Mitochondrial respiratory chain complexes as sources and targets of thiol-based redox-regulation

The respiratory chain of the inner mitochondrial membrane is a unique assembly of protein complexes that transfers the electrons of reducing equivalents extracted from foodstuff to molecular oxygen to generate a proton-motive force as the primary energy source for cellular ATP-synthesis. Recent evidence indicates that redox reactions are also involved in regulating mitochondrial function via redox-modification of specific cysteine-thiol groups in subunits of respiratory chain complexes. Vice versa the generation of reactive oxygen species (ROS) by respiratory chain complexes may have an impact on the mitochondrial redox balance through reversible and irreversible thiol-modification of specific target proteins involved in redox signaling, but also pathophysiological processes. Recent evidence indicates that thiol-based redox regulation of the respiratory chain activity and especially S-nitrosylation of complex I could be a strategy to prevent elevated ROS production, oxidative damage and tissue necrosis during ischemia–reperfusion injury. This review focuses on the thiol-based redox processes involving the respiratory chain as a source as well as a target, including a general overview on mitochondria as highly compartmentalized redox organelles and on methods to investigate the redox state of mitochondrial proteins. This article is part of a Special Issue entitled: Thiol-Based Redox Processes.     

Modulation of antioxidant defence system in brain of rainbow trout (Oncorhynchus mykiss) after chronic carbamazepine treatment

We investigated the effect of long-term exposure to CBZ on the antioxidant system in brain tissue of rainbow trout. Fish were exposed to sublethal concentrations of CBZ (1.0 μg/L, 0.2 mg/L or 2.0 mg/L) for 7, 21, and 42 days. Oxidative stress indices (LPO and CP) and activities of antioxidant enzymes (SOD, CAT, GPx and GR) in fish brain were measured. In addition, non-enzymatic antioxidant (GSH) was determined after 42 days exposure. Carbamazepine exposure at 0.2 mg/L led to significant increases (p < 0.05) of LPO and CP after 42 days and, at 2.0 mg/L, after 21 days. Activities of the antioxidant enzymes SOD, CAT, and GPx in CBZ-treated groups slightly increased during the first period (7 days). However, activities of all measured antioxidant enzymes were significantly inhibited (p < 0.05) at 0.2 mg/L exposure after 42 days and after 21 days at 2.0 mg/L. After 42 days, the content of GSH in fish brain was significantly lower (p < 0.05) in groups exposed to CBZ at 0.2 mg/L and 2.0 mg/L than in other groups. Prolonged exposure to CBZ resulted in excess reactive oxygen species formation, finally resulting in oxidative damage to lipids and proteins and inhibited antioxidant capacities in fish brain. In short, a low level of oxidative stress could induce the adaptive responses of antioxidant enzymes, but long-term exposure to CBZ could lead to serious oxidative damage in fish brain.     

Exposure of the blue mussel, Mytilus edulis, to gold nanoparticles and the pro-oxidant menadione

Relatively little is known about how gold nanoparticles (GNP) might interact in vivo with marine organisms. Mytilus edulis was exposed (24 h) to ~ 15 nm GNP, menadione and both compounds simultaneously (GNP/menadione). GNP was detected by inductively coupled plasma-optical emission spectroscopy mainly in digestive gland of samples exposed to GNP though not GNP/menadione, perhaps due to impaired feeding. Thioredoxin reductase activity and malondialdehyde levels were determined in all tissues. Thioredoxin reductase inhibition was detected only in digestive gland exposed to menadione whilst malondialdehyde levels did not vary in response to treatment in all tissues. GNP caused a decrease in the reduced/oxidized glutathione ratio in digestive gland, but no difference was found in other tissues or for other treatments. One dimensional electrophoresis of proteins containing thiol groups was performed in all tissues and revealed a reduction in protein thiols for all treatments in digestive gland. Two dimensional electrophoresis of digestive gland extracts, from GNP and control groups, showed decreased levels of thiol proteins in response to GNP which we attribute to oxidation. Our results suggest that GNP causes a modest level of oxidative stress sufficient to oxidize thiols in glutathione and proteins but without causing lipid peroxidation or induction of thioredoxin reductase activity.     

