Phosphorylation by Inorganic Phosphate of Sarcoplasmic Membranes

The calcium transport protein of the sarcoplasmic reticulum accepts inorganic phosphate rapidly when phosphorylation is initiated either by the addition of phosphate or magnesium ions to the calcium free protein. Phosphorylation proceeds much more slowly when it is initiated by the addition of the calcium chelatro ethyleneglycol-bis (beta-aminoethyl ether)-N,N'-tetraacetic acid (EGTA) to the phosphate and magnesium containing assay. The time course of phosphorylation following immediately calcium removal is monophasic at all temperatures between 20 degrees and 37 degrees C. In contrast, biphasic time course doses not only apply to net formation of phosphoprotein but also to its exchange with medium phosphate. On addition of calcium, 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 as well as of the extent of phosphoprotein formation indicate a discontinuity in the reactivity of the protein.