Second, glycine is added to the C-terminal of γ-glutamylcysteine. This condensation is catalyzed by glutathione synthetase.
While all animal cells are capable of synthesizing glutathione, glutathione synthesis in the liver has been shown to be essential. GCLC knockout mice die within a month of birth due to the absence of hepatic GSH synthesis.[4][5]
The unusual gamma amide linkage in glutathione protects it from hydrolysis by peptidases.[6]
Glutathione is the most abundant non-protein thiol (R−SH-containing compound) in animal cells, ranging from 0.5 to 10 mmol/L. It is present in the cytosol and the organelles.[6] In healthy cells and tissue, more than 90% of the total glutathione pool is in the reduced form (GSH), with the remainder in the disulfide form (GSSG).[7] 80-85% of cellular GSH is in the cytosol and 10-15% is in the mitochondria.[8]
Systemic availability of orally consumed glutathione has poor bioavailability because the tripeptide is the substrate of proteases (peptidases) of the alimentary canal, and due to the absence of a specific carrier of glutathione at the level of cell membrane.[11][12] The administration of N-acetylcysteine (NAC), a cysteine prodrug, helps replenish intracellular GSH levels.[13] The patented compound RiboCeine has been studied as a supplement that enhances production of glutathione which helps mitigate hyperglycemia.[14][15]
Glutathione exists in reduced (GSH) and oxidized (GSSG) states.[16] The ratio of reduced glutathione to oxidized glutathione within cells is a measure of cellular oxidative stress[17][8] where increased GSSG-to-GSH ratio is indicative of greater oxidative stress.
Aside from deactivating radicals and reactive oxidants, glutathione participates in thiol protection and redox regulation of cellular thiol proteins under oxidative stress by protein S-glutathionylation, a redox-regulated post-translational thiol modification. The general reaction involves formation of an unsymmetrical disulfide from the protectable protein (RSH) and GSH:[20]
RSH + GSH + [O] → GSSR + H2O
Glutathione is also employed for the detoxificationofmethylglyoxal and formaldehyde, toxic metabolites produced under oxidative stress. This detoxification reaction is carried out by the glyoxalase system. Glyoxalase I (EC 4.4.1.5) catalyzes the conversion of methylglyoxal and reduced glutathione to S-D-lactoylglutathione. Glyoxalase II (EC 3.1.2.6) catalyzes the hydrolysis of S-D-lactoylglutathione to glutathione and D-lactic acid.
It maintains exogenous antioxidants such as vitamins C and E in their reduced (active) states.[21][22][23]
Among the many metabolic processes in which it participates, glutathione is required for the biosynthesis of leukotrienes and prostaglandins. It plays a role in the storage of cysteine. Glutathione enhances the function of citrulline as part of the nitric oxide cycle.[24] It is a cofactor and acts on glutathione peroxidase.[25] Glutathione is used to produce S-sulfanylglutathione, which is part of hydrogen sulfide metabolism.[26]
The content of glutathione in must, the first raw form of wine, determines the browning, or caramelizing effect, during the production of white wine by trapping the caffeoyltartaric acid quinones generated by enzymic oxidation as grape reaction product.[32] Its concentration in wine can be determined by UPLC-MRM mass spectrometry.[33]
^Pompella A, Visvikis A, Paolicchi A, De Tata V, Casini AF (October 2003). "The changing faces of glutathione, a cellular protagonist". Biochemical Pharmacology. 66 (8): 1499–1503. doi:10.1016/S0006-2952(03)00504-5. PMID14555227.
^Scholz RW, Graham KS, Gumpricht E, Reddy CC (1989). "Mechanism of interaction of vitamin E and glutathione in the protection against membrane lipid peroxidation". Annals of the New York Academy of Sciences. 570 (1): 514–517. Bibcode:1989NYASA.570..514S. doi:10.1111/j.1749-6632.1989.tb14973.x. S2CID85414084.
^Vallverdú-Queralt A, Verbaere A, Meudec E, Cheynier V, Sommerer N (January 2015). "Straightforward method to quantify GSH, GSSG, GRP, and hydroxycinnamic acids in wines by UPLC-MRM-MS". Journal of Agricultural and Food Chemistry. 63 (1): 142–149. doi:10.1021/jf504383g. PMID25457918.