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 (125)I-labeled MGP-HA binding was examined. Nonlinear regression analysis of MGP binding yielded K(a) (association constant; approximately 8.0 x 10(4) M(-1)) and B(max) (maximum specific bound fraction of MGP; approximately 0.53). Anti-MGP antiserum reduced K(a) 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 approximately 1 mmol/L calcium increasing K(a) by a factor of 2. Phosphate ions decreased binding significantly in a dose-dependent fashion, with approximately 1 mmol/L PO(4) decreasing K(a) by a third. Magnesium at approximately 1 mmol/L decreased K(a) 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.