The pH dependent amelogenin solubility and its biological significance

Amelogenins are a group of extracellular enamel matrix proteins which are believed to be involved in the regulation of the size and habit of enamel crystals. The aim of this study was to compare the solubility properties of several amelogenins in various pH (4.0-9.0) solutions with an ionic strength (IS) of 0.15 M using the Micro BCA protein assay at 25 degrees C or 37 degrees C. The solubility of the recombinant amelogenin rM179 was lowest (0.7 mg/ml) close to its isoelectric point and it increased below and above this point. The solubility of the recombinant amelogenin rM166 remained almost the same (1-2 mg/ml) as the pH rose from 6.0 to 9.0 and it increased as the solution became more acidic. Synthetic "tyrosine-rich amelogenin polypeptide" (TRAP) was extremely insoluble (<0.2 mg/ml) in the pH range studied while synthetic "leucine-rich amelogenin polypeptide" (LRAP) was readily soluble (>3.3 mg/ml). The native porcine amelogenin with apparent molecular weight 25 kDa shared similar solubility behavior to rM179. The porcine 23 kDa amelogenin was only sparingly soluble (0.3-0.8 mg/ml) over a wide range of pH. Interestingly, the porcine 20 kDa amelogenin was remarkably soluble in the pH range of 4.0 to 6.0 (approximately 12 mg/ml), but the solubility dropped strikingly to only approximately 0.2 mg/ml at pH larger than approximately 7.0. The strong dependence of amelogenin solubility on solution pH may be involved in the regulation of aggregation, enzymatic degradation and the binding properties of amelogenins, thus playing an important role in enamel biomineralization.