The Role of S-Glutathionylation in Health and Disease: A Bird's Eye View

Protein glutathionylation is a reversible post-translational modification that involves the attachment of glutathione to cysteine residues. It plays a role in the regulation of several cellular processes and protection against oxidative damage. Glutathionylation (GS-ylation) modulates protein function, inhibits or enhances enzymatic activity, maintains redox homeostasis, and shields several proteins from irreversible oxidative stress. Aberrant GS-ylation patterns are thus implicated in various diseases, particularly those associated with oxidative stress and inflammation, such as cardiovascular diseases, neurodegenerative disorders, cancer, and many others. Research in the recent years has highlighted the potential to manipulate protein GS-ylation for therapeutic purposes with strategies that imply both its enhancement and inhibition according to different cases. Moreover, it has become increasingly evident that monitoring the GS-ylation status of selected proteins offers diagnostic potential in different diseases. In this review, we try to summarize recent research in the field with a focus on our current understanding of the molecular mechanisms related to aberrant protein GS-ylation.

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.

Redox signaling at the crossroads of human health and disease

Redox biology is at the core of life sciences, accompanied by the close correlation of redox processes with biological activities. Redox homeostasis is a prerequisite for human health, in which the physiological levels of nonradical reactive oxygen species (ROS) function as the primary second messengers to modulate physiological redox signaling by orchestrating multiple redox sensors. However, excessive ROS accumulation, termed oxidative stress (OS), leads to biomolecule damage and subsequent occurrence of various diseases such as type 2 diabetes, atherosclerosis, and cancer. Herein, starting with the evolution of redox biology, we reveal the roles of ROS as multifaceted physiological modulators to mediate redox signaling and sustain redox homeostasis. In addition, we also emphasize the detailed OS mechanisms involved in the initiation and development of several important diseases. ROS as a double-edged sword in disease progression suggest two different therapeutic strategies to treat redox-relevant diseases, in which targeting ROS sources and redox-related effectors to manipulate redox homeostasis will largely promote precision medicine. Therefore, a comprehensive understanding of the redox signaling networks under physiological and pathological conditions will facilitate the development of redox medicine and benefit patients with redox-relevant diseases.

Oxidative Modification of Proteins: From Damage to Catalysis, Signaling, and Beyond

Significance: The systematic investigation of oxidative modification of proteins by reactive oxygen species started in 1980. Later, it was shown that reactive nitrogen species could also modify proteins. Some protein oxidative modifications promote loss of protein function, cleavage or aggregation, and some result in proteo-toxicity and cellular homeostasis disruption. Recent Advances: Previously, protein oxidation was associated exclusively to damage. However, not all oxidative modifications are necessarily associated with damage, as with Met and Cys protein residue oxidation. In these cases, redox state changes can alter protein structure, catalytic function, and signaling processes in response to metabolic and/or environmental alterations. This review aims to integrate the present knowledge on redox modifications of proteins with their fate and role in redox signaling and human pathological conditions. Critical Issues: It is hypothesized that protein oxidation participates in the development and progression of many pathological conditions. However, no quantitative data have been correlated with specific oxidized proteins or the progression or severity of pathological conditions. Hence, the comprehension of the mechanisms underlying these modifications, their importance in human pathologies, and the fate of the modified proteins is of clinical relevance. Future Directions: We discuss new tools to cope with protein oxidation and suggest new approaches for integrating knowledge about protein oxidation and redox processes with human pathophysiological conditions. Antioxid. Redox Signal. 35, 1016-1080.

Role of protein S-Glutathionylation in cancer progression and development of resistance to anti-cancer drugs

The survival, functioning and proliferation of mammalian cells are highly dependent on the cellular response and adaptation to changes in their redox environment. Cancer cells often live in an altered redox environment due to aberrant neo-vasculature, metabolic reprogramming and dysregulated proliferation. Thus, redox adaptations are critical for their survival. Glutathione plays an essential role in maintaining redox homeostasis inside the cells by binding to redox-sensitive cysteine residues in proteins by a process called S-glutathionylation. S-Glutathionylation not only protects the labile cysteine residues from oxidation, but also serves as a sensor of redox status, and acts as a signal for stimulation of downstream processes and adaptive responses to ensure redox equilibrium. The present review aims to provide an updated overview of the role of the unique redox adaptations during carcinogenesis and cancer progression, focusing on their dependence on S-glutathionylation of specific redox-sensitive proteins involved in a wide range of processes including signalling, transcription, structural maintenance, mitochondrial functions, apoptosis and protein recycling. We also provide insights into the role of S-glutathionylation in the development of resistance to chemotherapy. Finally, we provide a strong rationale for the development of redox targeting drugs for treatment of refractory/resistant cancers.

Why is the novel Hb Miguel Servet visualised by CE-HPLC newborn and not by the CE-HPLC β-thalassaemia programme?

