FIG. 4.

Effects of pH (A), temperature (B), and metal cations (C) on the activity of O. sagaranesis Uca. In the temperature experiments the logarithm of specific activity was plotted against 1,000/T (K⁻¹) (Arrhenius plot), and the reaction mixtures used contained 50 mM HEPES buffer (pH 7.5), 50 mM KCl, 8 mM MgSO₄, 1 mM ATP, 1.5 mM phosphoenolpyruvate, 8 mM KHCO₃, 0.15 mM NADH, 10 mM urea, 10 U of pyruvate kinase, and 25 U of lactate dehydrogenase. In the pH experiments, the same reaction mixtures were used except that HEPES buffer was replaced with different buffers, as indicated. In the metal cation experiments, the same reaction mixtures were used except that MgSO₄ was replaced with the metal cations indicated at various concentrations.