Calcium Binding in Enamel Fluid and Driving Force for Enamel Mineralization in the Secretory Stage of Amelogenesis

Abstract

A large portion (about 85%) of the calcium present in the fluid of porcine enamel in the secretory stage of amelogenesis appears to be in a non-ionic form. Calcium is probably bound in the fluid by peptides derived from the enzymatic degradation of the amelogenins present in the matrix or by other ligands of an as-yet-undetermined nature. The apparent dissociation constant of the hypothesized complex was determined by titration of a small aliquot of fluid (50 μL) with 10 mmol/L CaCl₂ solution, adding 2 μL at a time. The initial composition of the fluid was determined analytically. The ionic calcium activity, (Ca ²⁺), and the pH value along the titration were determined by a specific ion and a glass electrode, respectively, with the same reference micro-electrode. The concentration of bound Ca, [B], was related to the (Ca²⁺) by the equation [B] = N(Ca²⁺)/ {K + (Ca²⁺)}, in which K is the dissociation constant of the Ca-ligand complex and N is the number of binding sites per liter of fluid. The values of K and N were obtained by a non-linear least-squares procedure. The value of K, 7.47 × 10^-5 mol/L, indicates that the binding bond is rather weak, a feature that seems advantageous if complexed calcium acts as a reservoir and/or fine control of calcium levels in the fluid during enamel formation. The effects that the total concentration of calcium, [Ca]_T, the total concentration of P, the concentration of ligand, and the pH value have on the degree of saturation (DS) were investigated by simulation models. It is concluded that the factor that most affects the DS is [Ca]T. Some physicochemical aspects of amelogenesis are discussed on the basis of present knowledge and the reported results.