Loss of SDHB Promotes Dysregulated Iron Homeostasis, Oxidative Stress, and Sensitivity to Ascorbate

Succinate dehydrogenase is a key enzyme in the tricarboxylic acid cycle and the electron transport chain. All four subunits of succinate dehydrogenase are tumor suppressor genes predisposing to paraganglioma, but only mutations in the SDHB subunit are associated with increased risk of metastasis. Here we generated an Sdhd knockout chromaffin cell line and compared it with Sdhb-deficient cells. Both cell types exhibited similar SDH loss of function, metabolic adaptation, and succinate accumulation. In contrast, Sdhb-/- cells showed hallmarks of mesenchymal transition associated with increased DNA hypermethylation and a stronger pseudo-hypoxic phenotype compared with Sdhd-/- cells. Loss of SDHB specifically led to increased oxidative stress associated with dysregulated iron and copper homeostasis in the absence of NRF2 activation. High-dose ascorbate exacerbated the increase in mitochondrial reactive oxygen species, leading to cell death in Sdhb-/- cells. These data establish a mechanism linking oxidative stress to iron homeostasis that specifically occurs in Sdhb-deficient cells and may promote metastasis. They also highlight high-dose ascorbate as a promising therapeutic strategy for SDHB-related cancers. SIGNIFICANCE: Loss of different succinate dehydrogenase subunits can lead to different cell and tumor phenotypes, linking stronger 2-OG-dependent dioxygenases inhibition, iron overload, and ROS accumulation following SDHB mutation.

Succinate dehydrogenase deficiency in a chromaffin cell model retains metabolic fitness through the maintenance of mitochondrial NADH oxidoreductase function

Mutations in succinate dehydrogenase (SDH) lead to the development of tumors in a restricted subset of cell types, including chromaffin cells and paraganglia. The molecular basis for this specificity is currently unknown. We show that loss of SDH activity in a chromaffin cell model does not perturb complex I function, retaining the ability to oxidize NADH within the electron transport chain. This activity supports continued oxidation of substrates within the tricarboxylic acid (TCA) cycle. However, due to the block in the TCA cycle at SDH, the high glutamine oxidation activity is only maintained through an efflux of succinate. We also show that although the mitochondria of SDH-deficient cells are less active per se, their higher mass per cell results in an overall respiratory rate that is comparable with wild-type cells. Finally, we observed that when their mitochondria are uncoupled, SDH-deficient cells are unable to preserve their viability, suggesting that the mitochondrial metabolic network is unable to compensate when exposed to additional stress. We therefore show that in contrast to models of SDH deficiency based on epithelial cells, a chromaffin cell model retains aspects of metabolic "health," which could form the basis of cell specificity of this rare tumor type.

Elevated Endogenous SDHA Drives Pathological Metabolism in Highly Metastatic Uveal Melanoma

Purpose

Metastatic uveal melanoma (UM) has a very poor prognosis and no effective therapy. Despite remarkable advances in treatment of cutaneous melanoma, UM remains recalcitrant to chemotherapy, small-molecule kinase inhibitors, and immune-based therapy.

Methods

We assessed two sets of oxidative phosphorylation (OxPhos) genes within 9858 tumors across 31 cancer types. An OxPhos inhibitor was used to characterize differential metabolic programming of highly metastatic monosomy 3 (M3) UM. Seahorse analysis and global metabolomics profiling were done to identify metabolic vulnerabilities. Analyses of UM TCGA data set were performed to determine expressions of key OxPhos effectors in M3 and non-M3 UM. We used targeted knockdown of succinate dehydrogenase A (SDHA) to determine the role of SDHA in M3 UM in conferring resistance to OxPhos inhibition.

Results

We identified UM to have among the highest median OxPhos levels and showed that M3 UM exhibits a distinct metabolic profile. M3 UM shows markedly low succinate levels and has highly increased levels of SDHA, the enzyme that couples the tricarboxylic acid cycle with OxPhos by oxidizing (lowering) succinate. We showed that SDHA-high M3 UM have elevated expression of key OxPhos molecules, exhibit abundant mitochondrial reserve respiratory capacity, and are resistant to OxPhos antagonism, which can be reversed by SDHA knockdown.

Conclusions

Our study has identified a critical metabolic program within poor prognostic M3 UM. In addition to the heightened mitochondrial functional capacity due to elevated SDHA, M3 UM SDHA-high mediate resistance to therapy that is reversible with targeted treatment.

Impacts of isopyrazam exposure on the development of early-life zebrafish (Danio rerio)

Isopyrazam (IPZ) is a broad spectrum succinate dehydrogenase inhibitor fungicide. Little is known about its potential ecological risks of aquatic organisms recently. The present study examined the embryonic development effects of zebrafish exposed to IPZ under static condition using a fish embryo toxicity test. The lowest observed effect concentration of IPZ was 0.025 mg/L in 4-day exposure. Developmental abnormalities, including edema, small head deformity, body deformation and decreased pigmentation, and mortality were observed in zebrafish embryos of 0.05 mg/L and higher concentrations, which shown concentration dependency. The heart rate of zebrafish was disrupted by IPZ. Moreover, enzyme and gene experiments shown that IPZ exposure caused oxidative stress of zebrafish. Furthermore, it induced a decrease of succinate dehydrogenase (SDH) enzyme activity and gene transcription level in zebrafish larvae. It can be speculated that IPZ may have a lethal effect on zebrafish, which is accompanied by decreased SDH activity, oxidative stress, and abnormality. These results provide toxicological data about the IPZ on aquatic non-target organisms, which could be useful for further understanding potential environmental risks.

Biogenesis and functions of mammalian iron-sulfur proteins in the regulation of iron homeostasis and pivotal metabolic pathways

Fe-S cofactors are composed of iron and inorganic sulfur in various stoichiometries. A complex assembly pathway conducts their initial synthesis and subsequent binding to recipient proteins. In this minireview, we discuss how discovery of the role of the mammalian cytosolic aconitase, known as iron regulatory protein 1 (IRP1), led to the characterization of the function of its Fe-S cluster in sensing and regulating cellular iron homeostasis. Moreover, we present an overview of recent studies that have provided insights into the mechanism of Fe-S cluster transfer to recipient Fe-S proteins.

Cloning and functional analysis of succinate dehydrogenase gene PsSDHA in Phytophthora sojae

Succinate dehydrogenase (SDH) is one of the key enzymes of the tricarboxylic acid cycle (TCA cycle) and a proven target of fungicides for true fungi. To explore the roles of the SDHA gene in Phytophthora sojae, we first cloned PsSDHA to construct the PsSDHA silenced expression vector pHAM34-PsSDHA, and then utilized PEG to mediate the P. sojae protoplast transformation experiment. Through transformation screening, we obtained the silenced mutants A1 and A3, which have significant suppressive effect. Further study showed that the hyphae of the silenced mutant strains were shorter and more bifurcated; the growth of the silenced mutants was clearly inhibited in 10% V8 agar medium containing sodium chloride (NaCl), hydrogen peroxide (H2O2) or Congo Red, respectively. The pathogenicity of the silenced mutants was significantly reduced compared with the wild-type strain and the mock. The results could help us better to understand the position and function of SDH in P. sojae and provide a proven target of fungicides for the oomycete.

