Effect of pH on protein adsorption to hydroxyapatite and tricalcium phosphate ceramics

Calcium phosphate ceramics used for clinical applications vary considerably in their phase composition. A range of hydroxyapatite and tricalcium phosphate powders, some of which are used in plasma-sprayed coatings for metal prostheses, were found to leach substances which change the pH of water and cell culture medium. The adsorption of serum proteins from cell culture medium to the same hydroxyapatite and tricalcium phosphate ceramics was compared by sodium dodecyl sulphate-polyacrylamide gel electrophoresis and individual bands identified by enhanced chemiluminescence immunodetection and gas chromatography sequencing. The iron transport protein transferrin showed distinct differences in adsorption to a variety of hydroxyapatite and tricalcium phosphate powders, with higher concentrations of transferrin adsorbed to powders which caused no change or a drop in the pH of cell culture medium or water. Transferrin was also adsorbed to Biotal HA 120 in greater concentrations at a lower pH than it was at a higher pH. These preliminary studies show that pH changes brought about by the leaching of soluble ions may have an effect on protein adsorption.