Studies of enamel proteins during maturation.

The solubility characteristics, amino acid composition and molecular size, and aggregation of the protein components of the organic matrix of bovine and rat enamel have been studied as a function of progressive stages in the maturation of the tissue. The age of the animal, type of tooth and location of the enamel on the crown of the tooth and the amount of calcification were used as indices of enamel maturation in the bovine species, whereas location of the anemel on the tooth crown was used as an index of maturation in the rat. The most immature enamel contains about 15–20% protein by weight, whereas the most mature enamel contains 0.1% protein or less. The proteins in the organic matrix of immature enamel are relatively insoluble in near neutral solutions of 0.5 M EDTA and are characterized by their high concentrations of proline, glutamic acid, leucine, and histidine (“immature” protein components). Conversely the proteins and peptides of fully mature enamel are soluble in 0.5 M EDTA and are characterized by their relatively high concentrations of aspartic and glutamic acids, serine and glycine (“mature” protein and peptide components). The very marked decrease in the protein content of maturing and mature enamel, and the differences in amino acid composition of mature enamel compared with immature enamel, suggest that maturation of enamel is characterized biochemically by the selective loss of certain peptides and the retention of certain other peptides, the latter possibly due to their interaction with the mineral ion constituents. The biochemical processes accompanying maturation appear to begin early in the development of the enamel at a time when the organic matrix and its protein constituents are immature as judged by their insolubility in EDTA and by their overall amino acid composition. In consists of the selective loss and the selective retention of certain components from the enamel which tend to decrease the proline and histidine contents and increase the serine, aspartic acid and glycine contents of the immature, EDTA-insoluble matrix, the latter components also becoming diffusible and suggesting protein and/or peptide degradation or depolymerization. Despite the changes in the overall composition of the immature, EDTA-insoluble enamel matrix which occur as a function of progressive stages in the maturation process, they all contain soluble, nondiffusible components of similar composition which are rich in proline, histidine and leucine. At stages of development intermediate between the most immature and the fully mature states, the enamel matrix contains a mixture of “immature” and “mature” proteins, the relative proportions of which are principally responsible for its overall amino acid composition. However, since the progressive biochemical changes of maturation are first observed in the immature, EDTA-insoluble enamel matrix, these changes also contribute to the overall differences in the amino acid composition. There is no evidence that the overall compositional changes in the enamel at any stage of development is due principally to compositional changes in the individual protein and peptide components per se. It is proposed that if peptide bond hydrolysis occurs during enamel maturation, it is catalysed in part by the enamel crystals per se.