Measuring Mitochondrial Protein Thiol Redox State

AIDA directly connects sympathetic innervation to adaptive thermogenesis by UCP1

The sympathetic nervous system-catecholamine-uncoupling protein 1 (UCP1) axis plays an essential role in non-shivering adaptive thermogenesis. However, whether there exists a direct effector that physically connects catecholamine signalling to UCP1 in response to acute cold is unknown. Here we report that outer mitochondrial membrane-located AIDA is phosphorylated at S161 by the catecholamine-activated protein kinase A (PKA). Phosphorylated AIDA translocates to the intermembrane space, where it binds to and activates the uncoupling activity of UCP1 by promoting cysteine oxidation of UCP1. Adipocyte-specific depletion of AIDA abrogates UCP1-dependent thermogenesis, resulting in hypothermia during acute cold exposure. Re-expression of S161A-AIDA, unlike wild-type AIDA, fails to restore the acute cold response in Aida-knockout mice. The PKA-AIDA-UCP1 axis is highly conserved in mammals, including hibernators. Denervation of the sympathetic postganglionic fibres abolishes cold-induced AIDA-dependent thermogenesis. These findings uncover a direct mechanistic link between sympathetic input and UCP1-mediated adaptive thermogenesis.

Silver Nanoparticles Induce Mitochondrial Protein Oxidation in Lung Cells Impacting Cell Cycle and Proliferation

Silver nanoparticles (AgNPs) are widely used nanomaterials in both commercial and clinical biomedical applications, due to their antibacterial properties. AgNPs are also being explored for the treatment of cancer in particular in combination with ionizing radiation. In this work, we studied the effects of AgNPs and ionizing radiation on mitochondrial redox state and function in a panel of lung cell lines (A549, BEAS-2B, Calu-1 and NCI-H358). The exposure to AgNPs caused cell cycle arrest and decreased cell proliferation in A549, BEAS-2B and Calu-1, but not in NCI-H358. The mitochondrial reactive oxygen species (ROS) and protein oxidation increased in a time- and dose-dependent manner in the more sensitive cell lines with the AgNP exposure, but not in NCI-H358. While ionizing radiation also induced changes in the mitochondrial redox profiles, in general, these were not synergistic with the effects of AgNPs with the exception of NCI-H358 and only at a higher dose of radiation.

Short-term high salt intake impairs hepatic mitochondrial bioenergetics and biosynthesis in SIRT3 knockout mice

High salt intake (HS) is an important factor in the development of many metabolic diseases. The liver is the metabolic center in the body. However, the effect of short-term HS on the liver mitochondria and its mechanism are still unclear. In this study, we investigated the effects of short-term HS on liver mitochondrial function. We found that HS reduced Sirtuin3 (SIRT3) protein level, increasing protein carbonylation in mice liver. HS intake decreased ATP production, mitochondrial transcription factor A (TFAM), and complex I level. SIRT3 knockout (SKO) mice exhibited similar results with HS-treated wild-type mice but with a less extent of carbonylation and ATP reduction. Our study shows that short-term HS led to increased hepatic oxidative state, impaired mitochondrial biosynthesis, and bioenergetics. HS-treated mice could still maintain hepatic glucose homeostasis by compensatory activation of Adenosine 5'-monophosphate-activated protein kinase (AMPK). However, in HS-treated SKO mice, AMPK was not activated, instead, the glycogen synthase activity increased, which caused an exceptionally increased glycogen accumulation. This study provides evidence that short-term HS intake could cause the early hepatic metabolic changes, highlighting the importance of controlling salt intake especially in those patients with defects in SIRT3. Highlights High salt intake down-regulates SIRT3 protein level and increases oxidation. High salt intake activates AMPK via AMP-dependent pathway. High salt intake impairs energy metabolism. High salt combined with SIRT3 knockout results in glycogen accumulation.

