Selective adsorption of porcine-amelogenins onto hydroxyapatite and their inhibitory activity on hydroxyapatite growth in supersaturated solutions

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Abstract
The selective adsorption of amelogenins onto synthetic hydroxyapatite (HA) and their inhibitory activity on the seeded HA crystal growth were investigated using enamel proteins obtained from the outer layer of immature porcine-enamel (soft, cheeselike in consistency) of developing permanent incisors. Special interests were paid to the effect of a postsecretory degradation of the original amelogenin(s) on their adsorption and inhibitory activity. In the adsorption studies, it was apparent that the originally secreted amelogenin (25 K), proline, and histidine-rich protein (2a), as well as the higher molecular weight components (60–90 K), showed a strong adsorption affinity onto the HA. This adsorption of protein 2a was related to its inhibition of the crystal growth of seeded HA in a dilute supersaturated solution. On the other hand, the partially degraded product (20 K) of amelogenins, protein 2b, lost the high adsorption affinity onto the HA, and consequently showed no significant inhibitory activity. The observed selective adsorption of protein 2a onto HA was apparent at pH 6.0 and pH 7.4 even in the presence of dissociative solvents, such as 3 M urea or 2 and 4 M guanidine-HCl; however, this selective behavior was sensitive to changes in pH, and was not displayed at pH values of 7.8 or 10.8. The results suggest that the originally secreted amelogenin 2a may play an active role in amelogenesis, and that enamel mineralization could be regulated by the secretion of amelogenins and their inactivation through partial enzymic degradation, prior to their complete removal.