Considerations on the role of aspartic acid racemization in the aging process.

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 stereochemical 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.