The Binding of Calcium and Magnesium to Sarcoplasmic Reticulum Vesicles as Studied by Manganese Electron Paramagnetic Resonance

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Abstract
The binding pattern of the biologically relevant ions Ca and Mg was investigated via the binding of the ion-analogue Mn. The binding parameters of Mn are obtained conveniently by standard EPR techniques; the binding of Ca and Mg is inferred from competition experiments. The quantitative analysis was conducted with a computer program which treated the competitiion of 3 kinds of ions for 3 classes of independent and 1 class of cooperative binding sites. Specific binding classes were correlated with biological functions of the sarcoplasmic reticulum membrane [rabbit muscle]. The Mg and Mn specific binding class of medium affinity is related to the catalytic function of these ions in the ATP-splitting. For the 1st time a Mn and Ca specific class of cooperative binding sites has been observed, and it can be related to the inhibition of the Ca transport at high concentrations of Mn or Ca. From the binding results a model can be developed which allows a full description of the role of the divalent ions and their specific binding sites during Ca transport.