Inhibition of hydroxyapatite-crystal growth by bone-specific and other calcium-binding proteins

Mineralization of bone matrix may be influenced by the presence of specific, noncollagenous bone proteins. The quantitative influence of two bone-specific proteins--bone gamma-carboxyglutamic acid (Gla) protein and osteonectin--and other proteins that decreased the rate of crystal growth was measured by adding seed crystals of hydroxyapatite to a solution of CaCl2 and KH2PO4, pH 7.4 at 37 degrees C. The molar concentrations of proteins needed to inhibit the rate of crystal growth by 50% were as follows: osteonectin, 0.15 microM; bone Gla protein, 0.8 microM; prothrombin, 0.9 microM; prothrombin fragment 1, 1.0 microM; soybean trypsin inhibitor, 3 microM; prethrombin 1, 9 microM; cytochrome c, 30 microM. Calmodulin and parvalbumin were found to be less active than prothrombin fragment 1 and had no activity in the micromolar range. The combination of two inhibitors resulted in a mixture with an inhibitory activity that was the sum of the two inhibitors. Decarboxylation of bone Gla protein significantly reduced its inhibitory activity. These results indicate that the inhibitory activity of a protein does not correlate with Ca2+-binding affinity under these conditions, that the mixture of inhibitors has an additive effect, and that gamma-carboxyglutamic acid residues enhance the ability of a protein to inhibit hydroxyapatite-seeded crystal growth.