Second, glycine is added to the C-terminal of γ-glutamylcysteine. This condensation is catalyzed byglutathione synthetase.
While all animal cells are capable of synthesizing glutathione, synthesis in the liver has been shown to be essential.GCLCknockout 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-proteinthiol (R−SH-containing compound) in animal cells, ranging from 0.5 to 10 mmol/L. It is present in thecytosol and theorganelles.[6] The concentration of glutathione in thecytoplasm is significantly higher (ranging from 0.5-10 mM) compared to extracellular fluids (2-20 μM), reaching levels up to 1000 times greater.[7][8] 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).[9] The cytosol holds 80-85% of cellular GSH and themitochondria hold 10-15%.[10]
Systemic availability of orally consumed glutathione is poor. It had low bioavailability because the tripeptide is the substrate ofproteases (peptidases) of thealimentary canal, and due to the absence of a specificcarrier of glutathione at the level of cell membrane.[13][14] The administration of N-acetylcysteine (NAC), a cysteine prodrug, helps replenish intracellular GSH levels.[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 cellularoxidative stress[17][10] 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 proteinS-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 thedetoxification ofmethylglyoxal andformaldehyde, toxic metabolites produced under oxidative stress. This detoxification reaction is carried out by theglyoxalase system.Glyoxalase I (EC 4.4.1.5) catalyzes the conversion of methylglyoxal and reduced glutathione toS-D-lactoylglutathione.Glyoxalase II (EC 3.1.2.6) catalyzes the hydrolysis ofS-D-lactoylglutathione to glutathione andD-lactic acid.
It maintains exogenous antioxidants such asvitamins C andE in their reduced (active) states.[21][22][23]
Among the many metabolic processes in which it participates, glutathione is required for the biosynthesis ofleukotrienes andprostaglandins. It plays a role in the storage of cysteine. Glutathione enhances the function ofcitrulline as part of thenitric oxide cycle.[24] It is acofactor and acts onglutathione peroxidase.[25] Glutathione is used to produce S-sulfanylglutathione, which is part ofhydrogen sulfide metabolism.[26]
Glutathione facilitatesmetabolism of xenobiotics.GlutathioneS-transferase enzymes catalyze its conjugation tolipophilic xenobiotics, facilitating their excretion or further metabolism.[27] The conjugation process is illustrated by the metabolism ofN-acetyl-p-benzoquinone imine (NAPQI). NAPQI is a reactivemetabolite formed by the action ofcytochrome P450 onparacetamol (acetaminophen). Glutathione conjugates to NAPQI, and the resulting ensemble is excreted. As a result of this reaction cellular glutathione concentration tends to be depleted in presence of acetaminophen.
Among various types ofcancer,lung cancer,larynx cancer,mouth cancer, andbreast cancer exhibit higher concentrations (10-40 mM) of GSH compared to healthy cells.[32] Thus,drug delivery systems containingdisulfide bonds, typically cross-linked micro-nanogels, stand out for their ability to degrade in the presence of high concentrations of glutathione (GSH).[33] This degradation process releases the drug payload specifically into cancerous or tumorous tissue, leveraging the significant difference in redox potential between the oxidizing extracellular environment and the reducing intracellular cytosol.[34][35]
When internalized byendocytosis, nanogels encounter high concentrations of GSH inside the cancer cell. GSH, a potent reducing agent, donates electrons to disulfide bonds in the nanogels, initiating a thiol-disulfide exchange reaction. This reaction breaks the disulfide bonds, converting them into two thiol groups, and facilitates targeted drug release where it is needed most. This reaction is called a thiol-disulfide exchange reaction.[36][37]
R−S−S−R′+ 2GSH →R−SH + R′−SH +GSSG
whereR andR' are parts of the micro-nanogel structure, andGSSG is oxidized glutathione (glutathione disulfide).
The breaking of disulfide bonds causes the nanogel to degrade into smaller fragments. This degradation process leads to the release of encapsulated drugs. The released drug molecules can then exert their therapeutic effects, such as inducingapoptosis in cancer cells.[38]
The content of glutathione inmust, the first raw form of wine, determines thebrowning, or caramelizing effect, during the production ofwhite wine by trapping thecaffeoyltartaric acid quinones generated by enzymic oxidation asgrape reaction product.[39] Its concentration in wine can be determined by UPLC-MRM mass spectrometry.[40]
^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.
^Cheng R, Feng F, Meng F, Deng C, Feijen J, Zhong Z (May 2011). "Glutathione-responsive nano-vehicles as a promising platform for targeted intracellular drug and gene delivery".J Control Release.152 (1):2–12.doi:10.1016/j.jconrel.2011.01.030.PMID21295087.
^Witschi A, Reddy S, Stofer B, Lauterburg BH (1992). "The systemic availability of oral glutathione".European Journal of Clinical Pharmacology.43 (6):667–9.doi:10.1007/bf02284971.PMID1362956.S2CID27606314.
^Noctor G, Foyer CH (June 1998). "Ascorbate and Glutathione: Keeping Active Oxygen Under Control".Annual Review of Plant Physiology and Plant Molecular Biology.49 (1):249–279.doi:10.1146/annurev.arplant.49.1.249.PMID15012235.
^Gilbert, H.F. (1995). "Thiol/disulfide exchange equilibria and disulfide bond stability".Biothiols, Part A: Monothiols and Dithiols, Protein Thiols, and Thiyl Radicals. Methods in Enzymology. Vol. 251. pp. 8–28.doi:10.1016/0076-6879(95)51107-5.ISBN978-0-12-182152-4.PMID7651233.
^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):142–9.Bibcode:2015JAFC...63..142V.doi:10.1021/jf504383g.PMID25457918.
