Glutamic acid (Glu, E) is an amino acid with the formula HOOC-CH2-CH2-CH(NH2)-COOH and has a molecular weight of 147.13 g mol−1. The L-isomer is one of the basic building blocks of proteins. Glutamic acid is consumed in many prepared foods as a flavor additive in the form of monosodium glutamate. L-glutamic acid is a non-essential amino acid approved as a nutraceutical dietary supplement or micronutrient. The term “L-glutamate” describes any salt of L-glutamic acid. The sodium salt is called monosodium glutamate. Furthermore, human and animal studies indicate that glutamate is the major oxidative fuel for the gut where dietary glutamate is extensively metabolized by the intestinal enterocytes. In addition glutamate is an important precursor for bioactive molecules, including glutathione, and functions as a key neurotransmitter. When taken orally, L-glutamate is absorbed from the lumen of the small intestine into the enterocytes. The absorption of this amino acid is efficient and occurs by an active transport mechanism. Enterocytes are intestinal absorptive cells that are present in the small intestines, colon and appendix. The glycocalyx surface coat consists of extracellular glycoproteins and also contains digestive enzymes. The surface area of these cells is increased through the presence of microvilli to allow for the digestion and transport of molecules from the intestinal lumen. L-glutamate is a very important oxidative substrate for the intestinal mucosa and is needed in the biosynthesis of the two essential amino acids proline and arginine. L-glutamate is a key factor responsible for protection of the mucosa and dietary glutamate appears to be an essential factor for the maintenance of mucosal health and functions. Furthermore, glutamate is one of the most important neurotransmitters needed for normal brain function and nearly all excitatory neurons in the central nervous system are glutamatergic. In addition, it is estimated that over half of all brain synapses release this amino acid
Glutamate plays an especially important role in clinical neurology because elevated concentrations of extracellular glutamate, released as a result of neural injury, are toxic to neurons. Free glutamic acid cannot cross the blood-brain barrier in the needed quantities but is instead converted into L-glutamine. The brain uses L-glutamine for fuel and protein synthesis. Glutamate is involved in cognitive functions like learning and memory in the brain. The neurotransmitter L-glutamate functions as an excitatory neurotransmitter and is a precursor for the synthesis of gamma-aminobutyric acid or γ-aminobutyric acid (GABA) in GABAergic neurons. GABAergic neurons transmit or secret γ-aminobutyric acid. L-Glutamate is also considered to be nature's "Brain food." L-glutamate helps to improve the mental capacities in humans and to speed up the healing of ulcers. In addition, L-glutamate has been reported to help control alcoholism, schizophrenia and the craving for sugar. However, excessive amounts of L-glutamate may cause neuronal damage and are associated with diseases such as amyotrophic lateral sclerosis, lathyrism, and Alzheimer's disease. Glutamate activates both ionotropic and metabotropic glutamate receptors. Ionotropic and metabotropic receptors are both ligand gated transmembrane proteins. Ionotropic receptors change shape when they are bound by a ligand. This change in shape creates a channel that allows ions to flow through. On the other hand, metabotropic receptors do not have channels and activate a G-protein that in turn activates a secondary messenger that may then activate a "secondary effector" whose effects depend on the particular secondary messenger system.
Glutamate can cause neuronal damage and eventual cell death, particularly when N-Methyl-D-aspartic acid or N-Methyl-D-aspartate (NMDA) receptors are activated. High dosages of glutamic acid may induce symptoms such as headaches and neurological problems. The pathological process called excitotoxicity damages and kills nerve cells. This is caused via excessive stimulation by neurotransmitters such as glutamate and similar substances. Unfortunately, glutamate is of transient nature in biological systems and is therefore extremely difficult to study in action. However, quantitative amino acid analysis can be used to determine the presence of L-glutamate in different tissue types, plasma and other bodily fluids including dietary supplements and foods.
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