The interaction of proteins with hydroxyapatite. I. Role of protein charge and structure

The criteria for elution of proteins from hydroxyapatite columns were examined as a function of (1) protein isoelectric point (22 proteins with isoelectric points between 3.5 and 11.0); (2) ionic nature of eluant (Na salts of PO4, F-, Cl-, SCN-, ClO-4, and CaCl2); and (3) structural differences between related proteins. It was found that proteins can be classified into three groups: (1) basic proteins, which elute at similar, moderate molarities of PO4, F-, Cl-, SCN-, and ClO-4, and low (less than 0.003 M) Ca2+; (2) acidic proteins which elute at about equal moderate molarities of PO4 and F-, but do not elute with Ca2+ and usually not with Cl-; (3) neutral proteins, which elute with PO4, F-, and Cl-, but show a strong anion specificity, and do not elute with Ca2+ or SCN-. Furthermore, individual specific polar groups are not in general crucial to binding or desorption, and variations in structure, other than major loosening, do not influence strongly the pattern of protein-hydroxyapatite interaction.