Modeling cardiac complexity: Advancements in myocardial models and analytical techniques for physiological investigation and therapeutic development in vitro

Cardiomyopathies, heart failure, and arrhythmias or conduction blockages impact millions of patients worldwide and are associated with marked increases in sudden cardiac death, decline in the quality of life, and the induction of secondary pathologies. These pathologies stem from dysfunction in the contractile or conductive properties of the cardiomyocyte, which as a result is a focus of fundamental investigation, drug discovery and therapeutic development, and tissue engineering. All of these foci require in vitro myocardial models and experimental techniques to probe the physiological functions of the cardiomyocyte. In this review, we provide a detailed exploration of different cell models, disease modeling strategies, and tissue constructs used from basic to translational research. Furthermore, we highlight recent advancements in imaging, electrophysiology, metabolic measurements, and mechanical and contractile characterization modalities that are advancing our understanding of cardiomyocyte physiology. With this review, we aim to both provide a biological framework for engineers contributing to the field and demonstrate the technical basis and limitations underlying physiological measurement modalities for biologists attempting to take advantage of these state-of-the-art techniques.

Glutathione compartmentalization and its role in glutathionylation and other regulatory processes of cellular pathways

Glutathione is considered the major non-protein low molecular weight modulator of redox processes and the most important thiol reducing agent of the cell. The biosynthesis of glutathione occurs in the cytosol from its constituent amino acids, but this tripeptide is also present in the most important cellular districts, such as mitochondria, nucleus, and endoplasmic reticulum, thus playing a central role in several metabolic pathways and cytoprotection mechanisms. Indeed, glutathione is involved in the modulation of various cellular processes and, not by chance, it is a ubiquitous determinant for redox signaling, xenobiotic detoxification, and regulation of cell cycle and death programs. The balance between its concentration and redox state is due to a complex series of interactions between biosynthesis, utilization, degradation, and transport. All these factors are of great importance to understand the significance of cellular redox balance and its relationship with physiological responses and pathological conditions. The purpose of this review is to give an overview on glutathione cellular compartmentalization. Information on its subcellular distribution provides a deeper understanding of glutathione-dependent processes and reflects the importance of compartmentalization in the regulation of specific cellular pathways. © 2018 BioFactors, 45(2):152-168, 2019.

Redox Equivalents and Mitochondrial Bioenergetics

Mitochondrial energy metabolism depends upon high-flux and low-flux electron transfer pathways. The former provide the energy to support chemiosmotic coupling for oxidative phosphorylation. The latter provide mechanisms for signaling and control of mitochondrial functions. Few practical methods are available to measure rates of individual mitochondrial electron transfer reactions; however, a number of approaches are available to measure steady-state redox potentials (E _h) of donor/acceptor couples, and these can be used to gain insight into rate controlling reactions as well as mitochondrial bioenergetics. Redox changes within the respiratory electron transfer pathway are quantified by optical spectroscopy and measurement of changes in autofluorescence. Low-flux pathways involving thiol/disulfide redox couples are measured by redox Western blot and mass spectrometry-based redox proteomics. Together, the approaches provide the opportunity to develop integrated systems biology descriptions of mitochondrial redox signaling and control mechanisms.

Mitochondria-targeted Probes for Imaging Protein Sulfenylation

Mitochondrial reactive oxygen species (ROS) are essential regulators of cellular signaling, metabolism and epigenetics underlying the pathophysiology of numerous diseases. Despite the critical function of redox regulation in mitochondria, currently there are limited methods available to monitor protein oxidation in this key subcellular organelle. Here, we describe compounds for imaging sulfenylated proteins in mitochondria: DCP-NEt₂-Coumarin (DCP-NEt₂C) and rhodamine-based DCP-Rho1. Side-by-side comparison studies are presented on the reactivity of DCP-NEt₂C and DCP-Rho1 with a model protein sulfenic acid (AhpC-SOH) and mitochondrial localization to identify optimized experimental conditions for labeling and visualization of protein sulfenylation that would be independent of mitochondria membrane potential and would not impact mitochondrial function. These probes are applied to image mitochondrial protein sulfenylation under conditions of serum starvation and in a cell culture model of lung cancer exposed to ionizing radiation and silver nanoparticles, agents serving dual functions as environmental stressors and cancer therapeutics.