Phytochrome-mediated regulation of plant respiration and photorespiration

The expression of genes encoding various enzymes participating in photosynthetic and respiratory metabolism is regulated by light via the phytochrome system. While many photosynthetic, photorespiratory and some respiratory enzymes, such as the rotenone-insensitive NADH and NADPH dehydrogenases and the alternative oxidase, are stimulated by light, succinate dehydrogenase, subunits of the pyruvate dehydrogenase complex, cytochrome oxidase and fumarase are inhibited via the phytochrome mechanism. The effect of light, therefore, imposes limitations on the tricarboxylic acid cycle and on the mitochondrial electron transport coupled to ATP synthesis, while the non-coupled pathways become activated. Phytochrome-mediated regulation of gene expression also creates characteristic distribution patterns of photosynthetic, photorespiratory and respiratory enzymes across the leaf generating different populations of mitochondria, either enriched by glycine decarboxylase (in the upper part) or by succinate dehydrogenase (in the bottom part of the leaf).

Clinical significance of insulin-growth factor 1 and insulin-growth factor 1 receptor expression in gastrointestinal stromal tumors

BACKGROUND/AIMS

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal tumors in the gastrointestinal tract and are mostly driven by KIT and PDGFRA-activation mutations. However, other signaling pathways are involved in pathogenesis and proliferation of GISTs. This study investigates the prognostic significance of insulin-like growth factor 1 (IGF1) and IGF1 receptor (IGF1R) and the role of succinate dehydrogenase subunit B (SDHB) in GISTs.

METHODOLOGY

Immunohistochemistry (IHC) for IGF1, IGF1R and SDHB was performed in total of 165 GISTs.

RESULTS

The overexpression of IGF1 was evident in tumors with high mitotic count, large tumor size and was correlated with high risk of malignant behavior. IGF1R overexpression was correlated with IGF overexpression, high mitotic count and high risk of malignant behavior. Loss of expression for SDHB was found in only 2 gastric GISTs.

CONCLUSIONS

The overexpression of IGF1 and IGF1R can be useful marker to predict relapse and aggressive behavior in GISTs and has prognostic implications.

SDH mutations establish a hypermethylator phenotype in paraganglioma

Paragangliomas are neuroendocrine tumors frequently associated with mutations in RET, NF1, VHL, and succinate dehydrogenase (SDHx) genes. Methylome analysis of a large paraganglioma cohort identified three stable clusters, associated with distinct clinical features and mutational status. SDHx-related tumors displayed a hypermethylator phenotype, associated with downregulation of key genes involved in neuroendocrine differentiation. Succinate accumulation in SDH-deficient mouse chromaffin cells led to DNA hypermethylation by inhibition of 2-OG-dependent histone and DNA demethylases and established a migratory phenotype reversed by decitabine treatment. Epigenetic silencing was particularly severe in SDHB-mutated tumors, potentially explaining their malignancy. Finally, inactivating FH mutations were identified in the only hypermethylated tumor without SDHx mutations. These findings emphasize the interplay between the Krebs cycle, epigenomic changes, and cancer.

A functional description of CymA, an electron-transfer hub supporting anaerobic respiratory flexibility in Shewanella

CymA (tetrahaem cytochrome c) is a member of the NapC/NirT family of quinol dehydrogenases. Essential for the anaerobic respiratory flexibility of shewanellae, CymA transfers electrons from menaquinol to various dedicated systems for the reduction of terminal electron acceptors including fumarate and insoluble minerals of Fe(III). Spectroscopic characterization of CymA from Shewanella oneidensis strain MR-1 identifies three low-spin His/His co-ordinated c-haems and a single high-spin c-haem with His/H(2)O co-ordination lying adjacent to the quinol-binding site. At pH 7, binding of the menaquinol analogue, 2-heptyl-4-hydroxyquinoline-N-oxide, does not alter the mid-point potentials of the high-spin (approximately -240 mV) and low-spin (approximately -110, -190 and -265 mV) haems that appear biased to transfer electrons from the high- to low-spin centres following quinol oxidation. CymA is reduced with menadiol (E(m) = -80 mV) in the presence of NADH (E(m) = -320 mV) and an NADH-menadione (2-methyl-1,4-naphthoquinone) oxidoreductase, but not by menadiol alone. In cytoplasmic membranes reduction of CymA may then require the thermodynamic driving force from NADH, formate or H2 oxidation as the redox poise of the menaquinol pool in isolation is insufficient. Spectroscopic studies suggest that CymA requires a non-haem co-factor for quinol oxidation and that the reduced enzyme forms a 1:1 complex with its redox partner Fcc3 (flavocytochrome c3 fumarate reductase). The implications for CymA supporting the respiratory flexibility of shewanellae are discussed.

Respiration of Escherichia coli in the mouse intestine

Mammals are aerobes that harbor an intestinal ecosystem dominated by large numbers of anaerobic microorganisms. However, the role of oxygen in the intestinal ecosystem is largely unexplored. We used systematic mutational analysis to determine the role of respiratory metabolism in the streptomycin-treated mouse model of intestinal colonization. Here we provide evidence that aerobic respiration is required for commensal and pathogenic Escherichia coli to colonize mice. Our results showed that mutants lacking ATP synthase, which is required for all respiratory energy-conserving metabolism, were eliminated by competition with respiratory-competent wild-type strains. Mutants lacking the high-affinity cytochrome bd oxidase, which is used when oxygen tensions are low, also failed to colonize. However, the low-affinity cytochrome bo(3) oxidase, which is used when oxygen tension is high, was found not to be necessary for colonization. Mutants lacking either nitrate reductase or fumarate reductase also had major colonization defects. The results showed that the entire E. coli population was dependent on both microaerobic and anaerobic respiration, consistent with the hypothesis that the E. coli niche is alternately microaerobic and anaerobic, rather than static. The results indicate that success of the facultative anaerobes in the intestine depends on their respiratory flexibility. Despite competition for relatively scarce carbon sources, the energy efficiency provided by respiration may contribute to the widespread distribution (i.e., success) of E. coli strains as commensal inhabitants of the mammalian intestine.

Role in anaerobiosis of the isoenzymes for Saccharomyces cerevisiae fumarate reductase encoded by OSM1 and FRDS1

Saccharomyces cerevisiae possesses both a cytoplasmic and a mitochondrial fumarate reductase, encoded by FRDS1 and OSM1, respectively. While previous studies have shown that mutants lacking FRDS1 and OSM1 cannot grow under anaerobiosis (Arikawa et al., 1998), the physiological role of fumarate reductase (FR) remains poorly understood. Here, we report that an osm1 frds1 mutant is unable to grow anaerobically, even with glutamate as a sole nitrogen source, when succinate can be produced by the TCA oxidative branch. We also show that the growth of the mutant is not restored by adding acetoin, an alternative sink for NADH oxidation, but it is at least partly restored by the addition of oxygen or menadione, which can oxidize FADH(2) in addition to NADH. These data indicate that the growth inhibition of the mutant is due to an inability to reoxidize FAD, rather than an indirect effect on NADH or an inability to produce succinate per se. During anaerobic growth, FRDS1 expression was two to eight times higher than that of OSM1, and fumarate reductase activity was higher in the osm1 mutant than in the frds1 mutant. FRDS1 expression was induced by anaerobiosis, and this induction was abolished in a rox1 mutant. We conclude that the formation of succinate is strictly required for the reoxidation of FADH(2) during anaerobiosis, and that it is regulated through the control of FRDS1 expression when oxygen is limiting. Based on these data, we discuss the potential role of fumarate reductase in the regeneration of the FAD-prosthetic group of essential flavoproteins.

