No abstract

The aspartic acid in human tooth enamel shows increasing racemization with age. This increase is not seen in the metabolically active protein hemoglobin. The rate constant for the racemization reaction of aspartic acid in human tooth enamel was found to be 8.29 X 10-4 yr-1. This rate constant suggests that in any protein with a long in vivo lifetime, D-aspartic acid will accumulate with age (about 8% of total aspartic acid in enamel will be the D-enantiomer after 60 years). Thus, racemization may play some role in the aging process affecting metabolically stable tissues in long-lived homeotherms. Aspartic acid racemization in tooth enamel also provides a biochronological tool for assessing the age of living mammals.Racemization is a natural process which will eventually-convert optically active compounds into a racemic mixture. The L-amino acids commonly found in living systems are the result of the stereochemical specificity of enzymes which utilize only the L-enantiomers. Racemization of amino acids in the proteins produced by living organisms would thus take place only after protein turnover has ceased. Recent studies (1-4) have shown that at 25°a period of about 100,000 years is required before all the L-amino acids present in living systems will have undergone complete racemization to an equilibrium mixture in which the ratio of D-to L-amino acids equals 1.0. (For amino acids with more than one asymmetric center, the ratio is different from 1.0, i.e., the ratio alloisoleucine/isoleucine equals approximately 1.3.)The extent of racemization of amino acids may be used to estimate the ages of various fossil materials, including deepsea sediments (5-7), shells (8-11), fossil bones (2-4, 12-15), and coprolites (16). Of the various amino-acid racemization reactions, the one involving aspartic acid has received the most attention. This amino acid has one of the fastest racemization rates of the stable amino acids (1). In bone, the half-life of aspartic acid racemization (the time required for the ratio of D-to L-enantiomers to reach 0.333) is about 15,000 years at 200 (3,15). Thus, for temperate to nearequatorial environments, significant racemization of aspartic acid will have taken place in a bone during the last 40,000 years (the upper limit of radiocarbon dating).The temperature dependence of the rate of the aminoacid racemization reaction can also be used to estimate the temperature history of a sample of known age. By determining the extent of aspartic acid racemization in a radiocarbon-dated bone, it is possible to calculate the in situ rate of racemization, and this rate constant can then be used to estimate the average temperature to which the bone has been exposed (17). Racemization analysis of -radiocarbon-dated bones with ages less than about 12,000 years (i.e., post-glacial) have yielded rate constants which are proportional to the present-day average annual air temperatures of the sites where the bones were found (15).Based on the correlations between aspartic acid racemiza-2891 METHODS Teeth ...

No abstract

Racemization of aspartyl residues in human dentine and enamel proteins has been shown to occur at a rate which corresponds to an enrichment in the D-aspartic acid content of 0.1 % per year. This rate can be used to calculate the ages of living people or the in vivo lifetimes of slowly turned over proteins. We present stereo chemical arguments for conformational changes in proteins as a consequence of racemized amino acid residues. In metabolically stable proteins, this phenomenon may play some part in the aging process. In renewed proteins, where certain factors may accelerate racemization, conformational changes induced by racemization could regulate protein degradation.