The role of cysteine in the regulation of blood glutathione-protein mixed disulfides in rats treated with diamide

Low molecular mass thiols, disulfides and protein mixed disulfides in rat tissues: influence of sample manipulation, oxidative stress and ageing

Most of the data in studies investigating the contribution of oxidative stress to some human diseases and to ageing derive from measurements carried out in blood, on the basis of the assumption that any alteration of the hematic thiol/disulfide balance should reflect a corresponding alteration in other less accessible tissues. But it is evident that the information that can be gleaned from a direct analysis in specific tissues is largely greater. Nevertheless, the accurate measurement of disulfides is frequently hampered by the artifactual oxidation occurring during sample manipulation as a consequence of the presence of heme-proteins. Therefore, the levels of disulfide forms of low molecular mass thiols in tissues are still poorly investigated, even if their measurements could represent a powerful index of the oxidative status. Here we have used an artifact-free procedure to measure low molecular mass thiols and their disulfides in different rat tissues. Our findings suggest that disulfides are a reliable biomarker of even slight oxidative damage. In tissues of aged rats we observed that either oxidative stress or glutathione depletion alone can occur in different tissues during ageing. Interestingly, among the investigated thiols, only homocysteine showed a tendency to increase in some organs with ageing.

Oxidative stress induces a reversible flux of cysteine from tissues to blood in vivo in the rat

Glutathione (GSH) plays a key role in defense against oxidative stress. The availability of GSH is ensured in tissues by systems devoted to its maintenance in the reduced state and by the flux of GSH and cysteine between sites of biosynthesis and sites of utilization. Little is known about the effect of oxidative stress on the distribution of low-molecular-mass thiols and their exchange rate between tissues. In this study, we found that a slow infusion of diamide (a specific thiol-oxidizing compound) evoked a dramatic increase in blood cysteine in rats. Our data suggest that inter-organ exchange of cysteine occurs, that cysteine derives from both glutathione via gamma-glutamyl transpeptidase and methionine via homocysteine and the trans-sulfuration pathway, and that these pathways are considerably influenced by oxidative stress.

Involvement of vertebrate hemoglobin in antioxidant protection: chicken blood as a model

Redox balance can be described as the equilibrium between oxidative and reductive forces within the cell. These forces control several cellular events, including the modulation of redox-sensitive receptors and signaling pathways. In cells, glutathione is the major non-protein thiol and is considered the main redox buffer. The ratio between the oxidized (GSSG) and reduced (GSH) forms reflects the cellular redox balance. Reactive protein thiols, including vertebrate hemoglobin (Hb), have been proposed as effective antioxidants that can contribute to the redox balance. To further explore this possibility, chicken ( Gallus gallus (L., 1758)) blood was used as a model system. The use of known oxidants (hydroperoxides, diamide, and a system generating reactive oxygen species) originated a pattern of glutathiolation in chicken erythrocytes that was fully reversed after removal of the oxidant, this being consistent with a physiological response. The glutathiolation sequence correlates to kinetic data on chicken Hb cysteine reactivity. The major chicken hemoglobin (Hb A) is responsible for most of the glutathiolated protein where the presence of externally positioned and fast-reacting cysteines is a contributing factor. The antioxidant potential of fast-reacting Hb cysteines is in line with the conservation of cysteine residues in Hb stereochemical positions in more than 95% of the available avian Hb sequences. This may represent an evolutionary trend for the antioxidant function of externally positioned and reactive cysteines in abundant proteins.

Biochemical and biological aspects of protein thiolation in cells and plasma

Protein thiolation is elicited by oxidation by different mechanisms and is involved in a variety of biological processes. Thiols, protein SH (PSH) and non-protein SH groups (NPSH, namely GSH), are in competition in all biological environments in the regulation of oxidant homeostasis because oxidants thiolate proteins, whereas GSH dethiolates them (e.g., GSSG + PSH --> GSSP + GSH). Although poorly investigated, the elimination of disulfides from thiolated proteins to regenerate critical PSH is important. These aspects are poorly known in cells, where glutaredoxin and peroxiredoxin operate as enzymes or potential chaperones to accelerate dethiolation. On the contrary, studies with plasma or albumin have highlighted the importance of protein conformation in dethiolation processes and have clarified the reason why homocysteine (thiol with potential toxicity) is preferentially bound to albumin as protein-thiol mixed disulfide with respect to other NPSH. Here we provide an overview of protein thiolation/dethiolation processes, with an emphasis on recent developments and future perspectives in this field.

