QUANTITATIVE CHARACTERIZATION OF THE BINDING OF HISTAMINE BY HEPARIN

Tissue histamine is stored in mast cell granules, presumably as a hist amine-heparin complex. Heparin is a polyelectrolyte, with a fraction o f its anionic charge neutralized by condensed counterions. The interac tion of heparin with histamine in aqueous solution was quantitatively characterized by H-1 nuclear magnetic resonance (NMR) spectroscopy. Bi nding constants were determined from chemical shift-pH titration data for the C2H proton of the imidazolium ring for a wide range of histami ne, heparin, and Na+ concentrations. The results indicate a binding st oichiometry of 1 histamine per heparin disaccharide repeat unit. The b inding 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 polyelectrolyte s, it was determined that the binding of H(2)A(2+) results in displace ment of 1.72 Na+ ions from the counterion condensation volume of hepar in and that H(2)A(2+) makes 2 ionic interactions with heparin. The dis placement of Na+ from the counterion condensation volume of heparin by H(2)A(2+) was also studied by Na-23 NMR. From Na-23 spin-lattice rela xation time data, it was determined directly that 1.78 Na+ ions are di splaced per H(2)A(2+) bound by heparin. The results are discussed in t erms of the ion exchange process which takes place when histamine is r eleased by mast cells.