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 approximately 75 residue percent of the protein. The second major organic phosphate-containing component was a peptide, Mr approximately 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 matrix-bound phosphophoryns, these data show that human dentin, like rat, hamster, rabbit, porcine, and bovine dentins, does contain a phosphophoryn as a major noncollagenous protein.