The effects of age and hyperhomocysteinemia on the redox forms of plasma thiols

We assayed the redox forms of cysteine (reduced [CSH], oxidized [CSSC], and bound to protein [CS-SP]), cysteinylglycine (CGSH; cysteinylgycine disulfide [CGSSGC] and cysteinylglycine-protein mixed disulfide [CGS-SP]), glutathione (GSH; glutathione disulfide [GSSG] and glutathione-protein mixed disulfide [GS-SP]), homocysteine (Hcy; homocystine [HcyS] and homocystine-protein mixed disulfides [bHcy]), and protein sulfhydryls in the plasma of healthy subjects (divided into 8 groups ranging in age from birth to 70 years) and patients with mild hyperhomocysteinemia associated with cardiovascular disease (heart-transplant patients) or vascular atherosclerosis, with or without renal failure. In healthy individuals, levels of disulfides and protein-mixed disulfides were more abundant than those of thiols, and those of protein-thiol mixed disulfides were higher than disulfides. Concentrations of CSH, GSH, and CGSH in the various groups had profiles characterized by a maximum over time. The concentration of Hcy was unchanged up to the age of 30 years, after which it increased. CSSC concentration increased gradually with age, whereas concentrations of the other disulfides were essentially unchanged. By contrast, the concentrations of all protein-thiol mixed disulfides, especially those with CSH, increased gradually with age. Ranks of distribution of the reduced forms changed with age (at birth, CSH > CGSH > GSH > Hcy; in 1- to 2-year-olds, CSH > GSH > CGSH > Hcy; and in 51- to 70-year-olds, CSH > CGSH = GSH > Hcy), whereas those of disulfides and protein-thiol mixed disulfides were substantially unchanged (in all age groups, CSSC > CGSSGC > GSSG = HcyS and CS-SP > CGS-SP > bHcy > GS-SP). In patients with pathologic conditions, plasma levels of disulfide forms CSSC, HcyS, CS-SP, and bHcy were significantly increased, whereas other redox forms of thiols were unchanged or showed variations opposite (increasing or decreasing) to control values. Maximal increases in disulfides and protein-thiol mixed disulfides were associated with renal failure. Our data suggest that increases in plasma bHcy concentrations in subjects with pathologic conditions were more likely the result of activation of thiol-disulfide exchange reactions between free reduced Hcy and CS-SP than of a direct action of reactive oxygen species.

Responses of thiols to an oxidant challenge: differences between blood and tissues in the rat

Treatment of rats with diamide (100 mg/kg i.p.) altered the thiol components of the blood to a very different extent than in tissues (liver, kidney, lung, spleen, heart and testis). A total consumption (10 min) and regeneration (120 min) of blood glutathione (GSH), matched by a parallel increase and decrease in glutathione-protein mixed disulfides (GS-SP) was observed. In contrast, no modification of non-protein SH groups (NPSH) and protein SH groups (PSH), GS-SP and malondialdehyde (MDA) was observed in liver, kidney, lung, testis spleen and heart within same time range. In particular, only glutathione disulfide (GSSG) levels and some activities of antioxidant enzymes were modified to a small extent and in an opposite direction in some organs. For example, GSSG, and glucose-6-phosphate dehydrogenase (G-6-PDH) and catalase (CAT) activities appeared up-regulated in one tissue and down-regulated in another. The least modified organ was the liver, whereas lung and spleen were the most affected (lung, GSSG, significantly increased whereas G-6-PDH, glutaredoxin (GRX), GPX, superoxide dimutase (SOD) levels were significantly lowered; spleen, GSSG and the activity of glutathione reductase (GR), G-6-PDH and glutathione transferase (GST) were significantly decreased). The different responses of erythrocytes and organs to diamide were explained by the high affinity of hemoglobin and by the relatively high potential of thiol regeneration in organs. The rapid reversibility of the process of protein S-thiolation in blood and the small effects in organs leads us to propose the existence of an inter-organ cooperation in the rat that regulates protein S-thiolation in blood. Plasma thiols may well play a role in this process.

