HPLC Methods for Determination of d-Aspartate and N-methyl-d-Aspartate

d -Amino acids are stereoisomers or optical isomers of naturally occurring l -amino acids and thus possess the same chemical structure, but may differ in their biological/physiological properties. Until a half century ago, d -amino acids had been considered to be unnatural substances found only in microorganisms. However, improvements in analytical instruments and methods have revealed that d -amino acids are present in invertebrates and vertebrates, including humans, and that they possess important physiological functions. d -Aspartate (d -Asp) and its methylated form N -methyl-d -aspartate (NMDA) possess neuroendocrine properties in many species. Several methods have been developed for determination of d - and l -enantiomers of amino acids by high performance liquid chromatography (HPLC). We report here improved HPLC methods for the specific determination of d -Asp and NMDA in biological tissues.