The Phosphoprotein of the Dentin Matrix

Purified, decalcified bovine dentin matrix contains a small amount of bound phosphorus but otherwise appears to have a composition typical of mammalian collagens. The bound phosphorus cannot be removed by nonhydrolytic or other nondegradative extractions. Prolonged oxidative degradation of the matrix with sodium metaperiodate resulted in the solubilization of about 30% of the matrix and this portion contained more than 75% of the phosphorus. Fractionation of the solubilized dentin led to the isolation of a rapidly moving, electrophoretically homogeneous anionic component (designated F component) at pH 5.3. Equilibrium ultracentrifuge studies showed the F component to be homogeneous with respect to molecular weight and to have a molecular weight of 38,0C0 ± 3000. Analyses showed F to be a protein, containing unusually large amounts of serine and aspartic acid plus smaller amounts of proline, hydroxyproline, and one residue of hydroxylysine per molecule. F contained 5.9% P by weight of 34 phosphate groups/molecule. Since hydroxylysine is destroyed under the conditions of periodate degradation unless either the hydroxyl or e-amino group is blocked, and since the F component contained almost all of the hydroxylysine of the dentin which survived the periodate degradation it was concluded that the F component phosphoprotein is attached to the collagen matrix via the hydroxylysine side chain. The phosphoprotein comprises less than 2 % of the total matrix on a weight basis or less than one molecule per four molecules of collagen. This highly anionic phosphoprotein, bound to the collagen matrix, may provide the sites for the epitactic nucleation of mineralization of the matrix.