title: Sodium-23 nuclear magnetic resonance as an indicator of sodium binding to calmodulin and tryptic fragments, in relation to calcium content.
abstract: The relaxation rate enhancements of the 23Na nuclei for NaHCO3 solutions of 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 approximately equal to 2, log KMg apoproximately equal to 4, log KCa approximately equal to 6 for TR-1; and log KNa approximately equal to 2, log KMg approximately equal to 3, log KCa approximately equal to 5 for TR-2. All the binding constants are systemically 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.
mesh_terms: D000818:Animals; D001665:Binding Sites; D001921:Brain; D002135:Calcium-Binding Proteins; D002147:Calmodulin; D002417:Cattle; D007700:Kinetics; D008274:Magnesium; D009682:Magnetic Resonance Spectroscopy; D010446:Peptide Fragments; D011485:Protein Binding; D012964:Sodium; D014357:Trypsin
chemical_list: D002135:Calcium-Binding Proteins; D002147:Calmodulin; D010446:Peptide Fragments; D012964:Sodium; D014357:Trypsin; D008274:Magnesium