Previous studies have shown that a peptide containing the N-terminal 21 residues of the salivary protein statherin (StN21) reduces the rate of demineralisation of enamel and permeable hydroxyapatite (HAp) under demineralising conditions by 45% and 20% respectively. The active HAp binding domain of StN21 is considered to be the negative charge cluster of amino acids at the N terminus. In human statherin the serines in this domain are phosphorylated. The aim was to investigate if phosphorylation of these serine residues is required for the ability of StN21 to reduce demineralisation.
Methods:
StN21 with and without phosphorylated serine residues was prepared by FMOC synthesis. One group of permeable HAp pellets were treated with StN21 with phosphorylated serines, another group with StN21 with unphosphorylated serines, both at concentrations of 1mg/ml in phosphate buffer, for 24hrs. The control group was HAp pellets treated with phosphate buffer only. All the pellets were then exposed to artificial demineralising solutions at pH=4.0 for 1-2 weeks, and the rate of mineral loss measured throughout using scanning microradiography (SMR).
Results:
SMR showed that both StN21 with phosphorylated serines and unphosphorylated serines reduced the rate of demineralisation of the HAp pellets under the demineralisation conditions used, compared to the untreated control HAp pellets.
Conclusion:
This suggests that phosphorylation of the serines may not be crucial for the action of StN21 in reducing the rate of demineralisation of HAp pellets under demineralisation conditions. The protective effect of StN21 may be related to the overall charge on the peptide at its N terminus.