Quantitative similarity of zinc and calcium binding to heparin in excess salt solution

Zn2+ binding to the anticoagulant heparin was examined using a dye spectrophotometric method, with added NaCl concentrations of 0.005, 0.0075, 0.01, 0.02 and 0.04 mol/l. The results are shown as Scatchard plots and demonstrate the entropy-driven anticooperativity of Zn2+ binding to heparin. From these Scatchard plots, intrinsic binding constants are determined and are compared to our earlier data for Mg2+ and Ca2+ binding to heparin at similar ionic strengths (J. Mattai and J.C.T. Kwak, Biochim. Biophys. Acta 677 (1981) 303), and to Manning's two-variable theory (G.S. Manning, Q. Rev. Biophys. 2 (1978) 179) for a generalized system of polyelectrolyte + divalent cations + univalent cations. While Mg2+ binding to heparin is purely electrostatic (delocalized or territorial), Zn2+ and Ca2+ binding is much stronger and more specific. Binding constants for these two cations are identical, suggesting similar mechanisms for Zn2+ and Ca2+ binding to heparin.