Influence of synthetic salivary cystatin SN segments on hydroxyapatite mineralization

Three segments of cystatin SN (CsnSN) have been synthesized which contain possible hydroxyapatite (HAP) binding sites. The influence of the fragments on HAP crystal growth has been studied using the constant composition (CC) method in solutions with a relative supersaturation with respect to HAP (σ(HAP)) of 3.6, pH 7.4, and 0.15 mol l^-1 ionic strength. The additives studied were CsnSN fragments containing residues 1-20, 21-40, and 82-101. At molar surface concentrations of (2-3) x 10^-7 mol m^-2, the two N-terminal CsnSN fragments inhibited HAP growth to a similar extent while the C-terminal segment of comparable size was less effective as a crystallization inhibitor. This indicates a more effective HAP surface coverage by CsnSN (1-20) and CsnSN (21-40) and that their charged residues are located in more favourable positions than CsnSN (82-101) for binding to the HAP surfaces. Also, relative inhibitory effects of salivary proteins and their fragments suggest that the degree to which an inhibitor changes the growth rates is also influenced by the nature of the rate-controlling crystal growth mechanism.