The mechanism of disulphide reduction by mitochondria

Redox-Driven Signaling: 2-Oxo Acid Dehydrogenase Complexes as Sensors and Transmitters of Metabolic Imbalance

SIGNIFICANCE

This article develops a holistic view on production of reactive oxygen species (ROS) by 2-oxo acid dehydrogenase complexes. Recent Advances: Catalytic and structural properties of the complexes and their components evolved to minimize damaging effects of side reactions, including ROS generation, simultaneously exploiting the reactions for homeostatic signaling.

CRITICAL ISSUES

Side reactions of the complexes, characterized in vitro, are analyzed in view of protein interactions and conditions in vivo. Quantitative data support prevalence of the forward 2-oxo acid oxidation over the backward NADH oxidation in feeding physiologically significant ROS production by the complexes. Special focus on interactions between the active sites within 2-oxo acid dehydrogenase complexes highlights the central relevance of the complex-bound thiyl radicals in regulation of and signaling by complex-generated ROS. The thiyl radicals arise when dihydrolipoyl residues of the complexes regenerate FADH₂ from the flavin semiquinone coproduced with superoxide anion radical in 1e^- oxidation of FADH₂ by molecular oxygen.

FUTURE DIRECTIONS

Interaction of 2-oxo acid dehydrogenase complexes with thioredoxins (TRXs), peroxiredoxins, and glutaredoxins mediates scavenging of the thiyl radicals and ROS generated by the complexes, underlying signaling of disproportional availability of 2-oxo acids, CoA, and NAD⁺ in key metabolic branch points through thiol/disulfide exchange and medically important hypoxia-inducible factor, mammalian target of rapamycin (mTOR), poly (ADP-ribose) polymerase, and sirtuins. High reactivity of the coproduced ROS and thiyl radicals to iron/sulfur clusters and nitric oxide, peroxynitrite reductase activity of peroxiredoxins and transnitrosylating function of thioredoxin, implicate the side reactions of 2-oxo acid dehydrogenase complexes in nitric oxide-dependent signaling and damage.

A novel modified carbon paste electrode based on NiO/CNTs nanocomposite and (9, 10-dihydro-9, 10-ethanoanthracene-11, 12-dicarboximido)-4-ethylbenzene-1, 2-diol as a mediator for simultaneous determination of cysteamine, nicotinamide adenine dinucleotide and folic acid

A carbon paste electrode (CPE) modified with (9, 10-dihydro-9, 10-ethanoanthracene-11, 12-dicarboximido)-4-ethylbenzene-1, 2-diol (DEDE) and NiO/CNTs nanocomposite was used for the sensitive voltammetric determination of cysteamine (CA), nicotinamide adenine dinucleotide (NADH) and folic acid (FA) for the first time. The synthesized materials were characterized with different methods such as XRD, cyclic voltammetry, electrochemical impedance spectroscopy (EIS) and square wave voltammetry (SWV). The modified electrode exhibited a potent and persistent electron mediating behavior followed by well-separated oxidation peaks of CA, NADH and FA. The peak currents were linearly dependent on CA, NADH and FA concentrations using square wave voltammetry (SWV) method in the ranges of 0.01-250, 1.0-500, and 3.0-550 µmol L⁻¹, with detection limits of 0.007, 0.6, and 0.9 µmol L⁻¹, respectively. The modified electrode was used for the determination of CA, NADH and FA in biological and pharmaceutical samples.

2-Oxo acid dehydrogenase complexes in redox regulation

A number of cellular systems cooperate in redox regulation, providing metabolic responses according to changes in the oxidation (or reduction) of the redox active components of a cell. Key systems of central metabolism, such as the 2-oxo acid dehydrogenase complexes, are important participants in redox regulation, because their function is controlled by the NADH/NAD+ ratio and the complex-bound dihydrolipoate/lipoate ratio. Redox state of the complex-bound lipoate is an indicator of the availability of the reaction substrates (2-oxo acid, CoA and NAD+) and thiol-disulfide status of the medium. Accumulation of the dihydrolipoate intermediate causes inactivation of the first enzyme of the complexes. With the mammalian pyruvate dehydrogenase, the phosphorylation system is involved in the lipoate-dependent regulation, whereas mammalian 2-oxoglutarate dehydrogenase exhibits a higher sensitivity to direct regulation by the complex-bound dihydrolipoate/lipoate and external SH/S-S, including mitochondrial thioredoxin. Thioredoxin efficiently protects the complexes from self-inactivation during catalysis at low NAD+. As a result, 2-oxoglutarate dehydrogenase complex may provide succinyl-CoA for phosphorylation of GDP and ADP under conditions of restricted NAD+ availability. This may be essential upon accumulation of NADH and exhaustion of the pyridine nucleotide pool. Concomitantly, thioredoxin stimulates the complex-bound dihydrolipoate-dependent production of reactive oxygen species. It is suggested that this side-effect of the 2-oxo acid oxidation at low NAD+in vivo would be overcome by cooperation of mitochondrial thioredoxin and the thioredoxin-dependent peroxidase, SP-22.

