Determination of glutathionyl hemoglobin in hemodialysis patients using electrospray ionization liquid chromatography-mass spectrometry

Glutathionyl Hemoglobin and Its Emerging Role as a Clinical Biomarker of Chronic Oxidative Stress

Hemoglobin is one of the proteins that are more susceptible to S-glutathionylation and the levels of its modified form, glutathionyl hemoglobin (HbSSG), increase in several human pathological conditions. The scope of the present review is to provide knowledge about how hemoglobin is subjected to S-glutathionylation and how this modification affects its functionality. The different diseases that showed increased levels of HbSSG and the methods used for its quantification in clinical investigations will be also outlined. Since there is a growing need for precise and reliable methods for markers of oxidative stress in human blood, this review highlights how HbSSG is emerging more and more as a good indicator of severe oxidative stress but also as a key pathogenic factor in several diseases.

Targeting protein modifications in metabolic diseases: molecular mechanisms and targeted therapies

The ever-increasing prevalence of noncommunicable diseases (NCDs) represents a major public health burden worldwide. The most common form of NCD is metabolic diseases, which affect people of all ages and usually manifest their pathobiology through life-threatening cardiovascular complications. A comprehensive understanding of the pathobiology of metabolic diseases will generate novel targets for improved therapies across the common metabolic spectrum. Protein posttranslational modification (PTM) is an important term that refers to biochemical modification of specific amino acid residues in target proteins, which immensely increases the functional diversity of the proteome. The range of PTMs includes phosphorylation, acetylation, methylation, ubiquitination, SUMOylation, neddylation, glycosylation, palmitoylation, myristoylation, prenylation, cholesterylation, glutathionylation, S-nitrosylation, sulfhydration, citrullination, ADP ribosylation, and several novel PTMs. Here, we offer a comprehensive review of PTMs and their roles in common metabolic diseases and pathological consequences, including diabetes, obesity, fatty liver diseases, hyperlipidemia, and atherosclerosis. Building upon this framework, we afford a through description of proteins and pathways involved in metabolic diseases by focusing on PTM-based protein modifications, showcase the pharmaceutical intervention of PTMs in preclinical studies and clinical trials, and offer future perspectives. Fundamental research defining the mechanisms whereby PTMs of proteins regulate metabolic diseases will open new avenues for therapeutic intervention.

Blood Thiol Redox State in Chronic Kidney Disease

Thiols (sulfhydryl groups) are effective antioxidants that can preserve the correct structure of proteins, and can protect cells and tissues from damage induced by oxidative stress. Abnormal levels of thiols have been measured in the blood of patients with moderate-to-severe chronic kidney disease (CKD) compared to healthy subjects, as well as in end-stage renal disease (ESRD) patients on haemodialysis or peritoneal dialysis. The levels of protein thiols (a measure of the endogenous antioxidant capacity inversely related to protein oxidation) and S-thiolated proteins (mixed disulphides of protein thiols and low molecular mass thiols), and the protein thiolation index (the molar ratio of the S-thiolated proteins to free protein thiols in plasma) have been investigated in the plasma or red blood cells of CKD and ESRD patients as possible biomarkers of oxidative stress. This type of minimally invasive analysis provides valuable information on the redox status of the less-easily accessible tissues and organs, and of the whole organism. This review provides an overview of reversible modifications in protein thiols in the setting of CKD and renal replacement therapy. The evidence suggests that protein thiols, S-thiolated proteins, and the protein thiolation index are promising biomarkers of reversible oxidative stress that could be included in the routine monitoring of CKD and ESRD patients.

Measurement of S-glutathionylated proteins by HPLC

S-glutathionylated proteins (GSSP), i.e., protein-mixed disulfides with glutathione (GSH), are considered a suitable biomarker of oxidative stress. In fact, they occur within cells at low level and their concentration increases markedly under pro-oxidant conditions. Plasma is something different, since it is physiologically rich in S-thiolated proteins (RSSP), i.e., protein-mixed disulfides with various types of low molecular mass thiols (LMM-SH). However, albumin, which is largely the most abundant plasma protein, possesses a cysteine residue at position 34 that is mostly reduced (about 60%) under physiological conditions, but easily involved in the formation of additional RSSP in the presence of oxidants. The quantification of GSSP requires special attention to sample handling, since their level can be overestimated as a result of artefactual oxidation of GSH. We have developed the present protocol to avoid this methodological problem. Samples should be treated as soon as possible after their collection with the alkylating agent N-ethylmaleimide that masks –SH groups and prevents their oxidation. The GSH released from mixed disulfides by reduction with dithiothreitol is then labeled with the fluorescent probe monobromobimane and quantified by HPLC. The method can be applied to many different biological samples, comprising blood components, red blood cell plasma membrane, cultured cells, and solid organs from animal models.

