Coupling Between ATP Splitting and Calcium Transport

The uptake of calcium is usually initiated by the addition of calcium ions to the otherwise complete assay medium. Before calcium is added, ATP is split with a low rate by most sarcoplasmic reticulum preparations. This ATPase which is active in the absence of calcium ions has been named basic ATPase . 

2 mM EGTA, 10 mM histidine pH 7.0, T = 20 °C. 14C labeled ADP and 7 P labeled ATP were used for the exchange and the phosphorylation experiments, respectively. Calcium uptake is illustrated as the removal of calcium from the assay medium. The arrow indicates the i 7 addition of 0.1 pmol calcium chlo- g1 ride ml-1 to the medium contain- c ing 0.05 mg protein ml-1. Note, 

S basic splitting and basic exchange -li prior to calcium addition. The rapidly formed phosphoprotein disappears when the calcium concentration declines 

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