Thiols in oxidative phosphorylation: inhibition and energy-potentiated uncoupling by monothiol and dithiol modifiers

The role of the myeloperoxidase-derived oxidant hypothiocyanous acid (HOSCN) in the induction of mitochondrial dysfunction in macrophages

A host of chronic inflammatory diseases are accelerated by the formation of the powerful oxidant hypochlorous acid (HOCl) by myeloperoxidase (MPO). In the presence of thiocyanate (SCN^-), the production of HOCl by MPO is decreased in favour of the formation of a milder oxidant, hypothiocyanous acid (HOSCN). The role of HOSCN in disease has not been fully elucidated, though there is increasing interest in using SCN^- therapeutically in different disease settings. Unlike HOCl, HOSCN can be detoxified by thioredoxin reductase, and reacts selectively with thiols to result in reversible modifications, which could potentially reduce the extent of MPO-induced damage during chronic inflammation. In this study, we show that exposure of macrophages, a key inflammatory cell type, to HOSCN results in the reversible modification of multiple mitochondrial proteins, leading to increased mitochondrial membrane permeability, decreased oxidative phosphorylation and reduced formation of ATP. The increased permeability and reduction in ATP could be reversed by pre-treatment of the macrophages with cyclosporine A, implicating a role for the mitochondrial permeability transition pore. HOSCN also drives cells to utilise fatty acids as an energetic substrate after the inhibition of oxidative phosphorylation. Raman imaging studies highlighted the ability of HOSCN to perturb the electron transport chain of mitochondria and redistribute these organelles within the cell. Taken together, these data provide new insight into the pathways by which HOSCN can induce cytotoxicity and cellular damage, which may have relevance for the development of inflammatory disease, and therapeutic strategies to reduce HOCl-induced damage by supplementation with SCN^-.

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HOSCN induces the oxidation of mitochondrial thiol proteins and cytochromes.

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HOSCN alters mitochondrial permeability and ATP production via MPTP formation.

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HOSCN increases the capacity of cells to use fatty acids as an energetic substrate.

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Raman imaging shows redistribution of mitochondria after cell exposure to HOSCN.

Oligomycin-induced proton uncoupling

Oligomycin is a classical mitochondrial reagent that binds to the proton channel on the F_o component of ATP synthase. As a result, oligomycin blocks mitochondrial ATP synthesis, proton translocation, and O₂ uptake. Here we show that oligomycin induces proton uncoupling subsequent to inhibition of ATP synthesis, as evidenced by recovery of O₂ uptake to near baseline levels. Uncoupling is uniquely rapid and readily observed in HepG2 cells but is also observed at longer times in the unrelated H1299 cell line. Proton fluxes plateau at oligomycin concentrations in the region 0.25-5 μM. At the plateau, fluxes are lower than expected for the classical mitochondrial permeability transition pore, although in H1229 cells, fluxes increase to levels consistent with pore opening at higher oligomycin concentrations. Uncoupling is observed in cells metabolizing either pyruvate or lactate and reversed by addition of glucose to restore ATP synthesis. Uncoupling is not sensitive to cyclosporin A and is not reversed by the ANT inhibitor bongkrekic acid. However, bongkrekic acid inhibits uncoupling if added before oligomycin, which we interpret in terms of maintenance of mitochondrial ATP levels.

Immunological Techniques to Assess Protein Thiol Redox State: Opportunities, Challenges and Solutions

To understand oxidative stress, antioxidant defense, and redox signaling in health and disease it is essential to assess protein thiol redox state. Protein thiol redox state is seldom assessed immunologically because of the inability to distinguish reduced and reversibly oxidized thiols by Western blotting. An underappreciated opportunity exists to use Click PEGylation to realize the transformative power of simple, time and cost-efficient immunological techniques. Click PEGylation harnesses selective, bio-orthogonal Click chemistry to separate reduced and reversibly oxidized thiols by selectively ligating a low molecular weight polyethylene glycol moiety to the redox state of interest. The resultant ability to disambiguate reduced and reversibly oxidized species by Western blotting enables Click PEGylation to assess protein thiol redox state. In the present review, to enable investigators to effectively harness immunological techniques to assess protein thiol redox state we critique the chemistry, promise and challenges of Click PEGylation.

Reversible Thiol Oxidation Inhibits the Mitochondrial ATP Synthase in Xenopus Laevis Oocytes

Oocytes are postulated to repress the proton pumps (e.g., complex IV) and ATP synthase to safeguard mitochondrial DNA homoplasmy by curtailing superoxide production. Whether the ATP synthase is inhibited is, however, unknown. Here we show that: oligomycin sensitive ATP synthase activity is significantly greater (~170 vs. 20 nmol/min^-1/mg^-1) in testes compared to oocytes in Xenopus laevis (X. laevis). Since ATP synthase activity is redox regulated, we explored a regulatory role for reversible thiol oxidation. If a protein thiol inhibits the ATP synthase, then constituent subunits must be reversibly oxidised. Catalyst-free trans-cyclooctene 6-methyltetrazine (TCO-Tz) immunocapture coupled to redox affinity blotting reveals several subunits in F₁ (e.g., ATP-α-F₁) and F_o (e.g., subunit c) are reversibly oxidised. Catalyst-free TCO-Tz Click PEGylation reveals significant (~60%) reversible ATP-α-F₁ oxidation at two evolutionary conserved cysteine residues (C²⁴⁴ and C²⁹⁴) in oocytes. TCO-Tz Click PEGylation reveals ~20% of the total thiols in the ATP synthase are substantially oxidised. Chemically reversing thiol oxidation significantly increased oligomycin sensitive ATP synthase activity from ~12 to 100 nmol/min^-1/mg^-1 in oocytes. We conclude that reversible thiol oxidation inhibits the mitochondrial ATP synthase in X. laevis oocytes.

Mitochondrial F-ATP Synthase and Its Transition into an Energy-Dissipating Molecular Machine

The mitochondrial F-ATP synthase is the principal energy-conserving nanomotor of cells that harnesses the proton motive force generated by the respiratory chain to make ATP from ADP and phosphate in a process known as oxidative phosphorylation. In the energy-converting membranes, F-ATP synthase is a multisubunit complex organized into a membrane-extrinsic F₁ sector and a membrane-intrinsic F_O domain, linked by central and peripheral stalks. Due to its essential role in the cellular metabolism, malfunction of F-ATP synthase has been associated with a variety of pathological conditions, and the enzyme is now considered as a promising drug target for multiple disease conditions and for the regulation of energy metabolism. We discuss structural and functional features of mitochondrial F-ATP synthase as well as several conditions that partially or fully inhibit the coupling between the F₁ catalytic activities and the F_O proton translocation, thus decreasing the cellular metabolic efficiency and transforming the enzyme into an energy-dissipating structure through molecular mechanisms that still remain to be defined.