Screening of haemoglobinopathies is indicated for the detection of sickle cell anaemia; thus, neonates can benefit from early and adequate treatment that prevents neurological damage, reduces morbidity and mortality associated with the disease. These types of programmes sometimes lead to unexpected findings. We present a new haemoglobin (Hb) variant (Hb Miguel Servet) detected by newborn screening. During neonatal screening of haemoglobinopathies by cation-exchange high-performance liquid chromatography (CE-HPLC) newborn, an Hb variant was detected. An analysis at 8 months of age using capillary zone electrophoresis (CZE) confirmed the presence of this new Hb. The molecular characterisation was performed by automatic sequencing of the α and β globin genes in an ABI PRISM 3100 Genetic Analyzer. Hb analysis by CE-HPLC β-thalassaemia short programmedid not indicate the presence of abnormal Hbs. By CZE showed a peak in the zone 12 zone comprising 3.3% of the total Hb. A new analysis by CE-HPLC on a Tosoh G8-2 (Horiba) shown a peak, in the region of HbA1b, did not interfere with the quantification of HbA1c. Sequencing of the β gene revealed the substitution of a guanine for a thymine (GGT >TGT) in codon 69 of the second exon, resulting in substitution of cysteine for the amino acid glutamine (HBB:c.208G>T). Hb Miguel Servet is a β-chain globin variant detected by CE-HPLC newborn (BioRad), by CZE and by CE-HPLC-CE Tosoh G8-2 (Horiba), but no by CE-HPLC-CE β-thalassaemia short programme (BioRad). In fact, for all the techniques that are visualised, what would be detected would be the glutathione variant of Hb (Miguel Servet).

Targeting oxidative stress in disease: promise and limitations of antioxidant therapy

Oxidative stress is a component of many diseases, including atherosclerosis, chronic obstructive pulmonary disease, Alzheimer disease and cancer. Although numerous small molecules evaluated as antioxidants have exhibited therapeutic potential in preclinical studies, clinical trial results have been disappointing. A greater understanding of the mechanisms through which antioxidants act and where and when they are effective may provide a rational approach that leads to greater pharmacological success. Here, we review the relationships between oxidative stress, redox signalling and disease, the mechanisms through which oxidative stress can contribute to pathology, how antioxidant defences work, what limits their effectiveness and how antioxidant defences can be increased through physiological signalling, dietary components and potential pharmaceutical intervention.

Characterization of cellular oxidative stress response by stoichiometric redox proteomics

The thiol redox proteome refers to all proteins whose cysteine thiols are subjected to various redox-dependent posttranslational modifications (PTMs) including S-glutathionylation (SSG), S-nitrosylation (SNO), S-sulfenylation (SOH), and S-sulfhydration (SSH). These modifications can impact various aspects of protein function such as activity, binding, conformation, localization, and interactions with other molecules. To identify novel redox proteins in signaling and regulation, it is highly desirable to have robust redox proteomics methods that can provide global, site-specific, and stoichiometric quantification of redox PTMs. Mass spectrometry (MS)-based redox proteomics has emerged as the primary platform for broad characterization of thiol PTMs in cells and tissues. Herein, we review recent advances in MS-based redox proteomics approaches for quantitative profiling of redox PTMs at physiological or oxidative stress conditions and highlight some recent applications. Considering the relative maturity of available methods, emphasis will be on two types of modifications: 1) total oxidation (i.e., all reversible thiol modifications), the level of which represents the overall redox state, and 2) S-glutathionylation, a major form of reversible thiol oxidation. We also discuss the significance of stoichiometric measurements of thiol PTMs as well as future perspectives toward a better understanding of cellular redox regulatory networks in cells and tissues.

S-glutathionylation, friend or foe in cardiovascular health and disease

Glutathione is a low molecular weight thiol that is present at high levels in the cell. The high levels of glutathione in the cell make it one of the most abundant antioxidants contributing to cellular redox homeostasis. As a general rule, throughout cardiovascular disease and progression there is an imbalance in redox homeostasis characterized by reactive oxygen species overproduction and glutathione underproduction. As research into these imbalances continues, glutathione concentrations are increasingly being observed to drive various physiological and pathological signaling responses. Interestingly in addition to acting directly as an antioxidant, glutathione is capable of post translational modifications (S-glutathionylation) of proteins through both chemical interactions and enzyme mediated events. This review will discuss both the chemical and enzyme-based S-glutathionylation of proteins involved in cardiovascular pathologies and angiogenesis.

N, K. S, C. VK, K. ND, C. SK, Maddu N. Smokeless tobacco induced biophysical and biochemical alterations in the plasma, erythrocytes, and platelets of panmasala users: Subsequent biological effects

AIM & BACKGROUND

Smokeless tobacco (SLT) products are extensively consumed throughout the world including India. These products act as the primary addictive agents, due to the presence of nicotine among other tobacco products to humans and animals and its quitting is difficult. Higher the exposure of SLT products more is the toxic effects and alterations in erythrocytes and platelets.

OBJECTIVES

The products of smokeless tobacco could cause increase in the concentrations of oxidants (free radicals), decrease the activities antioxidant enzymes, activate the process of programmed cell death through enhanced expression of inducible nitric oxide synthase. Smokeless tobacco products represent a major modifiable risk factor for the development of redox imbalance through the enhanced production of reactive oxygen species and diminished activities of antioxidant enzymes in plasma, bio-membranes of erythrocytes, and platelets and induction of apoptosis in the blood.

MATERIALS AND METHODS

The protein expression of inducible nitric oxide synthase (iNOS) was studied by western blot and gene expression of apoptotic proteins, tumor necrosis factor-alpha (TNF-α), interleukin-6 (IL-6) was evaluated by RT-PCR technique. Membrane fluidity of erythrocytes and platelets was studied by the fluorescence method.

