Isolation of phosphophoryn from human dentin organic matrix

Normal human dentin was demineralized in 0.6 N HCl and then extracted in 1.0 M NaCl, 0.5 M Tris/HCl, pH 7.6 in the presence of neutral protease inhibitors. All of the soluble phosphorus-containing proteins were extracted directly in the 0.6 N HCl demineralizing solution; none were collected in the 1.0 M NaCl neutral pH extraction. The principal phosphoprotein was precipitated from solution by 1.0 M CaCl₂ and subjected to further chromatographic purification. This fraction proved to be a typical phosphophoryn with Asp and Ser+PSer, in near equimolar amounts, accounting for ∼75 residue percent of the protein. The second major organic phosphate-containing component was a peptide, M₄ ∼2,000. It was calcium precipitable and its amino acid composition showed a relationship to phosphophoryn. The residual collagenous matrix, which also contained organic phosphate, was digested with CNBr and the phosphate-containing moiety isolated. This had a composition indicative of a complex of collagen and phosphophoryn. Thus, in spite of the reports by Leaver and colleagues that human dentin contains neither soluble nor matrixbound phosphophoryns, these data show that human dentin, like rat, hamster, rabbit, porcine, and bovine dentins, does contain a phosphophoryn as a major noncollagenous protein.