The Redox Potential of the β-93-Cysteine Thiol Group in Human Hemoglobin Estimated from In Vitro Oxidant Challenge Experiments

Glutathionyl hemoglobin is a minor form of hemoglobin with intriguing properties. The measurement of the redox potential of its reactive β-⁹³-Cysteine is useful to improve understanding of the response of erythrocytes to transient and chronic conditions of oxidative stress, where the level of glutathionyl hemoglobin is increased. An independent literature experiment describes the recovery of human erythrocytes exposed to an oxidant burst by measuring glutathione, glutathione disulfide and glutathionyl hemoglobin in a two-hour period. This article calculates a value for the redox potential E₀ of the β-⁹³-Cysteine, considering the erythrocyte as a closed system at equilibrium described by the Nernst equation and using the measurements of the literature experiment. The obtained value of E₀ of −121 mV at pH 7.4 places hemoglobin as the most oxidizing thiol of the erythrocyte. By using as synthetic indicators of the concentrations the electrochemical potentials of the two main redox pairs in the erythrocytes, those of glutathione–glutathione disulfide and of glutathionyl–hemoglobin, the mechanism of the recovery phase can be hypothesized. Hemoglobin acts as the redox buffer that scavenges oxidized glutathione in the oxidative phase and releases it in the recovery phase, by acting as the substrate of the NAD(P)H-cofactored enzymes.

Protein glutathionylation in cardiovascular diseases

The perturbation of thiol-disulfide homeostasis is an important consequence of many diseases, with redox signals implicated in several physio-pathological processes. A prevalent form of cysteine modification is the reversible formation of protein mixed disulfides with glutathione (S-glutathionylation). The abundance of glutathione in cells and the ready conversion of sulfenic acids to S-glutathione mixed disulfides supports the reversible protein S-glutathionylation as a common feature of redox signal transduction, able to regulate the activities of several redox sensitive proteins. In particular, protein S-glutathionylation is emerging as a critical signaling mechanism in cardiovascular diseases, because it regulates numerous physiological processes involved in cardiovascular homeostasis, including myocyte contraction, oxidative phosphorylation, protein synthesis, vasodilation, glycolytic metabolism and response to insulin. Thus, perturbations in protein glutathionylation status may contribute to the etiology of many cardiovascular diseases, such as myocardial infarction, cardiac hypertrophy and atherosclerosis. Various reports show the importance of oxidative cysteine modifications in modulating cardiovascular function. In this review, we illustrate tools and strategies to monitor protein S-glutathionylation and describe the proteins so far identified as glutathionylated in myocardial contraction, hypertrophy and inflammation.

Glutathione redox potential is low and glutathionylated and cysteinylated hemoglobin levels are elevated in maintenance hemodialysis patients

Glutathione (GSH), the most abundant intracellular low molecular mass thiol, protects cells from oxidative damage and regulates their function. Available information is inconsistent regarding levels of GSH and its disulfide (GSSG) in maintenance hemodialysis patients (HD). In addition, very limited data are available in HD about the relationship of GSH and GSSG with other measures of thiol metabolism and with the clinical profile. We tested the hypothesis that erythrocyte GSH/GSSG redox potential (Eh) is lower in HD than in healthy controls (C), and that Eh correlates with posttranslational thiolation of hemoglobin (Hb) and with standard clinical parameters in HD. In cross-sectional comparison of 33 stable HD and 21 C, we found a net loss of reducing capacity in HD as indicated by low erythrocyte GSH/GSSG Eh (-257 ± 5.5 vs -270 ± 5.6 mV, P = 0.002). Glutathionylated Hb (HbSSG) was 46% higher in HD than C (19.3 ± 4.80 vs 13.2 ± 2.79 pmol/mg Hb; P = 0.001) and cysteinylated Hb (HbSSCy) was >3-fold higher in HD than C [38.3 (29.0-63.3) vs 11.5 (9.6-17.2) pmol/mg Hb; P = 0.001]. In multiple regression analysis of the HD cases, statistically significant associations were found between the GSH/GSSG Eh and the blood urea nitrogen (P = 0.001), creatinine (P = 0.015) and normalized protein catabolic rate (P = 0.05), after adjusting for age, race/ethnicity, and etiology of end-stage renal disease. In conclusion, accurate and precise analysis of GSH, GSSG, and mixed disulfides reveals loss of erythrocyte GSH/GSSG Eh, rise of both HbSSG and HbSSCy, and correlation of these thiols with measures of uremia and dietary protein intake.

