GSSG

The FAD-dependent enzyme glutathione reductase plays a role in the antioxidant system. Glutathione reductase restores reduced glutathione (GSH), the most important antioxidant in erythrocytes, from oxidized glutathione (GSSG) [1, 2]. 

Figure 15.11 Possible scheme for the formation of free radicals from the metabolism of dopamine. Normally hydrogen peroxide formed from the deamination of DA is detoxified to H2O along with the production of oxidised glutathione (GSSG) from its reduced form (GSH), by glutathione peroxidase. This reaction is restricted in the brain, however, because of low levels of the peroxidase. By contrast the formation of the reactive OH-radical (toxification) is enhanced in the substantia nigra because of its high levels of active iron and the low concentration of transferin to bind it. This potential toxic process could be enhanced by extra DA formed from levodopa in the therapy of PD (see Olanow 1993 and Olanow et al. 1998)...

GSSG/GSH Liver Mallard duck No Increased Hoffman and Heinz (1998)... 

The possible role of cellular glutathione status in the controlling sarcolemmal protein activity has been addressed by studying the effect of GSH, GSSG and several other thiol and disulphide compounds on Na/K ATPase activity using (1) an isolated bovine ventricular Na/K ATPase preparation (2) crude sarcolemmal preparations (biochemical studies) (c) Langendorff-perfiised isolated hearts (cytochemical studies) and (4) isolated ventricular myocytes (electrophysiologjcal studies). 

If the cardiac redox state (reflected by fluctuations in the ratio of GSH and GSSG) is important in the regulation of the function of some enzymes, the manipulation of this ratio may result in parallel changes in enzyme activity. Thus, a reduction of Na/K ATPase activity associated with the application of 1 mM GSSG has been shown to be reversed and completely overcome, in a concentration-dependent manner, by the subsequent addition of GSH (O.l-l.OmM) (Haddock et al., 1991) (see Fig. 4.10). 

Figure 4.9 Effect of reduced glutathione (GSH) (0.25-1.0 ihm) and oxidized glutathione (GSSG) (0.25-1.0 mM) on ouabain-sensitive Na/K ATPase activity. An isolated Na/K ATPase preparation was prepared from fresh bovine ventricular tissue. Na/K ATPase activity was determined and quantified by the ouabain-sensitive hydrolysis of ATP to yield Inorganic phosphate. The rate of inorganic phosphate production was compared prior to and following the addition of either GSH or GSSG to the Incubation mixture. The data are presented as...

Additional studies have also demonstrated that the application of other disulphides, such as oxidized dithiothreitol and cystine, produce similar effects to GSSG over an identical concentration range (Haddock et al., 1991). Therefore the Na/K ATPase activity may be modified by disulphides derived from a variety of species. 

These data demonstrate that both GSH and GSSG have profound effects on Na/K ATPase activity and may act in concert to modify enzyme activity during oxidant stress. However, it should be recognized that the steric conformation of an isolated enzyme preparation in a chemically buffered solution may be considerably different to the native enzyme located in a dynamic lipid bilayer. For this reason, these investigations have been extended to include a variety of preparations in which the Na/K pump is in its native environment. 

Whilst experimentally it is relatively easy to investigate the eflFect of the exogenous application of GSH and GSSG on cardiac Na/K ATPase activity, one further approach that has been exploited in many aspects of oxidant-induced cell injury has been the depletion of cellular glutathione levels. The hypothesized importance of GSH in the cell s antioxidant armoury would be expected to be reflected in an increased susceptibility to oxidant stress-... 

Preliminary data, in which extracellular and intracellular glutathione status were altered in the absence of a ffee-radical-induced oxidant stress have demonstrated a similar effect on /Na/K- In these experiments the extracellular concentration of GSH was altered by varying the levels in the extracellular perfusate. In contrast, the intracellular GSH content was controlled by the inclusion of the required concentration of GSH in the patch pipette. After 5 min exposure to GSH there was an approximately 20% increase in /na/K at 0 mV. Conversely, in a separate group of cells, 5 min after the application of GSSG there was an approximately 15% decrease in /wa/K (Haddock, 1991). 

S-Thiolation of proteins may occur by two main processes as shown in Fig. 4.14. The first relies on an increase in GSSG levels while the second method depends on free radical production and the GSH concentration (Miller et al., 1990). Therefore, it is clear that a significant increase in tissue GSSG levels is not an absolute prerequisite for S-thiolation to occur. 

If cellular redox state, determined by the glutathione status of the heart, plays a role in the modulation of ion transporter activity in cardiac tissue, it is important to identify possible mechanisms by which these effects are mediated. Protein S-,thiolation is a process that was originally used to describe the formation of adducts of proteins with low molecular thiols such as glutathione (Miller etal., 1990). In view of the significant alterations of cardiac glutathione status (GSH and GSSG) and ion-transporter activity during oxidant stress, the process of S-thiolation may be responsible for modifications of protein structure and function. 

GSHPx Glutathione peroxidase GSSG Glutathione (oxidized)... 

Glutathione reductase (GR) catalyzes the reduction of oxidized glutathione (GSSG) to reduced glutathione (GSH) using NADPH provided from the hexose monophosphate pathway. GR, a ubiquitous flavoenzyme, maintains a high value of two for the GSH/GSSG ratio in the red blood cells. l,3-Bis(2-chloroethyl)-nitrosourea (BCNU) selectively inhibits cellular GR. GR is composed of two identical subunits, each of molecular mass 50 kDa (S8). The three-dimensional structure and mechanism of catalysis have been established for human GR (K17). 

Suzuki, H., Sugiyama, Y., Excretion of GSSG and glutathione conjugates mediated by MRP1 and cMOAT/ MRP2, Semin. Liver Dis. 1998, 18, 359-376. 

III. Glutathione reductase (EC 1.6.4.2) It is a flavoprotein that catalyzes the NADPH-dependent reduction of oxidized glutathione (GSSG) to glutathione (GSH). This enzyme is essential for the GSH redox cycle which maintains adequate levels of reduced cellular GSH. A high GSH/GSSG ratio is essential for protection against oxidative stress. 

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