Inhibition of calcium phosphate precipitation by human salivary statherin: Structure-activity relationships

Previous studies of human statherin showed the active region for inhibition of secondary calcium phosphate precipitation (crystal growth) to reside in the highly charged amino-terminal one-third of this molecule, and the neutral tyrosine-, glutamine- and proline-rich carboxy-terminal two-thirds of the molecule is required for maximal inhibition of primary (spontaneous) precipitation. The purpose of the present study was to define more clearly the activities of these different molecular segments of statherin with respect to the two kinds of inhibitory activities. Peptides from statherin were prepared by specific proteolysis using trypsin, endoproteinase Arg-C, and activated factor X to produce the amino-terminal hexa-, nona- and decapeptides, respectively, and carboxypeptidase-A was used to obtain a peptide extending from residue 1 to about residues 32–37. The peptides were purified by anion exchange and gel filtration chromatography, and characterized and quantified by amino-acid analysis. Serially diluted samples of statherin and derived peptides were assayed to determine the concentrations, giving a standard 50% inhibition of precipitation (C50%) in assay systems designed for this purpose using poly-aspartate as a standard. Results are expressed as (C50% statherin)/(C50% peptide). For inhibition of primary precipitation, these values were peptide(1–6), 0.20; peptide(1–9), 0.15; peptide(1–31/35), 0.24. For inhibition of secondary precipitation, the values were peptide(1–6), 3.8; peptide(1–9), 2.8; peptide(1–10), 1.9; peptide(1–32/37), 1.5. These quantitative findings show that maximum inhibition of primary precipitation by statherin requires the entire molecule. Thus, removal of a relatively small segment of its carboxy-terminal region results in a substantial reduction in inhibitory activity. Inhibition of secondary precipitation is associated only with the amino-terminal portion of the molecule; this activity increased with decreasing molecular size and increasing net negative charge, and is probably related to more efficient adsorption of the inhibitory peptide onto crystal growth sites.