Mercury and protein thiols: Stimulation of mitochondrial F1FO-ATPase and inhibition of respiration

In spite of the known widespread toxicity of mercury, its impact on mitochondrial bioenergetics is a still poorly explored topic. Even if many studies have dealt with mercury poisoning of mitochondrial respiration, as far as we are aware Hg²⁺ effects on individual complexes are not so clear. In the present study changes in swine heart mitochondrial respiration and F₁F_O-ATPase (F-ATPase) activity promoted by micromolar Hg²⁺ concentrations were investigated. Hg²⁺ was found to inhibit the respiration of NADH-energized mitochondria, whereas it was ineffective when the substrate was succinate. Interestingly, the same micromolar Hg²⁺ doses which inhibited the NADH-O₂ activity stimulated the F-ATPase, most likely by interacting with adjacent thiol residues. Accordingly, Hg²⁺ dose-dependently decreased protein thiols and all the elicited effects on mitochondrial complexes were reversed by the thiol reducing agent DTE. These findings clearly indicate that Hg²⁺ interacts with Cys residues of these complexes and differently modulate their functionality by modifying the redox state of thiol groups. The results, which cast light on some implications of metal-thiol interactions up to now not fully explored, may contribute to clarify the molecular mechanisms of mercury toxicity to mitochondria.

The Mitochondrial Permeability Transition Pore: Channel Formation by F-ATP Synthase, Integration in Signal Transduction, and Role in Pathophysiology

The mitochondrial permeability transition (PT) is a permeability increase of the inner mitochondrial membrane mediated by a channel, the permeability transition pore (PTP). After a brief historical introduction, we cover the key regulatory features of the PTP and provide a critical assessment of putative protein components that have been tested by genetic analysis. The discovery that under conditions of oxidative stress the F-ATP synthases of mammals, yeast, and Drosophila can be turned into Ca(2+)-dependent channels, whose electrophysiological properties match those of the corresponding PTPs, opens new perspectives to the field. We discuss structural and functional features of F-ATP synthases that may provide clues to its transition from an energy-conserving into an energy-dissipating device as well as recent advances on signal transduction to the PTP and on its role in cellular pathophysiology.

From ATP to PTP and Back: A Dual Function for the Mitochondrial ATP Synthase

Mitochondria not only play a fundamental role in heart physiology but are also key effectors of dysfunction and death. This dual role assumes a new meaning after recent advances on the nature and regulation of the permeability transition pore, an inner membrane channel whose opening requires matrix Ca(2+) and is modulated by many effectors including reactive oxygen species, matrix cyclophilin D, Pi (inorganic phosphate), and matrix pH. The recent demonstration that the F-ATP synthase can reversibly undergo a Ca(2+)-dependent transition to form a channel that mediates the permeability transition opens new perspectives to the field. These findings demand a reassessment of the modifications of F-ATP synthase that take place in the heart under pathological conditions and of their potential role in determining the transition of F-ATP synthase from and energy-conserving into an energy-dissipating device.

Triacylglycerol-induced impairment in mitochondrial biogenesis and function in J774.2 and mouse peritoneal macrophage foam cells

The aim of this study was to detect mitochondrial alterations in J774.2 macrophages and mouse peritoneal macrophages (MPM) foam cells. J774.2 and MPM cells were exposed to triacylglycerol (TG) emulsion (1 mg/ml) for induction of fat accumulation. Impairment of mitochondrial function was reflected by reduced cellular ATP production and decreased expression of subunits of mitochondrial complexes I and III. The expression of subunit IV of complex IV remained unchanged, however, the content of its precursor in cells increased. Inhibitors of mitochondrial complexes, rotenone (0.1 microM) and myxothiazol (25 nM), protected the viability in TG-loaded macrophages. The exposure to TG caused downregulation of PPARgamma coactivator (PGC)-1alpha and nuclear respiratory factor (NRF)-1. Activation of peroxisome proliferator-activated receptors attenuated reactive oxygen species production in the foam cells. Treatment with antioxidant N-acetylcysteine (NAC) prevented lipid-mediated mitochondrial and cellular damage. In conclusion, this study demonstrates the important role of mitochondrial biogenesis dysfunction in TG-induced lipotoxicity in macrophages.

Regulation of vascular smooth muscle cell bioenergetic function by protein glutathiolation

Protein thiolation by glutathione is a reversible and regulated post-translational modification that is increased in response to oxidants and nitric oxide. Because many mitochondrial enzymes contain critical thiol residues, it has been hypothesized that thiolation reactions regulate cell metabolism and survival. However, it has been difficult to differentiate the biological effects due to protein thiolation from other oxidative protein modifications. In this study, we used diamide to titrate protein glutathiolation and examined its impact on glycolysis, mitochondrial function, and cell death in rat aortic smooth muscle cells. Treatment of cells with diamide increased protein glutathiolation in a concentration-dependent manner and had comparably little effect on protein-protein disulfide formation. Diamide increased mitochondrial proton leak and decreased ATP-linked mitochondrial oxygen consumption and cellular bioenergetic reserve capacity. Concentrations of diamide above 200 microM promoted acute bioenergetic failure and caused cell death, whereas lower concentrations of diamide led to a prolonged increase in glycolytic flux and were not associated with loss of cell viability. Depletion of glutathione using buthionine sulfoximine had no effect on basal protein thiolation or cellular bioenergetics but decreased diamide-induced protein glutathiolation and sensitized the cells to bioenergetic dysfunction and death. The effects of diamide on cell metabolism and viability were fully reversible upon addition of dithiothreitol. These data suggest that protein thiolation modulates key metabolic processes in both the mitochondria and cytosol.