RESULTS

The results of the present study revealed that significantly elevated levels of iNOS enzyme in plasma, erythrocyte, and platelet membranes of panmasala users. We found that gene expression levels of Bcl2, Bax, IL-6, caspase proteins (Caspase 8, Caspase 10, and Caspase 12) are greater and decreased levels of TNF-α with no significant change in blood of smokeless tobacco users in comparison with normal controls. In addition, there were substantial significantly higher in concentrations of nicotine, cotinine, and epinephrine in the plasma of panmasala users than non-tobacco users. Panmasala can be caused a significant increase in nitroxidative stress marker (LPO, NO, and ONOO-) values and significant decrease in the levels of antioxidant enzymes in erythrocytes and platelets.

CONCLUSION

On the basis of the present study results, it may be concluded that the chronic use of panmasala than any smokeless tobacco products may be a contributory risk factor or may give conclusive idea and has been associated with the expansion of the development of structural and functional alterations in the erythrocyte and platelet membranes induced oxidative damage and apoptosis, possibly further enhanced by nicotine and tobacco-specific N-nitrosamines. SLT exposure had implicated a threat and enormous implications on public health and is required to prove that may not be viewed as a safe alternative to any tobacco products.

Analysis of cysteine glutathionylation in hemoglobin of gastric cancer patients using nanoflow liquid chromatography/triple-stage mass spectrometry

Glutathione is an intracellular antioxidant capable of scavenging free radicals and detoxifying electrophiles from endogenous and exogenous sources via the free thiol group. Post-translational glutathionylation at cysteine residues of proteins can affect the structure and cause a functional change of proteins. Protein glutathionylation has been proven to reflect the cellular redox status. Our previous report indicates that the levels of glutathionylation in hemoglobin from peripheral blood of smokers are significantly higher than in nonsmokers. In this study, a nanoflow liquid chromatography/nanospray ionization triple-stage mass spectrometric (nanoLC/NSI-MS³ ) method with a linear ion trap mass spectrometer was employed to quantify glutathionylated peptides in the trypsin digests of hemoglobin from gastric cancer patients. We compare the extent of glutathionylation in hemoglobin from nonsmoking gastric cancer patients with that from nonsmoking healthy adults. Using a carboxymethylated peptide as the reference peptide, the relative quantification of each glutathionylated peptide was measured as the peak area ratio of the modified peptide versus the sum of the peak areas of the modified and the carboxymethylated parent peptide in the selected reaction monitoring chromatograms. Using this method, we found that the extents of glutathionylation at Cys-104 of the α-globin and Cys-93 of β-globulin hemoglobin from 10 gastric cancer patients were significantly higher than those from 14 normal individuals with p values <0.0001. Our results suggest the possibility of using the extent of cysteine glutathionylation at β-93 of hemoglobin as an oxidative stress biomarker candidate for gastric cancer.

Structural analysis of glutathionyl hemoglobin using native mass spectrometry

Glutathionylation is an example of reversible post-translation modification of proteins where free and accessible cysteine residues of proteins undergo thiol-disulfide exchange with oxidized glutathione (GSSG). In general, glutathionylation occurs under the condition of elevated oxidative stress in vivo. In human hemoglobin, Cys93 residue of β globin chain was found to undergo this oxidative modification. Glutathionyl hemoglobin (GSHb) was reported to act as a biomarker of oxidative stress under several clinical conditions such as chronic renal failure, iron deficiency anemia, hyperlipidemia, diabetes mellitus, Friedreich's ataxia, atherosclerosis. Previously we showed that the functional abnormality associated with six-fold tighter oxygen binding of GSHb supposedly attributed to the conformational transition of the deoxy state of GSHb towards oxy hemoglobin like conformation. In the present study, we investigated the structural integrity and overall architecture of the quaternary structure of GSHb using native mass spectrometry and ion mobility mass spectrometry platforms. The dissociation equilibrium constants of both tetramer/dimer (K_d1) and dimer/monomer equilibrium (K_d2) was observed to increase by 1.91 folds and 3.64 folds respectively. However, the collision cross-section area of the tetrameric hemoglobin molecule remained unchanged upon glutathionylation. The molecular dynamics simulation data of normal human hemoglobin and GSHb was employed to support our experimental findings.

Glutathione "Redox Homeostasis" and Its Relation to Cardiovascular Disease

More people die from cardiovascular diseases (CVD) than from any other cause. Cardiovascular complications are thought to arise from enhanced levels of free radicals causing impaired "redox homeostasis," which represents the interplay between oxidative stress (OS) and reductive stress (RS). In this review, we compile several experimental research findings that show sustained shifts towards OS will alter the homeostatic redox mechanism to cause cardiovascular complications, as well as findings that show a prolonged antioxidant state or RS can similarly lead to such cardiovascular complications. This experimental evidence is specifically focused on the role of glutathione, the most abundant antioxidant in the heart, in a redox homeostatic mechanism that has been shifted towards OS or RS. This may lead to impairment of cellular signaling mechanisms and elevated pools of proteotoxicity associated with cardiac dysfunction.

