d-Aspartic acid in bovine dentine non-collagenous phosphoprotein

In tooth dentine, owing to its slow metabolism after its formation, racemized and transformed d-aspartic acid remains in the tissue and accumulates with age. However, no dentinal proteins which contain d-aspartic acid have been identified. In this study, a non-collagenous phosphoprotein was purified from bovine dentine. Its molecular mass was about 130 kDa and its amino acid composition was very similar to that of bovine dentine phosphophoryn. The purified protein contained a large proportion of aspartic acid residues and some of them were stereoinverted from the l-isomer to the d-isomer. The d-/l-aspartic acid ratio of dentine non-collagenous phosphoproteins purified from 8-month-old fetal, postnatal and 1-year-old bovine first incisors showed that the stereoinversion tended to increase with age. These results suggest that the purified non-collagenous phosphoprotein is a candidate for the protein in dentine containing d-aspartic acid.