D-amino acid in elderly tissues

All living organisms on earth are composed of L-amino acids and D-sugars. Therefore the presence and function of D-amino acids in living organisms have not been studied. Recently, however, D-aspartic acid (D-Asp) which is a D-amino acid, has been detected in various proteins from human tissue samples such as eye lens, brain, skin, bone, teeth, aorta from elderly individuals. It has been explained that the presence of D-Asp is the result of racemization of Asp residues in the protein during the life span, inasmuch as the proteins in these tissues are metabolically inert. The Asp-151 and Asp-58 residues in alpha A-crystallin from elderly human lens are especially stereochemically labile and the D/L ratios of these residues were greater than 1.0. A D/L ratio greater than 1.0 is not defined as racemization, but as the inversion of configuration. This was the first observation that inversion occurred in the configuration of amino acids in vivo during the natural aging process. In this review, we summarize the D-Asp in the various tissues reported by many researchers and describe the mechanism of D-Asp formation in protein. We suggest that a chiral reaction field exists in the native higher order structure of protein which induces the inversion of L-Asp to D-Asp residue.