FIGURE 2.

Ca²⁺-ATPase activity. The rate of Ca²⁺-activated ATP hydrolysis was determined at 37 °C in 50 mm TES/Tris (pH 7.0), 100 mm KCl, 7 mm MgCl₂, 1 mm EGTA, 0.9 mm CaCl₂ (3 μm free Ca²⁺), 5 mm ATP, and 1 μm Ca²⁺ ionophore A23187. Following subtraction of the background activity determined in the absence of Ca²⁺, the catalytic turnover rate was calculated as the molar ratio of P_i liberated per active site/s. The active-site concentration was determined by phosphorylation with 5 μm [γ-³²P]ATP for 10 s on ice in 40 mm MOPS/Tris (pH 7.0), 80 mm KCl, 5 mm MgCl₂, and 0.1 mm CaCl₂. The bars illustrate the catalytic turnover rates relative to the wild type (WT).