ATP synthase and the actions of inhibitors utilized to study its roles in human health, disease, and other scientific areas

ATP synthase, a double-motor enzyme, plays various roles in the cell, participating not only in ATP synthesis but in ATP hydrolysis-dependent processes and in the regulation of a proton gradient across some membrane-dependent systems. Recent studies of ATP synthase as a potential molecular target for the treatment of some human diseases have displayed promising results, and this enzyme is now emerging as an attractive molecular target for the development of new therapies for a variety of diseases. Significantly, ATP synthase, because of its complex structure, is inhibited by a number of different inhibitors and provides diverse possibilities in the development of new ATP synthase-directed agents. In this review, we classify over 250 natural and synthetic inhibitors of ATP synthase reported to date and present their inhibitory sites and their known or proposed modes of action. The rich source of ATP synthase inhibitors and their known or purported sites of action presented in this review should provide valuable insights into their applications as potential scaffolds for new therapeutics for human and animal diseases as well as for the discovery of new pesticides and herbicides to help protect the world's food supply. Finally, as ATP synthase is now known to consist of two unique nanomotors involved in making ATP from ADP and P(i), the information provided in this review may greatly assist those investigators entering the emerging field of nanotechnology.

Effect of antioxidant diets on mitochondrial gene expression in rat brain during aging

Age-related increase of reactive oxygen species (ROS) is particularly detrimental in postmitotic tissues. Calorie restriction (CR) has been shown to exert beneficial effects, consistent with reduced ROS generation by mitochondria. Many antioxidant compounds also mimic such effects. N-acetyl cysteine (NAC) provides thiol groups to glutathione and to mitochondrial respiratory chain proteins; thus, it may counteract both ROS generation and effects. In the present study we investigated, in different rat brain areas during aging (6, 12, and 28 months), the effect of 1-year treatment with CR and dietary supplementation with NAC on the expression of subunit 39 kDa and ND-1 (mitochondrial respiratory complex I), subunit IV (complex IV), subunit alpha of F0F1-ATP synthase (complex V) and of adenine nucleotide translocator, isoform 1 (ANT-1). The observed age-related changes of expression were prevented by the dietary treatments. The present study provides further evidence for the critical role of mitochondria in the aging process.

Modulation of expression of rat mitochondrial 2-oxoglutarate carrier in NRK-52E cells alters mitochondrial transport and accumulation of glutathione and susceptibility to chemically induced apoptosis

We previously showed that two anion carriers of the mitochondrial inner membrane, the dicarboxylate carrier (DIC; Slc25a10) and oxoglutarate carrier (OGC; Slc25a11), transport glutathione (GSH) from cytoplasm into mitochondrial matrix. In the previous study, NRK-52E cells, derived from normal rat kidney proximal tubules, were transfected with the wild-type cDNA for the DIC expressed in rat kidney; DIC transfectants exhibited increased mitochondrial uptake and accumulation of GSH and were markedly protected from chemically induced apoptosis. In the present study, cDNAs for both wild-type (WT) and a double-cysteine mutant of rat OGC (rOGC and rOGC-C221,224S, respectively) were expressed in Escherichia coli, purified, and reconstituted into proteoliposomes to assess their function. Although both WT rOGC and rOGC-C221,224S exhibited transport properties for GSH and 2-oxoglutarate that were similar to those found in mitochondria of rat kidney proximal tubules, rates of transport and mitochondrial accumulation of substrates were reduced by >75% in rOGC-C221,224S compared with the WT carrier. NRK-52E cells were stably transfected with the cDNA for WT-rOGC and exhibited 10- to 20-fold higher GSH transport activity than nontransfected cells and were markedly protected from apoptosis induced by tert-butyl hydroperoxide (tBH) or S-(1,2-dichlorovinyl)-L-cysteine (DCVC). In contrast, cells stably transfected with the cDNA for rOGC-C221,224S were not protected from tBH- or DCVC-induced apoptosis. These results provide further evidence that genetic manipulation of mitochondrial GSH transporter expression alters mitochondrial and cellular GSH status, resulting in markedly altered susceptibility to chemically induced apoptosis.

The characterization of mitochondrial permeability transition in clonal pancreatic beta-cells. Multiple modes and regulation

Mitochondrial permeability transition (MPT), which contributes substantially to the regulation of normal mitochondrial metabolism, also plays a crucial role in the initiation of cell death. It is known that MPT is regulated in a tissue-specific manner. The importance of MPT in the pancreatic beta-cell is heightened by the fact that mitochondrial bioenergetics serve as the main glucose-sensing regulator and energy source for insulin secretion. In the present study, using MIN6 and INS-1 beta-cells, we revealed that both Ca(2+)-phosphate- and oxidant-induced MPT is remarkably different from other tissues. Ca(2+)-phosphate-induced transition is accompanied by a decline in mitochondrial reactive oxygen species production related to a significant potential dependence of reactive oxygen species formation in beta-cell mitochondria. Hydroperoxides, which are indirect MPT co-inducers active in liver and heart mitochondria, are inefficient in beta-cell mitochondria, due to the low mitochondrial ability to metabolize them. Direct cross-linking of mitochondrial thiols in pancreatic beta-cells induces the opening of a low conductance ion permeability of the mitochondrial membrane instead of the full scale MPT opening typical for liver mitochondria. Low conductance MPT is independent of both endogenous and exogenous Ca(2+), suggesting a novel type of nonclassical MPT in beta-cells. It results in the conversion of electrical transmembrane potential into DeltapH instead of a decrease in total protonmotive force, thus mitochondrial respiration remains in a controlled state. Both Ca(2+)- and oxidant-induced MPTs are phosphate-dependent and, through the "phosphate flush" (associated with stimulation of insulin secretion), are expected to participate in the regulation in beta-cell glucose-sensing and secretory activity.

ADP- and oligomycin-sensitive redox behavior of F0 b thiol in ATPsynthase depends on neighbored primary structure: investigations using 14-C-labeled alpha lipoic acid

Purified ATPsynthase of bovine heart mitochondria has been analyzed for its mobility and reactivity of oligomycin-sensitive sulfhydryl regions in presence of the substrate ADP and oligomycin. Labeling of thiol groups at the hydrophobic F_0 region of the ATPsynthase was increased in the enzyme initially treated with SDS, N-ethylmaleimide and dithiothreitol (modified enzyme). After dialysis or gel permeation the ATPsynthase was treated with [14C] alpha lipoic acid at a molar ratio of 35-85/1 (lipoic acid/ATPsynthase) corresponding to 4-8.6 nmol/mg protein. Under these conditions, ATPase activity of the native enzyme was significantly decreased. After preincubation with ADP, PAGE of the native, [14C] labeled enzyme revealed an increase of radioactivity at a region of 25 kDa deduced to Cys 197 of subunit b. In the modified enzyme the increase in radioactivity was found at 10 kDa. In this context, the sequence Lys-Cys-Ile around Cys 197 of subunit b suggests excessive reactivity of this thiol, as well as ready reversibility by -SH-S-S- interchange. Therefore, previously observed reaction by thiol reagents and antioxidants from outside the mitochondrion can be interpreted with Cys 197 of F0 b. It accounts for sulfhydryl unmasked by binding of ADP at F1.

