Low Affinity Calcium Binding Sites of the Calcium Transport ATPase of Sarcoplasmic Reticulum Membranes

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Abstract
Ca binding sites having low affinity constants of < 103 M-1 were titrated in native sarcoplasmic reticulum vesicles as well as in lipid deprived membranes and in the isolated Ca transport ATPase. Short time Ca binding measurements and the determination of the Ca binding heat allow to distinguish low affinity Ca binding sites located on the external surface of the sarcoplasmic reticulum membranes from those present in the section of the transport molecule directed to the vesicular space. The same number of internal binding sites was found for preparations deprived of their lipid content and of preparations depleted of their lipids and of their accessorial proteins. Mg interferes with Ca binding to the external as well as to the internal low affinity Ca binding sites. The implications of the existence of the low affinity Ca binding sites in the internal section of the Ca transport ATPase are discussed.