Glutathionyl haemoglobin is not increased in diabetes nor related to glycaemia, complications, dyslipidaemia, inflammation or other measures of oxidative stress

Membrane Skeletal Protein S-Glutathionylation in Human Red Blood Cells as Index of Oxidative Stress

Glutathione (GSH) is one of the most well-studied biomarkers of oxidative stress. Under oxidizing conditions, GSH is transformed into its disulfide forms, glutathione disulfide (GSSG) and S-glutathionylated proteins (PSSG), which are considered to be reliable biomarkers of oxidative stress. In red blood cells (RBCs), the main targets of S-glutathionylation are hemoglobin and membrane-associated skeletal proteins, but S-glutathionylated hemoglobin (HbSSG) has been more thoroughly studied as a biomarker of oxidative stress than S-glutahionylated RBC membrane skeletal proteins. Here, we have investigated whether and how all these biomarkers are altered in human RBCs treated with a slow and cyclically intermittent flux of the oxidant tert-butyl hydroperoxide. To this aim, a new device for sample treatment and collection was developed. During and at the end of the treatment, GSH, GSSG, and PSSG (discriminating between HbSSG and membrane PSSG) were measured by the use of spectrophotometer (for GSSG) and HPLC (for GSH, HbSSG, and membrane PSSG). The main results of our study are as follows: (i) GSH decreased and GSSG increased, but only in the presence of the oxidant, and recovered their initial values at the end of the infusion; (ii) the increase in total PSSG concentration was lower than that of GSSG, but it kept on throughout the experiments; (iii) membrane skeletal proteins did not recover their initial values, whereas HbSSG levels recovered their initial values similarly to GSH and GSSG; (d) membrane skeletal PSSG were more stable and also more abundant than HbSSG. Western blot analysis indicated spectrin, ankyrin, and bands 3, 4.1, and 4.2 as the proteins most susceptible to S-glutathionylation in RBC membrane. These results suggest that S-glutathionylated membrane skeletal proteins can be considered as a suitable biomarker of oxidative stress. Mostly when the oxidant insult is slight and intermittent, PSSG in RBC membranes are worth measuring in addition to GSSG by virtue of their greater stability.

S-glutathionylation: relevance in diabetes and potential role as a biomarker

Glutathione is considered the main regulator of redox balance in the cellular milieu due to its capacity for detoxifying deleterious molecules. The oxidative stress induced as a result of a variety of stimuli promotes protein oxidation, usually at cysteine residues, leading to changes in their activity. Mild oxidative stress, which may take place in physiological conditions, induces the reversible oxidation of cysteines to sulfenic acid form, while pathological conditions are associated with higher rates of reactive oxygen species production, inducing the irreversible oxidation of cysteines. Among these, neurodegenerative disorders, cardiovascular diseases and diabetes have been proposed to be pathogenetically linked to this state. In diabetes-associated vascular complications, lower levels of glutathione and increased oxidative stress have been reported. S-glutathionylation has been proposed as a posttranslational modification able to protect proteins from over-oxidizing environments. S-glutathionylation has been identified in proteins involved in diabetic models both in vitro and in vivo. In all of them, S-glutathionylation represents a mechanism that regulates the response to diabetic conditions, and has been described to occur in erythrocytes and neutrophils from diabetic patients. However, additional studies are necessary to discern whether this modification represents a biomarker for the early onset of diabetic vascular complications.

All glutathione forms are depleted in blood of obese and type 1 diabetic children

BACKGROUND

Oxidative stress plays an important role in the pathogenesis of type 1 diabetes (T1D), where an increase in reactive oxygen species may contribute to the initial destruction of β-cells. Accumulating evidence also suggests a role for oxidative stress in obesity, where it may potentiate the development of complications.

OBJECTIVE

To analyze the in vivo homeostasis of glutathione in children with T1D at onset and in children who are obese, to evaluate the systemic content of all glutathione forms (total, reduced, oxidized, and protein-bound glutathione) and the balance among them. Moreover, since glutathione bound to hemoglobin is a clinical marker of oxidative stress in human blood, we analyzed glutathionyl-hemoglobin in T1D and in obese children.

SUBJECTS

Children with T1D at onset (n = 30) or obesity (n = 30) at the first observation, and 30 healthy subjects chosen from the children who attended the outpatient clinic for minor problems.

METHODS

We assessed circulating levels of various glutathione forms by performing reverse-phase high performance liquid chromatography. Glutathionyl-hemoglobin analysis was carried out by cation-exchange chromatography.

RESULTS

In children with T1D and in obese children, we found a significant decrease of all glutathione forms including, for the first time, the content of total glutathione and glutathionylated proteins. The comparison among forms shows no significant imbalance in T1D patients, whereas in obese children it seems to suggest an attempt to rebalance the glutathione system homeostasis.

CONCLUSIONS

Our findings consistently show in vivo evidence of glutathione depletion upon early onset of T1D and in obese children, thus evidencing glutathione as an early marker in these two metabolic conditions.

S-glutathionylation: from molecular mechanisms to health outcomes

Redox homeostasis governs a number of critical cellular processes. In turn, imbalances in pathways that control oxidative and reductive conditions have been linked to a number of human disease pathologies, particularly those associated with aging. Reduced glutathione is the most prevalent biological thiol and plays a crucial role in maintaining a reduced intracellular environment. Exposure to reactive oxygen or nitrogen species is causatively linked to the disease pathologies associated with redox imbalance. In particular, reactive oxygen species can differentially oxidize certain cysteine residues in target proteins and the reversible process of S-glutathionylation may mitigate or mediate the damage. This post-translational modification adds a tripeptide and a net negative charge that can lead to distinct structural and functional changes in the target protein. Because it is reversible, S-glutathionylation has the potential to act as a biological switch and to be integral in a number of critical oxidative signaling events. The present review provides a comprehensive account of how the S-glutathionylation cycle influences protein structure/function and cellular regulatory events, and how these may impact on human diseases. By understanding the components of this cycle, there should be opportunities to intervene in stress- and aging-related pathologies, perhaps through prevention and diagnostic and therapeutic platforms.

Cellular redox potential and hemoglobin S-glutathionylation in human and rat erythrocytes: A comparative study

The rat is commonly used to evaluate responses of red blood cells (RBCs) to oxidative stress. How closely the rat RBC model predicts the human RBC human response has not been well characterized. The objective of this study was to compare human and rat RBC responses to the thiol-specific oxidant tert-butylhydroperoxide by monitoring the intraerythrocyte glutathione redox potential and its correlation with hemoglobin S-glutathionylation. Changes in redox potential did not differ significantly between rat and human RBCs under the considered conditions, and both human and rat hemoglobins were apparently S-glutathionylated by a thiol-disulfide exchange mechanism with glutathione disulfide, though the extent of S-glutathionylation in rat erythrocytes was more than 10-fold higher than in human ones. On the contrary, human and rat hemoglobin S-glutathionylation differently correlated with redox potential for the glutathione redox couple, suggesting that the formation of S-glutathionylated hemoglobin was not simply a function of glutathione disulfide concentration or glutathione/glutathione disulfide ratio and that the content of reactive cysteines in hemoglobin beta globin can strongly influence intraerythrocyte glutathione metabolism and distribution between free and hemoglobin-bound forms. This study reveals fundamental physiological differences in rat and human RBCs because of differences in rat and human beta globin cysteine and reactivity, which can have important implications for the study of rat biology as a whole and for the use of rats as models for human beings under physiological and pathological circumstances and, therefore, highlights the need for caution when extrapolating rat responses to humans.