Glutathione: an overview of biosynthesis and modulation

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Abstract

Glutathione (GSH; γ-glutamylcysteinylglycine) is ubiquitous in mammalian and other living cells. It has several important functions, including protection against oxidative stress. It is synthesized from its constituent amino acids by the consecutive actions of γ-glutamylcysteine synthetase and GSH synthetase. γ-Glutamylcysteine synthetase activity is modulated by its light subunit and by feedback inhibition of the end product, GSH. Treatment with an inhibitor, buthionine sulfoximine (BSO), of γ-glutamylcysteine synthetase leads to decreased cellular GSH levels, and its application can provide a useful experimental model of GSH deficiency. Cellular levels of GSH may be increased by supplying substrates and GSH delivery compounds. Increasing cellular GSH may be therapeutically useful.

Introduction

This chapter seeks to give a brief overview of glutathione function, synthesis and modulation of its intracellular levels. There are too many papers on glutathione to cite them all, but for reviews see 1, 2, 3, 4, 5, 6. Many of the pioneering studies on glutathione synthesis, metabolism and function were led by the late Dr Alton Meister.

Glutathione (GSH) is a tripeptide of glutamate, cysteine and glycine that contains an unusual γ-peptide bond between glutamate and cysteine (Fig. 1). Such a bond prevents GSH from being hydrolyzed by most peptidases. GSH is less easily oxidized than its precursors, cysteine and γ-glutamylcysteine. GSH is found in most mammalian and many prokaryotic cells and is the most abundant intracellular thiol (0.2–10 mM). Intracellularly, GSH is kept in its thiol form by glutathione disulfide (GSSG) reductase, a NADPH-dependent enzyme. GSH has several important cellular functions (Fig. 2). GSH participates as a coenzyme and is involved in amino acid transport. It is involved in metabolism and the maintenance of the thiol moieties of proteins and low molecular weight compounds, such as cysteine and coenzyme A. GSH is also involved in maintaining ascorbic acid in its reduced form and in the formation of deoxyribonucleotides. GSH reacts enzymatically (GSH S-transferase family) or non-enzymatically with toxic compounds to form GSH conjugates. It also protects against oxidative damage caused by reactive oxygen species (ROS) that may be formed normally in metabolism. GSH may react with ROS non-enzymatically. Hydrogen peroxides and other peroxides are detoxified by GSH peroxidase.

Section snippets

GSH biosynthesis

GSH is synthesized intracellularly by the consecutive actions of γ-glutamylcysteine (reaction 1) and GSH (reaction 2) synthetases:

l-Glu+l-Cys+ATP⇔l-γ-Glu−l-Cys+ADP+Pi  (1)

l-γ-Glu-Cys+Gly+ATP⇔GSH+ADP+Pi  (2)

Cysteine is usually the limiting substrate in the synthesis of GSH. Intracellular GSH is exported from most cells, but it is not significantly taken up by cells under normal conditions [5]. Once outside of the cell, the γ-glutamyl bond of GSH may be cleaved by the membrane bound γ-glutamyl

Inborn errors of glutathione biosynthesis enzymes

Inborn metabolic deficiencies have been described for several GSH-related enzymes 41, 42, 43, 44, 45. While rare, there are cases of γ-glutamylcysteine synthetase deficiency. Deficiencies of GSH synthetase occur more frequently than with the first enzyme. Both γ-glutamylcysteine synthetase and GSH synthetase deficiencies are characterized by hemolytic anemia and neurological symptoms. It should be noted that these are deficiencies not complete absence of enzyme activities. For a more detailed

Methods for increasing cellular GSH levels

Increased cellular GSH levels may be beneficial in conditions where GSH levels are decreased. Administration of cysteine may raise cellular GSH levels because it is usually the limiting amino acid in GSH biosynthesis [5]. Besides the problems of oxidation to cystine which has low solubility, cysteine has been reported to be toxic to cultured cells [60]and to newborn mice [61]. While the mechanism of toxicity is not known, there are several possibilities 62, 63. Methionine is a precursor of

Acknowledgements

The support by (AI 31804) from the National Institutes of Health and Transcend Therapeutics is acknowledged.

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