Characterization of Mg2+-ATPase from slow-twitch muscle membranes

SR vesicles from rabbit slow-twitch muscle reveal high activity (0.7-0.9 mumol/mg X min) of "basic" or Mg2+-ATPase. This enzyme differs in its biochemical properties from the well characterized Ca2+ pump ATPase. It is active in millimolar concentration of magnesium or calcium. The activity is inhibited by various detergents except for digitonin. This enzyme seems to be an integral membrane protein since it remains in the membrane after removal of peripheral proteins with EDTA. It can be partially solubilized from the membrane using digitonin without a decrease in specific activity. Ion exchange chromatography on DEAE-Sephacel of the post digitonin supernatant allows us to obtain a 5-fold increase in Mg2+-ATPase specific activity concomitantly with the enrichment in two proteins of Mr = 30,000 and 150,000.