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Proton ejection as a major feature of the Ca(2+)-pump
1Centro de Biologia Celular e Departamento de Bioquímica, Coimbra, Portugal.
Biochimica Et Biophysica Acta
|January 19, 1994
Summary
Magnesium (Mg2+) enhances sarcoplasmic reticulum Ca2+-pump activity, promoting both calcium (Ca2+) uptake and proton (H+) ejection. This suggests the pump facilitates active Ca2+/H+ countertransport, a process influenced by Mg2+.
Area of Science:
- Biochemistry
- Molecular Biology
- Cell Physiology
Background:
- The sarcoplasmic reticulum Ca2+-pump (SERCA) is crucial for muscle contraction by regulating intracellular calcium levels.
- Understanding the factors that modulate SERCA activity, such as divalent cations, is essential for comprehending cellular calcium homeostasis.
Purpose of the Study:
- To investigate the effect of millimolar magnesium (Mg2+) on the activity of the sarcoplasmic reticulum Ca2+-pump.
- To elucidate the mechanism by which Mg2+ influences proton (H+) ejection and calcium (Ca2+) uptake by the Ca2+-pump.
Main Methods:
- Utilized sarcoplasmic reticulum (SR) vesicles to study Ca2+-pump activity.
- Manipulated Mg2+ and Mn2+ concentrations within and outside SR vesicles.
- Employed Ca2+ ionophores to control Ca2+ accumulation within vesicles.
Main Results:
- Millimolar Mg2+ similarly stimulates both H+ ejection and Ca2+ uptake mediated by the SR Ca2+-pump.
- High Mg2+ concentrations in SR vesicles do not impair H+ ejection.
- H+ ejection occurs independently of Ca2+ accumulation, even when prevented by Ca2+ ionophores.
- Mn2+ uptake does not inhibit H+ ejection during the process.
Conclusions:
- The Ca2+-pump actively promotes Ca2+/H+ countertransport, a function that is stimulated by Mg2+.
- The observed stimulation is not due to enhanced Ca2+ uptake or displacement of internal metal ions.
- A physico-chemical mechanism for Ca2+ translocation by the pump is proposed.