(a) kcat 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. Ki (PPM1A) = 0.30 ± 0.02 μM. Ki (PPM1G) = 0.17 ± 0.03 μM.