Numerous innovations in analytical methods have demonstrated the presence of D-amino acids in eukaryotes, including mammals (1-3). The most abundant D-amino acids in mammals are D-serine and D-aspartate (4). D-alanine is also present in mammals at moderate levels (4, 5). Recently, physiological roles of these D-amino acids in humans have been reported. For example, D-serine has been found to modulate the activity of N-methyl-D-aspartate (NMDA) receptor in the brain (6). It is postulated that D-serine binds to the coagonist binding site of the MDA receptor, which is involved in numerous physiological and pathological processes, including synaptic plasticity, learning, memory, neuronal cell migration, and neural diseases (7,8