Cysteine residues 244 and 458-459 within the catalytic subunit of Na,K-ATPase control the enzyme's hydrolytic and signaling function under hypoxic conditions

The effect of cytochrome c on Na,K-ATPase

Na,K-ATPase is a crucial enzyme responsible for maintaining Na+, K+-gradients across the cell membrane, which is essential for numerous physiological processes within various organs and tissues. Due to its significance in cellular physiology, inhibiting Na,K-ATPase can have profound physiological consequences. This characteristic makes it a target for various pharmacological applications, and drugs that modulate the pump's activity are thus used in the treatment of several medical conditions. Cytochrome c (Cytc) is a protein with dual functions in the cell. In the mitochondria, it is essential for ATP synthesis and energy production. However, in response to apoptotic stimuli, it is released into the cytosol, where it triggers programmed cell death through the intrinsic apoptosis pathway. Aside from its role in canonical intrinsic apoptosis, Cytc also plays additional roles. For instance, Cytc participates in certain non-apoptotic functions -those which are less well-understood in comparison to its role in apoptosis. Within this in vitro study, we have shown the impact of Cytc on Na,K-ATPase for the first time. Cytc has a biphasic action on Na,K-ATPase, with activation at low concentrations (0.06 ng/ml; 6 ng/ml) and inhibition at high concentration (120 ng/ml). Cytc moreover displays isoform/subunit specificity and regulates the Na+ form of the enzyme, while having no effect on the activity or kinetic parameters of the K+-dependent form of the enzyme. Changing the affinity of p-chloromercuribenzoic acid (PCMB) by Cytc is therefore both a required and sufficient condition for confirming that PCMB and Cytc share the same target, namely the thiol groups of cysteine in Na,K-ATPase.

Antioxidant Therapy Reduces Oxidative Stress, Restores Na,K-ATPase Function and Induces Neuroprotection in Rodent Models of Seizure and Epilepsy: A Systematic Review and Meta-Analysis

Epilepsy is a neurological disorder characterized by epileptic seizures resulting from neuronal hyperexcitability, which may be related to failures in Na,K-ATPase activity and oxidative stress participation. We conducted this study to investigate the impact of antioxidant therapy on oxidative stress, Na,K-ATPase activity, seizure factors, and mortality in rodent seizure/epilepsy models induced by pentylenetetrazol (PTZ), pilocarpine (PILO), and kainic acid (KA). After screening 561 records in the MEDLINE, EMBASE, Web of Science, Science Direct, and Scopus databases, 22 were included in the systematic review following the PRISMA guidelines. The meta-analysis included 14 studies and showed that in epileptic animals there was an increase in the oxidizing agents nitric oxide (NO) and malondialdehyde (MDA), with a reduction in endogenous antioxidants reduced glutathione (GSH) and superoxide dismutase (SO). The Na,K-ATPase activity was reduced in all areas evaluated. Antioxidant therapy reversed all of these parameters altered by seizure or epilepsy induction. In addition, there was a percentage decrease in the number of seizures and mortality, and a meta-analysis showed a longer seizure latency in animals using antioxidant therapy. Thus, this study suggests that the use of antioxidants promotes neuroprotective effects and mitigates the effects of epilepsy. The protocol was registered in the Prospective Register of Systematic Reviews (PROSPERO) CRD42022356960.

Hypoxic Stress-Dependent Regulation of Na,K-ATPase in Ischemic Heart Disease

In cardiomyocytes, regular activity of the Na,K-ATPase (NKA) and its Na/K pump activity is essential for maintaining ion gradients, excitability, propagation of action potentials, electro-mechanical coupling, trans-membrane Na⁺ and Ca²⁺ gradients and, thus, contractility. The activity of NKA is impaired in ischemic heart disease and heart failure, which has been attributed to decreased expression of the NKA subunits. Decreased NKA activity leads to intracellular Na⁺ and Ca²⁺ overload, diastolic dysfunction and arrhythmias. One signal likely related to these events is hypoxia, where hypoxia-inducible factors (HIF) play a critical role in the adaptation of cells to low oxygen tension. HIF activity increases in ischemic heart, hypertension, heart failure and cardiac fibrosis; thus, it might contribute to the impaired function of NKA. This review will mainly focus on the regulation of NKA in ischemic heart disease in the context of stressed myocardium and the hypoxia-HIF axis and argue on possible consequences of treatment.

