D-Amino acid metabolism in mammals: Biosynthesis, degradation and analytical aspects of the metabolic study

It was believed for long time that d-amino acids are not present in mammals. However, current technological advances and improvements in analytical instruments have enabled studies that now indicate that significant amounts of d-amino acids are present in mammals. The most abundant d-amino acids are d-serine and d-aspartate. d-Serine, which is synthesized by serine racemase and is degraded by d-amino-acid oxidase, is present in the brain and modulates neurotransmission. d-Aspartate, which is synthesized by aspartate racemase and degraded by d-aspartate oxidase, is present in the neuroendocrine and endocrine tissues and testis. It regulates the synthesis and secretion of hormones and spermatogenesis. d-Serine and d-aspartate bind to the N-methyl-d-aspartate (NMDA) subtype of glutamate receptors and function as a coagonist and agonist, respectively. The enzymes that are involved in the synthesis and degradation of these d-amino acids are associated with neural diseases where the NMDA receptors are involved. Knockout mice for serine racemase and d-aspartate oxidase have been generated, and natural mutations in the d-amino-acid oxidase gene are present in mice and rats. These mutant animals display altered behaviors caused by enhanced or decreased NMDA receptor activity. In this article, we review currently available studies on d-amino acid metabolism in mammals and discuss analytical methods used to assay activity of amino acid racemases and d-amino-acid oxidases.