Adsorption of Lysozyme onto Various Synthetic Hydroxyapatites

Abstract

The adsorption of lysozyme (LSZ) onto various kinds of synthetic hydroxyapatite (X10(PO4)6(OH)2, denoted as CaHAP, SrHAP, and CaSrHAP forX= Ca, Sr, and Ca + Sr, respectively) particles was investigated at 15°C in a 1 × 10−4mol dm−3KCl solution of pH 6.0. The adsorption isotherms of LSZ onto these HAPs exhibited the pseudo-Langmuir type as well as those of bovine serum albumin (BSA). The negative electrophoretic mobility of the particles was reversed to a positive one by adsorption of positively charged LSZ molecules. No remarkable relationship between saturated amounts of adsorbed LSZ (ns) and the cation/P molar ratio of the particles was observed on CaHAP systems. In the case of SrHAP and CaSrHAP, however,nsapproached zero at a cation/P molar ratio of ca. 1.70. Thensvalues of LSZ on CaHAP, SrHAP, and CaSrHAP particles are appreciably smaller than those of BSA. This study provides the first conclusive evidence that LSZ molecules specifically adsorb onto phosphate ions onacorbcfaces and/or on negatively charged sites formed by six oxygen atoms of three phosphate ions onabcrystal faces (Psites) on the HAP surface by using basic amino groups, though the low probability of the latter mode was insinuated.