KINETICS OF ATP HYDROLYSIS CATALYZED BY MG2+-ATPASE FROM SMOOTH-MUSCLE CELL PLASMA-MEMBRANES

The kinetic properties of Mg2+-ATPase (EC 3.6.1.3) from myometrium cell plasma membranes were studied. Under conditions of enzyme saturation with ATP (0.5-1.0 mM) or Mg2+ (1.0-5.0 mM) the initial maximal rates of Mg2+-dependent enzymatic ATP hydrolysis, V-0 ATP and V-0 Mg are 27.4 +/- 3.3 and 25.2 +/- 4.1 mu moles P-i/h per mg protein, respectively. The apparent Michaelis constant (K-m) for ATP and the apparent activation constant (K-a) for Mg2+ are 28.1 +/- 2.6 and 107.0 +/- 26.0 mu M, respectively. Bivalent metal ions (1 mM) suppress Mg-2+-dependent ATP hydrolysis with efficiency decreasing in the series: Cu2+ > Zn2+ = Ni2+ > Mn2+ > Ca2+ > Co2+. Alkalinization of the medium from pH 6.0 to pH 8.0 stimulates Mg2+-dependent ATP hydrolysis. The Mg2+-ATPase is a H+-sensitive enzymatic sensor having a linear dependence between the initial maximal reaction rate (V-0) and pH. The possible role of plasma membrane Mg2+-ATPase in reactions controlling proton and Ca2+ homeostasis in myometrium cells is discussed.