S-Adenosylmethionine Promotes Oxidative Stress and Decreases Na+, K+-ATPase Activity in Cerebral Cortex Supernatants of Adolescent Rats: Implications for the Pathogenesis of S-Adenosylhomocysteine Hydrolase Deficiency

S-Adenosylmethionine (AdoMet) concentrations are highly elevated in tissues and biological fluids of patients affected by S-adenosylhomocysteine hydrolase deficiency, who are clinically characterized by cerebral symptoms whose pathogenesis is still unknown. In the present work, we investigated the effects of AdoMet on redox homeostasis and on the activity of Na⁺, K⁺-ATPase in the cerebral cortex of young rats. AdoMet caused lipid peroxidation (increase of malondialdehyde concentrations) and protein oxidation (increase of carbonyl formation and decrease of sulfhydryl content). AdoMet also reduced the antioxidant defenses (reduced glutathione, GSH) and Na⁺, K⁺-ATPase activity. Furthermore, AdoMet-induced lipid peroxidation was fully prevented by the antioxidants trolox, melatonin, and resveratrol, and the decrease of GSH concentrations was abolished by trolox, suggesting the involvement of reactive oxygen species in these effects. In this context, AdoMet induced reactive oxygen (increase of 2',7'-dichloroflurescein-DCFH oxidation) but not nitrogen (nitrate and nitrite levels) species generation. Finally, the decrease of Na⁺, K⁺-ATPase activity provoked by AdoMet was totally prevented by trolox, implying a possible oxidation of cysteine groups of the enzyme that are critical for its function and highly susceptible to oxidative attack. It is also noted that adenosine and methionine did not alter the parameters evaluated, suggesting selective effects of AdoMet. Our data strongly indicate that disturbance of redox homeostasis caused by a major metabolite (AdoMet) accumulating in S-adenosylhomocysteine hydrolase deficiency may represent a deleterious mechanism of brain damage in this disease. Finally, reduction of Na⁺, K⁺-ATPase activity provoked by AdoMet may lead to impaired neurotransmission, but disturbance of this system should be better clarified in future studies.

Modulation of the specific glutathionylation of mitochondrial proteins in the yeast Saccharomyces cerevisiae under basal and stress conditions

The potential biological consequences of oxidative stress and changes in glutathione levels include the oxidation of susceptible protein thiols and reversible covalent binding of glutathione to the -SH groups of proteins by S-glutathionylation. Mitochondria are central to the response to oxidative stress and redox signaling. It is therefore crucial to explore the adaptive response to changes in thiol-dependent redox status in these organelles. We optimized the purification protocol of glutathionylated proteins in the yeast Saccharomyces cerevisiae and present a detailed proteomic analysis of the targets of protein glutathionylation in cells undergoing constitutive metabolism and after exposure to various stress conditions. This work establishes the physiological importance of the glutathionylation process in S. cerevisiae under basal conditions and provides evidence for an atypical and unexpected cellular distribution of the process between the cytosol and mitochondria. In addition, our data indicate that each oxidative condition (diamide, GSSG, H₂O₂, or the presence of iron) elicits an adaptive metabolic response affecting specific mitochondrial metabolic pathways, mainly involved in the energetic maintenance of the cells. The correlation of protein modifications with intracellular glutathione levels suggests that protein deglutathionylation may play a role in protecting mitochondria from oxidative stress. This work provides further insights into the diversity of proteins undergoing glutathionylation and the role of this post-translational modification as a regulatory process in the adaptive response of the cell.

Mitochondrial ROS regulate thermogenic energy expenditure and sulfenylation of UCP1

Brown adipose tissue (BAT) can dissipate chemical energy as heat through thermogenic respiration, which requires uncoupling protein 1 (UCP1)^1,2. Thermogenesis from BAT and beige adipose can combat obesity and diabetes³, encouraging investigation of factors that control UCP1-dependent respiration in vivo. Herein we show that acutely activated BAT thermogenesis is defined by a substantial increase in mitochondrial reactive oxygen species (ROS) levels. Remarkably, this process supports in vivo BAT thermogenesis, as pharmacological depletion of mitochondrial ROS results in hypothermia upon cold exposure, and inhibits UCP1-dependent increases in whole body energy expenditure. We further establish that thermogenic ROS alter BAT cysteine thiol redox status to drive increased respiration, and Cys253 of UCP1 is a key target. UCP1 Cys253 is sulfenylated during thermogenesis, while mutation of this site desensitizes the purine nucleotide inhibited state of the carrier to adrenergic activation and uncoupling. These studies identify BAT mitochondrial ROS induction as a mechanism that drives UCP1-dependent thermogenesis and whole body energy expenditure, which opens the way to develop improved therapeutic strategies for combating metabolic disorders.

