Sarcoplasmic membrane

The calcium-independent ATPase of the lipid modified preparations is not only different from the calcium-dependent ATPase but also from the calcium-independent ATPase of native preparations — the basic ATPase — which has a lower nucleotide specificity126. The experiments in which the lipid matrix of the sarcoplasmic membranes has been replaced by lipid compounds not present in native membranes reveal a high degree of functional flexibility of the enzyme. On the other hand, a few residual lipids in the protein are sufficient to prevent these changes in the structure of the enzyme and to preserve its calcium sensitivity. 

The transporting protein in the sarcoplasmic membrane can be phosphorylated by ATP as well as by inorganic phosphate (cf.2,174 ). In the forward running mode of the pump, i. e. when the calcium pump accumulates calcium and concomitantly hydrolyzes ATP, the terminal phosphate residue of ATP is transferred to the transport protein. The reaction depends on the presence of calcium ions in the external medium. In the reverse mode of the pump inorganic phosphate is incorporated into the transport protein. This reaction is inhibited when calcium ions are present in the external medium,... 

Immunoelectron microscopy of cardiac muscle revealed that immunogold was deposited on the mitochondria among the myofibrils as well as on the mitochondria beneath the sarcoplasmic membrane (Fig. 20, upper panel). Some immunogold was also seen on the secondary lysosomes and on lipofuscin, which contained heterogeneous substances and showed irregular shapes. In addition, the mitochondria of the endothelial cells of the blood capillaries reacted with this antibody (Fig. 20, lower panel). 

The Ca -Mg ATPase is the major protein constituent of the skeletal muscle sarcoplasmic membranes. This enzyme can be solubilized... 

In the presence of phosphate donating or accepting reactants, the translocation of calcium ions across the sarcoplasmic membranes is linked with phosphoryl transfer reactions leading to the phosphorylation of the transport protein. During calcium accumulation, the terminal phosphate group of ATP or of the other phosphate donors is rapidly transferred to the transport protein from which it is subsequently liberated by hydrolytic cleavage. The phosphoryl group in the protein is acid-stable and can therefore be stabilized in acidic quench media [112-114]. 

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