Osteocalcin-Controlled Dissolution−Reprecipitation of Calcium Phosphate under Biomimetic Conditions

We have examined the influence of osteocalcin, one of the 10 most abundant proteins of the human body, on hydroxyapatite (HAP, Ca 10 (PO 4 ) 6 (OH) 2 ) formation. Different functions in biomineralization are attributed to the bone-specific protein osteocalcin because of its Ca 2+ binding including HAP binding properties and its capability to inhibit HAP precipitation. To study nucleation and crystal growth, a model system with osteocalcin-controlled dissolution−reprecipitation of brushite (DCPD, CaHPO 4 ·2H 2 O) to HAP has been investigated. After DCPD crystals were grown from aqueous solution, they were exposed to an osteocalcin-containing buffer solution of pH 7.4. Thin apatite-like crystals with hexagonal symmetry grew on the (010) brushite planes. The apatite (0001) planes were fully covered with osteocalcin molecules. Thus, osteocalcin has been found to regulate HAP formation in two different ways: (i) it accelerates nucleation, and (ii) it acts as a specific inhibitor of the apatite (0001) plane, supressing crystal growth perpendicular to this plane. A stress-induced growth model was developed illustrating HAP growth along the brushite−HAP interface considering compressions in the protein-covered HAP crystals.