Desorption properties of BSA and LSZ on carbonate ion-modified apatite

Desorption behavior of two proteins; bovine serum albumin (BSA) and egg white lysozyme (LSZ) was followed on hydroxyapatite (HAp) after 72 h-adsorption in a static system, focusing modification effect of carbonate ion on the HAp surface. Desorbed amounts in a 5 mM - phosphate buffer solution (PBS) showed maxima in 2 h for BSA and 1 h for LSZ. The ratio of the maximal desorbed amount to the initially adsorbed one: the desorption rate was chosen as an index of the binding strength of protein to the surface. For LSZ, the desorption rate decreased as increase of the carbonate content, indicating stronger binding. This was ascribed to the increase of the strongly-adsorption site of LSZ: the P site of the c face because the increase of the carbonate in HAp inhibits crystal growth along the c axis and hence increases the c face. On the other hand, any clear dependency was not found for BSA. However, four samples of the various carbonate contents were divided into two groups; one with the higher desorption rate with nearly full and more than full surface coverages and the other with the lower one of 0.42-0.60, assuming to be side-on (multiple-site) adsorption. This suggested that more numbers of weaker binding sites were involved in BSA adsorption on HAp with the higher desorption rate than the lower one.