Glutathione: overview of its protective roles, measurement, and biosynthesis

Abstract

This review is the introduction to a special issue concerning, glutathione (GSH), the most abundant low molecular weight thiol compound synthesized in cells. GSH plays critical roles in protecting cells from oxidative damage and the toxicity of xenobiotic electrophiles, and maintaining redox homeostasis. Here, the functions and GSH and the sources of oxidants and electrophiles, the elimination of oxidants by reduction and electrophiles by conjugation with GSH are briefly described. Methods of assessing GSH status in the cells are also described. GSH synthesis and its regulation are addressed along with therapeutic approaches for manipulating GSH content that have been proposed. The purpose here is to provide a brief overview of some of the important aspects of glutathione metabolism as part of this special issue that will provide a more comprehensive review of the state of knowledge regarding this essential molecule.

Fig. 1

Glutathione structure. A stereochemical and ball and stick figure showing γ-glutamyl-cysteinyl-glycine are shown.

Fig. 2

Redox cycling of 1,4-naphthoquinones. A naphthoquinone with two variable groups (R) can be reduced by NADPH (or NADH, which is not shown) enzymatically to the semiquinone radical and then will react with oxygen to generate superoxide and restore the naphthoquinone.

Fig. 3

Reactions of glutathione with hypochlorous acid. GSH and HOCl can react to produce several different products.

Fig. 4

Formation of protein mixed disulfide. Both glutathione peroxidases and peroxiredoxin 6 can catalyze the oxidation of glutathione by hydrogen peroxide to glutathione disulfide and water. GSSG can then undergo an exchange reaction with protein sulfhydryl to form PSSG, which is usually catalyzed by a protein disulfide isomerase. An alternative mechanism is the oxidation of a protein thiolate to a sulfenic acid, which then will react with GSH to form PSSG and water.

Fig. 5

Glutathione conjugations with 4-hydroxynonenal. Glutathione S-transferases catalyze the conjugation of GSH with HNE. This is a Michael addition that can slowly occur non-enzymatically.

Fig. 6

Measurements of thiols. (a) Reaction of GSH with DTNB produces an adduct and TNB, which is measured spectrofluorometrically or spectrophometrically; (b) total glutathione can be determined by recycling of GSSG produced in the reaction in (a) and measuring the rate of TNB; (c) Glutathione and related compounds are first derivatized with iodoacetate followed by a second derivatization with 1-fluoro-2,4-dinitrophenol. The second products are then separated by HPLC and measured spectrofluorometrically; (d) Reaction of glutathione with orthophthaldehyde (OPT) yields a product that can be measured spectrofluorometrically.

Fig. 6

Measurements of thiols. (a) Reaction of GSH with DTNB produces an adduct and TNB, which is measured spectrofluorometrically or spectrophometrically; (b) total glutathione can be determined by recycling of GSSG produced in the reaction in (a) and measuring the rate of TNB; (c) Glutathione and related compounds are first derivatized with iodoacetate followed by a second derivatization with 1-fluoro-2,4-dinitrophenol. The second products are then separated by HPLC and measured spectrofluorometrically; (d) Reaction of glutathione with orthophthaldehyde (OPT) yields a product that can be measured spectrofluorometrically.

Fig. 7

Glutathione synthesis. The sequential ATP dependent formation of amide bonds between cysteine and the γ-carboxyl group of glutamate and then between glycine and cysteine are shown.