The reactivities and ionization properties of the active-site dithiol groups of mammalian protein disulphide-isomerase

Dithiol Based on l-Cysteine and Cysteamine as a Disulfide-Reducing Agent

We report the synthesis, chemical properties, and disulfide bond-reducing performance of a dithiol called NACMEAA, conceived as a hybrid of two biologically relevant thiols: cysteine and cysteamine. NACMEAA is conveniently prepared from inexpensive l-cystine in an efficient manner. As a nonvolatile, highly soluble, and neutral compound at physiological pH with the first thiol pK_a value of 8.0, NACMEAA is reactive and user-friendly. We also demonstrate that NACMEAA reduces disulfide bonds in GSSG and lysozyme.

Oxidative Cysteine Modification of Thiol Isomerases in Thrombotic Disease: A Hypothesis

Significance: Oxidative stress is a characteristic of many systemic diseases associated with thrombosis. Thiol isomerases are a family of oxidoreductases important in protein folding and are exquisitely sensitive to the redox environment. They are essential for thrombus formation and represent a previously unrecognized layer of control of the thrombotic process. Yet, the mechanisms by which thiol isomerases function in thrombus formation are unknown. Recent Advances: The oxidoreductase activity of thiol isomerases in thrombus formation is controlled by the redox environment via oxidative changes to active site cysteines. Specific alterations can now be detected owing to advances in the chemical biology of oxidative cysteine modifications. Critical Issues: Understanding of the role of thiol isomerases in thrombus formation has focused largely on identifying single disulfide bond modifications in isolated proteins (e.g., α_IIbβ₃, tissue factor, vitronectin, or glycoprotein Ibα [GPIbα]). An alternative approach is to conceptualize thiol isomerases as effectors in redox signaling pathways that control thrombotic potential by modifying substrate networks. Future Directions: Cysteine-based chemical biology will be employed to study thiol-dependent dynamics mediated by the redox state of thiol isomerases at the systems level. This approach could identify thiol isomerase-dependent modifications of the disulfide landscape that are prothrombotic.

AccPDIA6 from Apis cerana cerana plays important roles in antioxidation

PDIA6 is a member of the protein disulfide isomerase (PDI) family, shows disulfide isomerase activity and oxidoreductase activity, and can act as a molecular chaperone. Its biological functions include modulating apoptosis, regulating the proliferation and invasion of cancer cells, supporting thrombosis and modulating insulin secretion. However, the roles of PDIA6 in Apis cerana cerana are poorly understood. Herein, we obtained the PDIA6 gene from A. cerana cerana (AccPDIA6). We investigated the expression patterns of AccPDIA6 in response to oxidative stress induced by H₂O₂, UV, HgCl₂, extreme temperatures (4 °C, 42 °C) and pesticides (thiamethoxam and hexythiazox) and found that AccPDIA6 was upregulated by these treatments. Western blot analysis indicated that AccPDIA6 was also upregulated by oxidative stress at the protein level. In addition, a survival test demonstrated that the survival rate of E. coli cells expressing AccPDIA6 increased under oxidative stress, suggesting a possible antioxidant function of AccPDIA6. In addition, we tested the transcripts of other antioxidant genes and found that some of them were downregulated in AccPDIA6 knockdown samples. It was also discovered that the antioxidant enzymatic activity of superoxide dismutase (SOD) decreased in AccPDIA6-silenced bees. Moreover, the survival rate of AccPDIA6 knockdown bees decreased under oxidative stress, implying that AccPDIA6 may function in the oxidative stress response by enhancing the viability of honeybees. Taken together, these results indicated that AccPDIA6 may play an essential role in counteracting oxidative stress.

Basic Amino Acid Conjugates of 1,2-Diselenan-4-amine with Protein Disulfide Isomerase-like Functions as a Manipulator of Protein Quality Control

Protein disulfide isomerase (PDI) can assist immature proteins to correctly fold by controlling cysteinyl disulfide (SS)-relating reactions (i. e., SS-formation, SS-cleavage, and SS-isomerization). PDI controls protein quality by suppressing protein aggregation, as well as functions as an oxidative folding catalyst. Following the amino acid sequence of the active center in PDI, basic amino acid conjugates of 1,2-diselenan-4-amine (1), which show oxidoreductase- and isomerase-like activities for SS-relating reactions, were designed as a novel PDI model compound. By conjugating the amino acids, the diselenide reduction potential of compound 1 was significantly increased, causing improvement of the catalytic activities for all SS-relating reactions. Furthermore, these compounds, especially histidine-conjugated one, remarkably suppressed protein aggregation even at low concertation (0.3 mM∼). Thus, it was demonstrated that the conjugation of basic amino acids into 1 simultaneously achieves the enhancement of the redox reactivity and the capability to suppress protein aggregation.

drFrnE Represents a Hitherto Unknown Class of Eubacterial Cytoplasmic Disulfide Oxido-Reductases

AIMS

Living cells employ thioredoxin and glutaredoxin disulfide oxido-reductases to protect thiol groups in intracellular proteins. FrnE protein of Deinococcus radiodurans (drFrnE) is a disulfide oxido-reductase that is induced in response to Cd²⁺ exposure and is involved in cadmium and radiation tolerance. The aim of this study is to probe structure, function, and cellular localization of FrnE class of proteins.

RESULTS

Here, we show drFrnE as a novel cytoplasmic oxido-reductase that could be functional in eubacteria under conditions where thioredoxin/glutaredoxin systems are inhibited or absent. Crystal structure analysis of drFrnE reveals thioredoxin fold with an alpha helical insertion domain and a unique, flexible, and functionally important C-terminal tail. The C-tail harbors a novel 239-CX₄C-244 motif that interacts with the active site 22-CXXC-25 motif. Crystal structures with different active site redox states, including mixed disulfide (Cys22-Cys244), are reported here. The biochemical data show that 239-CX₄C-244 motif channels electrons to the active site cysteines. drFrnE is more stable in the oxidized form, compared with the reduced form, supporting its role as a disulfide reductase. Using bioinformatics analysis and fluorescence microscopy, we show cytoplasmic localization of drFrnE. We have found "true" orthologs of drFrnE in several eubacterial phyla and, interestingly, all these groups apparently lack a functional glutaredoxin system. Innovation and Conclusion: We show that drFrnE represents a new class of hitherto unknown intracellular oxido-reductases that are abundantly present in eubacteria. Unlike other well-known oxido-reductases, FrnE harbors an additional dithiol motif that acts as a conduit to channel electrons to the active site during catalytic turnover. Antioxid. Redox Signal. 28, 296-310.