The specific,submicromolar-Km ADP-ribose pyrophosphatase purified from human placenta is enzymically indistinguishable from recombinant NUDT9 protein,including a selectivity for Mn as activating cation and increase in Km for ADP-ribose,both elicited by H2O2

Free ADP-ribose is a putative second messenger and also a potentially toxic compound due to its non-enzymic reactivity towards protein side chains. ADP-ribose hydrolysis is catalysed by NDP-sugar/alcohol pyrophosphatases of differing specificity, including a highly specific, low-K_m ADP-ribose pyrophosphatase. In humans, a submicromolar-K_m ADP-ribose pyrophosphatase has been purified from placenta, while recombinant NUDT9 has been described as a similarly specific enzyme with a nudix motif, but with a 10²–10³ higher K_m. Here, a comparative study of both proteins is presented showing that they are in fact enzymically indistinguishable; crucially, they both have submicromolar K_m for ADP-ribose. This study firmly supports the view that the ADP-ribose pyrophosphatase present in human tissues is a product of the NUDT9 gene. In addition, this study reveals previously unknown properties of both enzyme forms. They display the same, differential properties in the presence of Mg²⁺ or Mn²⁺ as activating cations with respect to substrate specificity, ADP-ribose saturation kinetics, and inhibition by fluoride. Treatment with H₂O₂ alters the Mg²⁺/Mn²⁺ responses and increases the K_m values for ADP-ribose, changes that are reversed by DTT. The results are discussed in relation to the proposed roles for ADP-ribose in oxidative/nitrosative stress and for ADP-ribose pyrophosphatase as a protective enzyme whose function is to limit the intracellular accumulation of ADP-ribose.     

Glutathione redox regulates TGF-β-induced fibrogenic effects through Smad3 activation

Transforming growth factor-β (TGF-β) plays a pivotal role in the fibrogenic action involved in the induction of connective tissue growth factor (CTGF), extracellular matrix and fibroblast transformation. Smad3 mediates TGF-β signaling related to the fibrotic response. In human lung fibroblasts or bronchial smooth muscle cells, we demonstrated that an increase in the intracellular glutathione level suppressed TGF-β1-induced phosphorylation of Smad3, while inhibiting TGF-β1-induced expressions of CTGF, collagen type1, fibronectin and transformation into myofibroblasts, which are characterized by the expression of α-smooth muscle actin. These data indicate that the intracellular glutathione redox status regulates TGF-β-induced fibrogenic effects through Smad3 activation.     

Adrenomedullin protects against hypoxia/reoxygenation-induced cell death by suppression of reactive oxygen species via thiol redox systems

Adrenomedullin (ADM) functions as a survival factor against hypoxic cell death. However, molecular mechanisms underlying the cell survival pathway remain largely unknown. In this report, we showed that ADM suppressed reactive oxygen species (ROS) increase by inhibiting reduction of glutathione (GSH) level in hypoxia/reoxygenation (H/R) injury, and increased the activities of glutathione peroxidase and reductase. In addition, ADM maintained total and active reduced thioredoxin (Trx) levels against H/R. We also found that ADM blocked nuclear translocation of Trx induced by H/R. The results of the present study show that ADM regulates cellular ROS levels via the GSH and Trx system.     

The loops facing the active site of prolyl oligopeptidase are crucial components in substrate gating and specificity

