Inactivation of detergent-solubilized sarcoplasmic reticulum ATPase

Inactivation of sarcoplasmic ATPase in the solubilized state was studied in the absence and presence of Ca2+, Mg2+ and glycerol. The effects of the detergents octa(ethyleneglycol) mono-n-dodecyl ether (C12E8), 1-O-tetradecylpropanediol-(1,3)-3-phosphorylcholine and myristoylglycerophosphocholine were compared. All three detergents caused a rapid decline of the dinitrophenyl phosphatase activity of the unprotected enzyme. The stabilizing effect of Ca2+ ions was kinetically analysed. It was found that the stability of the solubilized enzyme depends on the Ca2+ concentration in a manner which is best explained by assuming rapid inactivation of Ca2+-free enzyme accompanied by slow inactivation of a calcium-enzyme complex (E1Ca). The apparent affinity constants obtained are in the order of 10(6)M-1, suggesting that high-affinity Ca2+ binding must be involved. No indications of a contribution were found, either of low-affinity Ca2+-binding sites of the conformational state E2 or of the high-affinity calcium complex E1Ca2. If Ca2+ was replaced by Mg2+, which exerts a weaker protection, the apparent affinity constants for Mg2+ are in the range of 1 mM-1. The stoichiometry of the effect of Mg2+ depends on the detergent.