NUCLEATION OF HYDROXYAPATITE BY BONE SIALOPROTEIN

Bone sialoprotein (BSP) and osteopontin, the major phosphorylated prot eins of mammalian bone, have been proposed to function in the initiati on of mineralization. To test this hypothesis, the effects of BSP and osteopontin on hydroxyapatite crystal formation were determined by usi ng a steady-state agarose gel system. At low calcium phosphate concent rations, no accumulation of calcium and phosphate occurred in control gels or gels containing osteopontin. Gels containing BSP at 1-5 mug/ml , however, exhibited a visible precipitation band and significantly el evated Ca + PO4 contents. By powder x-ray diffraction, the precipitate formed in the presence of BSP was shown to be hydroxyapatite. These f indings suggest that bone sialoprotein may be involved in the nucleati on of hydroxyapatite at the mineralization front of bone.