Purification and characterization of a Ca2+/Mg2+ ecto-ATPase from rat heart sarcolemma

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Abstract
The Ca2+/Mg2+ ATPase of the rat heart sarcolemmal particles was solublized with Triton X-100 after treating the membranes with trypsin and purified by high speed centrifugation, ammonium sulfate fractionation, hydrophobic chromatography and gel filtration. The purified enzyme was seen as a single protein band in nondenaturing polyacrylamide gel electrophoresis and its molecular weight by gel filtration was found to be about 240000. The enzyme utilized Ca-ATP or Mg-ATP as a substrate with high affinity sites (Km = 0.12 − 0.16 mM) and low affinity sites (Km = 1 mM). The enzyme also utilized CTP, GTP, ITP, UTP and ADP as substrates but at a lower rate in comparison to ATP. The enzyme was activated by Ca2+ (Ka = 0.4 mM) and Mg2+ (Ka = 0.2 mM) as well as by other cations in the order Ca2− > Mg2+ > Mn2+ > Sr2+ > Ba2+ > Ni2+ > Cu2+. The ATPase activity in the presence of Ca2+ was markedly inhibited by Mg2+, Mn2+, Ni2+ and Cu2+ whereas the monovalent cations such as Na+ and K+ were without effect. The enzyme did not exhibit Ca2+ stimulated Mg2+ dependent ATPase activity and was insensitive to calmodulin, ouabain, verapamil, D-600, oligomycin, azide and vanadate. Optimum pH for Ca2+ or Mg2+ ATPase activity was 8.5 − 9.0. In view of the possible ectoenzyme nature of the ATPase, its role in adenine nucleotide and Ca2+ metabolism in the myocardium is discussed.