Glutathione: A Research Overview of the Tripeptide and Its Redox Biology

Glutathione (GSH), a γ-glutamylcysteinylglycine tripeptide, has been described in the biochemical literature as ubiquitous in mammalian and other living cells, synthesised from its constituent amino acids by the consecutive actions of γ-glutamylcysteine synthetase and GSH synthetase [1]. Reviews of that biosynthetic pathway report that γ-glutamylcysteine synthetase activity is modulated by its light subunit and by feedback inhibition from the end product GSH, and that treatment of cells with the inhibitor buthionine sulfoximine lowers cellular GSH and has been used experimentally as a model of GSH deficiency [1]. Comparable synthesis, transport and degradation pathways have been reviewed in plant systems, where the literature reports that plants do not survive without glutathione or γ-glutamylcysteine-containing homologues [2]. The functions attributed to glutathione in cell and tissue models are multiple. In lens tissue, reviewed work describes GSH as synthesised locally from constituent amino acids and degraded through transpeptidation and hydrolysis, with turnover attributed to catabolism rather than to GSSG transport as reported for red blood cells and some other tissues [3]. The same overview summarises proposed roles in maintaining protein thiols in the reduced state, in protecting membrane sulfhydryl groups involved in cation transport and permeability, and in the detoxification of hydrogen peroxide and other organoperoxides via the glutathione redox cycle [3]. Biochemical reviews further describe GSH as the major low-molecular-weight thiol of mammalian cells, acting as a co-factor for glutathione peroxidases and glutathione S-transferases and as a nucleophile that conjugates reactive intermediates through its cysteine sulfhydryl group [4]. In vitro work summarised in that review reported the formation of glutathione adducts from lipid-peroxidation electrophiles such as 4-hydroxy-2(E)-nonenal and 4-oxo-2(E)-nonenal, including a thiadiazabicyclo-ONE-GSH adduct characterised in EA.hy926 endothelial cells during peroxide/Fe(II)-mediated oxidative stress and proposed as a quantitative marker of intracellular oxidative stress [4]. Glutathione also operates through enzyme systems that use it as a substrate or redox partner. The glutaredoxin literature describes small proteins with an active-site cysteine pair that catalyse glutathione-dependent glutathionylation and deglutathionylation, function within a system comprising glutathione, glutathione reductase and NADPH, and have been implicated in iron–sulfur cluster formation; these characterisations derive from biochemical and cell-based studies across organisms from E. coli to human isoforms [5]. The glyoxalase system has been reviewed as using intracellular thiols such as glutathione to convert α-ketoaldehydes including methylglyoxal into D-hydroxyacids, with the control of methylglyoxal levels framed as a biochemical requirement because that metabolite can covalently modify proteins, lipids and nucleic acid [6]. Plant studies form a substantial part of the glutathione literature. Reviews of GSH degradation in plants report that, alongside the long-dominant γ-glutamyl cycle model with γ-glutamyl transpeptidase as the principal degradative exoenzyme, cytosolic enzymes such as γ-glutamyl cyclotransferase and γ-glutamyl peptidase degrade GSH inside cells, and that a portion of GSH is degraded after conjugation to other molecules by vacuolar γ-glutamyl transpeptidase, γ-glutamyl peptidase or phytochelatin synthase [7]. Further plant reviews place glutathione at the interface between signalling pathways and metabolic reactions, describing roles in antioxidant chemistry, biosynthesis of sulfur-containing metabolites, inactivation of potentially deleterious compounds, hormonal signalling intensity, and outcomes of pathogen challenge [8]. Work on nuclear thiol redox systems in plants reports accumulation of glutathione in the nucleus, evidence that glutathione reduction is potentially active there, and enrichment of specific glutaredoxin and thioredoxin isoforms in that compartment [9]. Proteomic surveys in plants have catalogued redox post-translational modifications and note that redox-control proteins such as thioredoxin and glutaredoxin appear to serve both in oxidative stress resistance and in signal transduction, while flagging technical challenges in preserving in vivo redox states during sample preparation and mass spectrometry [10]. In cancer-focused preclinical literature, glutathione and glutathione transferases are described as participating in melanin biosynthesis and thiol homeostasis in melanoma cells, and redox traits of these cells are discussed as candidate targets in experimental settings [11]. Reviews of ferritinophagy in organ injury models list glutathione peroxidase 4 among the components of the iron- and reactive oxygen species–linked pathways examined in heart, liver, lung and kidney injury, with the mechanistic evidence drawn from cellular and animal models [12]. The elevated intracellular glutathione reported in tumour cells has been used as a chemical trigger in prodrug and nanomaterial design. Reviews of glutathione-triggered prodrugs catalogue recognition units including disulfides, diselenides, Michael acceptors and sulfonamides/sulfonates, spanning small molecules, polymers and organic–inorganic nanomaterials evaluated in laboratory studies [13]. A parallel review of GSH-responsive linkers in nanomedicine summarises their mechanisms of action and laboratory applications in sensing, diagnostics and drug delivery constructs [14]. Chemodynamic therapy reviews describe Fenton-type reactions triggered by endogenous chemical energy in the acidic, hydrogen peroxide–rich tumour microenvironment, together with strategies for modulating that microenvironment in preclinical systems [15]. Glutathione status is also central to the clinical literature on its precursor N-acetylcysteine. A narrative review of human clinical applications reports that NAC has been used for decades as a mucolytic and as an antidote in acetaminophen poisoning, where it is described as restoring the hepatic GSH pool depleted during drug detoxification, and summarises clinical study evidence across respiratory disease, central nervous system disorders, cardiovascular disease, contrast-induced nephropathy and ophthalmology [16]. Separately, in formulation research on oral delivery of therapeutic peptides and proteins, sulfhydryl compounds such as glutathione in the gastrointestinal tract are described as part of the sulfhydryl barrier, too hydrophilic to enter the lipophilic phase of lipid-based nanocarriers, which the review frames as protecting incorporated peptides from thiol/disulfide exchange; supporting evidence in that review comes from in vivo animal delivery studies [17].