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.

Toward an "omic" physiopathology of reactive chemicals: thirty years of mass spectrometric study of the protein adducts with endogenous and xenobiotic compounds

Cancer and degenerative diseases are major causes of morbidity and death, derived from the permanent modification of key biopolymers such as DNA and regulatory proteins by usually smaller, reactive molecules, present in the environment or generated from endogenous and xenobiotic components by the body's own biochemical mechanisms (molecular adducts). In particular, protein adducts with organic electrophiles have been studied for more than 30 [see, e.g., Calleman et al., 1978] years essentially for three purposes: (a) as passive monitors of the mean level of individual exposure to specific chemicals, either endogenously present in the human body or to which the subject is exposed through food or environmental contamination; (b) as quantitative indicators of the mean extent of the individual metabolic processing which converts a non-reactive chemical substance into its toxic products able to damage DNA (en route to cancer induction through genotoxic mechanisms) or key proteins (as in the case of several drugs, pesticides or otherwise biologically active substances); (c) to relate the extent of protein modification to that of biological function impairment (such as enzyme inhibition) finally causing the specific health damage. This review describes the role that contemporary mass spectrometry-based approaches employed in the qualitative and quantitative study of protein-electrophile adducts play in the discovery of the (bio)chemical mechanisms of toxic substances and highlights the future directions of research in this field. A particular emphasis is given to the measurement of often high levels of the protein adducts of several industrial and environmental pollutants in unexposed human populations, a phenomenon which highlights the possibility that a number of small organic molecules are generated in the human organism through minor metabolic processes, the imbalance of which may be the cause of "spontaneous" cases of cancer and of other degenerative diseases of still uncharacterized etiology. With all this in mind, it is foreseen that a holistic description of cellular functions will take advantage of new analytical methods based on time-integrated metabolomic measurements of a new biological compartment, the "adductome," aimed at better understanding integrated organism response to environmental and endogenous stressors.

Chromatographic and mass spectrometric analysis of glutathione in biological samples

Biological thiol compounds are classified into high-molecular-mass protein thiols and low-molecular-mass free thiols. Endogenous low-molecular-mass thiol compounds, namely, reduced glutathione (GSH) and its corresponding disulfide, glutathione disulfide (GSSG), are very important molecules that participate in a variety of physiological and pathological processes. GSH plays an essential role in protecting cells from oxidative and nitrosative stress and GSSG can be converted into the reduced form by action of glutathione reductase. Measurement of GSH and GSSG is a useful indicator of oxidative stress and disease risk. Many publications have reported successful determination of GSH and GSSG in biological samples. In this article, we review newly developed techniques, such as liquid chromatography coupled with mass spectrometry and tandem mass spectrometry, for identifying GSH bound to proteins, or for localizing GSH in bound or free forms at specific sites in organs and in cellular locations.

Detection of protein glutathionylation

Recent studies indicate that protein glutathionylation is an important regulatory mechanism. The develop-ment of redox proteomics techniques to identify proteins undergoing glutathionylation has a key role in defining the importance of this post-translational modification, although the available methods are not yet comparable to those for the study of other modifications like phosphorylation. We describe here methods that have been successfully employed to identify in vitro glutathionylated proteins.

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

Modified haemoglobin levels were quantified in 21 Type 1 and 21 Type 2 diabetic patients and two groups of 17 non-diabetic subjects. Glycated haemoglobin levels were increased in diabetes but glutathionyl haemoglobin (HbSSG) levels did not differ between groups, nor by complications; nor correlate with haemoglobin glycation or vascular risk factors.

Molecular mechanisms and potential clinical significance of S-glutathionylation

Protein S-glutathionylation, the reversible binding of glutathione to protein thiols (PSH), is involved in protein redox regulation, storage of glutathione, and protection of PSH from irreversible oxidation. S-Glutathionylated protein (PSSG) can result from thiol/disulfide exchange between PSH and GSSG or PSSG; direct interaction between partially oxidized PSH and GSH; reactions between PSH and S-nitrosothiols, oxidized forms of GSH, or glutathione thiyl radical. Indeed, thiol/disulfide exchange is an unlikely intracellular mechanism for S-glutathionylation, because of the redox potential of most Cys residues and the GSSG export by most cells as a protective mechanism against oxidative stress. S-Glutathionylation can be reversed, following restoration of a reducing GSH/GSSG ratio, in an enzyme-dependent or -independent manner. Currently, definite evidence of protein S-glutathionylation has been clearly demonstrated in few human diseases. In aging human lenses, protein S-glutathionylation increases; during cataractogenesis, some of lens proteins, including alpha- and beta-crystallins, form both mixed disulfides and disulfide-cross-linked aggregates, which increase with cataract severity. The correlation of lens nuclear color and opalescence intensity with protein S-glutathionylation indicates that protein-thiol mixed disulfides may play an important role in cataractogenesis and development of brunescence in human lenses. Recently, specific PSSG have been identified in the inferior parietal lobule in Alzheimer's disease. However, much investigation is needed to clarify the actual involvement of protein S-glutathionylation in many human diseases.

