Matrix Gla protein binding to hydroxyapatite is dependent on the ionic environment: calcium enhances binding affinity but phosphate and magnesium decrease affinity

Matrix Gla protein (MGP) is an inhibitor of mineralization found in bone, cartilage, developing tissues, smooth muscle, and atherosclerotic plaques. MGP interaction with hydroxyapatite (HA) has been inferred by its function, but has never been measured directly. In this study, the influence of MGP antibody (x-MGP) binding, plasmin digestion, and various ions, including calcium and phosphate, on 125I-labeled MGP-HA binding was examined. Nonlinear regression analysis of MGP binding yielded Ka (association constant; ≈8.0 × 104 M−1) and Bmax (maximum specific bound fraction of MGP; ≈0.53). Anti-MGP antiserum reduced Ka to less than half of control (0.33% x-MGP). Plasmin-digested MGP decreased HA binding parameters by almost a third, showing that protein binding and limited proteolysis greatly affected HA binding. The presence of free calcium ions significantly increased binding in a dose-dependent manner, with 1 mmol/L calcium increasing Ka by a factor of 2. Phosphate ions decreased binding significantly in a dose-dependent fashion, with 1 mmol/L PO4 decreasing Ka by a third. Magnesium at 1 mmol/L decreased Ka significantly by half, but the effect was not dose-dependent. Carbonate, sulfate, and sodium ions had no significant effect on binding. MGP binding to HA is sensitive to protein binding, limited proteolysis, and the surrounding ionic environment.