iHypoxia: An Integrative Database of Protein Expression Dynamics in Response to Hypoxia in Animals

Mammals have evolved mechanisms to sense hypoxia and induce hypoxic responses. Recently, high-throughput techniques have greatly promoted global studies of protein expression changes during hypoxia and the identification of candidate genes associated with hypoxia-adaptive evolution, which have contributed to the understanding of the complex regulatory networks of hypoxia. In this study, we developed an integrated resource for the expression dynamics of proteins in response to hypoxia (iHypoxia), and this database contains 2589 expression events of 1944 proteins identified by low-throughput experiments (LTEs) and 422,553 quantitative expression events of 33,559 proteins identified by high-throughput experiments from five mammals that exhibit a response to hypoxia. Various experimental details, such as the hypoxic experimental conditions, expression patterns, and sample types, were carefully collected and integrated. Furthermore, 8788 candidate genes from diverse species inhabiting low-oxygen environments were also integrated. In addition, we conducted an orthologous search and computationally identified 394,141 proteins that may respond to hypoxia among 48 animals. An enrichment analysis of human proteins identified from LTEs shows that these proteins are enriched in certain drug targets and cancer genes. Annotation of known posttranslational modification (PTM) sites in the proteins identified by LTEs reveals that these proteins undergo extensive PTMs, particularly phosphorylation, ubiquitination, and acetylation. iHypoxia provides a convenient and user-friendly method for users to obtain hypoxia-related information of interest. We anticipate that iHypoxia, which is freely accessible at https://ihypoxia.omicsbio.info, will advance the understanding of hypoxia and serve as a valuable data resource.

Na,K-ATPase Acts as a Beta-Amyloid Receptor Triggering Src Kinase Activation

Beta-amyloid (Aβ) has a dual role, both as an important factor in the pathology of Alzheimer's disease and as a regulator in brain physiology. The inhibitory effect of Aβ₄₂ oligomers on Na,K-ATPase contributes to neuronal dysfunction in Alzheimer's disease. Still, the physiological role of the monomeric form of Aβ₄₂ interaction with Na,K-ATPase remains unclear. We report that Na,K-ATPase serves as a receptor for Aβ₄₂ monomer, triggering Src kinase activation. The co-localization of Aβ₄₂ with α1- and β1-subunits of Na,K-ATPase, and Na,K-ATPase with Src kinase in SH-SY5Y neuroblastoma cells, was observed. Treatment of cells with 100 nM Aβ₄₂ causes Src kinase activation, but does not alter Na,K-ATPase transport activity. The interaction of Aβ₄₂ with α1β1 Na,K-ATPase isozyme leads to activation of Src kinase associated with the enzyme. Notably, prevention of Na,K-ATPase:Src kinase interaction by a specific inhibitor pNaKtide disrupts the Aβ-induced Src kinase activation. Stimulatory effect of Aβ₄₂ on Src kinase was lost under hypoxic conditions, which was similar to the effect of specific Na,K-ATPase ligands, the cardiotonic steroids. Our findings identify Na,K-ATPase as a Aβ₄₂ receptor, thus opening a prospect on exploring the physiological and pathological Src kinase activation caused by Aβ₄₂ in the nervous system.