Characterization of residue-specific glutathionylation of CSF proteins in multiple sclerosis - A MS-based approach

Reduction of a disulfide linkage between cysteine residues in proteins, a standard step in the preanalytical preparation of samples in conventional proteomics approach, presents a challenge to characterize S-glutathionylation of proteins. S-glutathionylation of proteins has been reported in medical conditions associated with high oxidative stress. In the present study, we attempted to characterize glutathionylation of CSF proteins in patients with multiple sclerosis which is associated with high oxidative stress. Using the nano-LC/ESI-MS platform, we adopted a modified proteomics approach and a targeted database search to investigate glutathionylation at the residue level of CSF proteins. Compared to patients with Intracranial hypertension, the following CSF proteins: Extracellular Superoxide dismutase (ECSOD) at Cys195, α1-antitrypsin (A1AT) at Cys232, Phospholipid transfer protein (PLTP) at Cys318, Alpha-2-HS-glycoprotein at Cys340, Ectonucleotide pyrophosphate (ENPP-2) at Cys773, Gelsolin at Cys304, Interleukin-18 (IL-18) at Cys38 and Ig heavy chain V III region POM at Cys22 were found to be glutathionylated in patients with multiple sclerosis during a relapse. ECSOD, A1AT, and PLTP were observed to be glutathionylated at the functionally important cysteine residues. In conclusion, in the present study using a modified proteomics approach we have identified and characterized glutathionylation of CSF proteins in patients with multiple sclerosis.

Measurement and Clinical Significance of Biomarkers of Oxidative Stress in Humans

Oxidative stress is the result of the imbalance between reactive oxygen species (ROS) formation and enzymatic and nonenzymatic antioxidants. Biomarkers of oxidative stress are relevant in the evaluation of the disease status and of the health-enhancing effects of antioxidants. We aim to discuss the major methodological bias of methods used for the evaluation of oxidative stress in humans. There is a lack of consensus concerning the validation, standardization, and reproducibility of methods for the measurement of the following: (1) ROS in leukocytes and platelets by flow cytometry, (2) markers based on ROS-induced modifications of lipids, DNA, and proteins, (3) enzymatic players of redox status, and (4) total antioxidant capacity of human body fluids. It has been suggested that the bias of each method could be overcome by using indexes of oxidative stress that include more than one marker. However, the choice of the markers considered in the global index should be dictated by the aim of the study and its design, as well as by the clinical relevance in the selected subjects. In conclusion, the clinical significance of biomarkers of oxidative stress in humans must come from a critical analysis of the markers that should give an overall index of redox status in particular conditions.

Molecular mechanisms of ROS production and oxidative stress in diabetes

Oxidative stress and chronic inflammation are known to be associated with the development of metabolic diseases, including diabetes. Oxidative stress, an imbalance between oxidative and antioxidative systems of cells and tissues, is a result of over production of oxidative-free radicals and associated reactive oxygen species (ROS). One outcome of excessive levels of ROS is the modification of the structure and function of cellular proteins and lipids, leading to cellular dysfunction including impaired energy metabolism, altered cell signalling and cell cycle control, impaired cell transport mechanisms and overall dysfunctional biological activity, immune activation and inflammation. Nutritional stress, such as that caused by excess high-fat and/or carbohydrate diets, promotes oxidative stress as evident by increased lipid peroxidation products, protein carbonylation and decreased antioxidant status. In obesity, chronic oxidative stress and associated inflammation are the underlying factors that lead to the development of pathologies such as insulin resistance, dysregulated pathways of metabolism, diabetes and cardiovascular disease through impaired signalling and metabolism resulting in dysfunction to insulin secretion, insulin action and immune responses. However, exercise may counter excessive levels of oxidative stress and thus improve metabolic and inflammatory outcomes. In the present article, we review the cellular and molecular origins and significance of ROS production, the molecular targets and responses describing how oxidative stress affects cell function including mechanisms of insulin secretion and action, from the point of view of possible application of novel diabetic therapies based on redox regulation.

Analysis of Chlorination, Nitration, and Nitrosylation of Tyrosine and Oxidation of Methionine and Cysteine in Hemoglobin from Type 2 Diabetes Mellitus Patients by Nanoflow Liquid Chromatography Tandem Mass Spectrometry

The post-translational modification (PTM) of proteins by endogenous reactive chlorine, nitrogen, and oxygen species is implicated in certain pathological conditions, including diabetes mellitus. Evidence showed that the extents of modifications on a number of proteins are elevated in diabetic patients. Measuring modification on hemoglobin has been used to monitor the extent of exposure. This study develops an assay for simultaneous quantification of the extent of chlorination, nitration, and oxidation in human hemoglobin and to examine whether the level of any of these modifications is higher in poorly controlled type 2 diabetic mellitus patients. This mass spectrometry-based assay used the bottom-up proteomic strategy. Due to the low amount of endogenous modification, we first characterized the sites of chlorination at tyrosine in hypochlorous acid-treated hemoglobin by an accurate mass spectrometer. The extents of chlorination, nitration, and oxidation of a total of 12 sites and types of modifications in hemoglobin were measured by nanoflow liquid chromatography-nanospray ionization tandem mass spectrometry under the selected reaction monitoring mode. Relative quantification of these PTMs in hemoglobin extracted from blood samples shows that the extents of chlorination at α-Tyr-24, nitration at α-Tyr-42, and oxidation at the three methionine residues are significantly higher in diabetic patients (n = 19) than in nondiabetic individuals (n = 18). After excluding the factor of smoking, chlorination at α-Tyr-24, nitration at α-Tyr-42, and oxidation at the three methionine residues are significantly higher in the nonsmoking diabetic patients (n = 12) than in normal nonsmoking subjects (n = 11). Multiple regression analysis performed on the combined effect of age, body-mass index (BMI), and HbA1c showed that the diabetes factor HbA1c contributes significantly to the extent of chlorination at α-Tyr-24 in nonsmokers. In addition, age contributes to oxidation at α-Met-32 significantly in all subjects and in nonsmokers. These results suggest the potential of using chlorination at α-Tyr-24-containing peptide to evaluate protein damage in nonsmoking type 2 diabetes mellitus.

