Proteomic analysis of enamel matrix using a two-dimensional protein fractionation system

Our objectives in this study were to perform separate proteomic analyses of porcine soft and hard enamel matrices, using the ProteomeLab PF-2D System, to compare the contents of the hard and soft enamel and to identify matrix constituents that are absent from the early maturation stage. Developing first permanent molars were dissected from 6-month-old pigs. Both immature and mature enamel samples were obtained by scraping the secretory-stage (soft) and maturation-stage (hard) enamel, respectively. Enamel matrix samples were sequentially extracted and fractionated with 50 mM phosphate buffer (pH 7.4) and then with 50 mM carbonate buffer (pH 10.8). The neutral enamel extract was separated into four fractions by successive ammonium sulfate precipitations. The alkaline enamel extract was separated into four fractions by ion-exchange chromatography. These eight extracts from both the soft and hard enamel were injected for chromatofocusing. Soft enamel fractions containing constituents absent from the hard enamel were further separated by reverse-phase high-performance liquid chromatography. The major soft enamel constituents absent from the hard enamel were acidic glycoproteins, corresponding to the 32-kDa enamelin, and the 29-, 27-, 15-, 13-, 8- and 6-kDa C-terminal fragments of ameloblastin. Loss of these glycoproteins is associated with a post-transition increase in enamel mineralization.