Specific S-thiolation of a 30-kDa cytosolic protein from rat liver under oxidative stress

Discovery of the first-in-class potent and isoform-selective human carbonic anhydrase III inhibitors

Considering the unrecognised physio-pathological role of human carbonic anhydrase III (hCA III), a structure-based drug design was set up to identify the first-in-class potent and selective inhibitors of this neglected isoform. hCA III targeting was planned considering a unique feature of its active site among the other hCA isoforms, i.e. the Leu198/Phe198 substitution which interferes with the binding of aromatic/heterocyclic sulfonamides and other inhibitors. Thus, new aliphatic primary sulfonamides possessing long and flexible (CH2)nSO2NH2 moieties were designed to coordinate the zinc(II) ion, bypassing the bulky Phe198 residue. They incorporate 1,2,3-triazole linkers which connect the tail moieties to the sulfonamide head, enhancing thus the contacts at the active site entrance. Some of these compounds act as nanomolar and selective inhibitors of hCA III over other isoforms. Docking/molecular dynamics simulations were used to investigate ligand/target interactions for these sulfonamides which might improve our understanding of the physio-pathological roles of hCA III.

Carbonic Anhydrases in Metazoan Model Organisms: Molecules, Mechanisms, and Physiology

During the past three decades, mice, zebrafish, fruit flies, and Caenorhabditis elegans have been the primary model organisms used for the study of various biological phenomena. These models have also been adopted and developed to investigate the physiological roles of carbonic anhydrases (CAs) and carbonic anhydrase-related proteins (CARPs). These proteins belong to eight CA families and are identified by Greek letters: α, β, γ, δ, ζ, η, θ, and ι. Studies using model organisms have focused on two CA families, α-CAs and β-CAs, which are expressed in both prokaryotic and eukaryotic organisms with species-specific distribution patterns and unique functions. This review covers the biological roles of CAs and CARPs in light of investigations performed in model organisms. Functional studies demonstrate that CAs are not only linked to the regulation of pH homeostasis, the classical role of CAs but also contribute to a plethora of previously undescribed functions.

Glutathione - From antioxidant to post-translational modifier

Helmut Sies is one of the leading investigators in the multiple roles of glutathione (GSH) in biology. He has pioneered work on the role of GSH in preventing oxidative stress, in transport of GSSG, in protection of protein thiols from irreversible oxidation through mixed disulfide formation and demonstrated a role of protein glutathionylation in response to hormonal stimulation well before redox signaling became a major subject of investigation. Here I will describe the roles of GSH in several aspects of biology, the work of my laboratory in those findings, and how Helmut Sies work influenced our studies.

L-Plastin S-glutathionylation promotes reduced binding to β-actin and affects neutrophil functions

Posttranslational modifications (PTMs) of cytoskeleton proteins due to oxidative stress associated with several pathological conditions often lead to alterations in cell function. The current study evaluates the effect of nitric oxide (DETA-NO)-induced oxidative stress-related S-glutathionylation of cytoskeleton proteins in human PMNs. By using in vitro and genetic approaches, we showed that S-glutathionylation of L-plastin (LPL) and β-actin promotes reduced chemotaxis, polarization, bactericidal activity, and phagocytosis. We identified Cys-206, Cys-283, and Cys-460as S-thiolated residues in the β-actin-binding domain of LPL, where cys-460 had the maximum score. Site-directed mutagenesis of LPL Cys-460 further confirmed the role in the redox regulation of LPL. S-Thiolation diminished binding as well as the bundling activity of LPL. The presence of S-thiolated LPL was detected in neutrophils from both diabetic patients and db/db mice with impaired PMN functions. Thus, enhanced nitroxidative stress may results in LPL S-glutathionylation leading to impaired chemotaxis, polarization, and bactericidal activity of human PMNs, providing a mechanistic basis for their impaired functions in diabetes mellitus.

