Sarcoplasmic reticulum calcium/magnesium ATPase

Sandwich complexes nickel. 5, 35 Sapphyrins, 2, 888 demetallation, 2, 891 metallation, 2, 891 reactions, 2, 891 synthesis, 2, 889 Sarcoplasmic reticulum calcium/magnesium ATPase, 6, 566 skeletal muscle... 

Stefanova, H. I. East, J. M. Gore, M. G. Lee, A. G. Labeling the calcium-magnesium-ATPase of sarcoplasmic reticulum with 4-(bromomethyl)-... 

ATP is cleaved by most sarcoplasmic reticulum preparations at low rates in the absence of calcium ions. This activity has been denoted as basal activity [20]. When calcium accumulation is initiated, ATP is rapidly hydrolyzed in a calcium-dependent activity, reaching its optimum at a calcium concentration of 10 jaM, and which is severely suppressed by the rising calcium concentrations in the interior of the vesicles [45,64,65]. The calcium-dependent activity was early characterized as the activity of an enzyme distinctly different from the calcium-independent enzyme. In contrast to the calcium-dependent ATPase, the calcium-independent enzyme is quite insensitive to thiol or amino group reagents. Conversely, the calcium-independent activity can be abolished by low concentrations of detergents which do not reduce the activity of the calcium-dependent enzyme [66]. The two enzymatic activities further differ in their nucleotide specificity and affinity, as well as in their magnesium and temperature dependences. The basal activity most likely originates from plasmalemma and T-tubules membranes [41]. 

Calcium plays a vitally important part in possibly as many as three sequences of the contraction—relaxation cycle of vertebrate muscle (Taylor, Lymn and Moll, 1970). In the sarcoplasmic reticulum, calcium is stored in calsequestrin, a protein which holds, with high affinity, 43 atoms of calcium per molecule. The calcium is released in response to nervous impulses and triggers a sequence of reactions that lead to contraction of the muscle. After the contraction, the calcium is pumped back into this reticulum by the calcium/magnesium-dependent ATPase. 

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