A soluble non-collagenous phosphoprotein has been isolated from the EDTA extracts of unerupted bovine dentin. The phosphoprotein was purified by a combination of molecular seive and ion-exchange chromatography and was homogenous by the criteria of acrylamide gel disc electrophoresis at pH 8.6 and analytical ultracentrifugation in 0.5 M KCl. The phosphoprotein is present as approximately 0.024% of the total weight of undecalcified dried teeth. The phosphoprotein is unusual in that it has an A260 nm/A280 nm ratio greater than 1. From the ratio of sedimentation and diffusion coefficients an approximate molecular weight of 39 000 was determined. The phosphoprotein is a conjugated protein, with the protein moiety accounting for only approx. 50% of the total. A striking feature of the protein portion is its high content of aspartic acid, serine and serine phosphate. These three residues account for approximately 75% of the protein moiety. Another interesting constituent is hydroxylysine, present as approx. 10 residues per 1000 amino acid residues, while no hydroxyproline is present. The phosphoprotein is also of interest because of the apparent identity of its protein moiety with that previously found covalently attached to dentin collagen.
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