On the substrate specificity of the red cell calcium pump.

ATP-dependent active calcium transport in inside-out human red cell membrane vesicles is stimulated by magnesium essentially parallel with an increase in MgATP concentration. At a constant, low (1 microM) calcium concentration, increasing ATP and magnesium increase the maximum calcium transport rate irrespective of the constant or decreasing concentrations of CaATP present. KCa for calcium pumping is practically unchanged at variable ATP and magnesium concentrations. Free magnesium above 1-2 mM inhibits active calcium transport, probably through a direct interaction with the transport enzyme. Based on the experimental findings reported we suggest that the true, physiological substrate of the red cell calcium pump is MgATP.