Pleiotropic functions of glutathione S-transferase P

Glutathione S-transferase P (GSTP) is one member of the GST superfamily that is prevalently expressed in mammals. Known to possess catalytic activity through deprotonating glutathione allowing formation of thioether bonds with electrophilic substrates, more recent discoveries have broadened our understanding of the biological roles of this protein. In addition to catalytic detoxification, other properties so far ascribed to GSTP include chaperone functions, regulation of nitric oxide pathways, regulation of a variety of kinase signaling pathways, and participation in the forward reaction of protein S-glutathionylation. The expression of GSTP has been linked with cancer and other human pathologies and more recently even with drug addiction. With respect to human health, polymorphic variants of GSTP may determine individual susceptibility to oxidative stress and/or be critical in the design and development of drugs that have used redox pathways as a discovery platform.

Influence of oxygen on wound healing dynamics in healing-impaired diabetic mice

A previous experiment using an in vivo mouse model has proved that hypoxia increased angiogenesis during wound healing. It was hypothesised that one of the mechanisms for wound healing impairment in diabetes includes insufficient angiogenic ability in response to hypoxia. The current study aims to investigate the influence of hypoxia on wound healing in diabetic mice. Oxygen-impermeable (hypoxic group) and -permeable membranes (normoxic group) were used to control topical oxygen tension. Membranes were applied to symmetrical excisional wounds on diabetic mice. Wound area, granulated tissue thickness, and vascular density were analyzed. As results, a decrease in wound size on day 7 was observed in the normoxic group (20.7 ± 3.64%) compared with the hypoxic group (34.1 ± 4.98%). The normoxic group also showed significantly thicker granulated tissue than the hypoxic group (225.7 ± 54.7 vs 128.7 ± 42.4 µm). There was no significant difference in mean vascular density between normoxic and hypoxic groups (0.046 ± 0.022 vs 0.038 ± 0.017 mm(2)/mm(2), p = 0.80). Contrary to healthy mice, diabetic mice have shown no enhancement of angiogenesis in hypoxic condition. The findings illustrate that neovascularisation in response to hypoxia is diminished in diabetic wounds.

Are nitric oxide-mediated protein modifications of functional significance in diabetic heart? ye'S, -NO', wh'Y-NO't?

Protein modifications effected by nitric oxide (NO) primarily in conjunction with reactive oxygen species (ROS) include tyrosine nitration, cysteine S-nitrosylation, and glutathionylation. The physiological and pathological relevance of these three modifications is determined by the amino acids on which these modifications occur -cysteine and tyrosine, for instance, ranging from altering structural integrity/catalytic activity of proteins or by altering propensity towards protein degradation. Even though tyrosine nitration is a well-established nitroxidative stress marker, instilled as a footprint of oxygen- and nitrogen-derived oxidants, newer data suggest its wider role in embryonic heart development and substantiate the need to focus on elucidating the underlying mechanisms of reversibility and specificity of tyrosine nitration. S-nitrosylation is a covalent modification in specific cysteine residues of proteins and is suggested as one of the ways in which NO contributes to its ubiquitous signalling. Several sensitive and specific techniques including biotin switch assay and mass spectrometry based analysis make it possible to identify a large number of these modified proteins, and provide a great deal of potential S-nitrosylation sites. The number of studies that have documented nitrated proteins in diabetic heart is relatively much less compared to what has been published in the normal physiology and other cardiac pathologies. Nevertheless, elucidation of nitrated proteome of diabetic heart has revealed the presence of many mitochondrial and cytosolic proteins of functional importance. But, the existence of different models of diabetes and analyses at diverse stages of this disease have impeded scientists from gaining insights that would be essential to understand the cardiac complications during diabetes. This review summarizes NO mediated protein modifications documented in normal and abnormal heart physiology including diabetes.

Mass spectrometry and redox proteomics: applications in disease

Proteomics techniques are continuously being developed to further understanding of biology and disease. Many of the pathways that are relevant to disease mechanisms rely on the identification of post-translational modifications (PTMs) such as phosphorylation, acetylation, and glycosylation. Much attention has also been focused on oxidative PTMs which include protein carbonyls, protein nitration, and the incorporation of fatty acids and advanced glycation products to amino acid side chains, amongst others. The introduction of these PTMs in the cell can occur due to the attack of reactive oxygen and nitrogen species (ROS and RNS, respectively) on proteins. ROS and RNS can be present as a result of normal metabolic processes as well as external factors such as UV radiation, disease, and environmental toxins. The imbalance of ROS and RNS with antioxidant cellular defenses leads to a state of oxidative stress, which has been implicated in many diseases. Redox proteomics techniques have been used to characterize oxidative PTMs that result as a part of normal cell signaling processes as well as oxidative stress conditions. This review highlights many of the redox proteomics techniques which are currently available for several oxidative PTMs and brings to the reader's attention the application of redox proteomics for understanding disease pathogenesis in neurodegenerative disorders and others such as cancer, kidney, and heart diseases.

