Age-related changes in the D-aspartic acid content of the teeth of the senescence-accelerated mouse

It is known that D-aspartic acid increases with age in dentine. Here, age-related changes in the D to L-aspartic acid (D/L) ratios of the lower teeth of two different sublines of the senescence-accelerated mouse (SAM), SAMP2/Iw (SAM, prone 2/Iwate) and SAMR1/Iw (SAM, resistant 1/Iwate) were measured by gas chromatography. The D/L ratio of the molars increased with advancing age, whereas that of the incisors did not. In mice younger than 6 months of age the D/L ratio of the molars from SAMP2/Iw tended to be higher than that from SAMR1/Iw, whereas the converse applied to older mice. Racemization in the molars occurred significantly faster in SAMR1/Iw than SAMP2/Iw (p = 0.01-0.001). Analysis according to the kind of tooth showed that the D/L ratio increased gradually in the order incisors < third molars < second molars < first molars, indicating that the ratio was higher the earlier the molars formed. As racemization depends upon the environmental temperature, the rectal temperatures of the mice were also examined. The rectal temperature of SAMP2/Iw was highest when they were 2 months old, but declined rapidly thereafter, whereas the rectal temperature of SAMRI/Iw was highest when they were 6 months old, after which it declined gradually. These results indicate that the D-aspartic acid contents of the molars of SAMR1/Iw and SAMP2/Iw increase with age in a different fashion and suggest that the fashion was determined by the body temperature, but not by the senescence-accelerated age.