Adsorption of catalase on hydroxyapatite

The adsorption of catalase on calcium hydroxyapatite is reported in this st udy. In all the solutions investigated, catalase adsorption takes place und er conditions where the adsorbent and the adorbate are both negatively char ged and is accompanied by the release of phosphate ions. This suggests that electrostatic attraction does not play a leading role in the adsorption pr ocess. However, electrostatic repulsion does play a role as evidenced by th e influence of the solution composition on the maximum amount adsorbed. In fact, the amount adsorbed increases when the surface charge tends to be les s negative (decrease of pH and phosphate concentration, presence of calcium ) or when the electrical charges are screened as a result of the ionic stre ngth increase. This was confirmed by the electrophoretic mobility which bec omes more negative after protein adsorption. The rate of catalase adsorptio n is extremely low; the time required to reach a stationary concentration i s 60 and 125 h in potassium nitrate and phosphate solution, respectively. T he reversibility of catalase was examined with respect to changes in the bu lk solution concentration. No significant desorption was obtained after sev eral days of observation. The adsorption, desorption, and kinetic aspects h ave been discussed in relation to changes in the the protein structure upon interaction and to the microstructural characteristics of the mineral. (C) 1998 Academic Press.