Alloxan-induced mitochondrial permeability transition triggered by calcium, thiol oxidation, and matrix ATP

In addition to their critical function in energy metabolism, mitochondria contain a permeability transition pore, which is regulated by adenine nucleotides. We investigated conditions required for ATP to induce a permeability transition in mammalian mitochondria. Mitochondrial swelling associated with mitochondria permeability transition (MPT) was initiated by adding succinate to a rat liver mitochondrial suspension containing alloxan, a diabetogenic agent. If alloxan was added immediately with or 5 min after adding succinate, MPT was strikingly decreased. MPT induced by alloxan was inhibited by EGTA and several agents causing thiol oxidation, suggesting that alloxan leads to permeability transition through a mechanism dependent on Ca(2+) uptake and sulfhydryl oxidation. Antimycin A and cyanide, inhibitors of electron transfer, carbonyl cyanide m-chlorophenylhydrazone, and oligomycin all inhibited MPT. During incubation with succinate, alloxan depleted ATP in mitochondria after an initial transient increase. However, in a mitochondrial suspension containing EGTA, ATP significantly increased in the presence of alloxan to a level greater than that of the control. These results suggest the involvement of energized transport of Ca(2+) in the MPT initiation. Addition of exogenous ATP, however, did not trigger MPT in the presence of alloxan and had no effect on MPT induced by alloxan. We conclude that alloxan-induced MPT requires mitochondrial energization, oxidation of protein thiols, and matrix ATP to promote energized uptake of Ca(2+).

Biochemical and clinical factors influencing oral malodor in periodontal patients

The amounts of volatile sulfur compounds (VSC) and methyl mercaptan/hydrogen sulfide ratio in mouth air from patients with periodontal involvement were 8 times greater than those of control subjects. Our studies demonstrated that, in patients with periodontal disease: 1) the concentration of disulfide, which is converted to VSC, increased in proportion to the total pocket depth; 2) 60% of the VSC was produced from the tongue surface; 3) the amount of tongue coating was 4 times greater than in control subjects; and 4) VSC production and the methyl mercaptan/hydrogen sulfide ratio of the tongue coating were increased. 2-Ketobutyrate, which is a byproduct of the metabolism of methionine to methyl mercaptan, was higher in the saliva of patients with periodontal disease. This implies that metabolism of methionine to methyl mercaptan increases in the oral cavity of patients with periodontal pockets. Since free L-methionine, rather than protein, is the main source for methyl mercaptan, we estimated the methionine supply from the gingival fluid into the oral cavity of patients with periodontal involvement. The results showed that the ratio of methionine to whole free amino acids was significantly higher than that of cysteine. Our studies suggest that not only microorganisms, but also the tongue coating and gingival fluid are factors which enhance VSC production in patients with periodontal disease.

Modulation of human gingival fibroblast cell metabolism by methyl mercaptan

Methyl mercaptan (CH3SH) is a malodorous compound whose levels are elevated in mouth and crevicular air of individuals with active periodontal disease. Since it may play a role in the disease process, its effects were evaluated using human gingival fibroblast cultures and viable porcine unkeratinized oral mucosal tissue sections. Results showed that the protein content of CH3SH-exposed cell cultures pulsed with [14C]-labelled glycine and proline was decreased by approximately 25%. Furthermore, this deleterious effect was irreversible in test cultures subsequently incubated for 24 h in a control 95% air/5% CO2 mercaptan-free environment. The supporting slab-gel electrophoresis profiles yielded evidence that exposure to CH3SH caused an alteration in collagen metabolism and a pooling of Type I procollagens. In addition, DNA synthesis was suppressed in CH3SH-exposed cultures by 44.1% at the 24 to 26 h peak of DNA synthesis. This is a true inhibition and not a shift in peak of maximum DNA synthesis as the shape and location of time-course curves of control and test systems is very similar. Proline transport study using [14C]-proline indicated a reduction in proline transport in the range of 40 to 50% in cultures exposed for 24 to 30 h to CH3SH. Significantly even 15 min exposure to 6.7 ng CH3SH/ml of incubating atmosphere suppressed proline transport by approximately 24%. This indicates that even brief exposure to low concentrations of CH3SH has a significant adverse effect on proline transport. Fluorescent staining of tissue sections exposed to mercaptan indicated that the agent elevated the number of cells stained with vital dye.(ABSTRACT TRUNCATED AT 250 WORDS)

