Magnesium dependence of sarcoplasmic reticulum calcium transport.

The activity of the calcium transport systems in the sarcoplasmic reticulum membranes operating in the forward and reverse modes depends on the presence of magnesium ions. When the system operates as an NTP-driven calcium pump, magnesium enters the reaction chain chelated with NTP. Magnesium is a component of the NDP-sensitive phosphoprotein species involved in the NDP-NTP exchange reaction. When the system operates in the reverse mode as a calcium efflux-driven NDP synthetase, magnesium together with inorganic phosphate rapidly forms the initial phosphorylated intermediate. The stability of the magnesium-phosphoprotein complex is considerably enhanced by an existing calcium gradient. Magnesium as well as calcium affects the partitioning of the energy in the reaction sequence.