Mechanistic Understanding of the Stage-specific Accumulation of Magnesium Ions in Developing Enamel: Simulation of Coupling Events at the Crystal/Solution Interface in an in vitro Precipitation Model

The present study investigated the mechanism of stage-specific Mg incorporation during enamel formation. To this end, a series of in vitro experiments were conducted using a precipitation model with a dialysis chamber. Synthetic hydroxyapatite and pig enamel proteins were used as seeds and regulators for precipitation, respectively. The results showed that the accumulation of Mg ion on and into apatite crystals was highly dependent on solution composition (Ca concentrations and the supersaturation levels) and protein coverages of the crystals. Fluoride at low concentrations efficiently promoted Mg intake into apatite crystals due to the accelerating effect on precipitation kinetics. Trypsin, used as a prototype of enamel resident proteases, also provided an increasing effect on Mg intake due to degradation and removal of proteins associated with crystals. From the data obtained, it is evident that stage-specific Mg incorporation during enamel formation is related to the multiple kinetic and thermodynamic aspects characteristic for enamel mineralization, namely, fluid composition and the competitive adsorption between Ca and Mg ions onto apatite surface, and accumulation of proteinaceous inhibitors of crystal growth and their removal after enzymatic degradation.