Glutathione disulfide bonds

Figure 42.7 Depiction of the performance of glutathione, disulfide bonds are reduced to a glutathione-responsive drug delivery system thiol groups, which causes the uncapping of consisting of MSNPs capped with PEG through pores and triggers payload release [131], disulfide bonds. In the presence of...

Disulfide bonds arise when the SH groups of two cysteine residues are covalently linked as a dithiol by oxidation. Bonds of this type are only found (with a few exceptions) in extracellular proteins, because in the interior of the cell glutathione (see p.284) and other reducing compounds are present in such high concentrations that disulfides would be reduc-... 

In lipoic acid (6), an intramolecular disulfide bond functions as a redox-active structure. As a result of reduction, it is converted into the corresponding dithiol. As a prosthetic group, lipoic acid is usually covalently bound to a lysine residue (R) of the enzyme, and it is then referred to as lipoamide. Lipoamide is mainly involved in oxidative decarboxylation of 2-0X0 acids (see p. 134). The peptide coenzyme glutathione is a similar disulfide/ dithiol system (not shown see p. 284). 

This enzyme [EC 1.8.4.2], also known as glutathione insulin transhydrogenase and insulin reductase, catalyzes the reaction of two glutathione with a disulfide bond in a protein to produce glutathione disulfide and a protein with two new thiol groups. The enzyme can reduce insulin and a number of other proteins. 

Mechanism of Action Asulfhydryl compound with similar properties to those of penicillamine and glutathione that undergoes thiol-disulfide exchange with cysteine to form tiopronin-cysteine, a mixed disulfide. This disulfide is water soluble, unlike cysteine, and does not crystallize in the kidneys. May break disulfide bonds present in bronchial secretions and break the mucus complexes. Therapeutic Effect Decreases cysteine excretion. 

Glutathione helps to maintain the sulfhydryl groups of proteins in a reduced state. An enzyme, protein-disulfide reductase, catalyzes sulfhydryl disulfide interchanges between glutathione and proteins. The reductase is important in insulin breakdown and may catalyze the reassortment of disulfide bonds during polypeptide chain folding. 

The unique characteristics of DTT and DTE are mainly reflected in their ability to form intramolecular ring structures upon oxidation. Disulfide reductants such as 2-mercaptoethanol, 2-mercaptoethylamine, glutathione, thioglycolate, and 2,3-dimercaptopropanol cleave disulfide bonds in a two-step reaction that involves the II formation of a mixed disulfide (Fig. 66). In the second stage of the reducing process, the... 

In neural cells, the redox status is controlled by the thioredoxin (Trx) and glutathione (GSH) systems that scavenge harmful intracellular ROS. Thioredoxins are antioxidants that serve as a general protein disulphide oxidoreductase (Saitoh et al., 1998). They interact with a broad range of proteins by a redox mechanism based on the reversible oxidation of 2 cysteine thiol groups to a disulphide, accompanied by the transfer of 2 electrons and 2 protons. These proteins maintain their reduced state through the thioredoxin system, which consists of NADPH, thioredoxin reductase (TR), and thioredoxin (Trx) (Williams, Jr. et al., 2000 Saitoh et al., 1998). The thioredoxin system is a system inducible by oxidative stress that reduces the disulfide bond in proteins (Fig. 7.4). It is a major cellular redox system that maintains cysteine residues in the reduced state in numerous proteins. 

Many, but not all, proteins are sensitive to alterations in the oxidation-reduction potential of their environment. The effect is caused in part by oxidation of sulfhydryl groups or reduction of disulfide bonds. Not all proteins are equally sensitive to such alterations, but when they are, it is critical to be aware of their sensitivity. The purification or assay of some proteins can be accomplished only by providing reducing conditions (reduced glutathione, free cysteine, dithiothreitol, or mercap-toethanol) in all buffer solutions. 

In in vitro experiments prolyl isomerase accelerates the oxidative folding of reduced RNase T1 (i.e., folding coupled with formation of the disulfide bonds) and the catalysis of disulfide bond formation by PDl is markedly improved when PPI is present simultaneously (Schonbrunner and Schmid, 1992). The oxidative folding of RNase T1 in the presence of a mixture of reduced and oxidized glutathione is a slow process and it can be followed by the increase in tryptophan fluorescence (Fig. 7). Folding is strictly linked to disulfide bond formation under the conditions... 

Fig. 7. Oxidative refolding of reduced RNase Tl. Reoxidation conditions were 0.1 M Tris-HCl, pH 7.8, 0.2 Af guanidinium chloride, 4 mM reduced glutathione, 0.4 mM oxidized glutathione, 0.2 mM EDTA, and 2.5 nM RNase Tl at 25°C. The kinetics of oxidative refolding were followed by the increase in tryptophan fluorescence intensity at 320 nm ( ), by an unfolding assay (Kiefhaber el ai, 1990b) that measures the formation of native protein molecules (A), and by the increase in the intensity of the band for native RNase Tl in native polyacrylamide gel electrophoresis ( ). Fluorescence emission in the presence of 10 mM reduced dithioerythritol to block disulfide bond formation (O). The small decrease in signal after several hours is caused by slight aggregation of the reduced and unfolded protein. (From Schonbrunner and Schmid (1992).

Acetyl CoA acetyltransferase, a key enzyme of ketogenesis, and 3-oxo-acyl CoA thiolase, involved in -oxidation, bind CoA by formation of a disulfide bond to cysteine, a reaction that can be reversed by glutathione and other sulfhydryl reagents. The physiological significance of this reaction with CoA, which inactivates the enzymes, is not clear (Quandt and Huth, 1984, 1985 Schwerdt and Huth, 1993). 

---