A Legionella pneumophila-translocated substrate that is required for growth within macrophages and protection from host cell death

Legionella pneumophila requires the Dot/Icm protein translocation system to replicate within host cells as a critical component of Legionnaire's pneumonia. None of the known individual substrates of the translocator have been shown to be essential for intracellular replication. We demonstrate here that mutants lacking the Dot/Icm substrate SdhA were severely impaired for intracellular growth within mouse bone marrow macrophages, with the defect absolute in triple mutants lacking sdhA and its two paralogs. The defect caused by the absence of the sdhA family was less severe during growth within Dictyostelium discoideum amoebae, indicating that the requirement for SdhA shows cell-type specificity. Macrophages harboring the L. pneumophila sdhA mutant showed increased nuclear degradation, mitochondrial disruption, membrane permeability, and caspase activation, indicating a role for SdhA in preventing host cell death. Defective intracellular growth of the sdhA(-) mutant could be partially suppressed by the action of caspase inhibitors, but caspase-independent cell death pathways eventually aborted replication of the mutant.

Succinate dehydrogenase functioning by a reverse redox loop mechanism and fumarate reductase in sulphate-reducing bacteria

Sulphate- or sulphur-reducing bacteria with known or draft genome sequences (Desulfovibrio vulgaris, Desulfovibrio desulfuricans G20, Desulfobacterium autotrophicum [draft], Desulfotalea psychrophila and Geobacter sulfurreducens) all contain sdhCAB or frdCAB gene clusters encoding succinate : quinone oxidoreductases. frdD or sdhD genes are missing. The presence and function of succinate dehydrogenase versus fumarate reductase was studied. Desulfovibrio desulfuricans (strain Essex 6) grew by fumarate respiration or by fumarate disproportionation, and contained fumarate reductase activity. Desulfovibrio vulgaris lacked fumarate respiration and contained succinate dehydrogenase activity. Succinate oxidation by the menaquinone analogue 2,3-dimethyl-1,4-naphthoquinone depended on a proton potential, and the activity was lost after degradation of the proton potential. The membrane anchor SdhC contains four conserved His residues which are known as the ligands for two haem B residues. The properties are very similar to succinate dehydrogenase of the Gram-positive (menaquinone-containing) Bacillus subtilis, which uses a reverse redox loop mechanism in succinate : menaquinone reduction. It is concluded that succinate dehydrogenases from menaquinone-containing bacteria generally require a proton potential to drive the endergonic succinate oxidation. Sequence comparison shows that the SdhC subunit of this type lacks a Glu residue in transmembrane helix IV, which is part of the uncoupling E-pathway in most non-electrogenic FrdABC enzymes.

There Is No Evidence That the SDHB Gene Is Involved in Neuroblastoma Development

Neuroblastoma and pheochromocytoma have the same embryonal origin. They originate from neural crest cells, and they usually affect suprarenal glands. The SDHB gene encodes the B subunit of succinate dehydrogenase, a protein implicated in the electron transport chain and Krebs cycle. Some mutations have been described in this gene in pheochromocytoma, and this gene could be an appropriate candidate for its study in neuroblastoma given its localization in 1p35-36. The aim of this study was to analyze neuroblastoma tumors in order to assess a possible implication of this gene in neuroblastoma development. We studied 28 neuroblastoma tumor samples from different stages. Mutation research in genomic DNA was carried out after individual amplification of each of the eight SDHB exons by SSCP analysis and sequencing of those samples with migration pattern variants. No variant was found except for three polymorphisms in four neuroblastoma samples. The first polymorphism was a synonymous A-->C change in the third position of codon 6 (exon 1). The other two polymorphisms were a TTC insert at the 5' flanking intron sequence of exon 5 in a stretch of seven TTC repeats. Upon the basis of posterior microsatellite instability and hypermethylation promoter studies, which were not significant, we can conclude that the SDHB gene, a positional candidate gene, is unlikely to be related to either initiation or tumoral progression in neuroblastoma.

The effect of chronic physical exercise on succinic dehydrogenase activity in the heart muscle of old rats

Succinic dehydrogenase (SDH) activity, preferentially evidenced by cytochemical methods, has been measured by computer-assisted morphometry in the heart muscle of old sedentary and age-matched animals chronically undergone physical exercise (20 min, twice a day, 5 days a week). The area of the SDH-positive mitochondria (MA) and the overall area of the cytochemical precipitates due to SDH activity (PA) were semiautomatically measured and the ratios PA/MA as well as MA/overall myocardial tissue area analysed (MA/TA) were the parameters taken into account. No significant difference was found between the two groups investigated as regards PA/MA, whereas the MA/TA value is significantly increased in the animals undergone physical training. The present findings document that chronic physical exercise significantly increases the overall mitochondrial area involved in energy provision in the old myocardial tissue. Considering that myocardial function highly relies on mitochondrial metabolism, our results support a beneficial effect of chronic physical exercise on the old heart muscle.

A mitochondrial NADH-dependent fumarate reductase involved in the production of succinate excreted by procyclic Trypanosoma brucei

Trypanosoma brucei is a parasitic protist responsible for sleeping sickness in humans. The procyclic stage of T. brucei expresses a soluble NADH-dependent fumarate reductase (FRDg) in the peroxisome-like organelles called glycosomes. This enzyme is responsible for the production of about 70% of the excreted succinate, the major end product of glucose metabolism in this form of the parasite. Here we functionally characterize a new gene encoding FRD (FRDm1) expressed in the procyclic stage. FRDm1 is a mitochondrial protein, as evidenced by immunolocalization, fractionation of digitonin-permeabilized cells, and expression of EGFP-tagged FRDm1 in the parasite. RNA interference was used to deplete FRDm1, FRDg, or both together. The analysis of the resulting mutant cell lines showed that FRDm1 is responsible for 30% of the cellular NADH-FRD activity, which solves a long standing debate regarding the existence of a mitochondrial FRD in trypanosomatids. FRDg and FRDm1 together account for the total NADH-FRD activity in procyclics, because no activity was measured in the double mutant lacking expression of both proteins. Analysis of the end products of 13C-enriched glucose excreted by these mutant cell lines showed that FRDm1 contributes to the production of between 14 and 44% of the succinate excreted by the wild type cells. In addition, depletion of one or both FRD enzymes results in up to 2-fold reduction of the rate of glucose consumption. We propose that FRDm1 is involved in the maintenance of the redox balance in the mitochondrion, as proposed for the ancestral soluble FRD presumably present in primitive anaerobic cells.