Succination of Protein Disulfide Isomerase Links Mitochondrial Stress and Endoplasmic Reticulum Stress in the Adipocyte During Diabetes

AIMS

Protein succination by fumarate increases in the adipose tissue of diabetic mice and in adipocytes matured in high glucose as a result of glucotoxicity-driven mitochondrial stress. The endoplasmic reticulum (ER) oxidoreductase protein disulfide isomerase (PDI) is succinated in adipocytes that are matured in high glucose, and in this study we investigated whether succination would alter PDI oxidoreductase activity, directly linking mitochondrial stress and ER stress.

RESULTS

Protein succination and the ER stress marker C/EBP homologous protein (CHOP) were diminished after pharmaceutical targeting of mitochondrial stress with the chemical uncoupler niclosamide in adipocytes matured in high-glucose concentrations. PDI was succinated by fumarate on both CXXC-containing active sites, contributing to reduced enzymatic activity. Succinated PDI decreased reductase activity in adipocytes matured in high glucose, and in db/db epididymal adipose tissue, in association with increased levels of CHOP. PDI succination was increased in fumarase knockdown adipocytes, leading to reduced PDI oxidoreductase activity, increased CHOP levels, and pro-inflammatory cytokine secretion, confirming the specific role of elevated fumarate levels in contributing to ER stress. In addition, PDI succination and ER stress were decreased, and PDI reductase activity was restored when exposure to chronic high glucose was limited, highlighting the importance of calorie restriction in the improvement of adipocyte metabolic function.

INNOVATION

These experiments identify PDI succination as a novel biochemical mechanism linking altered mitochondrial metabolism to ER stress in the adipocyte during diabetes.

CONCLUSION

The current study demonstrates that early biochemical changes in mitochondrial metabolism have important implications for the development of adipocyte stress. Antioxid. Redox Signal. 27, 1281-1296.

Challenges in the evaluation of thiol-reactive inhibitors of human protein disulfide Isomerase

This paper addresses how to evaluate the efficacy of the growing inventory of thiol-reactive inhibitors of mammalian protein disulfide Isomerase (PDI) enzymes under realistic concentrations of potentially competing thiol-containing peptides and proteins. For this purpose, we introduce a variant of the widely-used reductase assay by using a commercially-available cysteine derivative (BODIPY FL L-Cystine; BD-SS) that yields a 55-fold increase in fluorescence (excitation/emission; 490/513nm) on scission of the disulfide bond. This plate reader-compatible method detects human PDI down to 5-10nM, can utilize a range of thiol substrates (including 5µM dithiothreitol, 10µM reduced RNase thiols, and 5mM glutathione; GSH), and can operate from pH 6-9.5 in a variety of buffers. PDI assays often employ low micromolar levels of substrates leading to ambiguities when thiol-directed inhibitors are evaluated. The present work utilizes 5mM GSH for both pre-incubation and assay phases to more realistically reflect the high concentration of thiols that an inhibitor would encounter intracellularly. Extracellular PDI faces a much lower concentration of potentially competing thiols; to assess reductase activity under these conditions, the pre-reduced PDI is treated with inhibitor and then fluorescence increase upon reduction of BD-SS is followed in the absence of additional competing thiols. Both assay modes were tested with four mechanistically diverse PDI inhibitors. Two reversible reagents, 3,4-methylenedioxy-β-nitrostyrene (MNS) and the arsenical APAO, were found to be strong inhibitors of PDI in the absence of competing thiols, but were ineffective in the presence of 5mM GSH. A further examination of the nitrostyrene showed that MNS not only forms facile Michael adducts with GSH, but also with the thiols of unfolded proteins (K_d values of 7 and <0.1µM, respectively) suggesting the existence of multiple potential intracellular targets for this membrane-permeant reagent. The inhibition of PDI by the irreversible alkylating agent, the chloroacetamide 16F16, was found to be only modestly attenuated by 5mM GSH. Finally, the thiol-independent flavonoid inhibitor quercetin-3-O-rutinoside was found to show equal efficacy in reoxidation and turnover assay types. This work provides a framework to evaluate inhibitors that may target the CxxC motifs of PDI and addresses some of the complexities in the interpretation of the behavior of thiol-directed reagents in vivo.

Mechanistic insights on the reduction of glutathione disulfide by protein disulfide isomerase

We explore the enzymatic mechanism of the reduction of glutathione disulfide (GSSG) by the reduced a domain of human protein disulfide isomerase (hPDI) with atomistic resolution. We use classical molecular dynamics and hybrid quantum mechanics/molecular mechanics calculations at the mPW1N/6-311+G(2d,2p):FF99SB//mPW1N/6-31G(d):FF99SB level. The reaction proceeds in two stages: (i) a thiol-disulfide exchange through nucleophilic attack of the Cys53-thiolate to the GSSG-disulfide followed by the deprotonation of Cys56-thiol by Glu47-carboxylate and (ii) a second thiol-disulfide exchange between the Cys56-thiolate and the mixed disulfide intermediate formed in the first step. The Gibbs activation energy for the first stage was 18.7 kcal·mol^-1, and for the second stage, it was 7.2 kcal·mol^-1, in excellent agreement with the experimental barrier (17.6 kcal·mol^-1). Our results also suggest that the catalysis by protein disulfide isomerase (PDI) and thiol-disulfide exchange is mostly enthalpy-driven (entropy changes below 2 kcal·mol^-1 at all stages of the reaction). Hydrogen bonds formed between the backbone of His55 and Cys56 and the Cys56-thiol result in an increase in the Gibbs energy barrier of the first thiol-disulfide exchange. The solvent plays a key role in stabilizing the leaving glutathione thiolate formed. This role is not exclusively electrostatic, because an explicit inclusion of several water molecules at the density-functional theory level is a requisite to form the mixed disulfide intermediate. In the intramolecular oxidation of PDI, a transition state is only observed if hydrogen bond donors are nearby the mixed disulfide intermediate, which emphasizes that the thermochemistry of thiol-disulfide exchange in PDI is influenced by the presence of hydrogen bond donors.

