The dynamics of formation of a collagen-phosphophoryn conjugate in relation to the passage of the mineralization front in rat incisor dentin

Dentin and predentin matrices contain Type I collagen and phosphophoryns as major constituents. A collagen-phosphophoryn conjugate is also present in small amounts. This conjugate has been implicated in the deposition of mineral. Its formation has been followed in rat incisors. Rats were labeled for varied time intervals with [³H]proline, followed by a 2-h pulse of [³H] serine. The soluble α- and β-phosphophoryns were extracted under conditions minimizing degradation. The tooth residue was CNBr-treated and the collagen CNBr peptides α1(I)CB7 and α1(I)CB8 were collected along with the solubilized conjugate fraction. Each component was purified and the specific activities in [³H]phosphoserine were determined. The collagen and α-phosphophoryn accumulated proline label linearly at the same rate over the entire period of labeling. Entry of [³H]proline into the conjugate fraction was delayed by ~9-10 h and then the label accumulated also linearly at the same rate. [³H]Serine was present at a different but constant level in each fraction; the conjugate had the lowest activity. These data indicate an extracellular formation of the conjugate at the mineralization front from precursors which followed different secretory pathways.