Glutathione homeostasis and functions: potential targets for medical interventions

Glutathione (GSH) is a tripeptide, which has many biological roles including protection against reactive oxygen and nitrogen species. The primary goal of this paper is to characterize the principal mechanisms of the protective role of GSH against reactive species and electrophiles. The ancillary goals are to provide up-to-date knowledge of GSH biosynthesis, hydrolysis, and utilization; intracellular compartmentalization and interorgan transfer; elimination of endogenously produced toxicants; involvement in metal homeostasis; glutathione-related enzymes and their regulation; glutathionylation of sulfhydryls. Individual sections are devoted to the relationships between GSH homeostasis and pathologies as well as to developed research tools and pharmacological approaches to manipulating GSH levels. Special attention is paid to compounds mainly of a natural origin (phytochemicals) which affect GSH-related processes. The paper provides starting points for development of novel tools and provides a hypothesis for investigation of the physiology and biochemistry of glutathione with a focus on human and animal health.

Carbonic anhydrase VII is S-glutathionylated without loss of catalytic activity and affinity for sulfonamide inhibitors

Human carbonic anhydrase (CA, EC 4.2.1.1) VII is a cytosolic enzyme with high carbon dioxide hydration activity. Here we report an unexpected S-glutathionylation of hCA VII which has also been observed earlier in vivo for hCA III, another cytosolic isoform. Cys183 and Cys217 were found to be the residues involved in reaction with glutathione for hCA VII. The two reactive cysteines were then mutated and the corresponding variant (C183S/C217S) expressed. The native enzyme, the variant and the S-glutathionylated adduct (sgCA VII) as well as hCA III were fully characterized for their CO(2) hydration, esterase/phosphatase activities, and inhibition with sulfonamides. Our findings suggest that hCA VII could use the in vivo S-glutathionylation to function as an oxygen radical scavenger for protecting cells from oxidative damage, as the activity and affinity for inhibitors of the modified enzyme are similar to those of the wild type.

Molecular mechanisms and potential clinical significance of S-glutathionylation

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

Protein S-thiolation and depletion of intracellular glutathione in skin fibroblasts exposed to various sources of oxidative stress

It is well established that oxidative stress plays a central role in the onset and progression of over a 100 different diseases. Recently, a growing body of evidence has shown that chemicals/agents as diverse as aromatic compounds, UV radiation and redox-active metals also generate oxy-radicals in vivo and lead to cellular oxidative stress. In this study we have used mouse skin fibroblasts to study the effects of oxidative stress caused by organic aromatic compounds (xylene), UV radiation and redox-active metals. Specifically, we tested the effect of these treatments on intracellular GSH levels, as well as on protein S-thiolation. We show that acute exposure of these diverse set of conditions cause dramatic depletion of the intracellular GSH pool in mouse skin fibroblast cells. We also found evidence of synergistic effects of combined exposure to different sources of oxidative stress. Furthermore, we also found that these treatments also caused significant S-thiolation (protein mixed-disulfide formation) of a 70kDa cytosolic protein.

Carbonic anhydrase III is not required in the mouse for normal growth, development, and life span

Carbonic anhydrase III is a cytosolic protein which is particularly abundant in skeletal muscle, adipocytes, and liver. The specific activity of this isozyme is quite low, suggesting that its physiological function is not that of hydrating carbon dioxide. To understand the cellular roles of carbonic anhydrase III, we inactivated the Car3 gene. Mice lacking carbonic anhydrase III were viable and fertile and had normal life spans. Carbonic anhydrase III has also been implicated in the response to oxidative stress. We found that mice lacking the protein had the same response to a hyperoxic challenge as did their wild-type siblings. No anatomic alterations were noted in the mice lacking carbonic anhydrase III. They had normal amounts and distribution of fat, despite the fact that carbonic anhydrase III constitutes about 30% of the soluble protein in adipocytes. We conclude that carbonic anhydrase III is dispensable for mice living under standard laboratory husbandry conditions.