Oral digoxin effects on exercise performance, K+ regulation and skeletal muscle Na+ ,K+ -ATPase in healthy humans

Abstract

We investigated whether digoxin lowered muscle Na⁺,K⁺‐ATPase (NKA), impaired muscle performance and exacerbated exercise K⁺ disturbances. Ten healthy adults ingested digoxin (0.25 mg; DIG) or placebo (CON) for 14 days and performed quadriceps strength and fatiguability, finger flexion (FF, 105%_{peak‐workrate}, 3 × 1 min, fourth bout to fatigue) and leg cycling (LC, 10 min at 33% VO2peak and 67% VO2peak, 90% VO2peak to fatigue) trials using a double‐blind, crossover, randomised, counter‐balanced design. Arterial (a) and antecubital venous (v) blood was sampled (FF, LC) and muscle biopsied (LC, rest, 67% VO2peak, fatigue, 3 h after exercise). In DIG, in resting muscle, [³H]‐ouabain binding site content (OB‐F_ab) was unchanged; however, bound‐digoxin removal with Digibind revealed total ouabain binding (OB+F_ab) increased (8.2%, P = 0.047), indicating 7.6% NKA–digoxin occupancy. Quadriceps muscle strength declined in DIG (−4.3%, P = 0.010) but fatiguability was unchanged. During LC, in DIG (main effects), time to fatigue and [K⁺]_a were unchanged, whilst [K⁺]_v was lower (P = 0.042) and [K⁺]_a‐v greater (P = 0.004) than in CON; with exercise (main effects), muscle OB‐F_ab was increased at 67% VO2peak (per wet‐weight, P = 0.005; per protein P = 0.001) and at fatigue (per protein, P = 0.003), whilst [K⁺]_a, [K⁺]_v and [K⁺]_a‐v were each increased at fatigue (P = 0.001). During FF, in DIG (main effects), time to fatigue, [K⁺]_a, [K⁺]_v and [K⁺]_a‐v were unchanged; with exercise (main effects), plasma [K⁺]_a, [K⁺]_v, [K⁺]_a‐v and muscle K⁺ efflux were all increased at fatigue (P = 0.001). Thus, muscle strength declined, but functional muscle NKA content was preserved during DIG, despite elevated plasma digoxin and muscle NKA–digoxin occupancy, with K⁺ disturbances and fatiguability unchanged.

Key points

The Na⁺,K⁺‐ATPase (NKA) is vital in regulating skeletal muscle extracellular potassium concentration ([K⁺]), excitability and plasma [K⁺] and thereby also in modulating fatigue during intense contractions.

NKA is inhibited by digoxin, which in cardiac patients lowers muscle functional NKA content ([³H]‐ouabain binding) and exacerbates K⁺ disturbances during exercise.

In healthy adults, we found that digoxin at clinical levels surprisingly did not reduce functional muscle NKA content, whilst digoxin removal by Digibind antibody revealed an ∼8% increased muscle total NKA content.

Accordingly, digoxin did not exacerbate arterial plasma [K⁺] disturbances or worsen fatigue during intense exercise, although quadriceps muscle strength was reduced.

Thus, digoxin treatment in healthy participants elevated serum digoxin, but muscle functional NKA content was preserved, whilst K⁺ disturbances and fatigue with intense exercise were unchanged. This resilience to digoxin NKA inhibition is consistent with the importance of NKA in preserving K⁺ regulation and muscle function.

Effect of Fe3+ on Na,K-ATPase: Unexpected activation of ATP hydrolysis

Iron is a key element in cell function; however, its excess in iron overload conditions can be harmful through the generation of reactive oxygen species (ROS) and cell oxidative stress. Activity of Na,K-ATPase has been shown to be implicated in cellular iron uptake and iron modulates the Na,K-ATPase function from different tissues. In this study, we determined the effect of iron overload on Na,K-ATPase activity and established the role that isoforms and conformational states of this enzyme has on this effect. Total blood and membrane preparations from erythrocytes (ghost cells), as well as pig kidney and rat brain cortex, and enterocytes cells (Caco-2) were used. In E1-related subconformations, an enzyme activation effect by iron was observed, and in the E2-related subconformations enzyme inhibition was observed. The enzyme's kinetic parameters were significantly changed only in the Na⁺ curve in ghost cells. In contrast to Na,K-ATPase α2 and α3 isoforms, activation was not observed for the α1 isoform. In Caco-2 cells, which only contain Na,K-ATPase α1 isoform, the FeCl₃ increased the intracellular storage of iron, catalase activity, the production of H₂O₂ and the expression levels of the α1 isoform. In contrast, iron did not affect lipid peroxidation, GSH content, superoxide dismutase and Na,K-ATPase activities. These results suggest that iron itself modulates Na,K-ATPase and that one or more E1-related subconformations seems to be determinant for the sensitivity of iron modulation through a mechanism in which the involvement of the Na, K-ATPase α3 isoform needs to be further investigated.

