D-Aspartic acid is the D-isomer of aspartic acid. Since its discovery in invertebrates, free D-aspartate (D-Asp) has been identified in a variety of organisms, including microorganisms, plants, and lower animals, mammals and humans. D-Asp in mammalian tissues is present in specific cells, indicating the existence of specific molecular components that regulate D-Asp levels and localization in tissues. In the rat adrenal medulla, D-Asp is closely associated with adrenaline-cells (A-cells), which account for approximately 80% of the total number of chromaffin cells in the tissue, and which make and store adrenaline. D-Asp appears to be absent from noradrenaline-cells (NA-cells), which comprise approximately 20% of the total number of chromaffin cells in the adrenal medulla, and which make and store noradrenaline. D-aspartate oxidase (EC 1.4.3.1, D-AspO), which catalyzes oxidative deamination of D-Asp, appears to be present only in NA-cells, suggesting that the lack of D-Asp in these cells is due to D-Asp oxidase-mediated metabolism of D-Asp. In the rat adrenal cortex, the distribution of D-Asp changes during development. It has been suggested that developmental changes in the localization of D-Asp reflects the participation of D-Asp in the development and maturation of steroidogenesis in rat adrenal cortical cells. D-Asp is involved in steroid hormone synthesis and secretion in mammals as well. D-Asp is synthesized intracellularly, most likely by Asp racemase (EC 5.1.1.13). Endogenous D-Asp apparently has two different intracellular localization patterns: cytoplasmic and vesicular. D-Asp release can occur through three distinct pathways: 1) spontaneous, continuous release of cytoplasmic D-Asp, which is not associated with a specific stimulus; 2) release of cytoplasmic D-Asp via a volume-sensitive organic anion channel that connects the cytoplasm and extracellular space; 3) exocytotic discharge of vesicular D-Asp. D-Asp can be released via a mechanism that involves the L-Glu transporter. D-Asp is thus apparently in dynamic flux at the cellular level to carry out its physiological function(s) in mammals (PMID:16755369).