Different racemization ratios in dentin from different locations within a tooth.

It has been reported that the amount of D-aspartic acid in dentin is highly correlated with age. However, further studies are necessary to determine the details of this correlation. We quantitatively determined L- and D-aspartic acids in dentin, and from the data calculated the D/L ratio. We did not find any statistically significant difference in the D/L ratio between the same type of left and right teeth from the same jaw, and between the vestibular and lingual sides of the root dentin. However, the D/L ratio was significantly higher on the lingual side than on the labial side of the dentin. Although the D/L ratio in young subjects was comparatively high in the tooth crown and decreased toward the apex of the root, we did not always observe such a tendency in middle- to advanced-age individuals. These results indicate that the ratio of D/L aspartic acid in dentin varies between the lingual side and vestibular side of the crown dentin. Therefore, it appears that the racemization rate of aspartic acid is not uniform but differs with the region of the dentin, and may be affected by differences in factors such as temperature.