The Influence of Molecular Structure on Apatite Adsorption
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THE INFLUENCE OF MOLECULAR STRUCTURE ON APATITE ADSORPTION
Mats S.A. Johnsson, P.A. Raj, M.J. Levine and G.H. Nancollas, Dept's of Biomaterials and Oral Biology, SUNY at Buffalo. Buffalo NY 14214.
ABSTRACT The hydroxyapatite (HAP) adsorption of salivary statherin, cystatins, proline-rich proteins and histatins has been compared to the influence of these molecules on HAP crystallization in supersaturated solution. This may yield, in many cases, information about protein conformation in the adsorbed state. The results of studies involving both parent molecules and their fragments, indicated that statherin binds to HAP primarily with a 2-5 residues segment in the N-terminal part while the cystatins and proline-rich proteins bind through a segment 23 times larger. INTRODUCTION Serum and saliva are generally supersaturated with respect to the mineral of bones and teeth, hydroxyapatite (HAP). The degree of calcium phosphate supersaturation is normally higher in saliva than in serum allowing for a more efficient protection against variations in pH and concentration. At neutral pH, saliva is supersaturated with respect to hydroxyapatite (HAP), octacalcium phosphate (OCP) and, in the vicinity of carious lesions, often also with respect to dicalcium phosphate dihydrate (DCPD) [1]. Due to the high concentration of calcium and phosphate ions, an efficient inhibitor system is needed to protect the oral tissues from extensive precipitation. Several groups of proteins in saliva are efficient inhibitors of mineral dissolution or crystallization and binds to calcium phosphate minerals with high affinity [2-5]. A common feature among these proteins is the presence of domains with several phosphoserine, aspartic acid and glutamic acid residues. The localization of such groups in close vicinity has led to the suggestion that adsorption occurs by cooperative binding to specific sites at the crystal surface [6,7]. The salivary proteins found to have the highest affinity for HAP are statherin and the proline-rich proteins (PRPs), (Table 1) [8,9]. Both groups of molecules contain phosphoserine residues and have most of the acidic residues localized in the N-terminal part with the remainder of the molecule containing mostly uncharged groups [10,11]. All studies indicate that the
Mat. Res. Soc. Symp. Proc. Vol. 252. ©1992 Materials Research Society
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charged domains are the active binding segments to the HAP surface [8,9,12,13]. In this work, the uptake of salivary proteins by HAP has been compared to that of pyrophosphate in order to investigate the influence of size, charge and position of charges on the ability to bind to HAP. The studies have been performed by comparing the HAP adsorption and the growth inhibitory ability using HAP seed crystals in supersaturated solution. MATERIALS AND METHODS. All reagent were of analytical grade. The adsorption experiments were performed by equilibrating HAP crystallites, 2 1 having surface areas of 29.2+0.5 m g [14], with a solution of protein saturated with respect to HAP at an ionic strengt
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