Adsorption of two human parotid salivary macromolecules on hydroxy-, fluorhydroxy-and fluorapatites

Abstract

The adsorption isotherms (37 °C) for a tyrosine-rich peptide (statherin, mol. wt 5380) and a proline-rich protein (PRP-3, mol. wt 11,320), both isolated from human parotid saliva, were obtained by using hydroxyapatite, fluorapatite and two fluorhydroxyapatites containing 0.49 and 1.55 per cent fluoride, respectively, as adsorbents. An adsorption model based on the Langmuir isotherm adequately fitted the experimental data. The adsorption affinity of PRP-3 for the adsorbents tested was considerably higher than the affinity of statherin for these adsorbents. The reverse was true for the maximum number of adsorption sites. The adsorption affinities increased substantially with increased fluoride content in the crystalline lattice of the apatites, whereas only modest increases in the maximum number of sites were obtained. This behaviour is ascribed to a reduction in the surface free energy of the adsorbent with a concomitant decrease of the interaction with water and, as a consequence, an enhanced adsorption bond (presumably electrostatic) between the macromolecule and the adsorbent. As the relative increases in the adsorption affinities with increasing fluoride content of the adsorbents for the two macromolecules were markedly different, it is hypothesized that the composition of the acquired tooth pellicle may be different in fluoridated and non-fluoridated areas. This difference may be of significance in relation to the cariostatic effect of fluoride.