Multiprotein complex containing succinate dehydrogenase confers mitochondrial ATP-sensitive K+ channel activity

The mitochondrial ATP-sensitive K(+) (mitoK(ATP)) channel plays a central role in protection of cardiac and neuronal cells against ischemia and apoptosis, but its molecular structure is unknown. Succinate dehydrogenase (SDH) is inhibited by mitoK(ATP) activators, fueling the contrary view that SDH, rather than mitoK(ATP), is the target of cardioprotective drugs. Here, we report that SDH forms part of mitoK(ATP) functionally and structurally. Four mitochondrial proteins [mitochondrial ATP-binding cassette protein 1 (mABC1), phosphate carrier, adenine nucleotide translocator, and ATP synthase] associate with SDH. A purified IM fraction containing these proteins was reconstituted into proteoliposomes and lipid bilayers and shown to confer mitoK(ATP) channel activity. This channel activity is sensitive not only to mitoK(ATP) activators and blockers but also to SDH inhibitors. These results reconcile the controversy over the basis of ischemic preconditioning by demonstrating that SDH is a component of mitoK(ATP) as part of a macromolecular supercomplex. The findings also provide a tangible clue as to the structural basis of mitoK(ATP) channels.

Identification of the gene encoding the sole physiological fumarate reductase in Shewanella oneidensis MR-1

Shewanella oneidensis MR-1 is a Gram-negative, nonfermentative rod with a complex electron transport system which facilitates its ability to use a variety of terminal electron acceptors, including fumarate, for anaerobic respiration. CMTn-3, a mutant isolated by transposon (TnphoA) mutagenesis, can no longer use fumarate as an electron acceptor; it lacks fumarate reductase activity as well as a 65-kDa soluble tetraheme flavocytochrome c. The sequence of the TnphoA-flanking genomic DNA of CMTn-3 did not align to those for fumarate reductase or related electron transport genes from other bacteria. Sequence analysis of the MR-1 genomic database demonstrated that an open reading frame encoding a 65-kDa tetraheme cytochrome c with sequence similarity to the fumarate reductase from S. frigidimarina NCIMB400 was found 8 kb away from the TnphoA-flanking genomic DNA of CMTn-3. PCR analysis demonstrated that a large deletion (>or=9.2 kb and <or=11 kb) of genomic DNA occurred in CMTn-3 as a result of TnphoA insertion. This deletion included at least half of the fumarate reductase gene as well as approximately 8 kb of upstream DNA. Complementation of CMTn-3 with the fumarate reductase gene plus 0.5-kb of upstream DNA restored growth on fumarate. These studies explicitly define the sole physiological fumarate reductase gene from the several possibilities suggested by the genomic sequence of MR-1. Surprisingly, the fumarate reductase gene plus 0.77-kb upstream DNA from S. frigidimarina NCIMB400 did not complement CMTn-3.

Novel succinate dehydrogenase subunit B (SDHB) mutations in familial phaeochromocytomas and paragangliomas, but an absence of somatic SDHB mutations in sporadic phaeochromocytomas

Phaeochromocytomas arising in adrenal or extra-adrenal sites and paragangliomas of the head and neck, in particular of the carotid bodies, occur sporadically and also in a familial setting. In addition to mutations in RET and VHL in familial disease, germline mutations in SDHD and SDHB genes that encode subunits of mitochondrial complex II have also been associated with the development of familial phaeochromocytomas. To further investigate the role of SDHD and SDHB in the development of these tumours we determined the occurrence of germline SDHD and SDHB mutations in four patients with a family history of phaeochromocytoma with associated head and neck paraganglioma, one patient with a family history of phaeochromocytoma only and two patients with apparently sporadic extra-adrenal phaeochromocytoma, one of whom had early onset disease. Secondly, we investigated whether somatic SDHB mutations correlated with loss of heterozygosity at 1p36 in a subgroup of 11 sporadic and three MEN 2-associated RET-mutation-positive phaeochromocytomas. Novel SDHB mutations were identified in the probands from four families and two apparently sporadic cases (six of seven probands studied), including two missense mutations, a single nonsense and frameshift mutation, as well as two splice site mutations, one of which was shown to have partial penetrance resulting in 'leaky' splicing. Further, five intronic polymorphisms in SDHB were found. No SDHD mutations were identified. In addition, no somatic SDHB mutations were found in the remaining allele of the 11 sporadic adrenal phaeochromocytomas with allelic loss at 1p36 or the three MEN 2-associated RET-mutation-positive phaeochromocytomas. Therefore, we conclude that SDHB has a major role in the pathogenesis of familial phaeochromocytomas, but the possible role of SDHB in sporadic tumours showing allelic loss at 1p36 has yet to be ascertained.

A role for mitochondrial enzymes in inherited neoplasia and beyond

Mitochondrial defects have been associated with neurological disorders, as well as cancers. Two ubiquitously expressed mitochondrial enzymes--succinate dehydrogenase (SDH) and fumarate hydratase (FH, fumarase)--catalyse sequential steps in the Krebs tricarboxylic-acid cycle. Inherited heterozygous mutations in the genes encoding these enzymes cause predispositions to two types of inherited neoplasia syndromes that do not share any component tumours. Homozygous mutations in the same genes result in severe neurological impairment. Understanding this link between inherited cancer syndromes and neurological disease could provide further insights into the mechanisms by which mitochondrial deficiencies lead to tumour development.

Potential of fumarate reductase as a novel therapeutic target in Helicobacter pylori infection

Approximately 50% of the world's population carries Helicobacter pylori, a gastric bacterial pathogen linked to diseases including gastritis, ulcers and gastric cancer. Chemotherapies are being routinely used to treat systemic H. pylori infection. The common regimens consist of proton pump inhibitors (PPIs) or ranitidine bismuth citrate (RBC) and two antibiotics. Although these regimens efficiently eradicate H. pylori, the emergence of antibiotic-resistant H. pylori strains, their severe side effects and high costs are major drawbacks of these treatments. More efficient, economic and friendly drugs need to be developed. Fumarate reductase (FRD) catalyses the reduction of fumarate to succinate in the Krebs cycle and is also a key enzyme in anaerobic respiration with fumarate as the terminal electron acceptor for many facultative bacteria. H. pylori FRD contains three subunits, FrdA, FrdB and FrdC. Genome analysis and experimental evidence indicate that this enzyme appears to play an important role in the energy metabolism of H. pylori. In addition, FRD is essential for the colonisation of H. pylori in the acidic stomach as demonstrated in the mouse model of infection. Furthermore, three FRD inhibitors used to cure helminthic infection in animals and humans have both inhibitory and bactericidal effects on H. pylori. These lines of evidence indicate that FRD may be a promising chemotherapeutic target. Given that FrdA is strongly immunogenic in the sera from H. pylori-positive patients, this protein may also be used as a candidate for the development of an anti-H. pylori vaccine.

