Where is glutathione in the cell

Recent studies provide convincing data to support the view that cysteine is generally the limiting amino acid for GSH synthesis in humans, as in rats, pigs, and chickens 6 , 14 , Thus, factors e. In addition, increasing the supply of cysteine or its precursors e. Because cysteine generated from methionine catabolism via the transsulfuration pathway primarily in hepatocytes serves as a substrate for GCS, dietary methionine can replace cysteine to support GSH synthesis in vivo.

Cysteine is readily oxidized to cystine in oxygenated extracellular solutions. Cysteine and cystine are transported by distinct membrane carriers, and cells typically transport one more efficiently than the other 8. It is interesting that some cell types e. However, GSH that effluxes from the liver can reduce cystine to cysteine on the outer cell membrane, and the resulting cysteine is taken up by hepatocytes.

Other cell types e. Extracellular and intracellularly generated glutamate can be used for GSH synthesis Because dietary glutamate is almost completely utilized by the small intestine 16 , plasma glutamate is derived primarily from its de novo synthesis and protein degradation. Phosphate-dependent glutaminase, glutamate dehydrogenase, pyrrolinecarboxylate dehydrogenase, BCAA transaminase, and glutamine:fructosephosphate transaminase may catalyze glutamate formation Fig.

Indeed, glutamine is an effective precursor of the glutamate for GSH synthesis in many cell types, including enterocytes, neural cells, liver cells, and lymphocytes Thus, glutamine supplementation to total parenteral nutrition maintains tissue GSH levels and improves survival after reperfusion injury, ischemia, acetaminophen toxicity, chemotherapy, inflammatory stress, and bone marrow transplantation When extracellular glutamate concentrations are high, as in patients with advanced cancer, HIV infection, and spinal cord or brain injury as well as in cell culture medium containing high levels of glutamate, cystine uptake is competitively inhibited by glutamate, resulting in reduced GSH synthesis When intracellular glutamate concentrations are unusually high, as in canine erythrocytes, GSH synthesis is enhanced and its concentration is particularly high 9.

Glycine availability may be reduced in response to protein malnutrition, sepsis, and inflammatory stimuli 21 , When hepatic glycine oxidation is enhanced in response to high levels of glucagon or diabetes 23 , this amino acid may become a limiting factor for GSH synthesis.

In vivo studies show that glycine availability limits erythrocyte GSH synthesis in burned patients 7 and in children recovering from severe malnutrition The evidence indicates that the dietary amino acid balance has an important effect on protein nutrition and therefore on GSH homeostasis 8. In particular, the adequate provision of sulfur-containing amino acids as well as glutamate glutamine or BCAAs and glycine or serine is critical for the maximization of GSH synthesis.

Thus, in the erythrocytes of children with edematous protein-energy malnutrition and piglets with protein deficiency, GSH synthesis is impaired, leading to GSH deficiency 3.

Glutathione can be transported out of cells via a carrier-dependent facilitated mechanism 2. Plasma GSH originates primarily from the liver, but some of the dietary and intestinally derived GSH can enter the portal venous plasma 8. The extreme concentration gradient across the plasma membrane makes the transport of extracellular GSH or GSSG into cells thermodynamically unfavorable.

The kidney, lung, and intestine are major consumers of the liver-derived GSH 8. The interorgan metabolism of GSH functions to transport cysteine in a nontoxic form between tissues, and also helps to maintain intracellular GSH concentrations and redox state 8.

Glutathione participates in many cellular reactions. First, GSH effectively scavenges free radicals and other reactive oxygen species e. In addition, glutathione peroxidase a selenium-containing enzyme catalyzes the GSH-dependent reduction of H 2 O 2 and other peroxides Second, GSH reacts with various electrophiles, physiological metabolites e.

These reactions are initiated by glutathione- S -transferase a family of Phase II detoxification enzymes. In addition, both NO and GSH are necessary for the hepatic action of insulin-sensitizing agents 27 , indicating their critical role in regulating lipid, glucose, and amino acid utilization. The removal of formaldehyde a carcinogen is of physiological importance, because it is produced from the metabolism of methionine, choline, methanol alcohol dehydrogenase , sarcosine sarcosine oxidase , and xenobiotics via the cytochrome Pdependent monooxygenase system of the endoplasmic reticulum.