Protection of NRK-52E cells, a rat renal proximal tubular cell line, from chemical-induced apoptosis by overexpression of a mitochondrial glutathione transporter

The dicarboxylate carrier (DCC) is one of two carriers responsible for glutathione (GSH) transport into rat kidney mitochondria. The central hypothesis of the present study was that overexpression of this carrier in renal proximal tubular cells increases content of mitochondrial GSH, which in turn can protect these cells from chemical-induced injury. We first cloned the carrier protein and verified its properties. This was accomplished by reverse transcribing total rat kidney RNA and polymerase chain reaction amplification with primers based on the complete cDNA sequence for the mitochondrial DCC protein. DCC was expressed as a His(6)-tagged protein, purified from Escherichia coli inclusion bodies, and reconstituted into proteoliposomes for transport assays. Time- and concentration-dependent uptake of both L-[(3)H-glycyl]GSH and [2-(14)C]malonate was observed with kinetics, substrate specificity, and inhibitor sensitivities similar to those observed in rat kidney proximal tubules. We next transiently transfected NRK-52E cells with the cDNA for rat kidney DCC to overexpress the protein. The presence of the recombinant DCC-His(6) protein was confirmed by immunoblots. Transport of both GSH and malonate into the mitochondrial fraction of transfected cells was enhanced 2.45- to 11.3-fold, compared with that in wild-type cells. Transfected cells exhibited markedly less apoptosis from tert-butyl hydroperoxide or S-(1,2-dichlorovinyl)-L-cysteine than did wild-type cells, validating the central hypothesis and providing us with a valuable and novel tool with which to further study GSH and thiol redox status in renal mitochondria, and the function of GSH transport in regulation of processes such as apoptosis and oxidative phosphorylation.

Bivalent metal ions modulate Cd2+ effects on isolated rat liver mitochondria

We have studied Cd2+-induced effects on mitochondrial respiration and swelling in various media as a function of the [Cd2+] in the presence or absence of different bivalent metal ions or ruthenium red (RR). It was confirmed by monitoring oxygen consumption by isolated rat liver mitochondria that, beginning from 5 microM, Cd2+ decreased both ADP and uncoupler-stimulated respiration and increased their basal respiration when succinate was used as respiratory substrate. At concentrations higher than 5 microM, Cd2+ stimulated ion permeability of the inner mitochondrial membrane, which was monitored in this study by swelling of both nonenergized mitochondria in 125 mM KNO3 or NH4NO3 medium and succinate-energized mitochondria incubated in a medium containing 25 mM K-acetate and 100 mM sucrose. We have found substantial changes in the above-mentioned Cd2+ effects on mitochondria treated in sequence with 100 microM of Ca2+, Sr2+, Mn2+ or Ba2+(Me2+) and 7.5 microM RR, as well as the alterations in Cd2+ action on the uptake of 137Cs+ by succinate-energized mitochondria in the presence or absence of valinomycin in acetate medium (50 mM Tris-acetate and 140 mM sucrose) with or without Ca2+ or RR. The evidence obtained indicate that Ca2+ exhibits a synergestic action on all Cd2+ effects examined, whereas Sr2+ and Mn2+, conversely, are antagonistic. In the presence of RR, the Cd2+ effects on respiration [stimulation of State 4 respiration and inhibition of 2,4-dinitrophenol (DNP)-uncoupled respiration] still exist, but are observed at concentrations of cadmium more than one order higher; the inhibition of State 3 respiration by Cd2+ conversely, takes place under even lower cadmium concentrations than those determined without RR in the medium. In addition, RR added simultaneously with cadmium in the incubation medium prevents any swelling in the nitrate media, but induces an increment both in Cd2+-stimulated swelling and 137Cs+ (analog of K+) uptake in the acetate media. For the first time, we have shown that Cd2+-induced swelling in all media under study is susceptible to cyclosporin A (CSA), a high-potency inhibitor of the mitochondrial permeability transition (PT) pore. The observations are interpreted in terms of a dual effect of cadmium on respiratory chain activity and permeability transition.

Therapeutic potential of N-acetylcysteine in age-related mitochondrial neurodegenerative diseases

Increasing lines of evidence suggest a key role for mitochondrial damage in neurodegenerative diseases. Brain aging, Parkinson's disease, Alzheimer's disease, Huntington's disease and Friedreich's ataxia have been associated with several mitochondrial alterations including impaired oxidative phosphorylation. Mitochondrial impairment can decrease cellular bioenergetic capacity, which will then increase the generation of reactive oxygen species resulting in oxidative damage and programmed cell death. This paper reviews the mechanisms of N-acetylcysteine action at the cellular level, and the possible usefulness of this antioxidant for the treatment of age-associated neurodegenerative diseases. First, this thiol can act as a precursor for glutathione synthesis as well as a stimulator of the cytosolic enzymes involved in glutathione regeneration. Second, N-acetylcysteine can act by direct reaction between its reducing thiol group and reactive oxygen species. Third, it has been shown that N-acetylcysteine can prevent programmed cell death in cultured neuronal cells. And finally, N-acetylcysteine also increases mitochondrial complex I and IV specific activities both in vitro and in vivo in synaptic mitochondrial preparations from aged mice. In view of the above, and because of the ease of its administration and lack of toxicity in humans, the potential usefulness of N-acetylcysteine in the treatment of age-associated mitochondrial neurodegenerative diseases deserves investigation.