Mitochondrial DNA deletion mutations: a causal role in sarcopenia

Mitochondrial DNA (mtDNA) deletion mutations accumulate with age in tissues of a variety of species. Although the relatively low calculated abundance of these deletion mutations in whole tissue homogenates led some investigators to suggest that these mutations do not have any physiological impact, their focal and segmental accumulation suggests that they can, and do, accumulate to levels sufficient to affect the metabolism of a tissue. This phenomenon is most clearly demonstrated in skeletal muscle, where the accumulation of mtDNA deletion mutations remove critical subunits that encode for the electron transport system (ETS). In this review, we detail and provide evidence for a molecular basis of muscle fiber loss with age. Our data suggest that the mtDNA deletion mutations, which are generated in tissues with age, cause muscle fiber loss. Within a fiber, the process begins with a mtDNA replication error, an error that results in a loss of 25-80% of the mitochondrial genome. This smaller genome is replicated and, through a process not well understood, eventually comprises the majority of mtDNA within the small affected region of the muscle fiber. The preponderance of the smaller genomes results in a dysfunctional ETS in the affected area. As a consequence of both the decline in energy production and the increase in oxidative damage in the region, the fiber is no longer capable of self-maintenance, resulting in the observed intrafiber atrophy and fiber breakage. We are therefore proposing that a process contained within a very small region of a muscle fiber can result in breakage and loss of muscle fiber from the tissue.

Evolution of the enzymes of the citric acid cycle and the glyoxylate cycle of higher plants. A case study of endosymbiotic gene transfer

The citric acid or tricarboxylic acid cycle is a central element of higher-plant carbon metabolism which provides, among other things, electrons for oxidative phosphorylation in the inner mitochondrial membrane, intermediates for amino-acid biosynthesis, and oxaloacetate for gluconeogenesis from succinate derived from fatty acids via the glyoxylate cycle in glyoxysomes. The tricarboxylic acid cycle is a typical mitochondrial pathway and is widespread among alpha-proteobacteria, the group of eubacteria as defined under rRNA systematics from which mitochondria arose. Most of the enzymes of the tricarboxylic acid cycle are encoded in the nucleus in higher eukaryotes, and several have been previously shown to branch with their homologues from alpha-proteobacteria, indicating that the eukaryotic nuclear genes were acquired from the mitochondrial genome during the course of evolution. Here, we investigate the individual evolutionary histories of all of the enzymes of the tricarboxylic acid cycle and the glyoxylate cycle using protein maximum likelihood phylogenies, focusing on the evolutionary origin of the nuclear-encoded proteins in higher plants. The results indicate that about half of the proteins involved in this eukaryotic pathway are most similar to their alpha-proteobacterial homologues, whereas the remainder are most similar to eubacterial, but not specifically alpha-proteobacterial, homologues. A consideration of (a) the process of lateral gene transfer among free-living prokaryotes and (b) the mechanistics of endosymbiotic (symbiont-to-host) gene transfer reveals that it is unrealistic to expect all nuclear genes that were acquired from the alpha-proteobacterial ancestor of mitochondria to branch specifically with their homologues encoded in the genomes of contemporary alpha-proteobacteria. Rather, even if molecular phylogenetics were to work perfectly (which it does not), then some nuclear-encoded proteins that were acquired from the alpha-proteobacterial ancestor of mitochondria should, in phylogenetic trees, branch with homologues that are no longer found in most alpha-proteobacterial genomes, and some should reside on long branches that reveal affinity to eubacterial rather than archaebacterial homologues, but no particular affinity for any specific eubacterial donor.

Succinate:quinone oxidoreductases--what can we learn from Wolinella succinogenes quinol:fumarate reductase?

The structure of Wolinella succinogenes quinol:fumarate reductase by X-ray crystallography has been determined at 2.2-A resolution [Lancaster et al. (1999), Nature 402, 377-385]. Based on the structure of the three protein subunits A, B, and C and the arrangement of the six prosthetic groups (a covalently bound FAD, three iron-sulphur clusters, and two haem b groups) a pathway of electron transfer from the quinol-oxidising dihaem cytochrome b in the membrane to the site of fumarate reduction in the hydrophilic subunit A has been proposed. By combining the results from site-directed mutagenesis, functional and electrochemical characterisation, and X-ray crystallography, a residue was identified which is essential for menaquinol oxidation. [Lancaster et al. (2000), Proc. Natl. Acad. Sci. USA 97, 13051-13056]. The location of this residue in the structure suggests that the coupling of the oxidation of menaquinol to the reduction of fumarate in dihaem-containing succinate:quinone oxidoreductases could be associated with the generation of a transmembrane electrochemical potential. Based on crystallographic analysis of three different crystal forms of the enzyme and the results from site-directed mutagenesis, we have derived a mechanism of fumarate reduction and succinate oxidation [Lancaster et al. (2001) Eur. J. Biochem. 268, 1820-1827], which should be generally relevant throughout the superfamily of succinate:quinone oxidoreductases.

Generation of a proton potential by succinate dehydrogenase of Bacillus subtilis functioning as a fumarate reductase

The membrane fraction of Bacillus subtilis catalyzes the reduction of fumarate to succinate by NADH. The activity is inhibited by low concentrations of 2-(heptyl)-4-hydroxyquinoline-N-oxide (HOQNO), an inhibitor of succinate: quinone reductase. In sdh or aro mutant strains, which lack succinate dehydrogenase or menaquinone, respectively, the activity of fumarate reduction by NADH was missing. In resting cells fumarate reduction required glycerol or glucose as the electron donor, which presumably supply NADH for fumarate reduction. Thus in the bacteria, fumarate reduction by NADH is catalyzed by an electron transport chain consisting of NADH dehydrogenase (NADH:menaquinone reductase), menaquinone, and succinate dehydrogenase operating in the reverse direction (menaquinol:fumarate reductase). Poor anaerobic growth of B. subtilis was observed when fumarate was present. The fumarate reduction catalyzed by the bacteria in the presence of glycerol or glucose was not inhibited by the protonophore carbonyl cyanide m-chlorophenyl hydrazone (CCCP) or by membrane disruption, in contrast to succinate oxidation by O2. Fumarate reduction caused the uptake by the bacteria of the tetraphenyphosphonium cation (TPP+) which was released after fumarate had been consumed. TPP+ uptake was prevented by the presence of CCCP or HOQNO, but not by N,N'-dicyclohexylcarbodiimide, an inhibitor of ATP synthase. From the TPP+ uptake the electrochemical potential generated by fumarate reduction was calculated (Deltapsi = -132 mV) which was comparable to that generated by glucose oxidation with O2 (Deltapsi = -120 mV). The Deltapsi generated by fumarate reduction is suggested to stem from menaquinol:fumarate reductase functioning in a redox half-loop.

Mitochondrial mutations differentially affect aging, mutability and anesthetic sensitivity in Caenorhabditis elegans

In the nematode Caenorhabditis elegans, mutations have been previously isolated that affect the activities of Complex I (gas-1) and Complex II (mev-1), two of the five membrane-bound complexes that control electron flow in mitochondrial respiration. We compared the effects of gas-1 and mev-1 mutations on different traits influenced by mitochondrial function. Mutations in Complex I and II both increased sensitivity to free radicals as measured during development and in aging animals. However, gas-1 and mev-1 mutations differentially affected mutability and anesthetic sensitivity. Specifically, gas-1 was anesthetic hypersensitive but not hypermutable while mev-1 was hypermutable but displayed normal responses to anesthetics. These results indicate that Complexes I and II may differ in their effects on behavior and development, and are consistent with the wide variation in phenotypes that result from mitochondrial changes in other organisms.

