Radioautographic Visualization of the Deposition of a Phosphoprotein at the Mineralization Front in the Dentin of the Rat Incisor

A peptide that is rich in organically bound phosphorus and contains abundant serine residues has been identified in the dentin of man (1), fetal bovine (2, 3), and rat (4). This phosphoprotein may play a role in mineralization (5-9). Butler et al. (4) reported that the phosphoprotein of rat incisor dentin constituted 10.8% of the proteinaceous material recovered from decalcified incisor teeth while collagen comprised 84%. Since the phosphorus content of the phosphoprotein was estimated at 3.29% and that of collagen at 0.19% (4), much of the phosphorus taken up in organic form by the incisor would likely be present as phosphoprotein. With this in mind, it was decided to inject [~3P]phosphate into rats and examine the demineralized incisor teeth by radioautography in the hope of tracing phosphoprotein formation. The organic phosphorus of dentin phosphoprotein is believed to be attached to serine residues (6). In the rat incisor dentin, this amino acid comprises 35 residues per cent of the phosphoprotein and only four residues per cent of the cyanogen bromide peptides of collagen (4). Hence serine also appeared to be a suitable amino acid precursor to trace phosphoprotein formation by radioautography. Finally, the radioautographic pattern of the deposition of labeled phosphorus and serine was compared to that of [3H]proline. Proline may be used as a precursor to trace collagen, since it makes up 22.0 residues per cent of dentin collagen and only 2.4 residues per cent of the phosphoprotein (4). The results indicated that the pattern of phosphoprotein deposition into the dentin matrix is strikingly different from that of collagen.