Influence of porosity of HAp on protective effect of StN21

Adsorbed salivary proteins maintain enamel homeostasis by forming pellicle. Enamel surfaces are stabilised in the proximity of adsorbed proteins. This delays formation of demineralisation pits, reducing dissolution during acid attack. Enamel surface area and surface porosity are significant in controlling the adsorption of these proteins. Objectives: The overall aim is to understand the interactions of salivary proteins with enamel surfaces to develop novel therapeutic agents to reduce caries and erosion. Hydroxyapatite (HAp) is used in in vitro studies as a model because unlike enamel it does not have variability in structure, chemical composition, and/or porosity. The aim was to compare the effect of StN21 (a peptide containing the N-terminal 21 residues of statherin) on reducing demineralisation of HAp pellets with two known porosities. Methods: StN21 was produced using Fmoc synthesis. HAp pellets (5×5×2) mm of 12.5 and 17% porosity varnished leaving one surface exposed. Protein biofilms were created on each pellet using solutions containing StN21 at concentrations 0.094-1.88 × 10^-4 mol l^-1 in buffer for 24 h. Controls were HAp pellets immersed in buffer only. Demineralising solution (0.1 mol l^-1 acetic acid, pH 4.5, 1.0 mmol l^-1 calcium and 0.6 mmol l^-1 phosphate) was circulated past the HAp pellets at 0.4 ml min^-1. Scanning microradiography was used to measure the rate of demineralisation (RD_HAp) for 3 weeks. Results: The 12.5 % porous HAp pellets showed a ~35% decrease in RD_HAp for all StN21 concentrations used. The 17% porous pellets with 0.094-1.13 × 10^-4 mol l^-1 of StN21 showed a ~22% decrease in RD_HAp. Both HAp pellets showed ~35% decrease in RD_HAp for 1.88 × 10^-4 mol l^-1 of StN21. Conclusions: The influence of StN21 on reducing RD_HAp is dependant on HAp pellet surface porosity and surface area. This confirms that StN21 decreases RD_HAp by delaying formation of demineralisation pits.