In the course of aging of organisms, various modifications of side chains of amino acid residues in proteins affect their biological stability and turnover. The main alterations in protein structure are: changes in conformation of proteins, oxidation of amino acid residues (mixed-function oxidation) and racemization of L-amino acids (glutamic and aspartic acids and serine) to corresponding D-forms. D-amino acids were found in bones, teeth, brain and eye lenses. It is generally accepted that the content of D-amino acids slowly increases with age due to a lower metabolic turnover. The discovery of specific D-serine racemase brings a new view of this topic.