In the course of aging of organisms, various modifications of side chains o
f amino acid residues in proteins affect their biological stability and tur
nover. The main alterations in protein structure are: changes in conformati
on 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.