Figure 4. Dose-dependence of eosin inhibition on K⁺-dependent phosphatase activity.

Ouabain-sensitive, K⁺-dependent pNPPase activity was measured in the presence of the indicated concentrations of eosin (see Methods). Data were fit to the same equation show in Fig. 1. Interestingly, the IC₅₀ curve appears to plateau with a residual noneosin-sensitive 20% activity. The IC₅₀ for eosin-sensitive component was 3.8 ± 0.23 μM. Data points are means and bars are standard error of six separate experiments. B.) pNPP dose-dependence of eosin inhibition. K⁺-dependent phosphatase activity was measured with increasing [pNPP] in the absence (○) and presence (□) of 25μM eosin. Data were fit to the Michaelis-Menten equation. Control V_max = 1.17 ± 0.05; K_1/2 = 1.65 ± 0.21 μM; 25 μM eosin V_max = 0.38 ± 0.05; K_1/2 = 2.91 ± 0.54 μM. These data suggest that eosin does not compete with pNPP. Data points are means and bars are standard error of three separate experiments.