CALCIUM BINDING PROPERTIES OF SARCOPLASMIC RETICULUM MEMBRANES

Sarcoplasmic reticulum from rabbit skeletal muscle demonstrates two types of ATP-independent calcium binding. About 10% of the total calcium-binding sites are very little influenced by ionic strength and are readily inactivated by trypsin proteolysis. The remaining calcium-binding sites are relatively heat stable and are not inactivated by trypsin proteolysis. Calcium binding by these sites is dependent on the ionic strength and is completely inhibited by 0.01 M MgCl2, 0.6 M KCl or 0.6 M NaCl. Total calcium-binding capacity as tested in media of low ionic strength is 50 nmoles of calcium bound per mg of sarcoplasmic reticulum protein. Both binding systems have an apparent dissociation constant of 40 μM and are not influenced by hydrolysis of the membrane phospholipids by phospholipase C. © 1969.