Effects of divalent metal ions on the calcium pump and membrane phosphorylation in human red cells

In inside-out red cell membrane vesicles ATP-dependent calcium transport is activated by the divalent metal ions Mg2+, Mn2+, Co2+, Ni2+ and Fe2+. This activation is based on the formation of Me2+ -ATP complexes which can serve as energy-donor substrates for the calcium pump, and probably, satisfy the requirement for free Me2+ in this transport process. Higher Me2+ concentrations inhibit calcium transport with various efficiencies. Mn2+ directly competes with Ca2+ at the transport site, while other divalent metal ions investigated have no such effect. The formation of the hydroxylamine-sensitive phosphorylated intermediate (EP) of the red cell membrane calcium pump from [gamma-32P]ATP is induced by Ca2+ while rapid dephosphorylation requires the presence of Mg2+. At higher concentrations Mn2+ and Ni2+ inhibit predominantly the formation of EP, while Co2+ and Fe2+ block dephosphorylation. The possible sites and nature of the divalent metal interactions with the red cell calcium pump are discussed. Hydroxylamine-insensitive membrane phosphorylation in inside-out vesicles from [gamma-32P]ATP is significantly stimulated by Mn2+ and Co2+, as compared to that produced by Mg2+, Fe2+ and Ni2+. Part of this labelling is found in phospholipids, especially in phosphatidylinositol. The results presented for the metal dependency of protein and lipid phosphorylation in red cell membranes may help in the characterization of ATP consumptions directly related to the calcium pump and those involved in various regulatory processes.