Reversible and potent uncoupling of hog gastric (H(+)+K(+))-ATPase by prodigiosins

Prodigiosin, prodigiosin 25-C, and metacycloprodigiosin all strongly inhibited the acidification activity of (H(+)+K(+))-ATPase on membrane vesicles from hog gastric mucosa (IC(50) = 32 to 103 pmol/mg protein). But, the prodigiosins, unlike omeprazole, showed little inhibitory effect on K(+)-dependent ATPase (K(+)-ATPase) activity, although at higher concentrations they inhibited K(+)-ATPase activity with an IC(50) of 1.5 to 3.0 microM. Furthermore, the inhibitory effect of the prodigiosins was rapid and completely reversible unlike that of omeprazole, and the mode of inhibition was non-competitive with respect to ATP. Hog gastric (H(+)+K(+))-ATPase itself showed an absolute requirement of halide (effectively, chloride) for acidification activity. Prodigiosins also showed a chloride requirement for inhibition of vesicular acidification, and quickly reversed the acidification of vesicular pH to neutrality even in the presence of N, N'-dicyclohexylcarbodiimide (DCCD), showing their ionophoric nature of acidification inhibitory activity. In fact, tributyltin chloride (TBT, an OH(-)/Cl(-) exchange ionophore) also inhibited vesicular acidification, but it inhibited K(+)-ATPase activity too. Finally, the prodigiosins inhibited the acid secretion from parietal cells isolated from rabbit gastric mucosa. These results suggest that prodigiosins are potent reversible uncouplers of (H(+)+K(+))-ATPase that inhibit gastric acid secretion.

Inhibition of vascular NADH/NADPH oxidase activity by thiol reagents: lack of correlation with cellular glutathione redox status

Vascular NAD(P)H oxidase activity contributes to oxidative stress. Thiol oxidants inhibit leukocyte NADPH oxidase. To assess the role of reactive thiols on vascular oxidase, rabbit iliac/carotid artery homogenates were incubated with distinct thiol reagents. NAD(P)H-driven enzyme activity, assessed by lucigenin (5 or 250 microM) luminescence, was nearly completely (> 97%) inhibited by the oxidant diamide (1mM) or the alkylator p-chloromercuryphenylsulfonate (pCMPS, 0.5mM). Analogous inhibition was also shown with EPR spectroscopy using DMPO as a spin trap. The oxidant dithionitrobenzoic acid (0.5mM) inhibited NADPH-driven signals by 92% but had no effect on NADH-driven signals. In contrast, the vicinal dithiol ligand phenylarsine oxide (PAO, 1 microM) induced minor nonsignificant inhibition of NADPH-driven activity, but significant stimulation of NADH-triggered signals. The alkylator N-ethyl maleimide (NEM, 0.5mM) or glutathione disulfide (GSSG, 3mM) had no effect with each substrate. Coincubation of N-acetylcysteine (NAC, 3mM) with diamide or pCMPS reversed their inhibitory effects by 30-60%, whereas NAC alone inhibited the oxidase by 52%. Incubation of intact arterial rings with the above reagents disclosed similar results, except that PAO became inhibitor and NAC stimulator of NADH-driven signals. Notably, the cell-impermeant reagent pCMPS was also inhibitory in whole rings, suggesting that reactive thiol(s) affecting oxidase activity are highly accessible. Since lack of oxidase inhibition by NEM or GSSG occurred despite significant cellular glutathione depletion, change in intracellular redox status is not sufficient to account for oxidase inhibition. Moreover, the observed differences between NADPH and NADH-driven oxidase activity point to complex or multiple enzyme forms.

Role of voltage-dependent anion channels in glutathione transport into yeast mitochondria

Glutathione (GSH) is imported into mitochondria from the extra-mitochondrial cytoplasm. Translocation across the inner membrane of mitochondria is thought to occur via the dicarboxylate and 2-oxoglutarate carriers; however, the means by which GSH passes through the outer membrane is unknown. Disruption of the outer membrane of yeast mitochondria using either digitonin or osmotic shock did not alter GSH accumulation as compared with accumulation in intact mitochondria. These results suggested that passage across the outer membrane was not the rate-limiting step in GSH accumulation. Mitochondria isolated from yeast strains with a disruption in the major pore-forming protein of the outer membrane, VDAC1, accumulated GSH to a greater extent than mitochondria isolated from a wild-type strain. Disruption of the gene for VDAC2 did not affect GSH import. Thus, neither VDAC form is essential for GSH translocation into mitochondria, and the participation of another outer membrane channel in GSH import is possible.

N-acetylcysteine elicited increase in complex I activity in synaptic mitochondria from aged mice: implications for treatment of Parkinson's disease

It has been suggested that thiolic groups are essential for complex I activity and other respiratory mitochondrial enzymes. Recent experiments showed that the thiolic antioxidant N-acetylcysteine (NAC) can protect against age-related decrease in complex I activity in mice hepatic mitochondria. The present paper shows that NAC enhances complex I activity in vitro in synaptic mitochondria isolated from old mice. The optimum NAC concentration for maximum complex I activity was 10 mM in old synaptic preparations. Our data suggest that mitochondrial thiolic groups, which are essentials to oxidative phosphorylation, are impaired by aging. Based on the finding of decreased mitochondrial complex I activity in the substantia nigra of patients with Parkinson's disease, we propose that the thiol-containing antioxidant NAC could be beneficial for treatment of the disease.

N-Acetylcysteine delays age-associated memory impairment in mice: role in synaptic mitochondria

Mitochondrial oxidative damage is implicated in brain aging and in age-related neurodegenerative diseases. Since N-acetylcysteine (NAC) has recently been shown to prevent apoptotic death in neuronal cells and protect synaptic mitochondria proteins from oxidative damage in aged mice, we have investigated whether dietary administration of this thiolic antioxidant retards age-related memory loss. At 48 weeks of age, a control female OF-1 mice group was fed standard food pellets and another group received pellets containing 0.3% (w/w) of NAC. After 23 weeks of this diet, the NAC had partially restored the memory deficit associated with aging in mice. Moreover, the lipid peroxide and protein carbonyl contents of the synaptic mitochondria were significantly decreased in the NAC-supplemented animals in comparison with their age-matched controls. The antioxidant properties and probable action on mitochondrial bioenergetic ability in the synaptic terminals may explain, at least partially, the beneficial action of NAC administration.

