Inhibition of hydroxyapatite dissolution by ovalbumin as a function of pH, calcium concentration, protein...

Hydroxyapatite is the main constituent of the dental hard tissues, and in vivo its dissolution in acids leads to the pathological condition of dental erosion. Food proteins which inhibit hydroxyapatite dissolution may find application as erosion-reducing agents in food and drink products. The aim of this study was to investigate the egg protein ovalbumin as a potential inhibitor of hydroxyapatite dissolution in acidic solutions, with conditions representative of dental erosion. The dissolution rate of hydroxyapatite discs was measured in an acidic solution as a function of pH, calcium concentration, ovalbumin concentration and acid type. All experiments were performed in triplicate. 0.2% w/v ovalbumin significantly reduced the dissolution rate in citric acid by 50-75% over the pH range 2.80-4.00, and by 45-60% in solutions with calcium concentrations of up to 20 mM (p < 0.05). The effect was persistent for several rinses after the initial exposure to the protein. 0.02% w/v ovalbumin significantly reduced the dissolution of hydroxyapatite in citric acid by 30-55%. Ovalbumin did not, however, statistically significantly reduce the hydroxyapatite dissolution rate in malic or lactic acids. The effect is ascribed to adsorption and partial, reversible denaturation of ovalbumin on the hydroxyapatite surface. There may be some interaction between ovalbumin and the citrate ion which promotes the adsorption of protein in the presence of citric acid. Ovalbumin shows promise as a potential erosion-reducing additive to citrus-based drinks.