Role of Na+/K+-ATPase in ischemic stroke: in-depth perspectives from physiology to pharmacology

Na⁺/K⁺-ATPase (NKA) is a large transmembrane protein expressed in all cells. It is well studied for its ion exchanging function, which is indispensable for the maintenance of electrochemical gradients across the plasma membrane and herein neuronal excitability. The widely recognized pump function of NKA closely depends on its unique structure features and conformational changes upon binding of specific ions. Various Na⁺-dependent secondary transport systems are rigorously controlled by the ionic gradients generated by NKA and are essential for multiple physiological processes. In addition, roles of NKA as a signal transducer have also been unveiled nowadays. Plethora of signaling cascades are defined including Src-Ras-MAPK signaling, IP3R-mediated calcium oscillation, inflammation, and autophagy though most underlying mechanisms remain elusive. Ischemic stroke occurs when the blood flow carrying nutrients and oxygen into the brain is disrupted by blood clots, which is manifested by excitotoxicity, oxidative stress, inflammation, etc. The protective effect of NKA against ischemic stress is emerging gradually with the application of specific NKA inhibitor. However, NKA-related research is limited due to the opposite effects caused by NKA inhibitor at lower doses. The present review focuses on the recent progression involving different aspects about NKA in cellular homeostasis to present an in-depth understanding of this unique protein. Moreover, essential roles of NKA in ischemic pathology are discussed to provide a platform and bright future for the improvement in clinical research on ischemic stroke.

Biophysical Reviews' "Meet the Councilor"-a profile of Anastasia A. Anashkina

As one of the twelve Councilors of the International Union of Pure and Applied Biophysics elected in summer 2021, I have been asked to provide this short biographical sketch for the journal readers. I am a new member of the IUPAB Council. I hold a specialist degree in Applied Physics and Mathematics from the Moscow Institute of Physics and Technology and PhD in Biophysics from Moscow State University. I have spent my entire professional career at Engelhardt Institute of Molecular Biology of the Russian Academy of Sciences in Moscow, where I am currently a senior researcher. I am Associate Professor at the Digital Health Institute of the I.M. Sechenov First Moscow State Medical University since 2018, and have trained undergraduate students in structural biology, biophysics, and bioinformatics. In addition, I serve as the Guest Editor of special journal issues of International Journal of Molecular Sciences and Frontiers in Genetics BMC genomics. Now I joined Biophysical Reviews Editorial Board as IUPAB Councilor. I am a Secretary of National Committee of Russian Biophysicists, and have helped to organize scientific conferences and workshops, such as the VI Congress of Russian Biophysicists.

Redox regulation of glutathione peroxidase by thioredoxin in longan fruit in relation to senescence and quality deterioration

Thioredoxins (Trxs) are important redox regulators in organisms. However, their involvement in fruit senescence and quality deterioration remains unclear. In this study, one Trx (DlTrx1) and one NADPH-dependent Trx reductase (DlNRT1) cDNAs, were cloned from longan fruit. The DlTrx1 could be effectively reduced by the DlNTR1. Expression of DlTrx1 and DlNTR1 were up-regulated during fruit senescence and quality deterioration. We further identified 33 potential Trx target proteins in longan, including one glutathione peroxidase (DlGpx). DlTrx1 could physically interact with DlGpx. DlTrx1 in combination with DlNTR1 effectively activated DlGpx activity by regulating its redox state. Cys⁹⁰ in DlGPx could form a disulfide bond with either Cys⁴² or Cys⁷¹, which were the sites of redox modulation. Furthermore, DlGpx exhibited a higher ratio of disulfide bonds to sulfhydryl groups in senescent or deteriorative fruit. We propose that Trx-mediated redox regulation of DlGpx is involved in senescence or quality deterioration of harvested longan fruit.