Organocatalysts of oxidative protein folding inspired by protein disulfide isomerase

Organocatalysts derived from diethylenetriamine effect the rapid isomerization of non-native protein disulfide bonds to native ones. These catalysts contain a pendant hydrophobic moiety to encourage interaction with the non-native state, and two thiol groups with low pKa values that form a disulfide bond with a high E°' value.

Lactococcus lactis TrxD represents a subgroup of thioredoxins prevalent in Gram-positive bacteria containing WCXDC active site motifs

Three protein disulfide reductases of the thioredoxin superfamily from the industrially important Gram-positive Lactococcus lactis (LlTrxA, LlTrxD and LlNrdH) are compared to the "classical" thioredoxin from Escherichia coli (EcTrx1). LlTrxA resembles EcTrx1 with a WCGPC active site motif and other key residues conserved. By contrast, LlTrxD is more distantly related and contains a WCGDC motif. Bioinformatics analysis suggests that LlTrxD represents a subgroup of thioredoxins from Gram-positive bacteria. LlNrdH is a glutaredoxin-like electron donor for ribonucleotide reductase class Ib. Based on protein-protein equilibria LlTrxA (E°'=-259mV) and LlNrdH (E°'=-238mV) show approximately 10mV higher standard state redox potentials than the corresponding E. coli homologues, while E°' of LlTrxD is -243mV, more similar to glutaredoxin than "classical" thioredoxin. EcTrx1 and LlTrxA have high capacity to reduce insulin disulfides and their exposed active site thiol is alkylated at a similar rate at pH 7.0. LlTrxD on the other hand, is alkylated by iodoacetamide at almost 100 fold higher rate and shows no activity towards insulin disulfides. LlTrxA, LlTrxD and LlNrdH are all efficiently reduced by NADPH dependent thioredoxin reductase (TrxR) from L. lactis and good cross-reactivity towards E. coli TrxR was observed with LlTrxD as the notable exception.

An additional function of the rough endoplasmic reticulum protein complex prolyl 3-hydroxylase 1·cartilage-associated protein·cyclophilin B: the CXXXC motif reveals disulfide isomerase activity in vitro

Collagen biosynthesis occurs in the rough endoplasmic reticulum, and many molecular chaperones and folding enzymes are involved in this process. The folding mechanism of type I procollagen has been well characterized, and protein disulfide isomerase (PDI) has been suggested as a key player in the formation of the correct disulfide bonds in the noncollagenous carboxyl-terminal and amino-terminal propeptides. Prolyl 3-hydroxylase 1 (P3H1) forms a hetero-trimeric complex with cartilage-associated protein and cyclophilin B (CypB). This complex is a multifunctional complex acting as a prolyl 3-hydroxylase, a peptidyl prolyl cis-trans isomerase, and a molecular chaperone. Two major domains are predicted from the primary sequence of P3H1: an amino-terminal domain and a carboxyl-terminal domain corresponding to the 2-oxoglutarate- and iron-dependent dioxygenase domains similar to the α-subunit of prolyl 4-hydroxylase and lysyl hydroxylases. The amino-terminal domain contains four CXXXC sequence repeats. The primary sequence of cartilage-associated protein is homologous to the amino-terminal domain of P3H1 and also contains four CXXXC sequence repeats. However, the function of the CXXXC sequence repeats is not known. Several publications have reported that short peptides containing a CXC or a CXXC sequence show oxido-reductase activity similar to PDI in vitro. We hypothesize that CXXXC motifs have oxido-reductase activity similar to the CXXC motif in PDI. We have tested the enzyme activities on model substrates in vitro using a GCRALCG peptide and the P3H1 complex. Our results suggest that this complex could function as a disulfide isomerase in the rough endoplasmic reticulum.

Identification of the thiol isomerase-binding peptide, mastoparan, as a novel inhibitor of shear-induced transforming growth factor β1 (TGF-β1) activation

TGF-β1 is a disulfide-bonded homodimeric protein produced by platelets and other cells that plays a role in many physiologic and pathologic processes. TGF-β1 is secreted as an inactive large latent complex (LLC) comprised of TGF-β1, latency-associated peptide, and latent TGF-β binding protein 1. We previously demonstrated that shear force can activate LLC and that thiol-disulfide exchange contributes to the process. We have now investigated the role of thiol isomerases in the activation of LLC in platelet releasates (PR) and recombinant LLC. The wasp venom peptide mastoparan, which inhibits the chaperone activity of PDI, inhibited stirring- and shear-induced activation of latent TGF-β1 by 90 and 75% respectively. To identify the proteins that bind to mastoparan either directly or indirectly, PR were chromatographed on a mastoparan affinity column. Latent TGF-β binding protein 1, latency-associated peptide, TGF-β1, clusterin, von Willebrand factor, multimerin-1, protein disulfide isomerase (PDI), ERp5, ERp57, and ERp72 eluted specifically from the column. Anti-PDI RL90 attenuated the inhibitory effect of mastoparan on LLC activation. Furthermore, reduced PDI inhibited activation of PR LLC, whereas oxidized PDI had no effect. We conclude that thiol isomerases and thiol-disulfide exchange contribute to TGF-β1 activation and identify a number of molecules that may participate in the process.

Determination of acidity and nucleophilicity in thiols by reaction with monobromobimane and fluorescence detection

A method based on the differential reactivity of thiol and thiolate with monobromobimane (mBBr) has been developed to measure nucleophilicity and acidity of protein and low-molecular-weight thiols. Nucleophilicity of the thiolate is measured as the pH-independent second-order rate constant of its reaction with mBBr. The ionization constants of the thiols are obtained through the pH dependence of either second-order rate constant or initial rate of reaction. For readily available thiols, the apparent second-order rate constant is measured at different pHs and then plotted and fitted to an appropriate pH function describing the observed number of ionization equilibria. For less available thiols, such as protein thiols, the initial rate of reaction is determined in a wide range of pHs and fitted to the appropriate pH function. The method presented here shows excellent sensitivity, allowing the use of nanomolar concentrations of reagents. The method is suitable for scaling and high-throughput screening. Example determinations of nucleophilicity and pK(a) are presented for captopril and cysteine as low-molecular-weight thiols and for human peroxiredoxin 5 and Trypanosoma brucei monothiol glutaredoxin 1 as protein thiols.

