Inhibition of calcium phosphate precipitation by human salivary acidic proline-rich proteins: Structure-activity relationships

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Abstract
Absence of precipitation of calcium phosphate salts onto tooth surfaces from human saliva, which is supersaturated with respect to calcium phosphate salts, has been attributed in part to the presence in the salivary secretions of a group of acidic proline-rich phosphoproteins (PRP). These macromolecules are considered to act by adsorbing onto dental enamel where they inhibit surface-induced precipitation of calcium phosphate salts. The inhibitory activity is known to be associated primarily with the amino-terminal region of the PRP. The aim of this study was to determine the features of the primary structure of this molecular segment responsible for inhibitory activity. The 30-residue, amino-terminal segment of PRP-3, which contains the two phosphoserines and 11 of the 13 carboxyl groups present in PRP-3, was obtained by tryptic digestion. This peptide, designated PRP-3(Tl), was treated with thermolysin to give the monophosphopeptides, Val-PSer-Gln-Glu-Asp-Val-Pro and Leu-Val-Ile-Ser-Asp-Gly-Gly-Asp-PSer-Glu-Gln, and with alkaline phosphatase to give the dephosphorylated analog, PRP-3(Tl)DP. The inhibitory activities of PRP-3(Tl) and the derived peptides, a synthetic peptide, phosphoseryl-phosphoserine (PSer-PSer), and O-phosphoserine (PSer), were determined using an assay based on inhibition of seeded precipitation of calcium phosphate. Inhibitory activities, expressed as concentrations of inhibitors required to give standard inhibitory activities, were PRP-3(Tl), 0.59 μM; PSer-PSer, 3.5 μM; the two monophosphopeptides, 29 and 32.5 μM; PRP-3(Tl)DP, 56 μM; PSer, 329 μM. These results show the importance of the simultaneous presence of two phosphoserines in these precipitation inhibitors, and the contributions made by the carboxylic groups, but they also clearly show that a major part of the inhibitory activity is related to the structure of the intact molecule.