Quantitative characterization of the binding of histamine by heparin

Tissue histamine is stored in mast cell granules, presumably as a histamine-heparin complex. Heparin is a polyelectrolyte, with a fraction of its anionic charge neutralized by condensed counterions. The interaction of heparin with histamine in aqueous solution was quantitatively characterized by 1H nuclear magnetic resonance (NMR) spectroscopy. Binding constants were determined from chemical shift-pH titration data for the C2H proton of the imidazolium ring for a wide range of histamine, heparin, and Na+ concentrations. The results indicate a binding stoichiometry of 1 histamine per heparin disaccharide repeat unit. The binding is electrostatic, as indicated by the strong dependence of the binding constant on Na+ concentration. From an analysis of the binding constants using the counterion condensation theory of polyelectrolytes, it was determined that the binding of H2A2+ results in displacement of 1.72 Na+ ions from the counterion condensation volume of heparin and that H2A2+ makes 2 ionic interactions with heparin. The displacement of Na+ from the counterion condensation volume of heparin by H2A2+ was also studied by 23Na NMR. From 23Na spin-lattice relaxation time data, it was determined directly that 1.78 Na+ ions are displaced per H2A2+ bound by heparin. The results are discussed in terms of the ion exchange process which takes place when histamine is released by mast cells.