Understanding the pK(a) of redox cysteines: the key role of hydrogen bonding

Many cellular functions involve cysteine chemistry via thiol-disulfide exchange pathways. The nucleophilic cysteines of the enzymes involved are activated as thiolate. A thiolate is much more reactive than a neutral thiol. Therefore, determining and understanding the pK(a)s of functional cysteines are important aspects of biochemistry and molecular biology with direct implications for redox signaling. Here, we describe the experimental and theoretical methods to determine cysteine pK(a) values, and we examine the factors that control these pK(a)s. Drawing largely on experience gained with the thioredoxin superfamily, we examine the roles of solvation, charge-charge, helix macrodipole, and hydrogen bonding interactions as pK(a)-modulating factors. The contributions of these factors in influencing cysteine pK(a)s and the associated chemistry, including the relevance for the reaction kinetics and thermodynamics, are discussed. This analysis highlights the critical role of direct hydrogen bonding to the cysteine sulfur as a key factor modulating the equilibrium between thiol S-H and thiolate S(-). This role is easily understood intuitively and provides a framework for biochemical functional insights.

Modulation of conotoxin structure and function is achieved through a multienzyme complex in the venom glands of cone snails

The oxidative folding of large polypeptides has been investigated in detail; however, comparatively little is known about the enzyme-assisted folding of small, disulfide-containing peptide substrates. To investigate the concerted effect of multiple enzymes on the folding of small disulfide-rich peptides, we sequenced and expressed protein-disulfide isomerase (PDI), peptidyl-prolyl cis-trans isomerase, and immunoglobulin-binding protein (BiP) from Conus venom glands. Conus PDI was shown to catalyze the oxidation and reduction of disulfide bonds in two conotoxins, α-GI and α-ImI. Oxidative folding rates were further increased in the presence of Conus PPI with the maximum effect observed in the presence of both enzymes. In contrast, Conus BiP was only observed to assist folding in the presence of microsomes, suggesting that additional co-factors were involved. The identification of a complex between BiP, PDI, and nascent conotoxins further suggests that the folding and assembly of conotoxins is a highly regulated multienzyme-assisted process. Unexpectedly, all three enzymes contributed to the folding of the ribbon isomer of α-ImI. Here, we identify this alternative disulfide-linked species in the venom of Conus imperialis, providing the first evidence for the existence of a "non-native" peptide isomer in the venom of cone snails. Thus, ER-resident enzymes act in concert to accelerate the oxidative folding of conotoxins and modulate their conformation and function by reconfiguring disulfide connectivities. This study has evaluated the role of a number of ER-resident enzymes in the folding of conotoxins, providing novel insights into the enzyme-guided assembly of these small, disulfide-rich peptides.

Kinetic and thermodynamic features reveal that Escherichia coli BCP is an unusually versatile peroxiredoxin

In Escherichia coli, bacterioferritin comigratory protein (BCP) is a peroxiredoxin (Prx) that catalyzes the reduction of H(2)O(2) and organic hydroperoxides. This protein, along with plant PrxQ, is a founding member of one of the least studied subfamilies of Prxs. Recent structural data have suggested that proteins in the BCP/PrxQ group can exist as monomers or dimers; we report here that, by analytical ultracentrifugation, both oxidized and reduced E. coli BCP behave as monomers in solution at concentrations as high as 200 μM. Unexpectedly, thioredoxin (Trx1)-dependent peroxidase assays conducted by stopped-flow spectroscopy demonstrated that V(max,app) increases with increasing Trx1 concentrations, indicating a nonsaturable interaction (K(m) > 100 μM). At a physiologically reasonable Trx1 concentration of 10 μM, the apparent K(m) value for H(2)O(2) is ~80 μM, and overall, the V(max)/K(m) for H(2)O(2), which remains constant at the various Trx1 concentrations (consistent with a ping-pong mechanism), is ~1.3 × 10(4) M(-1) s(-1). Our kinetic analyses demonstrated that BCP can utilize a variety of reducing substrates, including Trx1, Trx2, Grx1, and Grx3. BCP exhibited a high redox potential of -145.9 ± 3.2 mV, the highest to date observed for a Prx. Moreover, BCP exhibited a broad peroxide specificity, with comparable rates for H(2)O(2) and cumene hydroperoxide. We determined a pK(a) of ~5.8 for the peroxidatic cysteine (Cys45) using both spectroscopic and activity titration data. These findings support an important role for BCP in interacting with multiple substrates and remaining active under highly oxidizing cellular conditions, potentially serving as a defense enzyme of last resort.

Functional in vitro analysis of the ERO1 protein and protein-disulfide isomerase pathway

Oxidative protein folding in the endoplasmic reticulum is supported by efficient electron relays driven by enzymatic reactions centering on the ERO1-protein-disulfide isomerase (PDI) pathway. A controlled in vitro oxygen consumption assay was carried out to analyze the ERO1-PDI reaction. The results showed the pH-dependent oxidation of PDI by ERO1α. Among several possible disulfide bonds regulating ERO1α activity, Cys(94)-Cys(131) and Cys(99)-Cys(104) disulfide bonds are dominant regulators by excluding the involvement of the Cys(85)-Cys(391) disulfide in the regulation. The fine-tuned species specificity of the ERO1-PDI pathway was demonstrated by functional in vitro complementation assays using yeast and mammalian oxidoreductases. Finally, the results provide experimental evidence for the intramolecular electron transfer from the a domain to the a' domain within PDI during its oxidation by ERO1α.

How proteins form disulfide bonds

The identification of protein disulfide isomerase, almost 50 years ago, opened the way to the study of oxidative protein folding. Oxidative protein folding refers to the composite process by which a protein recovers both its native structure and its native disulfide bonds. Pathways that form disulfide bonds have now been unraveled in the bacterial periplasm (disulfide bond protein A [DsbA], DsbB, DsbC, DsbG, and DsbD), the endoplasmic reticulum (protein disulfide isomerase and Ero1), and the mitochondrial intermembrane space (Mia40 and Erv1). This review summarizes the current knowledge on disulfide bond formation in both prokaryotes and eukaryotes and highlights the major problems that remain to be solved.

