Binding of calcium and phosphate ions to dentin phosphophoryn

The concomitant binding of calcium and inorganic phosphate ions by the highly phosphorylated rat dentin phosphophoryn (HP) was measured in the pH range of 7.4-8.5 using an ultrafiltration procedure. HP binds almost exclusively the triply charged PO4(3-)ion, and for each PO4(3-)ion bound, the protein binds about 1.5 additional Ca2+ ions. The protein-mineral ion complex can be described as a protein with two different ligands, Ca2+ ions and calcium phosphate clusters having a stoichiometry of about Ca1.5PO4. The stoichiometry of the bound clusters is similar to amorphous calcium phosphate, indicating that the protein does not sequester crystal embryos of octacalcium phosphate or hydroxyapatite. The protein-mineral ion complex is amorphous by electron diffraction analysis and does not catalyze the formation of a crystalline phase when aged in contact with its solution.