Protein binding properties of some histamine H2 antagonists: a 1H nuclear magnetic resonance study of antihistamine-calmodulin interactions

Oxmetidine (SK & F 92994) is a potent histamine H2 antagonist, which, however, also demonstrates cardiac effects consistent with its inhibiting transmembrane calcium fluxes. 1H nuclear magnetic resonance has been used to show that oxmetidine binds to a single site on the regulatory calcium-binding protein, calmodulin. Binding requires the presence of at least two equivalents of calcium per mol protein, is characterized by fast exchange behaviour and a dissociation constant of about 4 mM and is not affected by the presence of trifluoperazine. Protein-induced spectral changes and a limited study of structure-affinity relationships suggest the importance of the drug imidazole and benzyldioxymethylene groups in determining the strength of the interaction. Drug-induced perturbations in the spectrum of calmodulin indicate that the binding site is in the C-terminal half of the protein, and involves a hydrophobic area containing His-107, Met-144, Met-145 and possibly Phe-89, Phe-141, and calcium binding site III.