Protein glutathionylation and oxidative stress

Liquid chromatography/electrospray ionization-mass spectrometry (LC/ESI-MS) demonstrated that glutathionyl hemoglobin (Hb) levels are increased in patients with diabetes, hyperlipidemia, uremia and Friedreich's ataxia. Glutathionylation of Hb is enhanced by oxidative stress. High performance liquid chromatography (HPLC) and matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF-MS) have also been developed for the quantification of glutathionyl Hb. Glutathionyl-lens proteins were detected in uremic patients and cataractous aged subjects. Glutathionylation of numerous enzymes is induced by oxidative stress, reduces their catalytic activities and may be involved in protection from the damaging effects of oxidative agents. Thioredoxin, glutaredoxin (thioltransferase) and protein disulfide isomerase are the key enzymes in controlling cellular oxidative stress that catalyze reduction of glutathionyl protein disulfide bonds. Thus, protein glutathionylation is closely associated with oxidative stress.

Membrane skeletal protein S-glutathionylation and hemolysis in human red blood cells

In this work, protein-glutathione mixed disulfide formation in human red blood cells (RBCs) was evaluated in vitro by using the thiol-specific reagent diamide. We investigated what mechanism could lead to S-glutathionylation of membrane skeletal proteins, what are the main target proteins, and the correlation between protein S-glutathionylation and RBC hemolysis. Diamide caused a decrease in the reduced form of glutathione (GSH), which was accompanied by an increase in the basal level of glutathione disulfide (GSSG) and in S-glutathionylation of protein 4.2 and spectrin. The increase in membrane skeletal protein S-glutathionylation was correlated with a lower susceptibility of RBCs to osmotic hemolysis, suggesting that S-glutathionylation of protein 4.2 and spectrin could contribute to regulate RBC membrane stability.

A quantitative method for the analysis of glycated and glutathionylated hemoglobin by matrix-assisted laser desorption ionization-time of flight mass spectrometry

The quantization of glycated isoforms of hemoglobin has been increasingly used in clinical practice in recent years. Glycated hemoglobin is currently considered the most important measurement for long-term control of the glycemic state and it has become a reference tool for the management of diabetes. Glutathionylated hemoglobin is an increasingly clinically relevant covalent adduct of glutathione with beta chain of the globin and its concentration has been correlated with oxidative stress. We have developed an innovative technique based on linear mode matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectrometry for quantitative analysis of hemoglobin species. This method was applied to the quantification of glycated and glutathionylated hemoglobin. A rigorous comparison was pursued to evaluate the analytical performances in quantifying glycated hemoglobin in comparison to an established high-performance liquid chromatography method. Our results indicated a complete equivalence between the two methods. The same analysis enabled the quantitative determination of the glutathionylated hemoglobin fraction. This isoform was investigated in an adult Italian population (184 individuals, 101 males and 83 females), indicating a bimodal distribution of this species. In fact 65.22% of screened individuals had glutathionylated hemoglobin levels lower than 0.50% while 34.78% had glutathionylated hemoglobin levels higher than 0.50%. A semiautomatic robotic procedure was developed for fast analysis of a large number of samples. This is the first report of a quantitative application of linear MALDI-TOF mass spectrometry for the determination of glutathionylated hemoglobin in blood samples. This method allows fast screening of this hemoglobin isoform, therefore opening the route to explore its specificity and sensitivity as a molecular biomarker.

Analysis of glutathionyl hemoglobin levels in diabetic patients by electrospray ionization liquid chromatography-mass spectrometry: effect of vitamin E administration

By using electrospray ionization liquid chromatography-mass spectrometry, we demonstrated that glutathionyl hemoglobin (Hb)beta-chain levels are markedly increased in the erythrocytes of diabetic patients as compared with healthy subjects. The administration of vitamin E to the diabetic patients for 8 weeks significantly decreased the levels of glutathionyl Hbbeta, whereas it did not affect the levels of HbA1c, glycated Hbbeta or glycated Hbalpha. Glutathionyl Hb levels can be used as a new clinical marker of oxidative stress.