Selective covalent binding of acrylonitrile to Cys 186 in rat liver carbonic anhydrase III in vivo

Covalent binding of reactive chemical species to tissue proteins is a common, but poorly understood, mechanism of toxicity. Identification of the proteins and the specific amino acid residues within the proteins that are chemically modified will aid our understanding of the toxification/detoxification mechanisms involved in covalent binding. Acrylonitrile (AN) is a commercial vinyl monomer that is acutely toxic and readily binds to tissue proteins. Total covalent binding of AN to tissue proteins is highly correlated with acute toxicity. Two-dimensional PAGE and autoradiography were used to locate proteins in male rat liver cytosol that are radiolabeled following administration of [2,3-(14)C]AN in vivo. Four intensely labeled spots were prominent in the autoradiogram and formed an apparent "charge-train" at approximately 30 kDa. Tryptic peptide mapping by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) MS was used to identify all of the spots as carbonic anhydrase III (CAIII). HPLC of the tryptic digests combined with MALDI-TOF MS was used to localize the radiolabel to tryptic fragment T22 containing amino acids 171-187. This tryptic fragment contains two Cys residues (Cys181 and Cys186) in the rat CAIII sequence. Electrospray ionization ion-trap MS was used to sequence the peptide and establish that only Cys186 was labeled. Thus, although AN is considered to be highly reactive, our data indicate that it does not react indiscriminately with rat CAIII but rather is selective for one out of five Cys residues. Rat liver CAIII has previously been shown to protect cells against oxidative stress. Our data suggest that CAIII is also capable of scavenging reactive xenobiotics and may help prevent covalent binding to more critical macromolecules.

Protease activation during surgical stress in the rat small intestine

BACKGROUND

Surgical stress affects intestinal permeability and our earlier study using a rat model indicated that oxidative stress plays an important role in this process. Proteases are important mediators of cellular damage and are known to be activated in oxidative stress. This study looked at protease activity in enterocytes after surgical stress.

METHODS

Surgical stress was induced by opening the abdominal wall and handling the intestine as done during laparotomy, in normal and xanthine oxidase-deficient rats. Enterocytes at various stages of differentiation were isolated and protease activity and protection offered by xanthine oxidase inhibitors were determined. Mitochondria and cytosol were prepared from total isolated enterocytes at different periods after surgical stress and protease activation was studied.

RESULTS

Surgical stress induced activation of proteases in both the villus and crypt cells. Protease activation is seen in both mitochondria and cytosol, and similar to the other alterations in mucosal cells, protease activation was maximum 60 min after stress, returning to normal by 24 h. Thiol compounds modulate protease activity in both mitochondria and cytosol and the activation is not seen in xanthine oxidase-deficient animals.

CONCLUSIONS

Surgical stress induces activation of proteases in villus and crypt cells of the small intestine. Both mitochondrial and cytosolic proteases are activated and free radicals generated by xanthine oxidase may mediate protease activation after surgical stress in the intestine.

Thioltransferase activity of bovine lens glutathione S-transferase

A Mu-class glutathione S-transferase purified to electrophoretic homogeneity from bovine lens displayed thioltransferase activity, catalysing the transthiolation reaction between GSH and hydroxyethyldisulphide. The thiol-transfer reaction is composed of two steps, the formation of GSSG occurring through the generation of an intermediate mixed disulphide between GSH and the target disulphide. Unlike glutaredoxin, which is only able to catalyse the second step of the transthiolation process, glutathioneS-transferase catalyses both steps of the reaction. Data are presented showing that bovine lens glutathione S-transferase and rat liver glutaredoxin, which was used as a thioltransferase enzyme model, can operate in synergy to catalyse the GSH-dependent reduction of hydroxyethyldisulphide.

