Properties of phosphorylated 32 kd nonamelogenin proteins isolated from porcine secretory enamel

Enamel proteins were isolated from specific locations of permanent porcine incisors at various developmental stages, namely, the outer (young) and inner (old) secretory, and maturing (chalk-like in appearance) enamel. The selective adsorption of these matrix proteins onto hydroxyapatite (HA) crystals was investigated in the presence of dissociative agents. The results showed that the proteins with the highest adsorption affinity were present at the highest concentration in the vicinity of the ameloblasts, i.e., in the outer enamel layer; a substantial reduction of these proteins was observed in the older (inner) secretory enamel and in the tissue in the maturing stage. An interesting finding was that a group of proteins having molecular masses of 32 kd present only in the inner secretory enamel, adsorbed strongly onto the HA crystals and were potent inhibitors of HA crystal growth. This 32 kd group contains phosphorylated glycoproteins; they are rich in Pro, Glu, Gly, and Asp and the N-terminal sequence was LXQVPGRIPPGYGRPPTP-, having no resemblance to the reported sequences of amelogenins. It was also found that the 32 kd moieties remained only as trace constituents in the maturing enamel, suggesting that most of them were removed as soluble constituents in the tissue fluid or further degraded by enzymatic activity during the late secretory stage. The results obtained support the view that amelogenetic mineralization is regulated by the presence of various organic matter and, importantly, that their efficacy as inhibitors of mineralization may be modulated through their degradation.