A novel approach for predicting protein S-glutathionylation

Background

S-glutathionylation is the formation of disulfide bonds between the tripeptide glutathione and cysteine residues of the protein, protecting them from irreversible oxidation and in some cases causing change in their functions. Regulatory glutathionylation of proteins is a controllable and reversible process associated with cell response to the changing redox status. Prediction of cysteine residues that undergo glutathionylation allows us to find new target proteins, which function can be altered in pathologies associated with impaired redox status. We set out to analyze this issue and create new tool for predicting S-glutathionylated cysteine residues.

Results

One hundred forty proteins with experimentally proven S-glutathionylated cysteine residues were found in the literature and the RedoxDB database. These proteins contain 1018 non-S-glutathionylated cysteines and 235 S-glutathionylated ones. Based on 235 S-glutathionylated cysteines, non-redundant positive dataset of 221 heptapeptide sequences of S-glutathionylated cysteines was made. Based on 221 heptapeptide sequences, a position-specific matrix was created by analyzing the protein sequence near the cysteine residue (three amino acid residues before and three after the cysteine). We propose the method for calculating the glutathionylation propensity score, which utilizes the position-specific matrix and a criterion for predicting glutathionylated peptides.

Conclusion

Non-S-glutathionylated sites were enriched by cysteines in − 3 and + 3 positions. The proposed prediction method demonstrates 76.6% of correct predictions of S-glutathionylated cysteines. This method can be used for detecting new glutathionylation sites, especially in proteins with an unknown structure.

The Na/K-ATPase α1 and c-Src form signaling complex under native condition: A crosslinking approach

The protein-protein interactions amongst the Na/K-ATPase α1 subunit, c-Src, and caveolin-1 (cav-1) are essential for the Na/K-ATPase signaling functions. However, there are arguments concerning the interaction model. The present study aims to clarify the interactions amongst the endogenous native proteins in live cells under native resting condition. Under native condition, Blue Native-PAGE and Blue Native-PAGE/SDS-PAGE 2D analyses demonstrated co-existence of the α1 subunit and c-Src in same protein complex, as well as a direct interaction between the α1 subunit and c-Src. By comparison of cleavable and non-cleavable cysteine-cysteine crosslinked samples, capillary immunoblotting analysis demonstrated that depletion of Src kinase family members (c-Src, Yes, and Fyn) or cav-1 clearly reduced the interactions of the α1 subunit with proteins, but depletion of cav-1 did not affect the interaction of c-Src with the α1 subunit. The data indicated that there are direct interactions between the α1 subunit and c-Src as well as between the α1 subunit and cav-1, but argued about the interaction between c-Src and cav-1 under the condition. Furthermore, the data also indicated the existence of different protein complexes containing the α1 subunit and c-Src, which might have different signaling functions.

The Redox-Sensitive Na/K-ATPase Signaling in Uremic Cardiomyopathy

In recent years, Na/K-ATPase signaling has been implicated in different physiological and pathophysiological conditions, including cardiac hypertrophy and uremic cardiomyopathy. Cardiotonic steroids (CTS), specific ligands of Na/K-ATPase, regulate its enzymatic activity (at higher concentrations) and signaling function (at lower concentrations without significantly affecting its enzymatic activity) and increase reactive oxygen species (ROS) generation. On the other hand, an increase in ROS alone also regulates the Na/K-ATPase enzymatic activity and signaling function. We termed this phenomenon the Na/K-ATPase-mediated oxidant-amplification loop, in which oxidative stress regulates both the Na/K-ATPase activity and signaling. Most recently, we also demonstrated that this amplification loop is involved in the development of uremic cardiomyopathy. This review aims to evaluate the redox-sensitive Na/K-ATPase-mediated oxidant amplification loop and uremic cardiomyopathy.