Protein thiol modifications of human red blood cells treated with t-butyl hydroperoxide

Oxidative stress causes modification of cellular macromolecules and leads to cell damage. The objective of this study was to identify protein modifications that relate to thiol groups in human red blood cells under oxidative stress. With t-butyl hydroperoxide (t-BH) treatment, results of isoelectric focusing (IEF) analysis showed that two dithiothreitol-reversible modifications are observed, one toward the cathode and the other to the anode. Protein change toward the cathode was demonstrated to be hemoglobin oxidation, which gains a net positive charge, based on the same focus on IEF gels as hemoglobin and methemoglobin and molecular weight analysis by SDS-polyacrylamide gel electrophoresis (SDS-PAGE). Otherwise, the change toward the anode was the result of mixed disulfide formation between GSH and protein thiols. Based on the results of molecular weight analysis and its reversion from methemoglobin, protein formed mixed disulfides with GSH were also regarded as hemoglobin. As red blood samples were treated with diamide or GSSG, in addition to the mixed disulfides observed in t-BH-treated cells, additional hemoglobin-GSH mixed disulfide appeared. But the disappearance of this diamide-induced additional mixed disulfide by treating cells with t-BH after diamide treatment suggests that the increase of negative charges from GSH are offset by ferrohemoglobin oxidation to ferrihemoglobin. Additionally, other dithiothreitol-reversible modifications of one cell membrane protein, spectrin, were also observed from the formation of high molecular weight molecules as detected by SDS-PAGE. Results indicate that protein thiols in human red blood cells are susceptible to modification under oxidative stress. IEF analysis provides a useful tool to measure methemoglobin and hemoglobin GSH mixed disulfide formation.

Biochemistry and pathology of radical-mediated protein oxidation

Radical-mediated damage to proteins may be initiated by electron leakage, metal-ion-dependent reactions and autoxidation of lipids and sugars. The consequent protein oxidation is O2-dependent, and involves several propagating radicals, notably alkoxyl radicals. Its products include several categories of reactive species, and a range of stable products whose chemistry is currently being elucidated. Among the reactive products, protein hydroperoxides can generate further radical fluxes on reaction with transition-metal ions; protein-bound reductants (notably dopa) can reduce transition-metal ions and thereby facilitate their reaction with hydroperoxides; and aldehydes may participate in Schiff-base formation and other reactions. Cells can detoxify some of the reactive species, e.g. by reducing protein hydroperoxides to unreactive hydroxides. Oxidized proteins are often functionally inactive and their unfolding is associated with enhanced susceptibility to proteinases. Thus cells can generally remove oxidized proteins by proteolysis. However, certain oxidized proteins are poorly handled by cells, and together with possible alterations in the rate of production of oxidized proteins, this may contribute to the observed accumulation and damaging actions of oxidized proteins during aging and in pathologies such as diabetes, atherosclerosis and neurodegenerative diseases. Protein oxidation may also sometimes play controlling roles in cellular remodelling and cell growth. Proteins are also key targets in defensive cytolysis and in inflammatory self-damage. The possibility of selective protection against protein oxidation (antioxidation) is raised.

Chromatographic and electrophoretic methods related to the carbonic anhydrase isozymes

There are three gene families that encode zinc metalloenzymes that catalyze the reversible hydration of CO2. The encoded enzymes are termed carbonic anhydrases (CAs). The CA isozymes have been purified from representatives of all types of organisms. Most CAs are strongly inhibited by aromatic sulfonamides. Several chromatographic and electrophoretic methods have been devised to determine binding constants for sulfonamides to CAs, and these compounds have been extensively used for, often single-step, affinity chromatographic separation of CAs from complex matrixes. The purification of different CA isozymes from different organisms is reviewed, as are methods for detection of CAs during chromatography and electrophoresis.

Glutathione depletion impairs transcriptional activation of heat shock genes in primary cultures of guinea pig gastric mucosal cells

When primary cultures of guinea pig gastric mucosal cells were exposed to heat (43 degree C), ethanol, hydrogen peroxide (H2O2), or diamide, heat shock proteins (HSP90, HSP70, HSP60, and HSC73) were rapidly synthesized. The extent of each HSP induction varied with the type of stress. Ethanol, H2O2, and diamide increased the syntheses of several other undefined proteins besides the HSPs. However, none of these proteins were induced by exposure to heat or the reagents, when intracellular glutathione was depleted to <10% of the control level by pretreatment with DL-buthionine-[S,R]-sulfoximine. Gel mobility shift assay using a synthetic oligonucleotide coding HSP70 heat shock element showed that glutathione depletion inhibited the heat- and the reagent-initiated activation of the heat shock factor 1 (HSF1) and did not promote the expression of HSP70 mRNA. Immunoblot analysis with antiserum against HSF1 demonstrated that the steady-state level of HSF1 was not changed in glutathione-depleted cells, but glutathione depletion inhibited the nuclear translocation of HSF1 after exposure to heat stress. These results suggest that intracellular glutathione may support early and important biochemical events in the acquisition by gastric mucosal cells of an adaptive response to irritants.

