Phosphorylation by Inorganic Phosphate of Sarcoplasmic Membranes

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Abstract
The Ca transport protein of the rabbit skeletal muscle sarcoplasmic reticulum accepts Pi rapidly when phosphorylation is initiated either by the addition of phospphate or Mg2+ to the Ca-free protein. Phosphorylation proceeds much more slowly when it is initiated by the addition of the Ca chelator ethyleneglycol-bis (.beta.-aminoethyl ether)-N,N''-tetraacetic acid (EGTA) to the phosphate and Mg containing assay. The time course of phosphorylation immediately following Ca removal is monophasic at all temperatures between 20-37.degree. C. Phosphorylation of the Ca-free enzyme becomes biphasic at temperatures above 25.degree. C. The biphasic time course does not only apply to net formation of phosphoprotein but also to its exchange with medium phosphate. On addition of Ca, the phosphoprotein decays in a biphasic process the time constants of which are much longer than those observed for phosphoprotein formation. The temperature dependence of the rate and of the extent of phosphoprotein formation indicate a discontinuity in the reactivity of the protein.