Aspartic acid racemisation in dentine as a measure of ageing

Abstract

THE L-amino acids initially present in bone protein undergo slow racemisation over geological time at a rate which is proportional to temperature^1,2. We have shown³ that at the human body temperature of ∼37°C aspartyl residues in tooth enamel protein also undergo racemisation at a rate which corresponds to an enrichment in the D-aspartic acid content of ∼0.1% per year. No D-aspartic acid increase was detected in haemoglobin, a protein with a more rapid turnover. We concluded that D-aspartyl residues accumulate in the metabolically stable protein in tooth enamel during the human lifetime as a result of in situ racemisation. We proposed that the irreversible first-order rate equation calculated from the enamel results could be used to deduce the age of any stable protein from a long lived mammal and thus the age of the organism itself. The error among samples from old individuals was large, however, probably because attrition and caries reduce the amount of uncontaminated enamel, and thus tend to limit the usefulness of tooth enamel for age determinations. In this report, we show that tooth dentine is a more suitable material, and the extent of aspartic acid racemisation in this fraction can be used as a reliable indicator of mammalian age.