An overview on D-amino acids

More than half a century ago researchers thought that d-amino acids had a minor function compared to l-enantiomers in biological processes. Many evidences have shown that d-amino acids are present in high concentration in microorganisms, plants, mammals and humans and fulfil specific biological functions. In the brain of mammals, d-serine (d-Ser) acts as a co-agonist of the N-methyl-d-aspartate (NMDA)-type glutamate receptors, responsible for learning, memory and behaviour. d-Ser metabolism is relevant for disorders associated with an altered function of the NMDA receptor, such as schizophrenia, ischemia, epilepsy and neurodegenerative disorders. On the other hand, d-aspartate (d-Asp) is one of the major regulators of adult neurogenesis and plays an important role in the development of endocrine function. d-Asp is present in the neuroendocrine and endocrine tissues and testes, and regulates the synthesis and secretion of hormones and spermatogenesis. Also food proteins contain d-amino acids that are naturally originated or processing-induced under conditions such as high temperatures, acid and alkali treatments and fermentation processes. The presence of d-amino acids in dairy products denotes thermal and alkaline treatments and microbial contamination. Two enzymes are involved in the metabolism of d-amino acids: amino acid racemase in the synthesis and d-amino acid oxidase in the degradation.