Type I collagen shows a specific binding affinity for bovine dentin phosphophoryn.

Bovine dentin phosphophoryn was iodinated with 125I, then tested for binding to native monomeric collagen, to collagen fibrils, and to gelatin. The phosphophoryn was found to bind reversibly, but specifically, to both collagen monomers and fibrils, but not to denatured collagen (gelatin). Competitive binding studies showed that bovine serum albumin, fibronectin, and bovine bone 34K glycoprotein (osteonectin) did not compete with phosphophoryn and did not inhibit its binding to collagen fibrils. Phosvitin, a phosphoserine-rich protein, did compete, but sixfold higher concentrations of phosvitin than of unlabeled phosphophoryn were required to reduce iodinated phosphophoryn binding to the same extent. Quantitative analyses of the binding showed binding to be limited to the fibril surfaces. Bound phosphophoryn enhanced the uptake of 45Ca onto collagen fiber surfaces. These data support the hypothesis that, in dentin, the phosphophoryn plays an important role in localizing the calcium binding leading to the growth of collagen-oriented calcium hydroxyapatite crystals.