RNases Disrupt the Adaptive Potential of Malignant Cells: Perspectives for Therapy

Exogenous RNases are selectively toxic to tumor cells. The reasons for this selectivity are not quite clear and should be searched for in the properties that distinguish malignant from normal cells. During onco-transformation, cells acquire properties allowing them to adapt to the altered microenvironment, such as resistance to hypoxia, changes in intracellular pH, disruption of ion transport, reduced adhesion and increased mobility, and production of specific exosomes. These adaptation mechanisms distinguish malignant cells from normal ones and give them a competitive advantage, ensuring survival and spread in the organism. Here, we analyze if the directed cytotoxic effect of exogenous RNases is linked to the disruption of the adaptive potential of tumor cells and how it can be used in anticancer therapy.

Metabolic Syndrome and Salt-Sensitive Hypertension in Polygenic Obese TALLYHO/JngJ Mice: Role of Na/K-ATPase Signaling

We have demonstrated that Na/K-ATPase acts as a receptor for reactive oxygen species (ROS), regulating renal Na⁺ handling and blood pressure. TALLYHO/JngJ (TH) mice are believed to mimic the state of obesity in humans with a polygenic background of type 2 diabetes. This present work is to investigate the role of Na/K-ATPase signaling in TH mice, focusing on susceptibility to hypertension due to chronic excess salt ingestion. Age-matched male TH and the control C57BL/6J (B6) mice were fed either normal diet or high salt diet (HS: 2, 4, and 8% NaCl) to construct the renal function curve. Na/K-ATPase signaling including c-Src and ERK1/2 phosphorylation, as well as protein carbonylation (a commonly used marker for enhanced ROS production), were assessed in the kidney cortex tissues by Western blot. Urinary and plasma Na⁺ levels were measured by flame photometry. When compared to B6 mice, TH mice developed salt-sensitive hypertension and responded to a high salt diet with a significant rise in systolic blood pressure indicative of a blunted pressure-natriuresis relationship. These findings were evidenced by a decrease in total and fractional Na⁺ excretion and a right-shifted renal function curve with a reduced slope. This salt-sensitive hypertension correlated with changes in the Na/K-ATPase signaling. Specifically, Na/K-ATPase signaling was not able to be stimulated by HS due to the activated baseline protein carbonylation, phosphorylation of c-Src and ERK1/2. These findings support the emerging view that Na/K-ATPase signaling contributes to metabolic disease and suggest that malfunction of the Na/K-ATPase signaling may promote the development of salt-sensitive hypertension in obesity. The increased basal level of renal Na/K-ATPase-dependent redox signaling may be responsible for the development of salt-sensitive hypertension in polygenic obese TH mice.

Glutathione-related substances maintain cardiomyocyte contractile function in hypoxic conditions

Severe hypoxia leads to decline in cardiac contractility and induces arrhythmic events in part due to oxidative damage to cardiomyocyte proteins including ion transporters. This results in compromised handling of Ca2+ ions that trigger heart contractile machinery. Here, we demonstrate that thiol-containing compounds such as N-acetylcysteine (NAC), glutathione ethyl ester (et-GSH), oxidized tetraethylglutathione (tet-GSSG), oxidized glutathione (GSSG) and S-nitrosoglutathione (GSNO) are capable of reducing negative effects of hypoxia on isolated rat cardiomyocytes. Preincubation of cardiomyocytes with 0.1 mM GSNO, 0.5 mM et-GSH, GSSG, tet-GSSG or with 10 mM NAC allows cells 5-times longer tolerate the hypoxic conditions and elicit regular Ca2+ transients in response to electric pacing. The shape of Ca2+ transients generated in the presence of GSNO, et-GSH and NAC was similar to that observed in normoxic control cardiomyocytes. The leader compound, GSNO, accelerated by 34% the recovery of normal contractile function of isolated rat heart subjected to ischemia-reperfusion. GSNO increased glutathionylation of Na,K-ATPase alpha-2 subunit, the principal ion-transporter of cardiac myocyte sarcolemma, which prevents irreversible oxidation of Na,K-ATPase and regulates its function to support normal Ca2+ ion handling in hypoxic cardiomyocytes. Altogether, GSNO appears effective cardioprotector in hypoxic conditions worth further studies toward its cardiovascular application.