Volatile sulfur compounds in mouth air from clinically healthy subjects and patients with periodontal disease

Volatile sulfur compounds (VSC) in mouth air were estimated by gas chromatography. The amount of VSC and the methyl mercaptan/hydrogen sulfide ratio were significantly increased in patients with periodontal disease. These two parameters also increased in proportion to the bleeding index and probing depth. A study was also done on the effect of removal of tongue coating on VSC concentrations in mouth air from patients with periodontal involvement. VSC and the methyl mercaptan/hydrogen sulfide ratio were reduced to 49% and 35%, respectively, by removal of the tongue coating. The average amount of tongue coating removed from patients with periodontal disease was significantly higher than from controls (90.1 mg vs. 14.6 mg, p less than 0.01). Estimated production of VSC from tongue coating was 4 times higher than the control value, and the methyl mercaptan/hydrogen sulfide ratio was also markedly increased. However, a saliva putrefaction study suggested that saliva does not contribute to the elevated ratio of methyl mercaptan in mouth air. These results strongly suggest that, in addition to periodontal pockets, tongue coating has an important role in VSC production, in particular leading to an elevated concentration of methyl mercaptan, which is more pathogenic than hydrogen sulfide.

Studies on the mechanism of toxicity of the mycotoxin, sporidesmin. I. Generation of superoxide radical by sporidesmin

Sporidesmin (SDMS2), the mycotoxin responsible for 'facial eczema' in ruminants, contains a disulphide group which appears to be intimately involved in its toxic action. The reduced (dithiol) form of sporidesmin has been shown readily to undergo autoxidation in vitro in a reaction which generates superoxide radical (O2-). The autoxidation reaction, which takes place over a wide pH range, is strongly catalysed by trace amounts of copper, although the reaction was inhibited at high concentrations of this metal. Inhibition of the autooxidation of reduced sporidesmin (SDM(SH)2) was also observed in the presence of nickel, cobalt and manganese. Superoxide radical is also generated from SDMS2 itself in a cyclic reduction/autoxidation reaction with glutathione and other thiols; in view of the known toxicity of superoxide and its derivatives, it is suggested that oxygen-free-radicals may be involved in the initiation of the deleterious effects of the mycotoxin.

Cysteamine inhibition of [15N]-glycine turnover in cystinosis and of glycine cleavage system in vitro

In order to clarify the hyperglycinemic effect of cysteamine treatment in children with nephropathic cystinosis, we measured [15N]-glycine turnover in three affected patients. Administration of cysteamine lowered the glycine flux and the glycine metabolic clearance rate but did not alter the glycine pool size. Formation of [15N]-serine from [15N]-glycine was lower in untreated patients than in control subjects and was reduced still further by cysteamine. Studies in vitro with isolated rat liver mitochondria and acetone extracts of mitochondria indicated that even low cysteamine concentrations (0.1 mM) inhibited the glycine cleavage system in both the direction of glycine oxidation and glycine synthesis. Cysteamine was a more potent inhibitor of the glycine cleavage system than any other sulfhydryl containing compound. Although no ill effects of cysteamine treatment were immediately apparent, patients receiving cysteamine should be monitored carefully for the appearance of any neurologic symptoms which might be referable to inhibition of the glycine cleavage system.

