Purification and characterization of a Ca2+- or Mg2+-stimulated ATPase from plasma membrane enriched fractions of Dictyostelium discoideum

Evidence is presented for the presence of both diethylstilbestrol (DES)-sensitive and DES-insensitive Mg2+-ATPase activities in plasma membrane enriched fractions of Dictyostelium discoideum. When removed from the membrane, the DES-sensitive activity is markedly less stable than the DES-insensitive activity, and the two activities display a number of quite distinct properties. The DES-sensitive enzyme has a decided preference for Mg2+ over Ca2+, displays saturation kinetics in response to ATP as substrate (Km = 0.2 mM) and has a narrow pH optimum range. In contrast, the DES-insensitive activity is stimulated equally by Mg2+ or Ca2+, is not saturable by ATP within the mM concentration range and has a much broader pH optimum. The DES-insensitive activity has been purified extensively. The purified enzyme is inhibited by vanadate and fluoride, but is insensitive to N,N'-dicyclohexylcarbodiimide (DCCD), N-ethylmaleimide and thimerosal. In the absence of divalent cations, the enzyme displays a sigmoidal activity curve in response to substrate concentration, which is abolished by addition of either Mg2+ or Ca2+, suggesting a binding site for a divalent cation and a positive cooperative interaction. The enzyme is capable of hydrolyzing other nucleotide triphosphates and ADP, but is without activity on AMP, p-nitrophenyl phosphate and pyrophosphate. The enzyme has an apparent molecular weight of approximately 64,000.