A study of bone proteins which can prevent hydroxyapatite formation

Area of Science:

• Biochemistry
• Bone Biology
• Mineral Metabolism

Background:

• Noncollagenous bone proteins (NCBP) are known to inhibit calcium phosphate precipitation in vitro.
• The in vivo role and relative efficiency of these proteins in regulating bone mineral solubility remain to be fully elucidated.

Purpose of the Study:

• To investigate the in vivo activity and relative efficiency of different NCBP in inhibiting hydroxyapatite growth.
• To characterize the properties of bone proteins involved in calcium phosphate precipitation regulation.

Main Methods:

• Fractionation of NCBP from young adult sheep bones using non-degradative techniques.
• Measurement of relative inhibitory activity by assessing prevention of hydroxyapatite growth.
• Analysis of protein activity on a weight and molecular weight basis.
• Assessment of alkaline phosphatase sensitivity of inhibitory activity.

Main Results:

• On a weight basis, osteocalcin and a phosphorylated protein (similar to osteonectin) showed equal inhibitory activity.
• On a molecular weight basis, the phosphorylated protein exhibited nearly three times the activity of osteocalcin.
• The inhibitory activity of the phosphorylated protein was sensitive to alkaline phosphatase.

Conclusions:

• Both osteocalcin and osteonectin play significant roles in regulating bone mineral solubility.
• Osteonectin is a more potent inhibitor of hydroxyapatite growth on a per-molecule basis.
• The properties of osteonectin suggest a key role in maintaining calcium equilibrium between bone and blood.