Relationship between Ca2+-transport and ATP hydrolytic activities in guinea-pig pancreatic acinar plasma membranes |
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Authors: | Rajesh Mahey Michael A Bridges Sidney Katz |
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Institution: | (1) Division of Pharmacology and Toxicology, University of British Columbia, Vancouver, B. C., Canada;(2) Division of Clinical Pharmacy, Faculty of Pharmaceutical Sciences, University of British Columbia, Vancouver, B. C., Canada;(3) Departments of Paediatrics and Pathology, Faculty of Medicine, University of British Columbia, Vancouver, B. C., Canada |
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Abstract: | Partially purified plasma membrane fractions were prepared from guinea-pig pancreatic acini. These membrane preparations were found to contain an ATP-dependent Ca2+-transporter as well as a heterogenous ATP-hydrolytic activity. The Ca2+-transporter showed high affinity for Ca2+ (KCa
2+ = 0.04 ± 0.01 M), an apparent requirement for Mg2+ and high substrate specificity. The major component of ATPase activity could be stimulated by either Ca2+ or Mg2+ but showed a low affinity for these cations. At low concentrations, Mg2+ appeared to inhibit the Ca2+-dependent ATPase activity expressed by these membranes. However, in the presence of high Mg2+ concentration (0.5–1 mM), a high affinity Ca2+-dependent ATPase activity was observed (KCa
2+ = 0.08 ± 0.02 M). The hydrolytic activity showed little specificity towards ATP. Neither the Ca2+-transport nor high affinity Ca2+-ATPase activity were stimulated by calmodulin. The results demonstrate, in addition to a low affinity Ca2+ (or Mg+)-ATPase activity, the presence of both a high affinity Ca2+-pump and high affinity Ca2+-dependent ATPase. However, the high affinity Ca2+-ATPase activity does not appear to be the biochemical expression of the Ca2+-pump.Abbreviations Ca2+-ATPase
calcium-activated, magnesium-dependent adenosine triphosphatase
- CaM
calmodulin
- CDTA
trans-1,2-diaminocyclohexane-N,N,N,N-tetraacetate
- EDTA
ethylene-diaminetetraacetate
- EGTA
ethylene glycol bis(-aminoethyl ether)-N,N,N,N-tetraacetate
- NADPH
reduced form of nicotinamide adenine dinucleotide phosphate |
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Keywords: | Ca2+-pump Ca2+-ATPase high affinity Mg2+-dependent substrate specificity |
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