Na,K-ATPase α-subunit conformation determines glutathionylation efficiency

The functioning of the N, K-ATPase depends on the redox status of cells and its activity is inhibited by oxidative stress and hypoxia. We previously found that redox sensitivity of the Na,K-ATPase is mediated by glutathionylation of the α-subunit. An increase in the level of glutathionylation of cysteine residues in the Na,K-ATPase α-subunit under stressful conditions leads to a decrease in the activity of the enzyme and a change in its receptor function. The structure of the Na,K-ATPase undergoes significant conformational changes during functioning. The effects of enzyme conformation on its ability to undergo glutathionylation are not clear. Here we show that the highest level of glutathionylation in the α-subunit of Na,K-ATPase is achieved in the E1 (Na⁺-induced) conformation. The transition of the Na,K-ATPase to the E2 (K⁺-induced) conformation leads to a decrease in the efficiency of glutathionylation. The lowest efficiency of Na,K-ATPase glutathionylation was observed in the E2P and E2P ouabain states. According to molecular modelling data, the maximum number of cysteine residues available for glutathionylation are present in the E1P conformation. In the E2P conformation, the main functional cysteine residues (Cys204, Cys242, Cys452, and Cys456) are buried from the solvent, which makes them inaccessible for glutathionylation. Thus, the efficiency of α-subunit glutathionylation depends on enzyme conformation, which is altered by bound ligands and proteins. A shift in the E1/E2 equilibrium towards prevalence of E1 can lead to better access for the relevant ligands and proteins to the binding site located in the Na,K-ATPase α-subunit. Na,K-ATPase.

Glutathionylation of Na,K-ATPase Alpha-Subunit Alters Enzyme Conformation and Sensitivity to Trypsinolysis

We found earlier that Na,K-ATPase is purified from duck salt glands in partially glutathionylated state (up to 13 of the 23 cysteine residues of the Na,K-ATPase catalytic α-subunit can be S-glutathionylated). To determine the effect of glutathionylation on the enzyme conformation, we have analyzed the products of trypsinolysis of Na,K-ATPase α-subunit in different conformations with different extent of glutathionylation. Incubation of the protein in the E1 conformation with trypsin produced a large fragment with a molecular mass (MM) of 80 kDa with the following formation of smaller fragments with MM 40, 35.5, and 23 kDa. Tryptic digestion of Na,K-ATPase in the E2 conformation also resulted in the generation of the fragments with MM 40, 35.5, and 23 kDa. Deglutathionylation of Na,K-ATPase α-subunit increases the rate of proteolysis of the enzyme in both E1 and E2 conformations. The pattern of tryptic digestion of the α-subunit in E2 conformation additionally glutathionylated with oxidized glutathione is similar to that of partially deglutathionylated Na,K-ATPase. The pattern of tryptic digestion of the additionally glutathionylated α-subunit in E1 conformation is similar to that of the native enzyme. The highest rate of trypsinolysis was observed for the α-subunit in complex with ouabain (E2-OBN conformation). Additional glutathionylation increased the content of high-molecular-weight fragments among the digestion products, as compared to the native and deglutathionylated enzymes. The data obtained were confirmed using molecular modeling that revealed that number of sites accessible for trypsinolysis is higher in the E2P-OBN conformation than in the E1- and E2-conformations and that glutathionylation decreases the number of sites accessible for trypsin. Therefore, glutathionylation affects enzyme conformation and its sensitivity to trypsinolysis. The mechanisms responsible for the changes in the Na,K-ATPase sensitivity to trypsinolysis depending on the level of enzyme glutathionylation and increase in the enzyme sensitivity to proteolysis upon its binding to ouabain, as well as physiological role of these phenomena are discussed.