A, pH optimum of purified ECE-2 was determined with 10 μM McaBk2 in sodium acetate (circles), Tris acetate (squares), and sodium citrate (triangles) at the indicated pH values. The data represent means of quadruplicate determinations. Error bars show the standard error of the mean. B, the effect of metal ions on purified ECE-2 was determined with 10 μM McaBk2 in 0.2 M sodium acetate buffer, pH 5.5. The reaction was carried out in the presence of the indicated concentration of metal ion. “Relative units” represent fluorescence units generated per min, and the data represent means of triplicate determinations.