Comparative studies on Ca2+- and Mg2+-binding of sarcoplasmic reticulum and chromaffin granule membranes

The binding of calcium and magnesium ions to sarcoplasmic reticulum (SR) and chromaffin granule membranes was comparatively studied. The SR membranes are equipped with equal quantities of binding sites for both calcium and magnesium ions. The binding sites in presence of ATP combine specifically with calcium ions, while in the absence of ATP the binding sites react unspecifically with both ions. The trace amount of magnesium present in the SR membranes preparations is sufficient to drive ATP dependent calcium accumulation. Magnesium binding, however, is not affected by ATP. The chromaffin granule membranes bind calcium and magnesium in the same concentration range as observed for the SR membranes. Magnesium binding, however, is two times higher than that of calcium binding. In the absence of ATP, calcium and magnesium ions mutually compete. In the presence of ATP, magnesium binding values increase 3-5-fold, while the calcium binding isotherm remains unchanged. The appreciable contribution of the lipid phase to ions binding has been investigated, but was found to be of minor importance in this study.