Glutathionylated γG and γA subunits of hemoglobin F: a novel post-translational modification found in extremely premature infants by LC-MS and nanoLC-MS/MS

Oxidative stress plays an important role in the development of various disease processes and is a putative mechanism in the development of bronchopulmonary dysplasia, the most common complication of extreme preterm birth. Glutathione, a major endogenous antioxidant and redox buffer, also mediates cellular functions through protein thiolation. We sought to determine if post-translational thiol modification of hemoglobin F occurs in neonates by examining erythrocyte samples obtained during the first month of life from premature infants, born at 23 0/7 - 28 6/7 weeks gestational age, who were enrolled at our center in the Prematurity and Respiratory Outcomes Program (PROP). Using liquid chromatography-mass spectrometry (LC-MS), we report the novel finding of in vivo and in vitro glutathionylation of γG and γA subunits of Hgb F. Through tandem mass spectrometry (nanoLC-MS/MS), we confirmed the adduction site as the Cys-γ94 residue and through high-resolution mass spectrometry determined that the modification occurs in both γ subunits. We also identified glutathionylation of the β subunit of Hgb A in our patient samples; we did not find modified α subunits of Hgb A or F. In conclusion, we are the first to report that glutathionylation of γG and γA of Hgb F occurs in premature infants. Additional studies of this post-translational modification are needed to determine its physiologic impact on Hgb F function and if sG-Hgb is a biomarker for clinical morbidities associated with oxidative stress in premature infants.

Non-enzymatic glycation and glycoxidation protein products in foods and diseases: an interconnected, complex scenario fully open to innovative proteomic studies

The Maillard reaction includes a complex network of processes affecting food and biopharmaceutical products; it also occurs in living organisms and has been strictly related to cell aging, to the pathogenesis of several (chronic) diseases, such as diabetes, uremia, cataract, liver cirrhosis and various neurodegenerative pathologies, as well as to peritoneal dialysis treatment. Dozens of compounds are involved in this process, among which a number of protein-adducted derivatives that have been simplistically defined as early, intermediate and advanced glycation end-products. In the last decade, various bottom-up proteomic approaches have been successfully used for the identification of glycation/glycoxidation protein targets as well as for the characterization of the corresponding adducts, including assignment of the modified amino acids. This article provides an updated overview of the mass spectrometry-based procedures developed to this purpose, emphasizing their partial limits with respect to current proteomic approaches for the analysis of other post-translational modifications. These limitations are mainly related to the concomitant sheer diversity, chemical complexity, and variable abundance of the various derivatives to be characterized. Some challenges to scientists are finally proposed for future proteomic investigations to solve main drawbacks in this research field.

Biological markers of oxidative stress: Applications to cardiovascular research and practice

Oxidative stress is a common mediator in pathogenicity of established cardiovascular risk factors. Furthermore, it likely mediates effects of emerging, less well-defined variables that contribute to residual risk not explained by traditional factors. Functional oxidative modifications of cellular proteins, both reversible and irreversible, are a causal step in cellular dysfunction. Identifying markers of oxidative stress has been the focus of many researchers as they have the potential to act as an “integrator” of a multitude of processes that drive cardiovascular pathobiology. One of the major challenges is the accurate quantification of reactive oxygen species with very short half-life. Redox-sensitive proteins with important cellular functions are confined to signalling microdomains in cardiovascular cells and are not readily available for quantification. A popular approach is the measurement of stable by-products modified under conditions of oxidative stress that have entered the circulation. However, these may not accurately reflect redox stress at the cell/tissue level. Many of these modifications are “functionally silent”. Functional significance of the oxidative modifications enhances their validity as a proposed biological marker of cardiovascular disease, and is the strength of the redox cysteine modifications such as glutathionylation. We review selected biomarkers of oxidative stress that show promise in cardiovascular medicine, as well as new methodologies for high-throughput measurement in research and clinical settings. Although associated with disease severity, further studies are required to examine the utility of the most promising oxidative biomarkers to predict prognosis or response to treatment.

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Oxidative stress is a common mediator in pathobiology of risk factors for CVD.

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Oxidative modifications of proteins and lipids alter cellular function.

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Some oxidative biomarkers have been associated with severity of CVD.

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Pathophysiologically relevant biomarkers may integrate the effect of risk factors.

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Utility of oxidative biomarkers to guide prognosis/treatment merits further work.

Cadmium-induced glutathionylation of actin occurs through a ROS-independent mechanism: implications for cytoskeletal integrity

Cadmium disrupts the actin cytoskeleton in rat mesangial cells, and we have previously shown that this involves a complex interplay involving activation of kinase signaling, protein translocation, and disruption of focal adhesions. Here we investigate the role that glutathionylation of actin plays in Cd(2+)-associated cytoskeletal reorganization. Low concentrations of Cd(2+) (0.5-2 μM) caused an increase in actin glutathionylation by 6h, whereas at higher concentrations glutathionylation remained at basal levels. Although oxidation with diamide increased glutathionylation, reactive oxygen species (ROS) were not involved in the Cd(2+)-dependent effect, as only Cd(2+) concentrations above 2 μM were sufficient to increase ROS. However, low [Cd(2+)] increased total glutathione levels without affecting the ratio of reduced/oxidized glutathione, and inhibition of glutathione synthesis suppressed actin glutathionylation. Cadmium increased the activity of the enzyme glutaredoxin, which influences the equilibrium between glutathionylated and deglutathionylated proteins and thus may influence levels of glutathionylated actin. Together these observations show that cadmium-dependent effects on actin glutathionylation are affected by glutathione metabolism and not by direct effects of ROS on thiol chemistry. In vitro polymerization assays with glutathionylated actin show a decreased rate of polymerization. In contrast, immunofluorescence of cytoskeletal structure in intact cells suggests that increases in actin glutathionylation accompanying increased glutathione levels occurring under low Cd(2+) exposure are protective in vivo, with cytoskeletal disruption ensuing only when higher Cd(2+) concentrations increase ROS levels and prevent an increase in actin-glutathione conjugates.