Structure and function of DsbA, a key bacterial oxidative folding catalyst

Since its discovery in 1991, the bacterial periplasmic oxidative folding catalyst DsbA has been the focus of intense research. Early studies addressed why it is so oxidizing and how it is maintained in its less stable oxidized state. The crystal structure of Escherichia coli DsbA (EcDsbA) revealed that the oxidizing periplasmic enzyme is a distant evolutionary cousin of the reducing cytoplasmic enzyme thioredoxin. Recent significant developments have deepened our understanding of DsbA function, mechanism, and interactions: the structure of the partner membrane protein EcDsbB, including its complex with EcDsbA, proved a landmark in the field. Studies of DsbA machineries from bacteria other than E. coli K-12 have highlighted dramatic differences from the model organism, including a striking divergence in redox parameters and surface features. Several DsbA structures have provided the first clues to its interaction with substrates, and finally, evidence for a central role of DsbA in bacterial virulence has been demonstrated in a range of organisms. Here, we review current knowledge on DsbA, a bacterial periplasmic protein that introduces disulfide bonds into diverse substrate proteins and which may one day be the target of a new class of anti-virulence drugs to treat bacterial infection.

Nitrosative stress-induced Parkinsonian Lewy-like aggregates prevented through polyphenolic phytochemical analog intervention

Nitrosative stress has recently been demonstrated as a causal in a select sporadic variant of Parkinson's (PD) and Alzheimer's (AD) diseases. Specifically, elevated levels of NO disrupt the redox activity of protein-disulfide isomerase, a key endoplasmic reticulum-resident chaperone by S-nitroso modification of its redox-active cysteines. This leads to accumulation of misfolded AD- and PD-specific protein debris. We have recently demonstrated in vitro that polyphenolic phytochemicals, curcumin and masoprocol, can rescue S-nitroso-PDI formation by scavenging NOx. In this study, using dopaminergic SHSY-5Y cells, we have monitored the aggregation of green-fluorescent protein (GFP)-tagged synphilin-1 (a known constituent of PD Lewy neurites) as a function of rotenone-induced nitrosative stress. Importantly, we demonstrate a marked decrease in synphilin-1 aggregation when the cell line is previously incubated with 3,5-bis(2-flurobenzylidene) piperidin-4-one (EF-24), a curcumin analogue, prior to rotenone insult. Furthermore, our data also reveal that rotenone attenuates PDI expression in the same cell line, a phenomenon that can be mitigated through EF-24 intervention. Together, these results suggest that EF-24 can exert neuroprotective effects by ameliorating nitrosative stress-linked damage to PDI and the associated onset of PD and AD. Essentially, EF-24 can serve as a scaffold for the design and development of PD and AD specific prophylactics.

Molecular mechanisms regulating oxidative activity of the Ero1 family in the endoplasmic reticulum

Formation of disulfide bonds in the endoplasmic reticulum (ER) is catalyzed by the ER oxidoreductin (Ero1) family of sulfhydryl oxidases. Ero1 oxidizes protein disulfide isomerase (PDI), which, in turn, introduces disulfides into ER client proteins. To maintain an oxidized state, Ero1 couples disulfide transfer to PDI with reduction of molecular oxygen, forming hydrogen peroxide. Thus, Ero1 activity constitutes a potential source of ER-derived oxidative stress. Intricate feedback mechanisms have evolved to prevent Ero1 hyperactivity. Central to these mechanisms are noncatalytic cysteines, which form regulatory disulfides and influence catalytic activity of Ero1 in relation to local redox conditions. Here we focus on the distinct regulatory disulfides modulating Ero1 activities in the yeast and mammalian ER. In addition to considering effects on the Ero1 catalytic cycle, we consider the implications of these mechanisms with regard to function of Ero1 isoforms and the roles of Ero1 during responses to ER stress.

The oxidoreductase behavior of protein disulfide isomerase impedes fold maturation of endoplasmic reticulum-processed proteins in the pivotal structure-coupled step of oxidative folding: implications for subcellular protein trafficking

Protein disulfide isomerase (PDI), the chief endoplasmic reticulum (ER) resident oxidoreductase chaperone, is known to catalyze the maturation of disulfide bond-containing proteins primarily through oxidation and isomerization functions. The rate-determining step in the oxidative regeneration path of disulfide bond-containing proteins generally couples chemical thiol-disulfide-exchange reactions to a physical conformational folding reaction. We have determined the impact of PDI and its subdomains on the rate-determining step in ribonuclease A folding and on the physical structure-forming step of select ER-processed proteins including RNase A. This was facilitated through application of a novel chemical tool to exclusively populate native disulfide-containing intermediates in unstructured forms. The described biochemical inroad permits a deconvoluted study of the physical half-process in the rate-determining step from its chemical counterpart. Analysis of folding kinetics of RNase A and other proteins reveals that the highly evolved oxidoreductase activity of PDI masks its chaperone-like activity, impedes conformational folding of ER-processed proteins, and limits its potential to accelerate the rate-determining step in oxidative regeneration. Implications of the heretofore unknown and anomalous self-limiting behavior of PDI are discussed in the context of oxidative maturation and misfolding in vivo.

Rescue of ER oxidoreductase function through polyphenolic phytochemical intervention: implications for subcellular traffic and neurodegenerative disorders

Protein disulfide isomerase (PDI), the chief endoplasmic reticulum (ER) resident oxidoreductase chaperone that catalyzes maturation of disulfide-bond-containing proteins is involved in the pathogenesis of both Parkinson's (PD) and Alzheimer's (AD) diseases. S-nitrosylation of PDI cysteines due to nitrosative stress is associated with cytosolic debris accumulation and Lewy-body aggregates in PD and AD brains. We demonstrate that the polyphenolic phytochemicals curcumin and masoprocol can rescue PDI from becoming S-nitrosylated and maintain its catalytic function under conditions mimicking nitrosative stress by forming stable NOx adducts. Furthermore, both polyphenols intervene to prevent the formation of PDI-resistant polymeric misfolded protein forms that accumulate upon exposure to oxidative stress. Our study suggests that curcumin and masoprocol can serve as lead-candidate prophylactics for reactive oxygen species induced chaperone damage, protein misfolding and neurodegenerative disease; importantly, they can play a vital role in sustaining traffic along the ER's secretory pathway by preserving functional integrity of PDI.