Depletion of Intracellular Thiols and Increased Production of 4-Hydroxynonenal that Occur During Cryopreservation of Stallion Spermatozoa Lead to Caspase Activation, Loss of Motility, and Cell Death

Oxidative stress has been linked to sperm death and the accelerated senescence of cryopreserved spermatozoa. However, the molecular mechanisms behind this phenomenon remain poorly understood. Reactive oxygen species (ROS) are considered relevant signaling molecules for sperm function, only becoming detrimental when ROS homeostasis is lost. We hereby hypothesize that a major component of the alteration of ROS homeostasis in cryopreserved spermatozoa is the exhaustion of intrinsic antioxidant defense mechanisms. To test this hypothesis, semen from seven stallions was frozen using a standard technique. The parameters of sperm quality (motility, velocity, and membrane integrity) and markers of sperm senescence (caspase 3, 4-hydroxynonenal, and mitochondrial membrane potential) were assessed before and after cryopreservation. Changes in the intracellular thiol content were also monitored. Cryopreservation caused significant increases in senescence markers as well as dramatic depletion of intracellular thiols to less than half of the initial values (P < 0.001) postthaw. Interestingly, very high and positive correlations were observed among thiol levels with sperm functionality postthaw: total motility (r = 0.931, P < 0.001), progressive motility (r = 0.904, P < 0.001), and percentage of live spermatozoa without active caspase 3 (r = 0.996, P < 0.001). In contrast, negative correlations were detected between active caspase 3 and thiol content both in living (r = -0.896) and dead (r = -0.940) spermatozoa; additionally, 4-hydroxynonenal levels were negatively correlated with thiol levels (r = -0.856). In conclusion, sperm functionality postthaw correlates with the maintenance of adequate levels of intracellular thiols. The accelerated senescence of thawed spermatozoa is related to oxidative and electrophilic stress induced by increased production of 4-hydroxynoneal in thawed samples once intracellular thiols are depleted.

Pristanic acid provokes lipid, protein, and DNA oxidative damage and reduces the antioxidant defenses in cerebellum of young rats

Zellweger syndrome (ZS) and some peroxisomal diseases are severe inherited disorders mainly characterized by neurological symptoms and cerebellum abnormalities, whose pathogenesis is poorly understood. Biochemically, these diseases are mainly characterized by accumulation of pristanic acid (Prist) and other fatty acids in the brain and other tissues. In this work, we evaluated the in vitro influence of Prist on redox homeostasis by measuring lipid, protein, and DNA damage, as well as the antioxidant defenses and the activities of aconitase and α-ketoglutarate dehydrogenase in cerebellum of 30-day-old rats. The effect of Prist on DNA damage was also evaluated in blood of these animals. Some parameters were also evaluated in cerebellum from neonatal rats and in cerebellum neuronal cultures. Prist significantly increased malondialdehyde (MDA) levels and carbonyl formation and reduced sulfhydryl content and glutathione (GSH) concentrations in cerebellum of young rats. It also caused DNA strand damage in cerebellum and induced a high micronuclei frequency in blood. On the other hand, this fatty acid significantly reduced α-ketoglutarate dehydrogenase and aconitase activities in rat cerebellum. We also verified that Prist-induced increase of MDA levels was totally prevented by melatonin and attenuated by α-tocopherol but not by the nitric oxide synthase inhibitor N(ω)-nitro-L-arginine methyl ester, indicating the involvement of reactive oxygen species in this effect. Cerebellum from neonate rats also showed marked alterations of redox homeostasis, including an increase of MDA levels and a decrease of sulfhydryl content and GSH concentrations elicited by Prist. Finally, Prist provoked an increase of dichlorofluorescein (DCFH) oxidation in cerebellum-cultivated neurons. Our present data indicate that Prist compromises redox homeostasis in rat cerebellum and blood and inhibits critical enzymes of the citric acid cycle that are susceptible to free radical attack. The present findings may contribute to clarify the pathogenesis of the cerebellar alterations observed in patients affected by ZS and some peroxisomal disorders in which Prist is accumulated.

Disturbance of redox homeostasis by ornithine and homocitrulline in rat cerebellum: a possible mechanism of cerebellar dysfunction in HHH syndrome

AIMS

Cerebellar ataxia is commonly observed in hyperornithinemia-hyperammonemia-homocitrullinuria (HHH) syndrome, an inherited metabolic disorder biochemically characterized by ornithine (Orn), homocitrulline (Hcit) and ammonia accumulation. Since the pathophysiology of cerebellum damage in this disorder is still unknown, we investigated the effects of Hcit and Orn on important parameters of redox and energy homeostasis in cerebellum of young rats.