Succinate dehydrogenase and other respiratory pathways in thylakoid membranes of Synechocystis sp. strain PCC 6803: capacity comparisons and physiological function

Respiration in cyanobacterial thylakoid membranes is interwoven with photosynthetic processes. We have constructed a range of mutants that are impaired in several combinations of respiratory and photosynthetic electron transport complexes and have examined the relative effects on the redox state of the plastoquinone (PQ) pool by using a quinone electrode. Succinate dehydrogenase has a major effect on the PQ redox poise, as mutants lacking this enzyme showed a much more oxidized PQ pool. Mutants lacking type I and II NAD(P)H dehydrogenases also had more oxidized PQ pools. However, in the mutant lacking type I NADPH dehydrogenase, succinate was essentially absent and effective respiratory electron donation to the PQ pool could be established after addition of 1 mM succinate. Therefore, lack of the type I NADPH dehydrogenase had an indirect effect on the PQ pool redox state. The electron donation capacity of succinate dehydrogenase was found to be an order of magnitude larger than that of type I and II NAD(P)H dehydrogenases. The reason for the oxidized PQ pool upon inactivation of type II NADH dehydrogenase may be related to the facts that the NAD pool in the cell is much smaller than that of NADP and that the NAD pool is fully reduced in the mutant without type II NADH dehydrogenase, thus causing regulatory inhibition. The results indicate that succinate dehydrogenase is the main respiratory electron transfer pathway into the PQ pool and that type I and II NAD(P)H dehydrogenases regulate the reduction level of NADP and NAD, which, in turn, affects respiratory electron flow through succinate dehydrogenase.

Fumarate reductase is essential for Helicobacter pylori colonization of the mouse stomach

Fumarate reductase (FRD) is the key enzyme in fumarate respiration induced by anaerobic growth of bacteria. In Helicobacter pylori, this enzyme appears to be constitutively expressed under microaerobic conditions and is not essential for its survival in vitro. In this study, the role of FRD in the colonization of H. pylori was investigated using a mouse model. The frdA gene coding for subunit A of FRD, and two control genes, copA and copP associated with the export of copper out of H. pylori, were inactivated by insertion of the chloramphenicol acetyltransferase cassette into these individual genes. The isogenic mutants of H. pylori strain AH244 were obtained by natural transformation. Seventy-five ICR mice (15 mice/group) were orogastrically dosed with either the wild type H. pylori strain AH244, its isogenic mutants, or Brucella broth (negative control). Five mice from each group were killed at 2, 4 and 8 weeks post-inoculation (WPI), respectively. H. pylori colonization was not detected in mouse gastric mucosa infected with the frdA mutant at any time point in the study by both quantitative culture and PCR. In contrast, the mice inoculated with either wild type AH244, copA or copPH. pylori mutants became readily infected. These data indicate that FRD plays a crucial role in H. pylori survival in the gastric mucosa of mice. Given that FRD, present in all H. pylori strains, is immunogenic in H. pylori -infected patients and H. pylori growth in vitro can be inhibited by three anthelmintics (morantel, oxantel and thiabendazole), this enzyme could potentially be used both as a novel drug target as well as in the development of vaccines for H. pylori prevention and eradication.

The contribution of mitochondrial respiratory complexes to the production of reactive oxygen species

This work was focused on distinguishing the contribution of mitochondrial redox complexes to the production of reactive oxygen species (ROS) during cellular respiration. We were able to accurately measure, for the first time, the basal production of ROS under uncoupled conditions by using a very sensitive method, based on the fluorescent probe dichlorodihydrofluorescein diacetate. The method also enabled the detection of the ROS generated by the oxidation of the endogenous substrates in the mitochondrial preparations and could be applied to both mitochondria and live cells. Contrary to the commonly accepted view that complex III (ubiquinol:cytochrome c reductase) is the major contributor to mitochondrial ROS production, we found that complex I (NADH-ubiquinone reductase) and complex II (succinate-ubiquinone reductase) are the predominant generators of ROS during prolonged respiration under uncoupled conditions. Complex II, in particular, appears to contribute to the basal production of ROS in cells.

The electron transport chain in anaerobically functioning eukaryotes

Many lower eukaryotes can survive anaerobic conditions via a fermentation pathway that involves the use of the reduction of endogenously produced fumarate as electron sink. This fumarate reduction is linked to electron transport in an especially adapted, anaerobically functioning electron-transport chain. An aerobic energy metabolism with Krebs cycle activity is accompanied by electron transfer from succinate to ubiquinone via complex II of the respiratory chain. On the other hand, in an anaerobic metabolism, where fumarate functions as terminal electron acceptor, electrons are transferred from rhodoquinone to fumarate, which is the reversed direction. Ubiquinone cannot replace rhodoquinone in the process of fumarate reduction in vivo, as ubiquinone can only accept electrons from complex II and cannot donate them to fumarate. Rhodoquinone, with its lower redox potential than ubiquinone, is capable of donating electrons to fumarate. Eukaryotic fumarate reductases were shown to interact with rhodoquinone (a benzoquinone), whereas most prokaryotic fumarate reductases interact with the naphtoquinones menaquinone and demethylmenaquinone. Fumarate reductase, the enzyme essential for the anaerobic functioning of many eukaryotes, is structurally very similar to succinate dehydrogenase, the Krebs cycle enzyme catalysing the reverse reaction. In prokaryotes these enzymes are differentially expressed depending on the external conditions. Evidence is now emerging that also in eukaryotes two different enzymes exist for succinate oxidation and fumarate reduction that are differentially expressed.

Physiological function and regulation of flavocytochrome c3, the soluble fumarate reductase from Shewanella putrefaciens NCIMB 400

Shewanella putrefaciens produces a soluble flavocytochrome c under anaerobic growth conditions. This protein shares sequence similarity with the catalytic subunits of membrane-bound fumarate reductases from Escherichia coli and other bacteria and the purified protein has fumarate reductase activity. It is shown here that this enzyme, flavocytochrome c3, is essential for fumarate respiration in vivo since disruption of the chromosomal fccA gene, which encodes flavocytochrome c3, leads to a specific loss of the ability to grow with fumarate as terminal electron acceptor. Growth with nitrate, trimethylamine N-oxide (TMAO) and other acceptors was unaffected. The fccA gene is transcribed as a 2 kb monocistronic mRNA. An adjacent reading frame that bears limited sequence similarity to one of the membrane anchor subunits of E. coli fumarate reductase is not co-transcribed with fccA. Expression of the fccA gene is regulated by anaerobiosis and by the availability of alternative electron acceptors, particularly nitrate and TMAO. DNA sequences have been identified that are required for this regulation.

