The Key Role of GSH in Keeping the Redox Balance in Mammalian Cells: Mechanisms and Significance of GSH in Detoxification via Formation of Conjugates

Abstract

Glutathione (GSH) is a ubiquitous tripeptide that is biosynthesized in situ at high concentrations (1-5 mM) and involved in the regulation of cellular homeostasis via multiple mechanisms. The main known action of GSH is its antioxidant capacity, which aids in maintaining the redox cycle of cells. To this end, GSH peroxidases contribute to the scavenging of various forms of ROS and RNS. A generally underestimated mechanism of action of GSH is its direct nucleophilic interaction with electrophilic compounds yielding thioether GSH S-conjugates. Many compounds, including xenobiotics (such as NAPQI, simvastatin, cisplatin, and barbital) and intrinsic compounds (such as menadione, leukotrienes, prostaglandins, and dopamine), form covalent adducts with GSH leading mainly to their detoxification. In the present article, we wish to present the key role and significance of GSH in cellular redox biology. This includes an update on the formation of GSH-S conjugates or GSH adducts with emphasis given to the mechanism of reaction, the dependence on GST (GSH S-transferase), where this conjugation occurs in tissues, and its significance. The uncovering of the GSH adducts' formation enhances our knowledge of the human metabolome. GSH-hematin adducts were recently shown to have been formed spontaneously in multiples isomers at hemolysates, leading to structural destabilization of the endogenous toxin, hematin (free heme), which is derived from the released hemoglobin. Moreover, hemin (the form of oxidized heme) has been found to act through the Kelch-like ECH associated protein 1 (Keap1)-nuclear factor erythroid 2-related factor-2 (Nrf2) signaling pathway as an epigenetic modulator of GSH metabolism. Last but not least, the implications of the genetic defects in GSH metabolism, recorded in hemolytic syndromes, cancer and other pathologies, are presented and discussed under the framework of conceptualizing that GSH S-conjugates could be regarded as signatures of the cellular metabolism in the diseased state.

Keywords: GSH S-conjugates; GSH adducts; GSH metabolism; GSH redox cycle; GSH-hematin adducts; GSTs; hemolytic disorders; metabolic signatures; thioether bonds; xenobiotics.

Figure 1

Multifaceted intracellular metabolism of GSH. Cysteine, as the rate-limiting substrate for GSH’s biosynthesis, can be supplied to cells indirectly by the cystine/glutamate antiporter (xCT). Subsequently, the intracellularly imported cystine is reduced to cysteine. Moreover, GSH can itself supply cysteine by the sequential action of extracellular gamma-glutamyltranspeptidase (gamma-GT) and dipeptidases (DP) releasing cysteine that is transported inside cells by common amino-acid transporters (AATs). Glutamate cysteine ligase (GCL) and GSH synthetase (GSS) act in the cytosol to synthesize GSH, serving the coenzyme to glutathione S-transferases (GSTs) and glutathione peroxidase (GPx). GSH is transported in the endoplasmic reticulum (ER), nucleus and mitochondria via multidrug-associated resistance protein (MRP) and porins, respectively. In addition, GSSG reductase (GR) regenerates the GSH pool. Different oxidants, xenobiotics and anti-oxidative compounds induce the GSH biosynthesis, via the NRF2 transcription factor, by upregulating GCL and/or xCT. Parts of the figure have been acquired by Smart Servier Medical Art.

Figure 2

GSH employs multiple biochemical pathways to support key roles in mammalian cells and maintain the redox cycle. By taking into account all these functions and enzymes involved, GSH is indeed a key metabolite. GSH scavenges various ROS either directly or via the action of GSH peroxidases, where GSH is a co-factor. Moreover, GSH serves as a stable storage of the sulfhydryl group, while the self-regenerative capacity of GSH, by the GSH reductase, contributes to maintain its cellular levels. GSH biosynthesis occurs at high rates at many organs induced by oxidative stress or other stimuli. RBCs, liver, and skeletal muckle support plasma GSH’s levels, while liver (via the gamma-GT) is able to accelerate the uptake of GSH’s precursors. Finally, GSH forms various GSH S-conjugates with many different structurally compounds acting as the substrates, which is an event of vast importance in cell protection.

Figure 3

Representative compounds, of various origin, forming GSH S-conjugates are presented. In the figure, indicative pictures of the origin (source) as well as the structure of the reactant are presented to point out the diversified roles of GSH, reacting with several compounds. These include the pollutant, bisphenol A found in plasticizers and correlated with tumorigenesis, the drugs (cisplatin and simvastatin), whose conjugation with GSH is involved in chemoresistance (with diglutathionyl-platinum adducts) and elimination/clearance of the body, respectively. Furthermore, the list includes the derivatives (quinones) of 17 β-estradiol that act as promoters of tumorigenesis. The intratumor formation of catechol estrogen quinone–GSH conjugates is proposed to have a prognostic value. GSH–hematin conjugates have been detected in RBC hemolysates and may have an important role in the detoxification of free heme. Chemical structures were retrieved by PubChem, and parts of the figure were acquired by the Smart Servier Medical Art. Pubchem CIDs: Hematin (449355), Simvastatin hydroxyl acid D6 Sodium Salt (119090975), Platinum II aqua form (138393911, adjusted by ChemDraw Ultra 8.0, according to [249]), 4,5-Bisphenol-o-quinone (656690), and 17b estradiol 2–3 quinone (131769816).