Effect of bovine small intestine thioredoxin on aldose reductase activity

Under oxidative stress mediated by H(2)O(2), significant activation of purified aldose reductase from bovine small intestine was observed in the presence of purified thioredoxin from bovine small intestine.

Minor thiols cysteine and cysteinylglycine regulate the competition between glutathione and protein SH groups in human platelets subjected to oxidative stress

Changes in the concentrations of protein-mixed disulfides (XS-SP) of glutathione (GSH), cysteine (CSH), and cysteinylglycine (CGSH) were studied in human platelets treated with diamide and t-BOOH in timecourse experiments (time range, 1-30 min) in order to understand the contribution of minor thiols CSH and CGSH to the regulation of glutathione-protein mixed disulfides (GS-SP). Diamide was much more potent than t-BOOH in altering the platelet thiol composition of XS-SP (threshold dose: diamide, 0.03 mM; t-BOOH, 0.5 mM) and caused reversible XS-SP peaks whose magnitude was related to the concentration of free thiols in untreated cells. Thus maximum levels of GS-SP (8 min after 0.4 mM diamide) were about 16-fold higher than those of controls (untreated platelets, GS-SP = 0.374 nmol/10(9) platelets), whereas those of CS-SP and CGS-SP were only 4-fold increased (untreated platelets, CS-SP = 0.112 nmol/10(9) platelets; CGS-SP = 0.024 nmol/10(9) platelets). The greater effects of diamide with respect to t-BOOH were explained on the basis of the activities of fast reactive protein SH groups for diamide and glutathione reductase (GR) and glucose-6-phosphate dehydrogenase (G-6-PDH) for t-BOOH. The addition of cysteine (0.3 mM, at 4 min) after treatment of platelets with 0.4 mM diamide increased the rate of reversal of GS-SP peaks to normal values, but also caused a relevant change in CGS-SP with respect to that of platelets treated with diamide alone. An increased gamma-glutamyltranspeptidase activity was found in platelets treated with diamide. Moreover, untreated platelets were found to release and hydrolyze GSH to CGSH and CSH. Ratios of thiols/disulfides (XSH/XSSX) and activities of GR and G-6PDH were also related to a high reducing potential exerted by GSH but not by minor thiols. The lower mass and charge of minor thiols is a likely requisite of the regulation of GS-SP levels in platelets.

Chemopreventive effects of 2-(allylthio)pyrazine

A series of organosulfur compounds were synthesized with the aim of developing chemopreventive compounds active against hepatotoxicity and chemical carcinogenesis. 2-(Allylthio) pyrazine (2-AP) was effective in inhibiting cytochrome P450 2E1-mediated catalytic activities and protein expression, and in inducing microsomal epoxide hydrolase and major glutathione S-transferases. 2-AP reduced the hepatotoxicity caused by toxicants and elevated cellular GSH content. Development of skin tumors, pulmonary adenoma and aberrant crypt foci in colon by various chemical carcinogens was inhibited by 2-AP pretreatment. Anticarcinogenic effects of 2-AP at the stage of initiation of tumors were also observed in the aflatoxin B1 (AFB1)-induced three-step medium-term hepatocarcinogenesis model. Reduction of AFB1-DNA adduct by 2-AP appeared to result from the decreased formation of AFB1-8,9-epoxide via suppression of cytochrome P450, while induction of GST by 2-AP increases the excretion of glutathione-conjugated AFB1. 2-AP was a radioprotective agent effective against the lethal dose of total body irradiation and reduced radiation-induced injury in association with the elevation of detoxifying gene expression. 2-AP produces reactive oxygen species in vivo, which is not mediated with the thiol-dependent production of oxidants and that NF-kappa B activation is not involved in the induction of the detoxifying enzymes. The mechanism of chemoprotection by 2-AP may involve inhibition of the P450-mediated metabolic activation of chemical carcinogens and enhancement of electrophilic detoxification through induction of phase II detoxification enzymes which would facilitate the clearance of activated metabolites through conjugation reaction.