Dentin matrix collagen: Evidence for a covalently linked phosphoprotein attachment

Oxidative degradation of decalcified bovine dentin matrix with alkaline sodium metaperiodate solubilizes a substantial part of the matrix. The soluble portion includes a fraction containing more than 75% of the covalently-bound phosphorus present in the native dentin. Preparative scale electrophoresis of the solubilized dentin on polyacrylamide gels led to the isolation of a group of four strongly anionic components (I-IV). Analyses showed each component to be a protein characterized by a high content of phosphate groups, serine and aspartic acid, and, with one exception (II), small amounts of proline and hydroxyproline. Each component contained one residue of hydroxylysine per mole. In addition to the periodate resistant hydroxylysine, III and IV also contained periodate resistant glucose and galactose. On the basis of the presence of hydroxyproline in varying amount, and hydroxylysine on a mole per mole basis, it is suggested that components I, III and IV consist of collagen backbone fragments linked to a phosphoprotein moiety with a high content of serine and aspartic acid. The periodate-resistant hexoses and the hydroxylysine probably represent the linkage point. Component II is a phosphoprotein moiety essentially free of a collagen fragment. These data provide direct evidence that dentin collagen contains a covalently bound non-collagen polypeptide attachment of highly acidic character.