Ca 2+-stimulated, Mg 2+-independent ATP hydrolysis and the high affinity Ca 2+-pumping ATPase. Two different activities in rat kidney basolateral membranes

Abstract

The Mg 2+-dependency of Ca 2+-induced ATP hydrolysis is studied in basolateral plasma membrane vesicles from rat kidney cortex in the presence of CDTA and EGTA as Mg 2+- and Ca 2+-buffering ligands. ATP hydrolysis is strongly stimulated by Mg 2+ with a K m of 13 μ M in the absence or presence of 1 μ M free Ca 2+. At free Mg 2+ concentrations of 1 μ M and lower, ATP hydrolysis is Mg 2+ -independent, but is strongly stimulated by submicromolar Ca 2+ concentrations K m  0.25 μM, V max  24 μmol P i/h per mg protein). The Ca 2+-stimulated ATP hydrolysis strongly decreases at higher Mg 2+ concentrations. The Ca 2+-stimulated Mg 2+-independent ATP hydrolysis is not affected by calmodulin or trifluoperazine and shows no specificity for ATP over ADP, ITP and GTP. In contrast, at high Mg 2+ concentrations calmodulin and trifluoperazine affect the high affinity Ca 2+-ATPase activity significantly and ATP is the preferred substrate. Control studies on ATP-dependent Ca 2+-pumping in renal basolaterals and on Ca 2+-ATPase in erythrocyte ghosts suggest that the Ca 2+-pumping enzyme requires Mg 2+. In contrast, a role of the Ca 2+-stimulated Mg 2+-independent ATP hydrolysis in active Ca 2+ transport across basolateral membranes is rather unlikely.