MATERIAL AND METHODS

We determined thiobarbituric acid-reactive substance (TBA-RS) levels, carbonyl content, nitrate and nitrite production, hydrogen peroxide production, GSH concentrations, sulfhydryl content, as well as activities of respiratory chain complexes I-IV, creatine kinase, Na(+),K(+)-ATPase, aconitase and α-ketoglutarate dehydrogenase.

KEY FINDINGS

Orn and Hcit significantly increased TBA-RS levels (lipid oxidation), that was totally prevented by melatonin and reduced glutathione (GSH). We also found that nitrate and nitrite production was not altered by any of the metabolites, in contrast to hydrogen peroxide production which was significantly enhanced by Hcit. Furthermore, GSH concentrations were significantly reduced by Orn and Hcit and sulfhydryl content by Orn, implying an impairment of antioxidant defenses. As regards energy metabolism, Orn and Hcit provoked a significant reduction of aconitase activity, without altering the other parameters. Furthermore, Orn-elicited reduction of aconitase activity was totally prevented by GSH, indicating that the critical groups of this enzyme were susceptible to oxidation caused by this amino acid.

SIGNIFICANCE

Taken together, our data indicate that redox homeostasis is disturbed by the major metabolites accumulating in HHH syndrome and that this mechanism may be implicated in the ataxia and cerebellar abnormalities observed in this disorder.

Cardioprotection by S-nitrosation of a cysteine switch on mitochondrial complex I

Oxidative damage from elevated production of reactive oxygen species (ROS) contributes to ischemia-reperfusion injury in myocardial infarction and stroke. The mechanism by which the increase in ROS occurs is not known, and it is unclear how this increase can be prevented. A wide variety of nitric oxide donors and S-nitrosating agents protect the ischemic myocardium from infarction, but the responsible mechanisms are unclear. Here we used a mitochondria-selective S-nitrosating agent, MitoSNO, to determine how mitochondrial S-nitrosation at the reperfusion phase of myocardial infarction is cardioprotective in vivo in mice. We found that protection is due to the S-nitrosation of mitochondrial complex I, which is the entry point for electrons from NADH into the respiratory chain. Reversible S-nitrosation of complex I slows the reactivation of mitochondria during the crucial first minutes of the reperfusion of ischemic tissue, thereby decreasing ROS production, oxidative damage and tissue necrosis. Inhibition of complex I is afforded by the selective S-nitrosation of Cys39 on the ND3 subunit, which becomes susceptible to modification only after ischemia. Our results identify rapid complex I reactivation as a central pathological feature of ischemia-reperfusion injury and show that preventing this reactivation by modification of a cysteine switch is a robust cardioprotective mechanism and hence a rational therapeutic strategy.

Redox equivalents and mitochondrial bioenergetics

Mitochondrial energy metabolism depends upon high-flux and low-flux electron transfer pathways. The former provide the energy to support chemiosmotic coupling for oxidative phosphorylation. The latter provide mechanisms for signaling and control of mitochondrial functions. Few practical methods are available to measure rates of individual mitochondrial electron transfer reactions; however, a number of approaches are available to measure steady-state redox potentials (E (h)) of donor/acceptor couples, and these can be used to gain insight into rate-controlling reactions as well as mitochondrial bioenergetics. Redox changes within the respiratory electron transfer pathway are quantified by optical spectroscopy and measurement of changes in autofluorescence. Low-flux pathways involving thiol/disulfide redox couples are measured by redox western blot and mass spectrometry-based redox proteomics. Together, the approaches provide the opportunity to develop integrated systems biology descriptions of mitochondrial redox signaling and control mechanisms.

Maneb and paraquat-mediated neurotoxicity: involvement of peroxiredoxin/thioredoxin system