The phosphatase activity of carbonic anhydrase III is reversibly regulated by glutathiolation

Carbonic anhydrase isozyme III (CAIII) is unique among the carbonic anhydrases because it demonstrates phosphatase activity. CAIII forms a disulfide link between glutathione and two of its five cysteine residues, a process termed S-glutathiolation. Glutathiolation of CAIII occurs in vivo and is increased during aging and under acute oxidative stress. We show that glutathiolation serves to reversibly regulate the phosphatase activity of CAIII. Glutathiolation of Cys-186 is required for phosphatase activity, while glutathiolation of Cys-181 blocks activity. Phosphotyrosine is the preferred substrate, although phosphoserine and phosphothreonine can also be cleaved. Thus, glutathiolation is a reversible covalent modification that can regulate CAIII, a phosphatase that may function in the cellular response to oxidative stress.

Amino acid sequence of chicken Cu, Zn-containing superoxide dismutase and identification of glutathionyl adducts at exposed cysteine residues

The copper, zinc-containing superoxide dismutase electromorphs from chicken erythrocytes have been isolated, their complete amino acid sequence determined and the identity of the protein moieties established. All electromorphs are constituted by a polypeptide chain made of 153 amino acid residues, corresponding to a molecular mass of 15,598 Da. Accurate molecular mass determination by electrospray mass spectrometry of the separated electromorphs unequivocally proved that, in the chicken superoxide dismutase, either one or two cysteine residues/subunit are involved in a mixed disulfide with glutathione. The same post-translational modification has been proven to occur in human superoxide dismutase. A different rate of S-thiolation by endogenous glutathione was also demonstrated to be responsible for charge heterogeneity in cells. Effect of this modification on the catalytic and molecular properties of superoxide dismutases, and possible mechanisms for the S-thiolation process, were also investigated and discussed.

The detection of S-glutathionation of hepatic carbonic anhydrase III in rats treated with paraquat or diquat

Protein S-glutathionation has been demonstrated to be one of the cellular responses under oxidative stress and may be involved in many cellular metabolisms. In this study, the effect of redox cycling bipyridylium compounds, paraquat and diquat, on this protein modification was investigated. Male Sprague-Dawley rats were administered i.p. either paraquat at 20 or 40 mg/kg body wt. or diquat at 85 or 170 mg/kg body wt., respectively. The liver was examined at different time points for taking the measurement of the S-glutathionation of carbonic anhydrase III (CA III), thiobarbituric acid-reactive substances (TBARS), vitamin E depletion, glutathione (GSH) and glutathione disulfide (GSSG) contents. The extent of S-glutathionation of CA III was chosen as a marker and was determined by a method combining isoelectric focusing analysis with immunoblotting. Those results indicated that paraquat and diquat significantly increased the generation of TBARS and showed a time-dependent response. The significant effect on vitamin E depletion was only obtained in rats treated with a high dose of diquat for 2 h. Hepatic cellular GSSG contents did not increase but tended to decrease all of the treatments. Although oxidative damage was actually generated in liver, based on the increase of TBARS generation and vitamin E depletion, no increase of CA III S-glutathionation was observed. We propose that the reason for this observation under this circumstance is probably due to the reversible characteristic of CA III S-glutathionation, which has been demonstrated in our previous study (Chai et al., 1991) Arch. Biochem. Biophys. 384, 270-278) and named as dethiolation.

