Enamel protein chemistry--past, present and future.

Past progress in the chemistry of enamel proteins is reviewed and the current state of knowledge assessed. The matrix of young enamel is a complex system in which some 20 distinct components with molecular weights in the region of 3,000 to 16,000 are in dynamic equilibrium with much larger aggregates. During maturation, most of these components are selectively lost, more or less completely, from the enamel. 'Amelogenin' components rich in proline and histidine are removed first and 'tuft protein', characterized by high serine and glycine, is often partially retained in mature enamel. Some components have been isolated in a state approaching purity and a measure of agreement has been reached between laboratories concerning their characteristics. Partial amino acid sequences are known for two components, which contain phosphoserine. Though the mechanisms of mineralization and protein removal are not known, various possibilities are discussed. The essential unsolved problem is the nature of the overall protein system.