Glutamic acid is a nonessential amino acid, which means it is manufactured from other amino acids in the liver; it does not have to be obtained directly through the diet.
Glutamic acid is interconverted to glutamine, which is known to be a very important amino acid in preventing ammonia intoxication, and also a brain-active neurotransmitter substance. Adults may ingest 20-35g per day of this amino acid without any apparent ill effects. Glutamine has been used therapeutically rather than glutamate in the management of certain problems ie., alcoholism, liver problems, and brain biochemical problems.
Metabolism of glutamic acid can result in formation of gamma-amino butyric acid (GABA), which is known to be an inhibitory neurotransmitter in the brain. Deficiencies of GABA can result in excess activity of certain regions of the brain and seizures or behavioral hyperactivity. Dietary glutamic acid does not have a significant effect on GABA in the nervous system.
Recommended Dietary Allowances
An RDA has not been established for glutamic acid because it is a nonessential amino acid. Supplementation with purified glutamic acid on an empty stomach of 3g or more has resulted in intestinal disturbances.
The flavor-enhancing substance monosodium glutamate (MSG) is the sodium salt of glutamic acid and can produce a condition called "Chinese Restaurant Syndrome" which is associated with muscle cramping, abdominal spasms, and gastric distress. Generally, Caucasians are more susceptible to this condition in that they are unable to convert glutamate to glutamine as efficiently as Orientals. Higher levels of vitamin B-6 intake have been found useful for preventing "Chinese Restaurant Syndrome" due to enhanced conversion of glutamate to glutamine in those who are susceptible to this problem.
Method of Action
Aspartate and glutamate participate within the neurotransmitter family of substances. This family also includes acetylcholine, noradrenaline, and gamma-amino butyric acid (GABA). Glutamate is one of the most important excitatory transmitters in the central nervous system in lower animals and may also be important in humans.
Aspartate has been considered to be a neurotransmitter, whereas GABA and glycine are thought to be major inhibitory transmitters. Excitatory transmitters such as aspartate lead to depolarization of the nerves; on the other hand, inhibitory transmitters cause hyper-polarization, apparently by increasing the permeability within the nerve of potassium and chloride.
Glutamate and aspartate are also important in the tricarboxylic acid cycle (Kreb's cycle), from which most of the energy is produced by metabolism. Their reaction in this pathway is by what is called the malate-aspartate shuttle for the transportation of energy into the mitochondria.
Applebaum, A.E., Daabees, T.T., Filer, L.J., Jr., and Stegink, L.D., Hypothalamic Neuronal Necorsis in Neonatal Cats Administered Pharmacologic Doses of Monosodium L- Glutamate.
Society for Neuroscience, Abstracts, 7:89, 1981.
Blackburn, G.L., Grant, J.P., Young, V.R., ed. Amino Acids Metabolism and Medical Applications.
Munro, H.N. & Crim, M.C. The Proteins and Amino Acids. Modern Nutrition in Health and Disease, eds: R.S. Goodhart & M.E. Shils, 6 ed. Philadelphia: Lea and Febiger, 1980.
Olney, J.W., Labruyere, J., & DeGubareff, T. Brain Damage in Mice from Voluntary Ingestion of Glutamate and Aspartate. Neurobehav. Toxicol., 2:125-9, 1980.
Windmueller, H.G. & Spaeth, A.E. Intestinal Metabolism of Glutamine and Glutamate from the Lumen as Compared to Glutamine from Blood. Arch. Biochem. Biophys., 171:662-72, 1975.
Young, V.R., Meguid, M., Meredith, D.E. & Bier, D.M. Recent Developments in Knowledge of Human Amino Acid Requirements. Nitrogen Metabolism in Man, eds: J.C. Waterlow & J.M.L. Stephen, London: Applied Science Pubs, 1981.
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