Carbonic anhydrase III. Oxidative modification in vivo and loss of phosphatase activity during aging

Oxidative modification of DNA, lipids, and proteins occurs as a consequence of reaction with free radicals and activated oxygen. Oxidative modification of total cellular proteins has been described under many pathologic and experimental conditions, but no specific proteins have been identified as in vivo targets for oxidative modification. Utilizing an immunochemical method for detection of oxidatively modified proteins, we identified a protein in rat liver that was highly oxidized. It was purified to homogeneity and identified as carbonic anhydrase, isozyme III. Its characteristics match those previously described for a protein that was lost during aging of the rat, senescence marker protein-1. Carbonic anhydrase III was purified from rats aged 2, 10, and 18 months, and the proteins were characterized. All three preparations were highly oxidatively modified as assessed by their carbonyl content. The enzyme has three known catalytic activities, and the specific activities for carbon dioxide hydration and for ester hydrolysis decreased during aging by approximately 30%. However, the third activity, that of a phosphatase, was virtually lost during aging. While the physiologic role of carbonic anhydrase III is unknown, we suggest that it functions in an oxidizing environment, which leads to its own oxidative modification.

Transduction of reducing power across the plasma membrane by reduced glutathione. A 1H-NMR spin-echo study of intact human erythrocytes

The NMR signal of reduced glutathione (GSH) was monitored in intact human erythrocytes by the 1H spin-echo Carr-Purcell-Meiboom-Gill pulse sequence. Addition of GSH, which was unable to cross the erythrocyte membrane, produced an approximate twofold increase of the GSH signal in glucose-depleted cells. Addition of oxidised glutathione (GSSG), did not affect the signal, and addition of GSH to hemolysates gave a much smaller increase. Reduction of internal GSSG by NADPH-dependent enzymes was excluded by experiments with glucose-supplied or glucose-6-phosphate dehydrogenase deficient cells. Involvement of external thiol groups of the erythrocyte membrane was shown by the lack of effect in cells treated with an impermeable thiol-blocking compound. Involvement of spectrin was indicated by the proportional loss of the effect in erythrocytes with variable genetic deficiency of spectrin. Protein-glutathione mixed disulfides appeared to be the source of the NMR response since an increase of their content, by diamide treatment or aging procedures, produced a higher GSH signal, while their reduction by permeable reductants gave the opposite effect. It is concluded that GSH can transduce its reducing power by a thiol/disulfide exchange mechanism that sequentially involves sulfur-rich proteins spanning across the erythrocyte membrane.

A comparison of proteins S-thiolated by glutathione to those arylated by acetaminophen

This study was designed to evaluate whether the same proteins that irreversibly bind reactive electrophiles of drugs also bind glutathione (GSH) under oxidative conditions. Specifically, proteins that can be arylated by acetaminophen were compared to those that form glutathione-protein mixed disulfides (PSSG) after incubation with diamide. Data are presented which suggest that both GSH and acetaminophen bind to a subset of N-ethylmaleimide (NEM)-reactive protein thiols. To evaluate the pattern of proteins that bind GSH, PSSGs were formed in vitro by incubating cytosolic proteins with GSH and diamide. A sensitive procedure was developed in which PSSGs were first reduced with 0.1 mM dithiothreitol (DTT), and the newly exposed protein thiols were labeled with either [3H]NEM (for quantitative analysis) or with fluorescein-5-maleimide (for visual detection). Acetaminophen binding was achieved by incubating cytosolic proteins in vitro with the reactive acetaminophen metabolite, N-acetyl-p-benzoquinoneimine (NAPQI). Proteins from both assays were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and transferred to nitrocellulose for Western blot analysis. Acetaminophen binding was detected by immunoblotting with an affinity-purified antibody against acetaminophen, and PSSGs were visualized using anti-fluorescein antibodies. In both instances, binding to proteins was observed to be selective. A comparison of the proteins modified by GSH binding with those that bind acetaminophen indicates that the major cytosolic acetaminophen-binding protein of 58 kDa may also be modified by glutathiolation under oxidative conditions.