Enrichment and functional reconstitution of glutathione transport activity from rabbit kidney mitochondria: further evidence for the role of the dicarboxylate and 2-oxoglutarate carriers in mitochondrial glutathione transport

In previous studies, we provided evidence for uptake of glutathione (GSH) by the dicarboxylate and the 2-oxoglutarate carriers in rat kidney mitochondria. To investigate further the role of these two carriers, GSH transport activity was enriched from rabbit kidney mitochondria and functionally reconstituted into phospholipid vesicles. Starting with 200 mg of mitoplast protein, 2 mg of partially enriched proteins were obtained after Triton X-114 solubilization and hydroxyapatite chromatography. The reconstituted proteoliposomes catalyzed butylmalonate-sensitive uptake of [(14)C]malonate, phenylsuccinate-sensitive uptake of [(14)C]2-oxoglutarate, and transport activity with [(3)H]GSH. The initial rate of uptake of 5 mM GSH was approximately 170 nmol/min per mg protein, with a first-order rate constant of 0.3 min(-1), which is very close to that previously determined in freshly isolated rat kidney mitochondria. The enrichment procedure resulted in an approximately 60-fold increase in the specific activity of GSH transport. Substrates and inhibitors for the dicarboxylate and the 2-oxoglutarate carriers (i.e., malate, malonate, 2-oxoglutarate, butylmalonate, phenylsuccinate) significantly inhibited the uptake of [(3)H]GSH, whereas most substrates for the tricarboxylate and monocarboxylate carriers had no effect. GSH uptake exhibited an apparent K(m) of 2.8 mM and a V(max) of 260 nmol/min per mg protein. Analysis of mutual inhibition between GSH and the dicarboxylates suggested that the dicarboxylate carrier contributes a somewhat higher proportion to overall GSH uptake and that both carriers account for 70 to 80% of total GSH uptake. These results provide further evidence for the function of the dicarboxylate and 2-oxoglutarate carriers in the mitochondrial transport of GSH.

N-acetylcysteine elicited increase in cytochrome c oxidase activity in mice synaptic mitochondria

It has been suggested that thiolic groups are essential for cytochrome c oxidase (COX) activity and other respiratory mitochondrial enzymes. Recent experiments showed that the thiolic antioxidant N-acetylcysteine (NAC) can protect against age-related impairment in COX activity in mice hepatic mitochondria. The present paper shows that NAC enhances COX activity in vitro in synaptic mitochondria isolated from young and old mice. The optimum NAC concentration for maximum COX activity was 5 mM in young and 10 mM in old synaptic preparations. Our data suggest that mitochondrial thiolic groups, which are essentials to oxidative phosphorylation, are impaired by aging.

Phenylarsine oxide inhibits phosphate uptake in human ciliary non-pigmented epithelial cells

Phenylarsine oxide (PAO), a sulfhydryl modifying reagent and a widely used inhibitor for tyrosine phosphatases and endocytosis, was tested on the level of phosphorylation in human nonpigmented ciliary epithelial ocular (HNPE) cells. Pretreatment with (PAO, 10 microM) for 30 min followed by incubation with 32Pi to stimulate endogenous phosphorylation surprisingly resulted in a total reduction in 32Pi labeled proteins. PAO (10-50 microM) dose-dependently inhibited both sodium-dependent and -independent phosphate uptake in cells. p-Hydroxymercuribenzoate (pHMB, 10 microM), another sulfhydryl modifying reagent failed to mimic PAO effects. However, metabolic inhibitors (iodoacetamide (0.1 mM) and 2,4-dinitrophenol (DNP, 0.5 mM) also mimicked PAO effects, suggesting that the inhibition of ATP production may be responsible for attenuation of both phosphate uptake mechanisms. However, sodium-dependent phosphate uptake in isolated plasma membrane vesicles pretreated with PAO was also significantly lower than control vesicles treated with dimethlysulfoxide (DMSO), suggesting that PAO may be directly targeting a component of the sodium-dependent cotransporter. It is suggested that PAO is a novel inhibitor of phosphate uptake in HNPE cells that acts indirectly by inhibiting ATP production and directly by inhibiting the Na-dependent cotransporter.

Triphenyltin as an inhibitor of membrane-bound pyrophosphatase of Rhodospirillum rubrum

The effect of triphenyltin on the activity of membrane-bound pyrophosphatase of Rhodospirillum rubrum was investigated. Triphenyltin inhibits the hydrolysis of chromatophore membrane-bound pyrophosphatase in a pH-dependent pattern, being maximal at pH 9-10. At basic pH values, the inhibition produced by this organotin on membrane-bound pyrophosphatase is very similar to that produced on the chromatophore H+ATPase (I50 = 14.4 and 10 microM, respectively). Detergent-solubilized membrane-bound pyrophosphatase is also inhibited by triphenyltin, but the cytoplasmic enzyme of R. rubrum is inhibited only slightly. The inhibitory effect of triphenyltin on membrane-bound pyrophosphatase is the same with Mg-PPi or Zn-PPi, and is dependent on the chromatophore membrane concentration. Triphenyltin modified mainly the Vmax of the enzyme, and only slightly its Km. Free Mg2+ does not reverse the inhibition. Reducing agents prevent triphenyltin inhibition of the membrane-bound pyrophosphatase, but their effect is due to an alteration of the inhibitor, and not to a modification of thiol groups of the enzyme. The most likely site for triphenyltin inhibition in chromatophore membrane-bound pyrophosphatase is a component either within or closely associated with the membrane.

Substrate-dependent cadmium toxicity affecting energy-linked K+/86Rb transport in Staphylococcus aureus

Bacteria accumulate high amounts of potassium in the cytoplasm. For studying transport of K+ (with 86Rb as a marker) in bacteria (Staphylococcus aureus 17810S), the cells were depleted of the internal K+ pool by a DNP treatment. Kinetics and energetics of 86Rb transport was assayed with glucose as an exogenous energy source. It was shown that 86Rb uptake proceeded via a low affinity K+ transport system with an apparent K(m) of 2.3 mmol/L Rb+. Studies with the lipophilic cation TPP+ (tetraphenylphosphonium), the protonophore CCCP (carbonyl cyanide 3-chlorophenylhydrazone) and inhibitors (HQNO--2-heptyl-4-hydroxyquinoline N-oxide; iodoacetate) indicated that 86Rb transport was driven by delta psi (membrane potential) generated via the respiratory chain. The effect of Cd2+ on 86Rb transport was assayed with two energy donors--glucose and L-lactate. It was found that Cd2+ strongly inhibited delta psi-dependent 86Rb transport energized by cadmium-sensitive glucose oxidation, but was not toxic when cadmium-insensitive L-lactate was used as an energy source. The mechanism of these differential, substrate-dependent effects of Cd2+ on 86Rb transport is discussed.

N-acetylcysteine protects against age-related increase in oxidized proteins in mouse synaptic mitochondria

Since it has been proposed that oxidized protein accumulation plays a critical role in brain aging, we have investigated the effect of a thiolic antioxidant on protein carbonyl content in synaptic mitochondria from female OF-1 mice. At 48 weeks of age, a control group was fed standard food pellets and another group received pellets containing 0.3% (w/w) of N-acetylcysteine. A 24-week treatment resulted in a significant decrease in protein carbonyl content in synaptic mitochondria of the N-acetylcysteine-treated animals as compared to age-matched controls.

