Purification and characterization of a Ca2+-dependent ATPase from rat heart sarcolemma

The Ca2+-dependent ATPase was solubilized from rat heart sarcolemmal membranes upon digestion with trypsin and was found to be different from Ca2+-stimulated Mg2+-dependent ATPase (Dhalla, N. S., Anand-Srivastava, M. B., Tuana, B. S., and Khandelwal, R. L. (1981) J. Mol. Cell. Cardiol. 13, 413-423). The enzyme was purified by high speed centrifugation, ammonium sulfate fractionation, and column chromatography and was seen as a single protein band in nondenaturing polyacrylamide gel electrophoresis. In sodium dodecyl sulfate-acrylamide gels, the enzyme dissociated into two subunits or fragments with molecular weights of about 55,000 and 12,000. The molecular weight of the enzyme, estimated by gel filtration on a Sephadex G-100 column, was found to be about 67,000. The enzyme utilized ATP with a Km of 0.20-0.26 mM but was also able to utilize ITP, CTP, GTP, and ADP as substrates at much lower rates. It was activated by Ca2+ with a Ka of 0.13-0.21 mM; it was also activated by other cations in the order Ca2+ greater than Mn2+ greater than Sr2+ greater than Ba2+ greater than Mg2+. Divalent cations like Cu2+, Ni2+, and Mg2+ were potent inhibitors. The enzyme was insensitive to ouabain, verapamil, oligomycin, cyanide, and vanadate but was markedly inhibited by N-ethylmaleimide. Calmodulin failed to stimulate Ca2+-dependent ATPase and instead inhibited slightly. Unlike K+, Na+ produced a marked inhibition of the Ca2+-dependent ATPase activity, and this inhibition was associated with an 8- 10-fold decrease in the affinity of the enzyme for Ca2+. The competitive action of Na+ indicates that the Ca2+-dependent ATPase may be a site of Na+-Ca2+ antagonism in the cell membrane.