Mutations in sdh (succinate dehydrogenase genes) alter the thiamine requirement of Salmonella typhimurium

Mutants lacking the first enzyme in de novo purine synthesis (PurF) can synthesize thiamine if increased levels of pantothenate are present in the culture medium (J. L. Enos-Berlage and D. M. Downs, J. Bacteriol. 178:1476-1479, 1996). Derivatives of purF mutants that no longer required pantothenate for thiamine-independent growth were isolated. Analysis of these mutants demonstrated that they were defective in succinate dehydrogenase (Sdh), an enzyme of the tricarboxylic acid cycle. Results of phenotypic analyses suggested that a defect in Sdh decreased the thiamine requirement of Salmonella typhimurium. This reduced requirement correlated with levels of succinyl-coenzyme A (succinyl-CoA), which is synthesized in a thiamine pyrophosphate-dependent reaction. The effect of succinyl-CoA on thiamine metabolism was distinct from the role of pantothenate in thiamine synthesis.

Inhibition of succinate:ubiquinone reductase and decrease of ubiquinol in nephrotoxic cysteine S-conjugate-induced oxidative cell injury

The role of complex II in the cellular protection against oxidative stress was investigated in freshly isolated rat renal proximal tubular cells (PTC) with the use of the nephrotoxin S-(1,2-dichlorovinyl)-L-cysteine (DCVC). DCVC caused oxidative stress in PTC as determined by flow cytometry with dihydrorhodamine-123; this fluorescent probe is readily oxidized by primary hydroperoxides such as those formed during lipid peroxidation. The oxidative stress could be prevented by inhibition of the beta-lyase-mediated formation and covalent binding to cellular macromolecules of reactive DCVC metabolites, with amino oxyacetic acid (AOA), or by the antioxidant N,N'-diphenyl-p-phenylenediamine. Both AOA and DPPD also prevented cell death. The DCVC-induced oxidative stress was associated with a decrease in the succinate:ubiquinone reductase (SQR) activity of complex II, whereas NADH:ubiquinone reductase activity of complex I remained unaffected. AOA prevented the effect on SQR activity, whereas N,N'-diphenyl-p-phenylenediamine did not. Inhibition of SQR activity with thenoyl trifluoracetone (TTFA) potentiated the DCVC-induced oxidative cell injury, suggesting the involvement of SQR activity in an antioxidant pathway. To investigate this in greater detail, PTC were treated with an inhibitor of cytochrome-c-oxidase, KCN, in a buffer containing glycine, which prevents cell death by KCN. Glycine did not affect cell death by DCVC. KCN prevented the DCVC-induced oxidative stress and cell death. KCN cytoprotection could be prevented by inhibition of SQR activity with oxaloacetate or TTFA, whereas inhibition of either complex I or III with rotenone and antimycin, respectively, did not prevent it. The effect of DCVC on complex II was associated with a decrease in the cellular amount of reduced ubiquinone (QH2); the KCN-mediated cytoprotection was related to a 60% increase of cellular QH2. Rotenone almost completely inhibited ubiquinone reduction even in the presence of KCN, whereas oxaloacetate in combination with KCN resulted in QH2 levels comparable to control. This suggests that the SQR activity by complex II rather than the cellular content of reduced ubiquinone (QH2) is important as a part of the cellular antioxidant machinery in the cyto-protection against oxidative stress.

Oxidative enzyme activity in the rat soleus muscle and its motoneurons during postnatal maturation

The effect of postnatal maturation on changes in the oxidative enzyme (succinate dehydrogenase) activity in the rat soleus muscle and its motoneurons was examined at 3, 6, and 12 weeks of age. The motoneurons innervating the soleus muscle were identified using the fluorescent retrograde neuronal tracer Nuclear Yellow. An inverse relationship between soma size and oxidative enzyme activity of soleus motoneurons was observed at 3 and 6 weeks of age, whereas there was no correlation between them at 12 weeks. Although the oxidative enzyme activity in the soleus muscle increased during postnatal maturation, it showed a decrease in the soleus motoneurons. These data demonstrate that the inverse relationship between soma size and oxidative enzyme activity of rat soleus motoneurons can only be detected in the early postnatal period and that the oxidative enzyme activity in the rat soleus muscle and its motoneurons can change independently during postnatal maturation.

Characterization of the excitotoxic potential of the reversible succinate dehydrogenase inhibitor malonate

Although the mechanism of neuronal death in neurodegenerative diseases remains unknown, it has been hypothesized that relatively minor metabolic defects may predispose neurons to N-methyl-D-aspartate (NMDA) receptor-mediated excitotoxic damage in these disorders. To further investigate this possibility, we have characterized the excitotoxic potential of the reversible succinate dehydrogenase (SDH) inhibitor malonate. After its intrastriatal stereotaxic injection into male Sprague-Dawley rats, malonate produced a dose-dependent lesion when assessed 3 days after surgery using cytochrome oxidase histochemistry. This lesion was attenuated by coadministration of excess succinate, indicating that it was caused by specific inhibition of SDH. The lesion was also prevented by administration of the noncompetitive NMDA antagonist MK-801. MK-801 did not induce hypothermia, and hypothermia itself was not neuroprotective, suggesting that the neuroprotective effect of MK-801 was due to blockade of the NMDA receptor ion channel and not to any nonspecific effect. The competitive NMDA antagonist LY274614 and the glycine site antagonist 7-chlorokynurenate also profoundly attenuated malonate neurotoxicity, further indicating an NMDA receptor-mediated event. Finally, the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) antagonist NBQX (2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(f)-quinoxaline) was ineffective at preventing malonate toxicity at a dose that effectively reduced S-AMPA toxicity, indicating that non-NMDA receptors are involved minimally, if at all, in the production of the malonate lesion. We conclude that inhibition of SDH by malonate results in NMDA receptor-mediated excitotoxic neuronal death. If this mechanism of "secondary" or "weak" excitotoxicity plays a role in neurodegenerative disease, NMDA antagonists and other "antiexcitotoxic" strategies may have therapeutic potential for these diseases.

Effect of acute ethanol exposure on cultured fetal rat hepatocytes: relation to mitochondrial function

Studies from our laboratory have shown that short-term ethanol exposure inhibits epidermal growth factor-dependent replication of cultured fetal rat hepatocytes, along with a drop in ATP level, and that these effects could be caused, at least in part, by ethanol-induced oxidative stress. In these prior studies, mitochondrial morphology was abnormal and membrane lipid peroxidation products were increased, along with reduced transmembrane potential and enhanced permeability to sucrose. To define the effects of ethanol on mitochondrial function further, the present study examines the impact of ethanol exposure on mitochondrial electron transport chain components. A 24-hr exposure of cultured fetal rat hepatocytes to ethanol (2.5 mg/ml) reduced mitochondrial complex I activity by 16% (p < 0.05), complex IV by 28% (p < 0.05), and succinate dehydrogenase by 23% (p < 0.05). This reduction was paralleled by lower ADP translocase activity (24%, p < 0.05) and diminished mitochondrial glutathione (GSH) (20%, p < 0.05). Pretreatment with 0.1 mM S-adenosyl methionine, before ethanol exposure, normalized mitochondrial GSH along with activities of complex I, complex IV, and succinate dehydrogenase. A 3-hr exposure of isolated mitochondria (which do not metabolize ethanol) to ethanol (2.5 mg/ml), inhibited the activities of complex I (19%, p < 0.05), complex IV (24%, p < 0.05), and of ATP synthesis (20%, p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Expression and functional properties of fumarate reductase

