Possible Roles of Partial Sequences at N- and C-termini of Amelogenin in Protein-Enamel Mineral Interaction

Abstract

The purpose of this study was to assess the functional significance of homologous sequences of mammalian amelogenins at their N- and C-termini. A porcine 5-kDa fragment corresponding to the N-terminal 45 residues of amelogenins was purified from the secretory enamel. The decapeptide TDKTKREEVD corresponding to the C-terminal 10 residues of amelogenins was synthesized according to conventional solid-phase procedures. The inhibitory activity of both moieties on apatite crystal growth was determined in a supersaturated solution having an ionic composition similar to that of the fluid phase separated from porcine secretory enamel. The 5-kDa amelogenin fragment was sparingly soluble in neutral solutions and (in condensed forms because of aggregation) showed no significant inhibition of crystal growth, whereas the fragment molecules pre-adsorbed onto the seed crystals yielded modest inhibition of hydroxyapatite precipitation. However, their inhibitory activity was significantly lower than that of parent porcine amelogenin (25-kDa molecular mass). The high solubility of synthesized decapeptide allowed us to determine the adsorption isotherm onto hydroxyapatite at 37°C, at an ionic strength similar to that of the enamel fluid. The obtained adsorption isotherm was described by a Langmuir model; the adsorption affinity and the maximum adsorption sites were 6.2 mL/μmol and 0.53 μmol/m², respectively. As expected from the low adsorption affinity, the peptide showed a much weaker inhibition of apatite crystal growth than the parent amelogenin. All the foregoing results suggest that the adsorption onto apatite crystals and any inhibition of crystal growth by the amelogenin macromolecules cannot be associated with either partial molecular sequence, but may be determined by the whole molecular structure, including both segments at the N- and C-termini.