Untersuchungen über eine Mg 2+ ‐(Ca 2+ )‐aktivierte ATPase aus Rinderhirnmikrosomen

1 The steady state kinetics of the Mg 2+ ‐dependent ATPase of beef brain microsomes are assayed by means of a rapid photometric method. Omitting K + from the reaction medium, the Na+, K+ ‐Mg 2+ ‐dependent ATPase is not active. 2 The Mg 2+ ‐dependent ATPase is also active with Ca 2+ and inactive in the absence of Mg 2+ or Ca 2+ when 40 mM Na+ is present. Hence the name “Mg 2+ ‐(Ca 2+ )‐ATPase” is proposed. 3 The complexes MgATP 2 ‐ and CaATP 2− are evidently the true substrates of the ATPase. MgATP 2 ‐ is split with a 2.2‐fold higher velocity than CaATP 2− . 4 The Mg 2+ ‐(Ca 2+ )‐ATPase is inhibited competitively by an excess of free Mg 2+ ‐ or Ca 2+ ‐ions and probably uncompetitively by an excess of free ATP 4− . 5 The reaction rate of the Mg 2+ ‐(Ca 2+ )‐ATPase as a function of the concentrations of substrate, Mg 2+ ‐ resp. Ca 2+ ‐ions and ATP 4− can be expressed as[MeATP 2− ] has to be calculated from the ATP 4− and the metal ion concentrations and the dissociation constants of MgATP 2− and CaATP 2− . 6 MgATP 2− acting as substrate yields the Michaelis‐constant K m = 3.6 ± 0.4 × 10 −5 M and the dissociation constants K Mg = 8.3 ± 0.8 × 10 −4 M and K ATP = 2.0 ± 0.6 × 10 −4 M; CaATP 2− yields K m = 1.3 ± 0.15 × 10 −5 M, K Ca = 8.0 ± 0.9 × 10 −4 M and K ATP = 4.5 ± 1.2 × 10 −5 M. On account of the magnitude of the reaction velocity constants k 1 , k −1 and k 2 , the Michaelis‐constant seems to approximate k 2 / k 1 for both substrates.