Modulation of an active-site cysteine pKa allows PDI to act as a catalyst of both disulfide bond formation and isomerization

Protein disulfide isomerase (PDI) plays a central role in disulfide bond formation in the endoplasmic reticulum. It is implicated both in disulfide bond formation and in disulfide bond reduction and isomerization. To be an efficient catalyst of all three reactions requires complex mechanisms. These include mechanisms to modulate the pK(a) values of the active-site cysteines of PDI. Here, we examined the role of arginine 120 in modulating the pK(a) values of these cysteines. We find that arginine 120 plays a significant role in modulating the pK(a) of the C-terminal active-site cysteine in the a domain of PDI and plays a role in determining the reactivity of the N-terminal active-site cysteine but not via direct modulation of its pK(a). Mutation of arginine 120 and the corresponding residue, arginine 461, in the a' domain severely reduces the ability of PDI to catalyze disulfide bond formation and reduction but enhances the ability to catalyze disulfide bond isomerization due to the formation of more stable PDI-substrate mixed disulfides. These results suggest that the modulation of pK(a) of the C-terminal active cysteine by the movement of the side chain of these arginine residues into the active-site locales has evolved to allow PDI to efficiently catalyze both oxidation and isomerization reactions.

Protein disulfide isomerase: a critical evaluation of its function in disulfide bond formation

Disulfide bond formation is probably involved in the biogenesis of approximately one third of human proteins. A central player in this essential process is protein disulfide isomerase or PDI. PDI was the first protein-folding catalyst reported. However, despite more than four decades of study, we still do not understand much about its physiological mechanisms of action. This review examines the published literature with a critical eye. This review aims to (a) provide background on the chemistry of disulfide bond formation and rearrangement, including the concept of reduction potential, before examining the structure of PDI; (b) detail the thiol-disulfide exchange reactions that are catalyzed by PDI in vitro, including a critical examination of the assays used to determine them; (c) examine oxidation and reduction of PDI in vivo, including not only the role of ERo1 but also an extensive assessment of the role of glutathione, as well as other systems, such as peroxide, dehydroascorbate, and a discussion of vitamin K-based systems; (d) consider the in vivo reactions of PDI and the determination and implications of the redox state of PDI in vivo; and (e) discuss other human and yeast PDI-family members.

Oxidative folding and reductive activities of EhPDI, a protein disulfide isomerase from Entamoeba histolytica

PDI enzymes are oxidoreductases that catalyze oxidation, reduction and isomerization of disulfide bonds in polypeptide substrates. We have previously identified an E. histolytica PDI enzyme (EhPDI) that exhibits oxidase activity in vivo. However, little is known about the specific role of its redox-related structural features on the enzymatic activity. Here, we have studied the in vivo oxidative folding of EhPDI by mutagenic analysis and functional complementation assays as well as the in vitro oxidative folding and reductive activities by comparative kinetics using functional homologues in standard assays. We have found that the active-site cysteine residues of the functional domains (Trx-domains) are essential for catalysis of disulfide bond formation in polypeptides and proteins, such as the bacterial alkaline phosphatase. Furthermore, we have shown that the recombinant EhPDI enzyme has some typical properties of PDI enzymes: oxidase and reductase activities. These activities were comparable to those observed for other functional equivalents, such as bovine PDI or bacterial thioredoxin, under the same experimental conditions. These findings will be helpful for further studies intended to understand the physiological role of EhPDI.

Participation of low molecular weight electron carriers in oxidative protein folding

Oxidative protein folding is mediated by a proteinaceous electron relay system, in which the concerted action of protein disulfide isomerase and Ero1 delivers the electrons from thiol groups to the final acceptor. Oxygen appears to be the final oxidant in aerobic living organisms, although the existence of alternative electron acceptors, e.g. fumarate or nitrate, cannot be excluded. Whilst the protein components of the system are well-known, less attention has been turned to the role of low molecular weight electron carriers in the process. The function of ascorbate, tocopherol and vitamin K has been raised recently. In vitro and in vivo evidence suggests that these redox-active compounds can contribute to the functioning of oxidative folding. This review focuses on the participation of small molecular weight redox compounds in oxidative protein folding.

pH dependence of the isomerase activity of protein disulfide isomerase: insights into its functional relevance

The isomerase efficacy of the oxidoreductase, protein disulfide isomerase (PDI), has been examined by a simple method. Using this technique, the pH-dependence of relative efficiency of isomerization reactions by PDI has been evaluated and its impact on a key structure-forming step in the oxidative folding pathway of a model protein determined. Results reveal that PDI has a greater relative impact on thiol-disulfide reshuffling (isomerization) reactions and consequently the structure-forming step in oxidative folding at pH 7, as opposed to pH's 8 and 9. These results suggest that PDI, which possesses an anomalously low thiol pKa, is fine-tuned to catalyze oxidative folding in the lumen of the endoplasmic reticulum where the ambient pH of approximately 7 would otherwise retard thioldisulfide exchange reactions and hinder acquisition of the native fold. The pH-dependent impact on isomerization catalysis has important implications for the development of synthetic chaperones for in vivo and in vitro applications.

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.

Acceleration of oxidative protein folding by curcumin through novel non-redox chemistry

Curcumin, the major constituent of turmeric is a known antioxidant. We have examined the oxidative folding of the model four-disulfide-bond-containing protein bovine pancreatic ribonuclease A (RNase A) in its presence; results indicate that RNase A regeneration rate increases in a curcumin-dependent manner. Examination of the native tendency of the fully-reduced polypeptide and the stability of key folding intermediates suggests that the increased oxidative folding rate can be attributed to native-like elements induced within the fully-reduced polypeptide and the stabilization of native-like species by this non-redox-active natural product. Our results provide a template for the design of curcuminoid-based synthetic small-molecule fold catalysts that accelerate the folding of ER-processed proteins; this assumes significance given that nitrosative stress and dysfunction of the ER-resident oxidoreductase protein disulfide isomerise due to S-nitrosylation are factors associated with the pathogenesis of Alzheimer's and Parkinson's diseases.

