Interaction of human salivary mucins with hydroxyapatite

The interactions between hydroxyapatite (HAP) and several types of mucins (human whole salivary mucins, HWSM; ovine submandibular mucin, OSM; porcine gastric mucin, PGM) were compared using a quantitative assay. Of these mucins, HWSM displayed by far the highest binding to hydroxyapatite, followed by PGM and OSM, respectively. HWSM binding to hydroxyapatite was measured at pH 6.3 and 7.0. Data obtained appeared to fit empirically the Langmuirian adsorption isotherm. Apparent affinity constant (K-value) and maximum binding capacity (N-value), derived from these isotherms, indicated that upon lowering the pH the K-value increased from 0.7 to 0.8 ml/mg and the N-value from 2,050 to 2,640 micrograms/m2 hydroxyapatite. The pH-dependence of the HWSM-HAP interaction, measured over a large pH-range, at a fixed concentration of HWSM, indicated a two- to three-fold increase in binding upon lowering the pH from 7.5 to 5.5 At pH 5.5 the presence of HWSM induced an increased solubilization of hydroxyapatite, as deduced from the amounts of Ca2+ and phosphate-ions released. Addition of Ca2+ - and Mg2+ -ions (1mM) resulted in a decrease, both of HWSM binding and of the hydroxyapatite solubilization. These effects were more pronounced at the lower pH-values (pH less than 6.0).