Characterization of volatile sulphur production by pathogenic and non-pathogenic strains of oral Bacteroides

Marked differences were observed in intermediate sulphur metabolism between non-pathogenic strains of Bacteroides melaninogenicus var melaninogenicus (CP-) and pathogenic Bacteroides melaninogenicus asaccharolyticus (CP+). The CP+ strains, which produced collagenase and protease and caused formation of abscesses when injected subcutaneously into groins of guinea pigs, produced copious amounts of volatile sulphur compounds (VSC) which consisted predominantly of CH3SH and (CH3S)2. Hydrogen sulphide occurred in considerably lesser amounts. CP+ cultures yielded 8-fold more total volatile S, 15-fold more CH3SH and 260-fold more (CH3S)2 during 24 h of incubation in trypticase-yeast extract medium. Whereas H2S accounted for 60 per cent of the total volatile S content of the head-space of CP- cultures, it represented only 8 per cent of the volatile S in CP + systems. Although the CP-organisms did not grow as well as CP +, the differences in concentration of VSC may be only partly related to the disparity in growth rates. When the VSC concentrations were calculated on the basis of equivalent optical density of 1.0, the CP + strains still produced over 3-fold more total volatile S, 6-fold more CH3SH and 100-fold more (CH3S)2. A similar allowance for growth rate suggests that CP-strains may possess a greater potential to produce H2S. Both groups metabolized S-containing amino acids and serine, resulting in appreciable increases in H2S production by CP-. However, the two groups appeared to metabolize the carbon moiety of cystine an cysteine by different pathways. The addition of glucose to the medium depressed total volatile S production by both CP+ and CP-strains, attributable mostly to lower H2S levels. Whereas the omission of yeast extract and charcoal treatment of trypticase did not adversely effect the activity of CP+, it further markedly reduced the capacity of CP-cultures to produce VSC. These results suggest that VSC analysis offers a convenient means of assessing strain differences and pathogenic potential of B. melaninogenicus.

Cystinosis. Intracellular cystine depletion by aminothiols in vitro and in vivo

Certain aminothiols rapidly deplete cultured cystinotic skin fibroblasts of their abnormally high free (nonprotein) cystine pool. The free cystine content of these cells if reduced by over 90% in 1 h with 0.1 mM cysteamine. This is more rapid than previously known methods of removing free cystine from cystinotic fibroblasts. The disulfide, cystamine, is also able to deplete cystinotic cells of free cystine. A patient with nephropathic cystinosis and end-stage renal disease was treated with cysteamine, both intravenously and orally. Both methods of administration rapidly lowered the free cystine content of the patient's peripheral leukocytes. Study of the patient's urinary sulfur excretion did not conclusively determine the effect of this therapy on the total body cystine pool. Her renal status remained at end stage after 1 mo of oral cysteamine, when an episode of grand mal seizures prompted cessation of the study. Determination of the proper place of aminothiol therapy in this disease will depend upon further clinical trial with patients whose kidney function has not deteriorated to the point of irreversible change, accompanied by careful monitoring of plasma aminothiol levels.

Effect of 60Co gamma-irradiation on the reaction of mixed disulphides of mercaptoethylguanidine with enzymes of rat-liver cytoplasm

A theoretical assumption of Eldjarn and Pihl suggests that mixed disulphides formed by radioprotective aminothiols and protein SH-groups can be broken down by enzymes in the organism, and the native structure of the macromolecules restored. Irradiation should enhance this effect. In our experiments, mixed disulphides of mercaptoethylguanidine (MEG) and albumin/haemoglobin were split by the soluble enzyme fraction of rat-liver homogenate (cytosol). The liberation of the radioprotector MEG is brought about by small molecules; dialysed cytosol has no effect, nor has the suspension of particles of mitochondria. On irradiation with doses in the 0-1--5-0 Mrad range, the mixed disulphide bridge is stabilized and made more resistant to splitting. Increased resistance up to 700 per cent with albumin-MEG and 160 per cent with haemoglobin (Hb)-MEG mixed disulphide was observed compared with the unirradiated control.

Some properties of mitochondrial glutathione

1. The presence of GSH in rat liver mitochondria is confirmed. GSH diffuses from the suspended particles in the presence of phosphate but respiratory inhibitors inhibit the diffusion. 2. GSH is oxidised in situ by oxidants including t-butyl hydroperoxide. The products formed include GSSG and GSS-protein mixed disulphides. The oxidation occurs at lower oxidant concentrations if phosphate or oxaloacetate are also present. Respiratory inhibitors abolish their effect. 3. With phosphate, the GSSG produced is found chiefly outside the mitochondria whereas with oxaloacetate, it is found chiefly inside. 4. The GSSG formed by the oxidation is reduced by Krebs-cycle acids with the exception of the ketoacids. Exogenous GSSG is reduced by these substrates only after lysis. Intact particles, however, catalyse the reduction of GSSG by either NADH2 or NADPH2.