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 alpha- and beta-phosphophoryns were extracted under conditions minimizing degradation. The tooth residue was CNBr-treated and the collagen CNBr peptides alpha 1(I)CB7 and alpha 1(I)CB8 were collected along with the solubilized conjugate fraction. Each component was purified and the specific activities in [3H] proline, [3H]hydroxyproline, [3H]serine, and [3H]phosphoserine were determined. The collagen and alpha-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 approximately 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.