Epidemiological and in vivo studies have demonstrated that exposure to the pesticides paraquat (PQ) and maneb (MB) increase the risk of developing Parkinson's disease (PD) and cause dopaminergic cell loss, respectively. PQ is a well-recognized cause of oxidative toxicity; therefore, the purpose of this study was to determine if MB potentiates oxidative stress caused by PQ, thus providing a mechanism for enhanced neurotoxicity by the combination. The results show that PQ alone at a moderately toxic dose (20-30% cell death in 24 h) caused increased reactive oxygen species (ROS) generation, oxidation of mitochondrial thioredoxin-2 and peroxiredoxin-3, lesser oxidation of cytoplasmic thioredoxin-1 and peroxiredoxin-1, and no oxidation of cellular GSH/GSSG. In contrast, MB alone at a similar toxic dose resulted in no ROS generation, no oxidation of thioredoxin and peroxiredoxin, and an increase in cellular GSH after 24 h. Together, MB increased GSH and inhibited ROS production and thioredoxin/peroxiredoxin oxidation observed with PQ alone, yet resulted in more extensive (> 50%) cell death. MB treatment resulted in increased abundance of nuclear Nrf2 and mRNA for phase II enzymes under the control of Nrf2, indicating activation of cell protective responses. The results show that MB potentiation of PQ neurotoxicity does not occur by enhancing oxidative stress and suggests that increased toxicity occurs by a combination of divergent mechanisms, perhaps involving alkylation by MB and oxidation by PQ.

Factors affecting protein thiol reactivity and specificity in peroxide reduction

Protein thiol reactivity generally involves the nucleophilic attack of the thiolate on an electrophile. A low pK(a) means higher availability of the thiolate at neutral pH but often a lower nucleophilicity. Protein structural factors contribute to increasing the reactivity of the thiol in very specific reactions, but these factors do not provide an indiscriminate augmentation in general reactivity. Notably, reduction of hydroperoxides by the catalytic cysteine of peroxiredoxins can achieve extraordinary reaction rates relative to free cysteine. The discussion of this catalytic efficiency has centered in the stabilization of the thiolate as a way to increase nucleophilicity. Such stabilization originates from electrostatic and polar interactions of the catalytic cysteine with the protein environment. We propose that the set of interactions is better described as a means of stabilizing the anionic transition state of the reaction. The enhanced acidity of the critical cysteine is concurrent but not the cause of catalytic efficiency. Protein stabilization of the transition state is achieved by (a) a relatively static charge distribution around the cysteine that includes a conserved arginine and the N-terminus of an α-helix providing a cationic environment that stabilizes the reacting thiolate, the transition state, and also the anionic leaving group; (b) a dynamic set of polar interactions that stabilize the thiolate in the resting enzyme and contribute to restraining its reactivity in the absence of substrate; but upon peroxide binding these active/binding site groups switch interactions from thiolate to peroxide oxygens, simultaneously increasing the nucleophilicity of the attacking sulfur and facilitating the correct positioning of the substrate. The switching of polar interaction provides further acceleration and, importantly, confers specificity to the thiol reactivity. The extraordinary thiol reactivity and specificity toward H(2)O(2) combined with their ubiquity and abundance place peroxiredoxins, along with glutathione peroxidases, as obligate hydroperoxide cellular sensors.

Proteomic approaches to the characterization of protein thiol modification

Protein cysteine residues are central to redox signaling and to protection against oxidative damage through their interactions with reactive oxygen and nitrogen species, and electrophiles. Although there is considerable evidence for a functional role for cysteine modifications, the identity and physiological significance of most protein thiol alterations are unknown. One way to identify candidate proteins involved in these processes is to utilize the proteomic methodologies that have been developed in recent years for the identification of proteins that undergo cysteine modification in response to redox signals or oxidative damage. These tools have proven effective in uncovering novel protein targets of redox modification and are important first steps that allow for a better understanding of how reactive molecules may contribute to signaling and damage. Here, we discuss a number of these approaches and their application to the identification of a variety of cysteine-centered redox modifications.

Measuring the redox state of cellular peroxiredoxins by immunoblotting

The peroxiredoxins (Prxs) are a family of thiol peroxidases that scavenge hydroperoxides and peroxynitrite. The abundance and reactivity of these proteins makes them primary targets for cellular H(2)O(2). The catalytic cycle of typical 2-Cys Prxs involves formation of an intermolecular disulfide bond between peroxidatic and resolving cysteines on opposing subunits. Rapid alterations in the ratio of reduced monomer and oxidized dimer have been detected in the cytoplasm and mitochondria of cultured cells exposed to various exogenous and endogenous sources of oxidative stress. Here we describe immunoblot methods to monitor the interconversion of individual 2-Cys Prxs in cultured cells. We also outline an adaptation of this method to measure the extent to which individual 2-Cys Prxs become hyper oxidized in treated cells. Together, these methods enable the redox status of cellular Prxs to be assessed and quantified in a rapid and robust manner.