A study of bone proteins which can prevent hydroxyapatite formation

The ability of noncollagenous bone proteins (NCBP) to inhibit calcium phosphate precipitation in vitro raises the question as to the nature and the relative efficiency of such proteins in vivo. To investigate this question NCBP from young adult sheep bones were fractionated using nondegradative techniques. Their relative activity was measured by their efficiency in preventing hydroxyapatite growth. On a weight (of protein recovered) basis, the activity is about equally divided between the Gla-containing protein, osteocalcin, and a phosphorylated protein essentially the same as osteonectin. On a molecular weight basis, the activity of the phosphorylated protein is almost three times higher than that of the Gla-containing protein. Finally, the inhibitory activity of the phosphorylated protein is alkaline phosphatase sensitive. The properties of this protein could provide a means of regulating the solubility of bone mineral and maintaining an equilibrium of calcium between bone and blood.