Reduction (dethiolation) of protein mixed-disulfides; distribution and specificity of dethiolating enzymes and N,N'-bis(2-chlorethyl)-N-nitrosourea inhibition of an NADPH-dependent cardiac dethiolase

The S-thiolated proteins phosphorylase b (Phb) and carbonic anhydrase III (CAIII) were prepared with [3H]glutathione in a reaction initiated with diamide. These substrates were used to measure the rate of reduction (dethiolation) of protein mixed-disulfides by enzymes with properties similar to those of thioredoxin and glutaredoxin. This enzyme activity is termed a dethiolase since the identities of the enzymes are still unknown. The dethiolation of either S-[3H]glutathiolated Phb or S-[3H]glutathiolated CAIII was employed in tissue assays and for study of two partially purified dethiolases from cardiac tissue. NADPH-dependent dethiolase activity was most abundant except in rat liver and muscle. Total dethiolase activity was approximately 10-fold higher in neutrophils, 3T3-L1 cells, and Escherichia coli than in other sources. Rat skeletal muscle had 3- to 4-fold higher dethiolase activity than rat heart or liver. These data indicate that protein dethiolase activity is ubiquitous and that normal expression of the two dethiolase activities varies considerably. A partially purified cardiac NADPH-dependent dethiolase acted on Phb approximately 1.5 times faster than CAIII, and a glutathione (GSH)-dependent dethiolase acted on Phb 3 times faster than CAIII. The Km for glutathione for the GSH-dependent dethiolase was 15 microM with Phb as substrate and 10 microM with CAIII. Thus, the GSH-dependent dethiolase is probably not affected by normal changes in the cardiac glutathione content (normally approximately 3 mM). Partially purified cardiac NADPH-dependent dethiolase was inactivated by BCNU (N,N'-bis(2-chloroethyl)-N-nitrosourea) and the GSH-dependent dethiolase was unaffected under similar conditions. In a soluble extract from bovine heart, 200 microM BCNU inhibited NADPH-dependent dethiolase by more than 60% but did not affect GSH-dependent activity. These results demonstrate that BCNU is a selective inhibitor of the NADPH-dependent dethiolase.

Identification of an abundant S-thiolated rat liver protein as carbonic anhydrase III; characterization of S-thiolation and dethiolation reactions

An S-thiolated 30-kDa protein has been purified from rat liver by two steps of ion-exchange chromatography. This monomeric protein has two "reactive" sulfhydryls that can be S-thiolated by glutathione (form a mixed disulfide with glutathione) in intact liver. The protein has been identified as carbonic anhydrase III by sequence analysis of tryptic peptides from the pure protein. The two "reactive" sulfhydryls on this protein can produce three different S-thiolated forms of the protein that can be separated by isoelectric focusing. Using this technique it was possible to study the S-thiolation and dethiolation reactions of the pure protein. The reduced form of this protein was S-thiolated both by thiol-disulfide exchange with glutathione disulfide and by oxyradical-initiated S-thiolation with reduced glutathione. The S-thiolation rate of this 30-kDa protein was somewhat slower than that of glycogen phosphorylase b by both S-thiolation mechanisms. The S-thiolated form of this protein was poorly dethiolated (i.e., reduced) by glutathione, cysteine, cysteamine, or coenzyme A alone. Enzymatic catalysis by two different enzymes (glutaredoxin and thioredoxin-like) greatly enhanced the dethiolation rate. These experiments suggest that carbonic anhydrase III is a major participant in the liver response to oxidative stress, and that the protein may be S-thiolated by two different non-enzymatic mechanisms and dethiolated by enzymatic reactions in intact cells. Thus, the S-thiolation/dethiolation of carbonic anhydrase III resembles glycogen phosphorylase and not creatine kinase.

