Figure 1. Profiles of the effects of metal ions on the phosphatase activities of PPM1A and PPM1G, for which cadmium is a potent and competitive inhibitor.

(a) k_cat of the metal ion-dependent phosphatase activities of PPM1A and PPM1G. All assays were conducted in Tris/Bis-Tris/acetate buffer at pH 8.0 and 25°C. (b–c) Inhibition of PPM1A- and PPM1G-catalyzed pNPP hydrolysis by different divalent metals ions. Colors: Cd²⁺ (red), Zn²⁺ (yellow), Hg²⁺ (green), Ca²⁺ (light blue), Sr²⁺ (dark blue), Cr²⁺ (pink), Ba²⁺ (orange), and Li⁺ (purple). (d–e) Double-reciprocal plot (1/V vs. 1/[Mn²⁺]) of the effect of Cd²⁺ on PPM1A/PPM1G-catalyzed pNPP hydrolysis. Mn²⁺ was the variable substrate, and Cd²⁺ was the inhibitor (0, 0.25, 0.5, 1.0 μM). The pNPP concentration was held constant at 20 mM. K_i (PPM1A) = 0.30 ± 0.02 μM. K_i (PPM1G) = 0.17 ± 0.03 μM.