The Ca²⁺-transporting ATPases of rabbit and trout exhibit different pH- and temperature dependences

The phosphorylation of the trout sarcoplasmic-reticulum Ca²⁺-ATPase by P(i) differs in its temperature- and pH-dependence from the rabbit ATPase. In the trout enzyme, the apparent affinity for P(i) and maximum phosphoenzyme values do not vary over a pH and temperature ranges that have a pronounced effect on the rabbit enzyme. The lack of temperature-dependence for phosphorylation is observed at pH 6.8. At pH 8.0, the temperature profile for phosphorylation of the trout enzyme resembles that of the rabbit at pH 6.8. The rabbit ATPase is no longer phosphorylated by P(i) after solubilization with the detergent C₁₂E₉. In contrast, the trout enzyme can be phosphorylated by P(i) after solubilization with C(l2)E₉, and the same levels of phosphoenzyme were obtained with the soluble and membrane-bound ATPase at both 0° and 25°C. In the range of 0-20 °C, the rates of ATP synthesis and of Ca²⁺ uptake by the trout ATPase are less temperature-dependent than for the rabbit enzyme. However, both isoenzymes catalyse ATP hydrolysis with similar temperature-dependences. The results raise the possibility that protonation of specific amino acid residues may contribute to the lack of temperature-dependence for phosphorylation of the trout Ca²⁺-ATPase.