Mechanism of histamine binding II: effect of alkali metal and alkaline earth cations on histamine binding to peptide H.

The association constants of histamine with Mg2+, Ca2+, Sr2+, and Ba2+ were determined in buffer solutions at constant pH using an ion selective electrode. These cations enabled histamine to bind to peptide H. A minimum cation binding concentration was required for histamine binding. A linear relationship existed between the minimum cation binding concentration and the log of the equilibrium constant for the histamine-cation complexes, indicating that the specificity of the alkaline earth cations in promoting histamine binding was due to the difference in their ability to complex with histamine. The monovalent cations, Nat, Kt, and Cst inhibited histamine binding to peptide H, with the extent of inhibition dependent on cation concentration. An ion-exchange mechanism or a conformation change in the peptide may account for the inhibition.