Inhibition of apatite formation by vitronectin

This study investigated the concentration-dependent effect of vitronectin (VN), a glycoprotein of the bone matrix, on apatite formation and growth. Precipitation trials in metastable solution and in a pH-controlled solution system showed an inhibition of apatite microcrystal formation by VN. In the presence of biphasic calcium-phosphate ceramic, transmission electron microscopy showed a reduction of precipitated microcrystal size: precipitates were significantly smaller than in ionic simulated body fluid without proteins or in the presence of type I collagen as a negative control. Moreover, the size of the precipitated microcrystals was reduced in a dose-dependent manner. Two indirect methods showed that calcium-phosphate precipitation was inhibited by VN. It would appear that VN prevents apatite formation by inhibiting the growth of apatite crystals rather than by secondary nucleation, as in the case of osteopontin, a bone-specific protein.