d-Amino acids in mammalian endocrine tissues

d-Aspartate,d-serine andd-alanine are a regular occurrence in mammalian endocrine tissues, though in amounts varying with the type of gland. The pituitary gland, pineal gland, thyroid, adrenal glands and testis contain relatively large amounts ofd-aspartate in all species examined.d-alanine is relatively abundant in the pituitary gland and pancreas. High levels ofd-serine characterize the hypothalamus.d-leucine,d-proline andd-glutamate are generally low. The current knowledge of physiological roles ofd-amino acids in endocrine tissues is far from exhaustive, yet the topic is attracting increasing interest because of its potential in pharmacological application.d-aspartate is known to act at all levels of the hypothalamus-pituitary-testis axis, playing a key role in reproductive biology in several vertebrate classes. An involvement ofd-amino acids in the endocrine function of the pancreas is emerging.d-Aspartate has been immunolocalized in insulin-containing secretory granules in INS-1 E clonal beta cells and is co-secreted with insulin by exocytosis. Specific immunolocalization ofd-alanine in pituitary ACTH-secreting cells and pancreatic beta-cells suggests that this amino acid participates in blood glucose regulation in mammals. By modulating insulin secretion,d-serine probably participates in the control of systemic glucose metabolism by modulating insulin secretion. We anticipate that future investigation will significantly increase the functional repertoire ofd-amino acids in homeostatic control.