Reactive cysteine in proteins: protein folding, antioxidant defense, redox signaling and more

Cysteine plays structural roles in proteins and can also participate in electron transfer reactions, when some structural folds provide appropriated environments for stabilization of its sulfhydryl group in the anionic form, called thiolate (RS(-)). In contrast, sulfhydryl group of free cysteine has a relatively high pK(a) (8,5) and as a consequence is relatively inert for redox reaction in physiological conditions. Thiolate is considerable more powerful as nucleophilic agent than its protonated form, therefore, reactive cysteine are present mainly in its anionic form in proteins. In this review, we describe several processes in which reactive cysteine in proteins take part, showing a high degree of redox chemistry versatility.

Role of protein disulfide isomerase and other thiol-reactive proteins in HIV-1 envelope protein-mediated fusion

Cell-surface protein disulfide isomerase (PDI) has been proposed to promote disulfide bond rearrangements in HIV-1 envelope protein (Env) that accompany Env-mediated fusion. We evaluated the role of PDI in ways that have not been previously tested by downregulating PDI with siRNA and by overexpressing wild-type or variant forms of PDI in transiently and stably transfected cells. These manipulations, as well as treatment with anti-PDI antibodies, had only small effects on infection or cell fusion mediated by NL4-3 or AD8 strains of HIV-1. However, the cell-surface thiol-reactive reagent 5, 5'-dithiobis(2-nitrobenzoic acid) (DTNB) had a much stronger inhibitory effect in our system, suggesting that cell-surface thiol-containing molecules other than PDI, acting alone or in concert, have a greater effect than PDI on HIV-1 Env-mediated fusion. We evaluated one such candidate, thioredoxin, a PDI family member reported to reduce a labile disulfide bond in CD4. We found that the ability of thioredoxin to reduce the disulfide bond in CD4 is enhanced in the presence of HIV-1 Env gp120 and that thioredoxin also reduces disulfide bonds in gp120 directly in the absence of CD4. We discuss the implications of these observations for identification of molecules involved in disulfide rearrangements in Env during fusion.

The pressure effect on the structure and functions of protein disulfide isomerase

Studying on the pressure effects of the structure and functions of the multidomain protein, protein disulfide isomerase (PDI), the intrinsic Trp fluorescence spectra of PDI were measured under high pressure. PDI has 5 Trp residues and the two of all Trp residues are located at the neighborhood of the active site (WCGHC) for isomerase activity. On the basis of the red shift of center of spectral mass (CSM) of the intrinsic Trp fluorescence and the decrease in its fluorescence intensity, the changes in tertiary structure of PDI were observed above 100 MPa. These structural changes were completed at 400 MPa. The CSM of 400 MPa denatured PDI was comparable to that of 6.0 M GdnHCl denatured one. All of the Trp residues included in PDI are completely exposed to aqueous medium at 400 MPa. However, there is the significant difference between the pressure and GdnHCl-denatured PDI. The Trp fluorescence intensity was decreased with increasing pressure, but increased with the increase of the GdnHCl concentration. It is implied that the pressure-denatured state of PDI might remain compact not to be extensively unfolded. In the point of view about the reversibility of pressure-treated PDI, the tertiary structure was completely recovered after released to ambient pressure. The disulfide reduction and chaperone activity of 400 MPa-treated PDI were also recovered to be comparable to those of native one. Despite of a multidomain protein, the excellence in both structural and functional recovery of pressure-denatured PDI is quite remarkable. These unique properties of PDI against high pressure provide the insights into understanding the pressure-induced denaturation of PDI.

An introduction to thiol redox proteins in the endoplasmic reticulum and a review of current electrochemical methods of detection of thiols

This aim of this paper is to expound the complexity of thiol redox systems in the endoplasmic reticulum of eukaryotic cells to the electroanalytical community. A summary of the state of the art in electrochemical methods for detection of thiols gives an insight into the challenges that need to be addressed to bridge the disparity between current analytical techniques and applications in a 'real' biological scenario.

Analysis of protein covalent modification by xenobiotics using a covert oxidatively activated tag: raloxifene proof-of-principle study

Numerous xenobiotics, including therapeutics agents, are substrates for bioactivation to electrophilic reactive intermediates that may covalently modify biomolecules. Selective estrogen receptor modulators (SERMs) are in clinical use for long-term therapy of postmenopausal syndromes and chemoprevention and provide a potential alternative for hormone replacement therapy (HRT). Raloxifene, in common with many SERMs and other xenobiotics, is a polyaromatic phenol that has been shown to be metabolically bioactivated to electrophilic and redox active quinoids. Nucleic acid and glutathione adduct formation have been reported, but little is known about protein covalent modification. A novel COATag (covert oxidatively activated tag) was synthesized in which raloxifene was linked to biotin. The COATag was reactive toward a model protein, human glutathione-S-transferase P1-1, in the presence but not the absence of monooxygenase. The covalent modification of proteins in rat liver microsomal incubations was NADPH-dependent implicating cytochrome P450 oxidase. The COATag facilitated isolation and identification of covalently modified microsomal proteins: cytosolic glucose regulated protein (GRP78/BiP), three protein disulfide isomerases, and microsomal glutathione S-transferase 1. Oxidative metabolism of raloxifene produces reactive intermediates of sufficient lifetimes to covalently modify proteins in tissue microsomes, behavior anticipated for other polyaromatic phenol xenobiotics that can be tested by the COATag methodology. The combined use of a COATag with a simple biotin-linked electrophile (such as an iodoacetamide tag) is a new technique that allows quantification of protein covalent modification via alkylation vs oxidation in response to xenobiotic reactive intermediates. The identification of modified proteins is important for defining pathways that might lead alternatively to either cytotoxicity or cytoprotection.

