Sodium-23 Nuclear Magnetic Resonance as an Indicator of Sodium Binding to Calmodulin and Tryptic Fragments, in Relation to Calcium Content

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Abstract
The relaxation rate enhancements of the 23Na nuclei for NaHCO3 solutions of [bovine brain] calmodulin and its tryptic peptides TR-1 and TR-2 indicate true binding of Na+ ions to these biomolecules. With both TR-1 and TR-2, Na+ binding occurs in competition with Ca2+ and Mg2+ binding: log KNa .apprxeq. 2, log KMg .apprxeq. 4, log KCa .apprxeq. 6 for TR-1; and log KNa .apprxeq. 2, log KMg .apprxeq. 3, log KCa .apprxeq. 5 for TR-2. All the binding constants are systematically greater for binding to TR-1, as compared to TR-2. There is also an increase in KNa for TR-1 of calmodulin as compared to the homologous tryptic fragment of troponin C. The increased binding is identified tentatively with site I of calmodulin. The binding constants KNa, KCa and KMg of calmodulin appear to be finely tuned to the intracellular concentrations of these cations.