Otherwise known as glutamate, glutamic acid is the most common excitatory neurotransmitter in the central nervous system. It serves as an energy source for brain cells and plays a critical role in brain metabolism. In the brain, glutamate can regulate ammonia levels by taking up nitrogen in its conversion to glutamine, another amino acid that functions as a neurotransmitter. Glutamate serves the same function in the periphery, taking up ammonia and then carrying it via the blood back to the liver for ultimate conversion to urea, which is then excreted. Glutamic acid is also important in the synthesis of gamma-aminobutyric acid (GABA), an inhibitory neurotransmitter that has the opposite effect of glutamic acid and helps to decrease activity within the central nervous system.