Catalysis of protein disulfide bond isomerization in a homogeneous substrate

Protein disulfide isomerase (PDI) catalyzes the rearrangement of nonnative disulfide bonds in the endoplasmic reticulum of eukaryotic cells, a process that often limits the rate at which polypeptide chains fold into a native protein conformation. The mechanism of the reaction catalyzed by PDI is unclear. In assays involving protein substrates, the reaction appears to involve the complete reduction of some or all of its nonnative disulfide bonds followed by oxidation of the resulting dithiols. The substrates in these assays are, however, heterogeneous, which complicates mechanistic analyses. Here, we report the first analysis of disulfide bond isomerization in a homogeneous substrate. Our substrate is based on tachyplesin I, a 17-mer peptide that folds into a beta hairpin stabilized by two disulfide bonds. We describe the chemical synthesis of a variant of tachyplesin I in which its two disulfide bonds are in a nonnative state and side chains near its N and C terminus contain a fluorescence donor (tryptophan) and acceptor (N(epsilon)-dansyllysine). Fluorescence resonance energy transfer from 280 to 465 nm increases by 28-fold upon isomerization of the disulfide bonds into their native state (which has a lower E(o') = -0.313 V than does PDI). We use this continuous assay to analyze catalysis by wild-type human PDI and a variant in which the C-terminal cysteine residue within each Cys-Gly-His-Cys active site is replaced with alanine. We find that wild-type PDI catalyzes the isomerization of the substrate with kcat/K(M) = 1.7 x 10(5) M(-1) s(-1), which is the largest value yet reported for catalysis of disulfide bond isomerization. The variant, which is a poor catalyst of disulfide bond reduction and dithiol oxidation, retains virtually all of the activity of wild-type PDI in catalysis of disulfide bond isomerization. Thus, the C-terminal cysteine residues play an insignificant role in the isomerization of the disulfide bonds in nonnative tachyplesin I. We conclude that catalysis of disulfide bond isomerization by PDI does not necessarily involve a cycle of substrate reduction/oxidation.

Functional analysis of the CXXC motif using phage antibodies that cross-react with protein disulphide-isomerase family proteins

Polyclonal antibodies that had been raised against particular PDI (protein disulphide-isomerase) family proteins did not cross-react with other PDI family proteins. To evade immune tolerance to the important self-motif Cys-Xaa-Xaa-Cys, which is present in PDI family proteins, we used the phage display library [established by Griffiths, Williams, Hartley, Tomlinson, Waterhouse, Crosby, Kontermann, Jones, Low, Allison et al. (1994) EMBO J. 13, 3245-3260] to isolate successfully the phage antibodies that can cross-react with human and bovine PDIs, human P5, human PDI-related protein and yeast PDI. By measuring the binding of scFv (single-chain antibody fragment of variable region) to synthetic peptides and to mutants of PDI family proteins in a surface plasmon resonance apparatus, we identified clones that recognized sequences containing the CGHC motif or the CGHCK sequence. By using the isolated phage antibodies, we demonstrated for the first time that a lysine residue following the CXXC motif significantly increases the isomerase activities of PDI family proteins. Moreover, we demonstrated that the affinity of isolated scFvs for mutant PDI family proteins is proportional to the isomerase activities of their active sites.

Inactivation of protein disulfide isomerase by alkylators including alpha,beta-unsaturated aldehydes at low physiological pHs

Protein disulfide isomerase (PDI) is known to contain the thioredoxin box motif with a low pKa cysteine residue. To investigate the reactivity of PDI with thiol modifiers at low physiological pHs, either the reduced (PDIred) or oxidized form (PDIoxid) of PDI was exposed to various alkylating ragents. When PDI was incubated with iodoacetamide at pH 6.3 for 30 min at 38 degrees C, a remarkable inactivation (>90%) of PDIred was caused by iodoacetamide (IC50=8 microM). However, PDIoxid was only slightly inactivated (approximately 18%) by iodoacetamide. Similarly, PDIred was significantly inactivated by N-ethylmaleimide (NEM), but PDIoxid was not. When the inactivation by these alkylators was analyzed by pseudo-first order kinetics, NEM (k3=1.75x10(-2) s(-1); K(i)=124 microM) was observed to be more potent than iodoacetamide (k3=9.1x10(-3) s(-1); K(i)=311 microM). Interestingly, the inactivation of PDIred by iodoacetamide was greater at pH 6.3 than pH 7.0, in contrast to a similar inactivation potency of NEM at both pHs. Moreover, the maximal inactivation of PDIred or PDIoxid by iodoacetamide was mainly observed around pH 6.0. In addition, PDIred was found to be inactivated by acrolein (IC50=10 microM) at pH 6.3, and this inactivation was also greater at pH 6.3 than at pH 7. Based on these results, we suggest that PDIred is susceptible to inactivation by alkylators including endogenous alpha,beta-unsaturated aldehydes at low physiological pHs.

Identification of alkylation-sensitive target chaperone proteins and their reactivity with natural products containing Michael acceptor

Molecular chaperones have a crucial role in the folding of nascent polypeptides in endoplasmic reticulum. Some of them are known to be sensitive to the modification by electrophilic metabolites of organic pro-toxicants. In order to identify chaperone proteins sensitive to alkyators, ER extract was subjected to alkylation by 4-acetamido-4'-maleimidyl-stilbene-2,2'-disulfonate (AMS), and subsequent SDS-PAGE analyses. Protein spots, with molecular mass of 160, 100, 57 and 36 kDa, were found to be sensitive to AMS alkylation, and one abundant chaperon protein was identified to be protein disulfide isomerase (PDI) in comparison with the purified PDI. To see the reactivity of PDI with cysteine alkylators, the reduced form (PDIred) of PDI was incubated with various alkylators containing Michael acceptor structure for 30 min at 38 degrees C at pH 6.3, and the remaining activity was determined by the insulin reduction assay. Iodoacetamide or N-ethylmaleimide at 0.1 mM remarkably inactivated PDIred with N-ethylmaleimide being more potent than iodoacetamide. A partial inactivation of PDIoxid was expressed by iodoacetamide, but not N-ethylmaleimide (NEM) at pH 6.3. Of Michael acceptor compounds tested, 1,4-benzoquinone (IC50, 15 microM) was the most potent, followed by 4-hydroxy-2-nonenal and 1,4-naphthoquinone. In contrast, 1,2-naphthoquinone, devoid of a remarkable inactivation action, was effective to cause the oxidative conversion of PDIred to PDIoxid. Thus, the action of Michael acceptor compounds differed greatly depending on their structure. Based on these, it is proposed that PDI, one of chaperone proteins in ER, could be susceptible to endogenous or xenobiotic Michael acceptor compounds in vivo system.