title: Protein adsorption characteristics of calcium hydroxyapatites modified with pyrophosphoric acids.
abstract: Protein adsorption characteristics of calcium hydroxyapatite (Hap) modified with pyrophosphoric acids (PP(a)) were examined. The PP(a) modified Hap particles (abbreviated as PP-Hap) possessed anchored polyphosphate (PP: P-{O-PO(OH)}(n)-OH) branches on their surfaces. The proteins of bovine serum albumin (BSA: isoelectric point (iep)=4.7, molecular mass (M(s))=67,200 Da, acidic protein), myoglobin (MGB: iep=7.0, M(s)=17,800 Da, neutral protein), and lysozyme (LSZ: iep=11.1, M(s)=14,600 Da, basic protein) were examined. The zeta potential (zp) of PP-Hap particles as a function of pH overlapped; zp-pH curves were independent of the concentration of pyrophosphoric acids (abbreviated as [PP(a)]) used for modifying Hap surface. The saturated amounts of adsorbed BSA (Delta n(ads)(BSA)) were increased three-fold by the surface modification with PP(a) though they were independent of the [PP(a)]. Furthermore, the fraction of BSA desorption was independent of the [PP(a)]. This enhancement of BSA adsorption onto the PP-Hap is due to the hydrogen bonding between oxygen and OH groups of the PP-branches and functional groups of BSA molecules. In the case of LSZ, a more higher adsorption enhancement was observed; the saturated amount of adsorbed LSZ (Delta n(ads)(LSZ)) for Hap modified at [PP(a)]=6 mmol/dm(3) was nine-fold than that for Hap unmodified. This remarkable adsorption enhancement was explained by a three-dimensional binding mechanism; LSZ molecules were trapped inside of the PP-branches. Hence, a fraction of LSZ desorption was decreased with an increase in the [PP(a)]; as more PP-branches are presented on the surface the higher retardation of LSZ desorption was induced. It was expected from their small size that MGB adsorb between the PP-branches as well as LSZ. However, the amounts of adsorbed MGB (Delta n(ads)(MGB)) did not vary and were independent of the [PP(a)] due to the small numbers of functional groups of MGB. In addition, no dependence of the fraction of MGB desorption on the [PP(a)] was observed. The results of zp for all the protein systems supported the mode of protein adsorption discussed. The anchored structure of the PP-branches developed on the Hap surface to provide three-dimensional protein adsorption spaces was proved by a comparative experiment that was elucidating the effect of pyrophosphate ions for BSA adsorption onto Hap.
authors.affiliation: School of Chemistry, Osaka University of Education, 4-698-1 Asahigaoka, Kashiwara, 582-8582 Osaka, Japan. kandori@c.osaka-kyoiku.ac.jp
authors.forename: Kazuhiko
authors.initials: K
authors.lastname: Kandori
authors.forename: Shintaro
authors.initials: S
authors.lastname: Tsuyama
authors.forename: Hidekazu
authors.initials: H
authors.lastname: Tanaka
authors.forename: Tatsuo
authors.initials: T
authors.lastname: Ishikawa
mesh_terms: D001665:Binding Sites; D002851:Chromatography, High Pressure Liquid / Q000295:instrumentation; D017886:Durapatite / Q000737:chemistry*; D010756:Phosphoric Acids / Q000737:chemistry*; D011506:Proteins / Q000493:pharmacokinetics*
chemical_list: D010756:Phosphoric Acids; D011506:Proteins; D017886:Durapatite