N-acetylcysteine protects against age-related decline of oxidative phosphorylation in liver mitochondria

Since it has been proposed that oxygen radical inactivation of key enzymes plays a critical role in cell aging, we have investigated the effects of a thiolic free radical scavenger on the oxidative phosphorylation enzymes of liver mitochondria from female OF-1 mice. At 48 weeks of age a control group was fed standard food pellets and another group received pellets containing 0.3% (w/w) of N-acetylcysteine. A 24-week treatment resulted in a significant increase in the specific activities of complex I, IV and V in the hepatic mitochondria of the N-acetylcysteine-treated animals as compared to aged controls.

ATP synthase complex. Proximities of subunits in bovine submitochondrial particles

The catalytic sector, F1, and the membrane sector, F0, of the mitochondrial ATP synthase complex are joined together by a 45-A-long stalk. Knowledge of the composition and structure of the stalk is crucial to investigating the mechanism of conformational energy transfer between F0 and F1. This paper reports on the near neighbor relationships of the stalk subunits with one another and with the subunits of F1 and F0, as revealed by cross-linking experiments. The preparations subjected to cross-linking were bovine heart submitochondrial particles (SMP) and F1-deficient SMP. The cross-linkers were three reagents of different chemical specificities and different lengths of cross-linking from zero to 10 A. Cross-linked products were identified after gel electrophoresis of the particles and immunoblotting with subunit-specific antibodies to the individual subunits alpha, beta, gamma, delta, OSCP, F6, A6L, a (subunit 6), b, c, and d. The results suggested that the two b subunits form the principal stem of the stalk to which OSCP, d, and F6 are bound independent of one another. Subunits b, OSCP, d, and F6 cross-linked to alpha and/or beta, but not to gamma or delta. The COOH-terminal half of A6L, which is extramembranous, cross-linked to d but not to any other stalk or F1 subunit. No cross-links of subunits a and c with any stalk or F1 subunits were detected. In F1-deficient SMP, cross-linked b+b and d+F6 dimers appeared, and the extent of cross-linking between b and OSCP diminished greatly. The addition of F1 to F1-deficient particles appeared to reverse these changes. Treatment of F1-deficient particles with trypsin rapidly hydrolyzed away OSCP and F6, fragmented b to membrane-bound 18-, 12-, and 8-9-kDa antigenic fragments, which cross-linked to d and/or with one another. Trypsin also removed the COOH-terminal part of A6L, but the remainder still cross-linked to subunit d. Models showing the near neighbor relationships of the stalk subunits with one another and with the alpha and beta subunits at a level near the proximal end (bottom) of F1 and at the membrane-matrix interface are presented.

A cysteine in the C-terminal region of alanyl-tRNA synthetase is important for aminoacylation activity

Alanyl-tRNA synthetase (AlaRS) from Escherichia coli is a multimeric enzyme that catalyzes the esterification of alanine to tRNA(Ala) in the ATP-dependent aminoacylation reaction. The functional binding of all three substrates follows Michaelis-Menten kinetics. The role of cysteines in this enzyme has been evaluated via modification of these residues with p-(hydroxymercuri)phenylsulfonic acid, monobromobimane, and 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB). The former two reagents induce nearly complete inactivation of AlaRS aminoacylation activity and the release of all tightly bound zinc. In the case of mild DTNB treatment, only two of the six cysteines in AlaRS are modified, with release of all zinc and partial loss of aminoacylation activity. These experiments indicate the importance of one or more cysteines, other than those thought to be coordinated with zinc, in the aminoacylation reaction. Substitution of each of the cysteine residues outside the zinc-binding motif with serine does not disrupt zinc binding. However, the cysteine most removed in primary sequence from the active site (Cys665) is identified as important in the aminoacylation step. Mutation of Cys665 to serine induces a 120-fold decrease in the catalytic efficiency of this enzyme, primarily through a kcat effect, and introduces sigmoidal kinetics (nH = 1.8) with respect to the RNA substrate. The results demonstrate that a simple manipulation in the C-terminal region can introduce positive cooperativity in this otherwise noncooperative enzyme.

Deleterious effects of disulfiram on the respiratory electron transport system of liver mitochondria

1. The mechanism of action of disulfiram on the respiratory electron transport system of the liver mitochondria was studied in vitro. 2. Disulfiram inhibited the respiration supported by malate-glutamate as well as succinate. 3. Mitochondrial respiration inhibition was dependent upon alteration of -SH groups. 4. The inhibitory action of disulfiram might be related to the crosslinking of several proteins of the inner mitochondrial membrane. 5. The effects described above could be attributed to disulfiram per se and not to the main metabolite diethyldithiocarbamate.

Role of F0 and F1 subunits in the gating and coupling function of mitochondrial H(+)-ATP synthase. The effect of dithiol reagents

A study is presented on the role of F0 and F1 subunits in oligomycin-sensitive H+ conduction and energy transfer reactions of bovine heart mitochondrial F0F1 H(+)-ATP synthase. Mild treatment with azodicarboxylic acid bis(dimethylamide) (diamide) enhanced oligomycin-sensitive H+ conduction in submitochondrial particles containing F1 attached to F0. This effect was associated with stimulation of the ATPase activity, with no effect on its inhibition by oligomycin, and depression of the 32Pi-ATP exchange. The stimulatory effect of diamide on H+ conduction decreased in particles from which F1 subunits were partially removed by urea. The stimulatory effect exerted by diamide in the submitochondrial particles with F1 attached to F0 was directly correlated with a decrease of the original electrophoretic bands of a subunit of F0 (F0I-PVP protein) and the gamma subunit of F1, with corresponding formation of their cross-linking product. In F0 liposomes, devoid of gamma subunit, diamide failed to stimulate H+ conduction and to cause disappearance of F0I-PVP protein, unless purified gamma subunit was added back. The addition to F0 liposomes of gamma subunit, but not that of alpha and beta subunits, caused per se inhibition of H+ conduction. It is concluded that F0I-PVP and gamma subunits are directly involved in the gate of the F0F1 H(+)-ATP synthase. Data are also presented indicating contribution to the gate of oligomycin-sensitivity conferral protein and of another protein subunit of F0, F6.