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An evaluation of the measurement of the activities of complexes I-IV in the respiratory chain of human skeletal muscle mitochondria

The measurement of individual respiratory chain complexes is an important component of the investigation of diseases due to mitochondrial dysfunction. We have evaluated assays which measure complexes I to IV in human skeletal muscle mitochondria and in addition optimized these assays to provide sensitive and reliable diagnostic techniques, particularly in situations where a partial interruption at a single complex needs to identified. Using several established methods of membrane disruption we have found that optimal activities of complexes I and II are obtained by freeze-thawing the mitochondria in hypotonic potassium phosphate buffer, whereas complex III and IV activities are markedly increased by the addition of the detergent n-dodecyl-beta-D-maltoside. Complex I activity is measured in the presence of 2.5 mg.ml-1 bovine serum albumin, which increases rotenone sensitivity, and we have shown that NADH-cytochrome b5 reductase makes an important contribution to the rotenone-insensitive NADH-ubiquinone oxidoreductase activity. Complex II activity is measured after preincubation of the mitochondrial fraction with succinate to fully activate the complex. Complex I and III activities are dependent upon the length of the isoprenoid chain of the ubiquinone and ubiquinol, respectively. These assays have been used to establish a control range.

Hypertrophic cardiomyopathy with mitochondrial myopathy. A new phenotype of complex II defect

Two brothers, 25 and 19 years old, were affected by asymmetrical hypertrophic cardiomyopathy. The older brother had waddling gait and weakness of the proximal girdle muscles, while the younger had a broad-based gait and weakness of selected limb girdle muscles. EMG exam was myopathic. Serum enzyme, CPK and aldolase were elevated. Histochemical reactions in muscle revealed "core-like" areas, subsarcolemmal rims of mitochondria and lipid accumulation. Succinate-dehydrogenase stain showed a lack of activity in both biopsies, with the exception of intrafusal fibers. Microphotometric quantitative measurements confirmed the defect in both biopsies. Biochemical measurements of several mitochondrial enzymes in muscle showed a reduced activity of succinate-dehydrogenase (33%) and succinate-cytochrome C reductase (36-47%) which are both components of complex II. On myocardial biopsy lipid and mitochondrial abnormalities were found. This mitochondriopathy represents a new phenotype of partial complex II defect.

Diode-like behaviour of a mitochondrial electron-transport enzyme

In mitochondria, electrons derived from the oxidation of succinate by the tricarboxylic acid cycle enzyme succinate-ubiquinone oxido-reductase are transferred directly to the quinone pool. Here we provide evidence that the soluble form of this enzyme (succinate dehydrogenase) behaves as a diode that essentially allows electron flow in one direction only. The gating effect is observed when electrons are exchanged rapidly and directly between fully active succinate dehydrogenase and a graphite electrode. Turnover is therefore measured under conditions of continuously variable electrochemical potential. The otherwise rapid and efficient reduction of fumarate (the reverse reaction) is severely retarded as the driving force (overpotential) is increased. Such behaviour can arise if a rate-limiting chemical step like substrate binding or product release depends on the oxidation state of a redox group on the enzyme. The observation provides, for a biological electron-transport system, a simple demonstration of directionality that is enforced by kinetics as opposed to that which is assumed from thermodynamics.

Identification of a second gene involved in global regulation of fumarate reductase and other nitrate-controlled genes for anaerobic respiration in Escherichia coli

Fumarate reductase catalyzes the final step of anaerobic electron transport in Escherichia coli when fumarate is used as a terminal electron acceptor. Transcription of the fumarate reductase operon (frdABCD) was repressed when cells were grown in the presence of either of the preferred terminal electron acceptors, oxygen or nitrate, and was stimulated modestly by fumarate. We have previously identified a locus called frdR which pleiotropically affects nitrate repression of fumarate reductase, trimethylamine N-oxide reductase, and alcohol dehydrogenase gene expression and nitrate induction of nitrate reductase expression (L. V. Kalman and R. P. Gunsalus, J. Bacteriol. 170:623-629, 1988). Transformation of various frdR mutants with plasmids identified two complementation groups, indicating that the frdR locus is composed of two genes. One class of mutants was not completely restored to wild-type frdA-lacZ expression or nitrate reductase induction when complemented with multicopy narX+ plasmids, whereas low-copy narX+ plasmid-containing strains were. A second class of frdR mutants was identified and shown to correspond to a previously described gene, narL (frdR2). Complementation of these strains with multicopy narL+ plasmids resulted in superrepression of frdA-lacZ expression and moderate elevation of nitrate reductase expression. Multicopy plasmids containing both narL+ and narX+ or only narL+ were able to complement narL mutants, whereas narX+ plasmids complemented narX mutants only when present in a copy number approximately equal to that of narL. Both narL and narX mutants retained normal oxygen control of frdA-lacZ expression. Both types of mutants are pleiotropic, as evidenced by derepressed levels of the fumarate reductase and trimethylamine N-oxide reductase enzymes and by defective induction of nitrate reductase when cells were grown in the presence of nitrate. These results indicate that both the narL and narX gene products must be present in a defined ratio in the cell. We conclude that these proteins interact to effect normal nitrate control of the anaerobic electron transport-associated operons. From these studies, we propose that narX encodes a nitrate sensor protein while narL encodes a DNA-binding regulatory protein which together function in a manner analogous to other two-component regulatory systems.

The effects of recessive lethal Notch mutations of Drosophila melanogaster on flavoprotein enzyme activities whose inhibitions cause Notch-like phenocopies

The biochemical action of the Notch locus whose mutants cause morphological aberrations in flies, viz., notches of wings and bristle multiplication, has been analyzed by the addition to the food medium of enzyme inhibitors causing phenocopies of Notch and by comparison of enzyme activity patterns of Notch mutants with different degrees of phenotypic expression. Notch phenocopies were induced by inhibitors of enzyme activities in two biochemical pathways: the de novo pyrimidine synthesis by 5-methylorotate (inhibitor of dihydroorotate dehydrogenase) and the choline shunt by amobarbital (inhibits choline dehydrogenase) and methoxyacetate (inhibits sarcosine dehydrogenase). The inhibition of de novo pyrimidine synthesis prevents the production of deoxyuridine-5-phosphate, the substrate for the synthesis of thymidine-5-phosphate via thymidylate synthase, whereas the inhibition of the choline shunt prevents the production of HCHO groups and glycine, both of which are involved in the synthesis of 5,10-methylenetetrahydrofolate, which is a cofactor of thymidylate synthase. It was already known that the inhibition of the latter enzyme in vivo induces Notch phenocopies. Notch mutants with a strong morphological expression show low enzyme activities for dihydroorotate dehydrogenase and choline dehydrogenase. Both are flavoprotein enzymes linked to the respiratory chain. The correspondence between the low enzyme activities in Notch mutants with a strong morphological expression and the phenocopying effect of antimetabolites on these enzymes in the two biochemical pathways involved strongly suggests that the morphological effects of Notch on flies are a consequence of lowered activities of choline dehydrogenase and dihydroorotate dehydrogenase.