Phosphorylase and creatine kinase modification by thiol-disulfide exchange and by xanthine oxidase-initiated S-thiolation

The reaction of glycogen phosphorylase b and creatine kinase with glutathione disulfide, cystine, and cystamine was compared by direct analysis on electrofocusing gels. This method was useful for individual proteins or for mixtures of the proteins. Millimolar concentrations of glutathione disulfide were required for both proteins and the rate of modification of each protein was similar. The reaction of glutathione disulfide with creatine kinase was inhibited by reduced glutathione (GSH), but the effect on the reaction with phosphorylase was minimal. Cystine and cystamine were required in micromolar amounts to effectively form the disulfide adducts. Both proteins were modified by cystine but cystamine reacted only with phosphorylase. Cystamine (10 microM) was an effective inhibitor of the reaction of phosphorylase b with 2 mM glutathione disulfide. S-thiolation of creatine kinase inactivated the enzyme and a direct assay of the enzyme activity could be used to quantitate S-thiolation of this protein by each of the disulfides. The effect of each disulfide on enzyme activity confirmed the results obtained by gel electrofocusing. Glutathione disulfide and cystine both inactivated the enzyme while cystamine had no effect on the activity. S-thiolation of phosphorylase had no observable effect on any activity parameter, but it effectively prevented binding of phosphorylase to high-molecular-weight glycogen, probably at the glycogen storage site of phosphorylase. The rate of S-thiolation of a mixture of phosphorylase and creatine kinase by thiol-disulfide exchange with glutathione disulfide was compared to the rate of S-thiolation of these proteins by a xanthine oxidase-initiated process (presumably due to protein sulfhydryl activation by reactive oxygen species). The xanthine oxidase-initiated mechanism was somewhat faster than thiol-disulfide exchange with both proteins. It was shown that GSH inhibited S-thiolation of creatine kinase by this mechanism as well as by thiol-disulfide exchange. It is suggested that both mechanisms may play a role in protein S-thiolation in vivo. For proteins that are typified by creatine kinase, the concentration of GSH in the cells may determine whether the S-thiolated form of the protein accumulates. For proteins typified by phosphorylase b, the accumulation of S-thiolated forms may be more independent of GSH.

S-(4-azidophenacyl)[35S]glutathione photoaffinity labeling of rat liver plasma membrane-associated proteins

A method for the synthesis of the glutathione conjugate S-(4-azidophenacyl)[35S]glutathione is described. The compound was used for photoaffinity labeling of proteins present in canalicular membrane vesicles (CMV), sinusoidal membrane vesicles (SMV), mitochondria and microsomes from rat liver. Most of the radioactivity introduced by photoaffinity labeling of CMV appeared in the 25-29 kDa range. Further labeled proteins were observed in bands at 37, 105 and about 120 kDa. 79% of the 25-29 kDa associated radioactivity was recovered in the supernatant after extensive revesiculation (washing) of the vesicles, together with the 37 kDa protein. CMV and SMV contained glutathione S-transferase (GST) activity which in CMV was decreased by 75% by washing. Photolabeling of a mixture of purified basic GST subunits from rat liver resulted in a band pattern at 25-29 kDa similar to that in the membrane preparations. Isoelectric focusing of the CMV indicated the presence of basic soluble GST subunits. S-Hexylglutathione-Sepharose affinity chromatography showed reversible binding of photolabeled proteins at 25-29 kDa. Difference photoaffinity labeling with GSSG, S-hexylglutathione, taurocholate and phenylmethylsulfonyl fluoride decreased the radioactivity bound by GST, but not that introduced into the 105 kDa protein band present in CMV. It is concluded that membrane-associated basic GST isoenzymes are present in standard membrane vesicle preparations. In the cell, the function may be transport of GST-bound compounds across the membrane and protection of the membranes against electrophiles.

[Biochemistry of thiol groups: the role of glutathione]

Glutathione (GSH) comprises the bulk of the pool of free thiol groups in biological systems. Since its first description as philothione 100 years ago, there have been repeated surprises in discoveries of novel functions. Just recently the important role of thioethers with products of the lipoxygenase reaction, i.e., the leukotrienes, was revealed as mediator of physiological and pathophysiological processes. Another major function resides in detoxication, GSH being cosubstrate in the GSH-peroxidase reaction for the reduction of hydroperoxides in the defense against oxidative stress. Interest also focuses on reactions of glutathionyl radicals in protection by thiols against DNA damage resulting from ionizing radiation.