Enamel Formation-Biochemical Aspect-

Abstract

This review shows the biochemical aspect of enamel formation focused on porcine amelogenesis.Volumetric changes in the chemical components of secretory stage enamel intimate that enamel crystals thicken as amelogenin, the major protein component, decreases. Solubility behaviors and hydrophobicity analyses of amelogenin derivatives suggest the hypothesis that cylindrical amelogenin forms micelles. This model explains how amelogenin constructs a large structure, which forms a matrix, and continuously yields space as the crystals thicken. In porcine secretory enamel, mineral volume increases with depth, gaining space for thickening the existing crystallites by shrinking amelogenin micelles through the removal of C-terminal and 13-kDa peptides from the prototype 25-kDa amelogenin, the most abundant porcine amelogenin gene product, by the action of enamelysin (MMP-20) and enamel matrix serine proteinase (EMSP1), respectively.During the secretory stage, the crystallites thicken in the deeper secretory enamel, although calcium ions are supplied by ameloblasts. Another morphological feature of the crystallites is that they are all nearly the same size within the same developmental stage. There are several enamel proteins and their cleavage products have calcium binding activity and binding affinity for crystallites. The 25-kDa amelogenin and 89-kDa enamelin, both neutral insoluble, are involved in slow crystallite growth by trapping calcium ions secreted by ameloblasts, and may control by inhibiting newly nucleated crystals between existing crystallites. The calcium binding proteins derived from the C-terminal side of sheathlin (ameloblastin/amelin) and 32-kDa enamelin, belonging to the neutral soluble fraction, act as a calcium carrier to transport calcium ions to deeper secretory stage enamel. Sheath protein derived from the N-terminal side of sheathlin is the main protein constituent of the enamel sheath.Enamelin may be involved in initial crystal formation as it is immuno-histochemically shown to be localized on or around the initial nucleated crystals in a presecretory stage matrix. This is confirmed by no crystal formation in the enamel of enamelin knock-out mice. This conflicts with the inhibition of new nucleation between the existing crystallites in advanced secretory stage enamel. Enamelin may have multi-functions which are involved in the induction of nucleation in a particular phase of the molecule and inhibition in other phases, or change its function with the products produced during each degradation step.