The fructose diphosphate concentration versus velocity curve is sigmoidal at the lower MgC12 concentrations.