The brain has the highest concentration of glutamic acid, gamma-aminobutyric acid (GABA) and aspartic acid, and these compounds do not occur to any significant extent in the other tissues of mammals.

Gamma-aminobutyric acid (GABA) is synthesized by two isoforms of the pyridoxal 5′-phosphate–dependent enzyme glutamic acid ... have determined the crystal structures of N-terminal truncations ...