Uptake of glutathione by renal cortical mitochondria

Transport of GSH into renal cortical mitochondria was studied. Mitochondria were highly enriched with little contamination from other subcellular organelles (as assessed by marker enzymes), they exhibited coupled respiration (respiratory control ratio greater than 3.0), and they had initial GSH concentrations of 5.71 +/- 0.65 nmol/mg protein (n = 47). Incubation of mitochondria with GSH in a triethanolamine, pH 7.4, buffer containing sucrose, potassium phosphate, MgCl2, and KCl, produced time- and concentration-dependent increases in intramitochondrial GSH content. Uptake was linear versus time for at least 2 min and exhibited kinetics consistent with one low-affinity, high-capacity process (Km = 1.3 mM, Vmax = 5.59 nmol/min per mg protein), although the results cannot exclude the presence of other, less quantitatively significant pathways. The initial rate of uptake of 5 mM GSH was not significantly altered by uncouplers (0.1 mM 2,4-dinitrophenol and 25 microM carbonyl cyanide m-chlorophenylhydrazone) or by 1 mM ADP. In contrast, incubation with 1 mM ATP, 1 mM KCN, 0.1 mM or 1 mM CaCl2 inhibited uptake by 41, 39, 43, or 55%, respectively. GSH uptake was markedly inhibited by gamma-glutamylglutamate and by a series of S-alkyl GSH derivatives. Strong interactions (i.e., both cis and trans effects) were observed with other dicarboxylates (i.e., succinate, malate, glutamate) but not with monocarboxylates (i.e., lactate, pyruvate). Preincubation of mitochondria with GSH protected against tert-butyl hydroperoxide- or methyl vinyl ketone-induced inhibition of state 3 respiration. These results demonstrate uptake of GSH into renal cortical mitochondria that appears to involve electroneutral countertransport (exchange) with other dicarboxylates. Functionally, GSH uptake into mitochondria can protect these organelles from various forms of injury, such as oxidative stress.

Animal and plant mitochondria contain specific thioredoxins

Thioredoxins have been purified from pig heart and potato tuber mitochondria which differ in chromatographic behaviour, enzyme activating capacity, and slightly higher molecular mass (Mr = 12,500) from the major thioredoxin(s) present in mitochondria-free fractions of the same tissue. Both mt-thioredoxins can serve as hydrogen donor for E. coli ribonucleotide reductase but only the plant protein activates spinach chloroplast NADP malate dehydrogenase in vitro. Mitochondrial target enzymes specifically activated by thioredoxin have not as yet been identified.

Careful consideration of the effects induced by glutathione depletion in rat liver and heart. The involvement of cytosolic and mitochondrial glutathione pools

One of the most widely used mechanisms by which the role of glutathione (GSH) in cellular functions has been withdrawn, is to deplete GSH intracellularly. The importance of the procedure and xenobiotic chosen to get it is discussed. Mitochondrial GSH plays certainly an important role in maintaining cellular homeostasis. This contribution varies depending on the tissue and the conclusions obtained about the functions of this GSH pool in one organ may not be applied to others. Original data on the subcellular distribution of GSH in myocardial tissue of the rat are presented, and the effect of phorone on both cardiac GSH pools is compared with the effect in liver. The mechanical failure of myocardium after ischemic or reperfusion damage might involve mitochondrial GSH, in view of the literature data referring to the role of thiol groups in energy transfer from mitochondria to cytosol.

Inhibition of oxidative phosphorylation by organotin thiocarbamates

A series of triphenyl-, tricyclohexyl- and tribenzyltin compounds have been synthesized and examined as inhibitors of mitochondrial oxidative phosphorylation. All compounds tested inhibit oxidative phosphorylation linked to succinate oxidation by potato tuber mitochondria. All of the organotin compounds inhibit ADP-stimulated O2 uptake linked to succinate oxidation with concentrations for 50% inhibition in the range 2-50 microM. This inhibition is not due to inhibition of electron transport from succinate to O2 per se: none of the organotin compounds at 50 microM substantially inhibit the rate of succinate oxidation in the presence of 2,4-dinitrophenol. Representative organotin compounds at 0.5-50 microM do not act as uncouplers of succinate oxidation. It is concluded that the organotin compounds act as energy transfer inhibitors to inhibit oxidative phosphorylation in potato tuber mitochondria. A similar mode of action of representative organotin compounds was found with rat liver mitochondria. These organotin compounds inhibit a hydrophobic Ca2+-dependent plant protein kinase in the absence but not in the presence of thiols.

Thiols in oxidative phosphorylation: thiols in the F0 of ATP synthase essential for ATPase activity

It was shown previously that the ATP synthase complex of bovine heart mitochondria contains an essential set of thiols or dithiols in its membrane sector (F0), whose modification by various reagents results in uncoupling [Yagi, T., and Hatefi, Y. (1984) Biochemistry 23, 2449-2455]. The sensitivity to modifiers was increased by membrane energization, and the uncoupling was reversed by membrane-permeable thiol compounds when modifiers other than alkylating agents were used to uncouple. The present paper demonstrates that there exists in the F0 of bovine ATP synthase another set of essential thiols, whose modification results in reversible inhibition of ATPase activity. These thiols are most susceptible to modification by mercurials (p-chloromercuribenzoate greater than p-chloromercuribenzene sulfonate) and do not appear to be modified by N-ethylmaleimide. The reversible modification of these thiols by mercurials protects the ATP synthase against irreversible inhibition in F0 by N,N-dicyclohexylcarbodiimide. The possible location of these two sets of thiols in the F0 of bovine ATP synthase is discussed.

Evidence for a role of a vicinal dithiol in catalysis and proton pumping in mitochondrial nicotinamide nucleotide transhydrogenase

The effect of glutathione, glutathione disulfide and the dithiol reagent phenylarsine oxide on purified soluble as well as reconstituted mitochondrial nicotinamide nucleotide transhydrogenase from beef heart was investigated. Glutathione disulfide and phenylarsine oxide caused an inhibition of transhydrogenase, the extent of which was dependent on the presence of either of the transhydrogenase substrates. In the absence of NADPH glutathione protected partially against inactivation by glutathione disulfide and phenylarsine oxide. In the presence of NADPH glutathione also inhibited transhydrogenase. Reconstituted transhydrogenase vesicles behaved differently as compared to the soluble transhydrogenase and was partially uncoupled by GSSG. It is concluded that transhydrogenase contains a dithiol that is essential for catalysis as well as for proton translocation.