Fifth, GSH is required for the conversion of prostaglandin H 2 a metabolite of arachidonic acid into prostaglandins D 2 and E 2 by endoperoxide isomerase 8. Sixth, GSH is involved in the glyoxalase system, which converts methylglyoxal to d -lactate, a pathway active in microorganisms.

Finally, glutathionylation of proteins e. Thus, GSH serves vital functions in animals Table 1. Adequate GSH concentrations are necessary for the proliferation of cells, including lymphocytes and intestinal epithelial cells Glutathione also plays an important role in spermatogenesis and sperm maturation 1. In addition, GSH is essential for the activation of T-lymphocytes and polymorphonuclear leukocytes as well as for cytokine production, and therefore for mounting successful immune responses when the host is immunologically challenged 2.

Further, both in vitro and in vivo evidence show that GSH inhibits infection by the influenza virus Technically N-L-gamma-glutamyl-cysteinyl glycine or L-glutathione, the molecule has a sulfhydryl SH group on the cysteinyl portion, which accounts for its strong electron-donating character. As electrons are lost, the molecule becomes oxidized, and two such molecules become linked dimerized by a disulfide bridge to form glutathione disulfide or oxidized glutathione GSSG.

This linkage is reversible upon re-reduction. GSH is under tight homeostatic control both intracellularly and extracellularly. First, cysteine and glutamate are combined by gamma-glutamyl cysteinyl synthetase. Fasting, protein-energy malnutrition, or other dietary amino acid deficiencies limit GSH synthesis.

The reducing power of ascorbate helps conserve systemic GSH. GSH is used as a cofactor by 1 multiple peroxidase enzymes, to detoxify peroxides generated from oxygen radical attack on biological molecules; 2 transhydrogenases, to reduce oxidized centers on DNA, proteins, and other biomolecules; and 3 glutathione S-transferases GST to conjugate GSH with endogenous substances e.

Direct attack by free radicals and other oxidative agents can also deplete GSH. The GSH levels were measured using a colorimetric assay kit. FHC cells as determined by analysis of variance and Tukey's tests. GSH, glutathione. The ratios indicate the expression levels in each cancer cell line relative to those in the normal FHC cell line. The expression levels of GSH were then examined in the normal and cancerous colon tissue samples of 15 patients undergoing treatment at the Jeju National University Hospital.

In nine out of 15 patients, GSH expression levels were higher in the tumor tissue samples, as compared with the corresponding normal tissue samples Fig.

Furthermore, immunohistochemical analyses of the colon tissue sections revealed low protein expression levels of GCLC and GSS in normal mucosa, whereas diffuse and high expression levels of these proteins were detected in the cancerous tissue Fig. GSH expression levels tissue from patients with colon cancer. The GSH expression levels were measured using a colorimetric assay kit.

GSH, glutathione; N, normal; T, tumor. Expression levels of GSH synthetic enzymes in tissue from patients with colon cancer. GSH is a major cellular antioxidant and is crucial for maintaining the balance between oxidation and antioxidation 2 , GSH is also important in cellular detoxification and is associated with numerous aspects of the immune response 2.

Enhanced resistance to chemotherapeutic drugs and radiotherapy is associated with GSH-conjugation and detoxification 5 — 7.

Previous studies have reported that the expression levels of GSH are elevated in numerous types of human cancer tissue, and increased expression levels of GCLC have also been identified in lung, breast, liver, and other types of cancer 19 , Furthermore, a previous study demonstrated that drug-resistance is associated not only with elevated GSH levels, but also with increased activity of GCLC Similar results were also observed in cancerous and normal tissue samples, which were collected from 15 patients with colon cancer.

The relatively high expression levels of GCLC and GSS in colon cancer tissue, as compared with normal tissue, were confirmed by an immunohistochemical analysis. In conclusion, the results of the present study suggest that the expression levels of GSH and GSH metabolizing enzymes may serve as clinically useful biomarkers of colon cancer, as well as potential targets for anticancer drugs.

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