Causes and consequences of cysteine S-glutathionylation

Post-translational S-glutathionylation occurs through the reversible addition of a proximal donor of glutathione to thiolate anions of cysteines in target proteins, where the modification alters molecular mass, charge, and structure/function and/or prevents degradation from sulfhydryl overoxidation or proteolysis. Catalysis of both the forward (glutathione S-transferase P) and reverse (glutaredoxin) reactions creates a functional cycle that can also regulate certain protein functional clusters, including those involved in redox-dependent cell signaling events. For translational application, S-glutathionylated serum proteins may be useful as biomarkers in individuals (who may also have polymorphic expression of glutathione S-transferase P) exposed to agents that cause oxidative or nitrosative stress.

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.

Antioxidant functions for the hemoglobin β93 cysteine residue in erythrocytes and in the vascular compartment in vivo

The β93 cysteine (β93Cys) residue of hemoglobin is conserved in vertebrates but its function in the red blood cell (RBC) remains unclear. Because this residue is present at concentrations more than 2 orders of magnitude higher than enzymatic components of the RBC antioxidant network, a role in the scavenging of reactive species was hypothesized. Initial studies utilizing mice that express human hemoglobin with either Cys (B93C) or Ala (B93A) at the β93 position demonstrated that loss of the β93Cys did not affect activities nor expression of established components of the RBC antioxidant network (catalase, superoxide dismutase, peroxiredoxin-2, glutathione peroxidase, GSH:GSSG ratios). Interestingly, exogenous addition to RBCs of reactive species that are involved in vascular inflammation demonstrated a role for the β93Cys in hydrogen peroxide and chloramine consumption. To simulate oxidative stress and inflammation in vivo, mice were challenged with lipopolysaccharide (LPS). Notably, LPS induced a greater degree of hypotension and lung injury in B93A versus B93C mice, which was associated with greater formation of RBC reactive species and accumulation of DMPO-reactive epitopes in the lung. These data suggest that the β93Cys is an important effector within the RBC antioxidant network, contributing to the modulation of tissue injury during vascular inflammation.

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 signaling in cell biology: progress and prospects

S-Glutathionylation is a mechanism of signal transduction by which cells respond effectively and reversibly to redox inputs. The glutathionylation regulates most cellular pathways. It is involved in oxidative cellular response to insult by modulating the transcription factor Nrf2 and inducing the expression of antioxidant genes (ARE); it contributes to cell survival through nuclear translocation of NFkB and activation of survival genes, and to cell death by modulating the activity of caspase 3. It is involved in mitotic spindle formation during cell division by binding cytoskeletal proteins thus contributing to cell proliferation and differentiation. Glutathionylation also interfaces with the mechanism of phosphorylation by modulating several kinases (PKA, CK) and phosphatases (PP2A, PTEN), thus allowing a cross talk between the two processes of signal transduction. Also, skeletal RyR1 channels responsible of muscle excitation-contraction coupling appear to be sensitive to glutathionylation. Members of the ryanodine receptor super family, responsible for Ca(2) release from endoplasmic reticulum stores, contain sulfhydryl groups that function as a redox "switch", which either induces or inhibits Ca(2) release. Finally, but very importantly, glutathionylation of proteins may also act on cell metabolism by modulating enzymes involved in glycosylation, in the Krebs cycle and in mitochondrial oxidative phosphorylation. In this review, we propose a greater role for glutathionylation in cell biology: not only a cellular response to oxidative stress, but an elegant and sensitive mechanism able to respond even to subtle changes in redox balance in the different cellular compartments. Given the wide spectrum of redox-sensitive proteins, we discuss the possibility that different pathways light up by glutathionylation under various pathological conditions. The feature of reversibility of this process also makes it prone to develop targeted drug therapies and monitor the pharmacological effectiveness once identified the sensor proteins involved.

Endothelial dysfunction and diabetes: effects on angiogenesis, vascular remodeling, and wound healing

Diabetes mellitus (DM) is a chronic metabolic disorder characterized by inappropriate hyperglycemia due to lack of or resistance to insulin. Patients with DM are frequently afflicted with ischemic vascular disease or wound healing defect. It is well known that type 2 DM causes amplification of the atherosclerotic process, endothelial cell dysfunction, glycosylation of extracellular matrix proteins, and vascular denervation. These complications ultimately lead to impairment of neovascularization and diabetic wound healing. Therapeutic angiogenesis remains an attractive treatment modality for chronic ischemic disorders including PAD and/or diabetic wound healing. Many experimental studies have identified better approaches for diabetic cardiovascular complications, however, successful clinical translation has been limited possibly due to the narrow therapeutic targets of these agents or the lack of rigorous evaluation of pathology and therapeutic mechanisms in experimental models of disease. This paper discusses the current body of evidence identifying endothelial dysfunction and impaired angiogenesis during diabetes.

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.

