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.