ACIDIC AMINO ACID-RICH SEQUENCES AS BINDING-SITES OF OSTEONECTIN TO HYDROXYAPATITE CRYSTALS

Osteonectin, an acidic noncollagenous protein of bone and dentin, has affinity to hydroxyapatite crystals. Binding sites to hydroxyapatite o f this protein were determined by a proteolytic experiment and an in v itro binding experiment using synthetic peptide analogues. Osteonectin was adsorbed on hydroxyapatite crystals and digested with trypsin. A peptide was left adsorbed on the crystal even after the digestion. The peptide was identified as an amino terminal peptide containing glutam ic acid-rich sequences, which have been assumed to be possible hydroxy apatite-binding sites. Poly glutamic acid sequences were synthesized a s models of the binding sites. Glu(6) peptide was bound to the hydroxy apatite with a dissociation constant of 2.4 mu M. Peptides containing fewer glutamic acids had lower affinity to the crystal. Effects of the se peptides on in vitro mineralization were examined by a gel system i n microtiter plates. The Glu(6) peptide had a positive effect on the m ineralization in this system, whereas Asp(6) peptide had a negative ef fect. These effects indicate the presence of an interaction between th ese peptides and mineral crystals.