Prolyl oligopeptidase (POP) has emerged as a drug target for neurological diseases. A flexible loop structure comprising loop A (res. 189–209) and loop B (res. 577–608) at the domain interface is implicated in substrate entry to the active site. Here we determined kinetic and structural properties of POP with mutations in loop A, loop B, and in two additional flexible loops (the catalytic His loop, propeller Asp/Glu loop). POP lacking loop A proved to be an inefficient enzyme, as did POP with a mutation in loop B (T590C). Both variants displayed an altered substrate preference profile, with reduced ligand binding capacity. Conversely, the T202C mutation increased the flexibility of loop A, enhancing the catalytic efficiency beyond that of the native enzyme. The T590C mutation in loop B increased the preference for shorter peptides, indicating a role in substrate gating. Loop A and the His loop are disordered in the H680A mutant crystal structure, as seen in previous bacterial POP structures, implying coordinated structural dynamics of these loops. Unlike native POP, variants with a malfunctioning loop A were not inhibited by a 17-mer peptide that may bind non-productively to an exosite involving loop A. Biophysical studies suggest a predominantly closed resting state for POP with higher flexibility at the physiological temperature. The flexible loop A, loop B and His loop system at the active site is the main regulator of substrate gating and specificity and represents a new inhibitor target.     

The fairytale of the GSSG/GSH redox potential

Background

The term GSSG/GSH redox potential is frequently used to explain redox regulation and other biological processes.

Scope of review

The relevance of the GSSG/GSH redox potential as driving force of biological processes is critically discussed. It is recalled that the concentration ratio of GSSG and GSH reflects little else than a steady state, which overwhelmingly results from fast enzymatic processes utilizing, degrading or regenerating GSH.

Major conclusions

A biological GSSG/GSH redox potential, as calculated by the Nernst equation, is a deduced electrochemical parameter based on direct measurements of GSH and GSSG that are often complicated by poorly substantiated assumptions. It is considered irrelevant to the steering of any biological process. GSH-utilizing enzymes depend on the concentration of GSH, not on [GSH]², as is predicted by the Nernst equation, and are typically not affected by GSSG. Regulatory processes involving oxidants and GSH are considered to make use of mechanistic principles known for thiol peroxidases which catalyze the oxidation of hydroperoxides by GSH by means of an enzyme substitution mechanism involving only bimolecular reaction steps.

General significance

The negligibly small rate constants of related spontaneous reactions as compared with enzyme-catalyzed ones underscore the superiority of kinetic parameters over electrochemical or thermodynamic ones for an in-depth understanding of GSH-dependent biological phenomena. At best, the GSSG/GSH potential might be useful as an analytical tool to disclose disturbances in redox metabolism. This article is part of a Special Issue entitled Cellular Functions of Glutathione.     

Protein disulfide isomerase and glutathione are alternative substrates in the one Cys catalytic cycle of glutathione peroxidase 7

Background

Mammalian GPx7 is a monomeric glutathione peroxidase of the endoplasmic reticulum (ER), containing a Cys redox center (CysGPx). Although containing a peroxidatic Cys (C_P) it lacks the resolving Cys (C_R), that confers fast reactivity with thioredoxin (Trx) or related proteins to most other CysGPxs.

Methods

Reducing substrate specificity and mechanism were addressed by steady-state kinetic analysis of wild type or mutated mouse GPx7. The enzymes were heterologously expressed as a synuclein fusion to overcome limited expression. Phospholipid hydroperoxide was the oxidizing substrate. Enzyme–substrate and protein–protein interaction were analyzed by molecular docking and surface plasmon resonance analysis.

Results

Oxidation of the C_P is fast (k_+ 1 > 10³ M^− 1 s^− 1), however the rate of reduction by GSH is slow (k′_+ 2 = 12.6 M^− 1 s^− 1) even though molecular docking indicates a strong GSH–GPx7 interaction. Instead, the oxidized C_P can be reduced at a fast rate by human protein disulfide isomerase (HsPDI) (k_+ 1 > 10³ M^− 1 s^− 1), but not by Trx. By surface plasmon resonance analysis, a K_D = 5.2 μM was calculated for PDI–GPx7 complex. Participation of an alternative non-canonical C_R in the peroxidatic reaction was ruled out. Specific activity measurements in the presence of physiological reducing substrate concentration, suggest substrate competition in vivo.

Conclusions

GPx7 is an unusual CysGPx catalyzing the peroxidatic cycle by a one Cys mechanism in which GSH and PDI are alternative substrates.

General significance

In the ER, the emerging physiological role of GPx7 is oxidation of PDI, modulated by the amount of GSH.