Racemization of aspartic acid in the extracellular matrix proteins of primary and secondary dentin.

The accumulation with age of D-aspartic acid in primary and secondary human dentin was determined. In primary dentin, the plot of the D/L Asp ratio vs. age was found to fit first-order kinetics in accordance with the literature. But the secondary dentin behaved in an irregular manner and showed, in the great majority of cases, significantly increased D/L Asp ratios. Possible reasons for these findings, such as differences in protein composition and/or in the prevailing temperature, are discussed.