Differential rates of glutathione oxidation for assessment of cellular redox status and antioxidant capacity by capillary electrophoresis-mass spectrometry: an elusive biomarker of oxidative stress

Glutathione metabolism plays a fundamental role in maintaining homeostasis and regulating the redox environment of a cell. Despite the widespread interest in quantifying glutathione metabolites in oxidative stress research, conventional techniques are hampered by complicated sample handling procedures to prevent significant oxidation artifacts generated during sample collection, sample pretreatment, and/or chemical analysis. In this report, a simple and validated method for glutathione analysis from filtered red blood cell (RBC) lysates was developed using capillary electrophoresis-electrospray ionization-mass spectrometry (CE-ESI-MS) in conjunction with fingerprick microsampling and ultrafiltration. About a 3-fold improvement in precision with nanomolar detection limits was achieved when using online sample preconcentration with CE-ESI-MS via a modified injection sequence, which permitted accurate determination of the intracellular reduced/oxidized glutathione ratio (GSH/GSSG), as well as other glutathione species, including protein-bound glutathione mixed disulfide (PSSG), free glutathione mixed disulfides (GSSR) and glutathione thioether conjugates (GSX). In this work, the redox status of filtered hemolysates was determined by the equilibrium half-cell reduction potential for glutathione (E(GSSG/2GSH)), whereas its intrinsic antioxidant capacity was assessed by the apparent rate of metal-catalyzed oxidation of glutathione. In-vitro incubation studies of intact RBCs with 1-chloro-2,4-dinitrobenzene (CDNB) and N-acetyl-L-cysteine (NAC) were found to significantly alter E(GSSG/2GSH) and/or glutathione oxidation kinetics (e.g., k(GSSG)) relative to normal controls based on their function as a toxic electrophilic compound and a competitive free radical scavenging/reducing agent, respectively. Differential rates of glutathione oxidation (DIRGO) using CE-ESI-MS offers a novel strategy for global assessment of the impact of intrinsic metabolite constituents (i.e., metabolome) and/or extrinsic perturbants on cellular redox status that is relevant to improved understanding of aging and the pathogenesis of acute or chronic disease states.

S-glutathionylation regulates inflammatory activities of S100A9

Reactive oxygen species generated by activated neutrophils can cause oxidative stress and tissue damage. S100A8 (A8) and S100A9 (A9), abundant in neutrophil cytoplasm, are exquisitely sensitive to oxidation, which may alter their functions. Murine A8 is a neutrophil chemoattractant, but it suppresses leukocyte transmigration in the microcirculation when S-nitrosylated. Glutathione (GSH) modulates intracellular redox, and S-glutathionylation can protect susceptible proteins from oxidative damage and regulate function. We characterized S-glutathionylation of A9; GSSG and GSNO generated S-glutathionylated A8 (A8-SSG) and A9 (A9-SSG) in vitro, whereas only A9-SSG was detected in cytosol of neutrophils activated with phorbol myristate acetate (PMA) but not with fMLP or opsonized zymosan. S-Glutathionylation exposed more hydrophobic regions in Zn(2+)-bound A9 but did not alter Zn(2+) binding affinity. A9-SSG had reduced capacity to form heterocomplexes with A8, but the arachidonic acid binding capacities of A8/A9 and A8/A9-SSG were similar. A9 and A8/A9 bind endothelial cells; S-glutathionylation reduced binding. We found little effect of A9 or A9-SSG on neutrophil CD11b/CD18 expression or neutrophil adhesion to endothelial cells. However, A9, A9-SSG and A8/A9 promoted neutrophil adhesion to fibronectin but, in the presence of A8, A9-mediated adhesion was abrogated by glutathionylation. S-Glutathionylation of A9 may protect its oxidation to higher oligomers and reduce neutrophil binding to the extracellular matrix. This may regulate the magnitude of neutrophil migration in the extravasculature, and together with the functional changes we reported for S-nitrosylated A8, particular oxidative modifications of these proteins may limit tissue damage in acute inflammation.

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.

Characterization of S-glutathionyl hemoglobin in homozygous sickle cell disease

S-glutathionyl hemoglobin is a proposed biomarker of oxidative stress but has not been measured in sickle cell disease patients. Unlike the S-glutathionyl adduct of normal adult hemoglobin, S-glutathionyl sickle hemoglobin (HbSSG) cannot be directly measured by capillary isoelectric focusing, because it coelutes with fetal hemoglobin (HbF). This suggests that HbF, measured in sickle cell patients with or without hydroxyurea therapy, might contain endogenous HbSSG. As S-glutathionyl hemoglobin can form during sample storage, HbSSG could falsely elevate HbF levels in stored samples. We measured HbSSG based on the quantitative difference in the heterogeneous HbF/HbSSG peak before and after hemolysates were treated with dithiothreitol. Paired t tests showed that dithiothreitol reduced HbF/HbSSG in blood from pediatric sickle cell patients (n=25, mean decrease+/-SD=1.0%+/-0.6, P<0.05) but not in normal infants (n=25). Higher HbF levels in hydroxyurea-treated patients (n=5) were not attributable to HbSSG. HbSSG increased significantly within 1 day in samples stored at -20 degrees C but was unchanged in samples stored 60 days at-70 degrees C. We conclude that blood from sickle cell patients contained up to 2.2% HbSSG, and that endogenous HbSSG is a minor interferent in the measurement of HbF in fresh blood but a major interferent in improperly stored samples.