Dictyostelium Differentiation-Inducing Factor 1 Promotes Glucose Uptake via Direct Inhibition of Mitochondrial Malate Dehydrogenase in Mouse 3T3-L1 Cells

Differentiation-inducing factor 1 (DIF-1), found in Dictyostelium discoideum, has antiproliferative and glucose-uptake-promoting activities in mammalian cells. DIF-1 is a potential lead for the development of antitumor and/or antiobesity/antidiabetes drugs, but the mechanisms underlying its actions have not been fully elucidated. In this study, we searched for target molecules of DIF-1 that mediate the actions of DIF-1 in mammalian cells by identifying DIF-1-binding proteins in human cervical cancer HeLa cells and mouse 3T3-L1 fibroblast cells using affinity chromatography and liquid chromatography-tandem mass spectrometry and found mitochondrial malate dehydrogenase (MDH2) to be a DIF-1-binding protein in both cell lines. Since DIF-1 has been shown to directly inhibit MDH2 activity, we compared the effects of DIF-1 and the MDH2 inhibitor LW6 on the growth of HeLa and 3T3-L1 cells and on glucose uptake in confluent 3T3-L1 cells in vitro. In both HeLa and 3T3-L1 cells, DIF-1 at 10-40 μM dose-dependently suppressed growth, whereas LW6 at 20 μM, but not at 2-10 μM, significantly suppressed growth in these cells. In confluent 3T3-L1 cells, DIF-1 at 10-40 μM significantly promoted glucose uptake, with the strongest effect at 20 μM DIF-1, whereas LW6 at 2-20 μM significantly promoted glucose uptake, with the strongest effect at 10 μM LW6. Western blot analyses showed that LW6 (10 μM) and DIF-1 (20 μM) phosphorylated and, thus, activated AMP kinase in 3T3-L1 cells. Our results suggest that MDH2 inhibition can suppress cell growth and promote glucose uptake in the cells, but appears to promote glucose uptake more strongly than it suppresses cell growth. Thus, DIF-1 may promote glucose uptake, at least in part, via direct inhibition of MDH2 and a subsequent activation of AMP kinase in 3T3-L1 cells.

Inhibition of Malate Dehydrogenase-2 Protects Renal Tubular Epithelial Cells from Anoxia-Reoxygenation-Induced Death or Senescence

Ischemia-reperfusion injury is the leading cause of acute kidney injury. Reactive oxygen species (ROS) production causes cell death or senescence. In cultures of primary human renal tubular epithelial cells (RPTECs) subjected to anoxia-reoxygenation, inhibition of the Krebs cycle at the level of malate dehydrogenase-2 (MDH-2) decreases hypoxia-inducible factor-1α and oxidative stress and protects from apoptotic or ferroptotic cell death. Inhibition of MDH-2 decreased reoxygenation-induced upregulation of p53 and p21, restored the levels of the proliferation marker Ki-67, and prevented the upregulation of the senescence marker beta-galactosidase and interleukin-1β production. MDH-2 inhibition reduced the reoxygenation-induced upregulation of ATP, but the alterations of critical cell metabolism enzymes allowed enough ATP production to prevent cell energy collapse. Thus, inhibition of the Krebs cycle at the level of MDH-2 protects RPTECs from anoxia-reoxygenation-induced death or senescence. MDH-2 may be a promising pharmaceutical target against ischemia-reperfusion injury.

Malic enzyme 2 connects the Krebs cycle intermediate fumarate to mitochondrial biogenesis

Mitochondria have an independent genome (mtDNA) and protein synthesis machinery that coordinately activate for mitochondrial generation. Here, we report that the Krebs cycle intermediate fumarate links metabolism to mitobiogenesis through binding to malic enzyme 2 (ME2). Mechanistically, fumarate binds ME2 with two complementary consequences. First, promoting the formation of ME2 dimers, which activate deoxyuridine 5'-triphosphate nucleotidohydrolase (DUT). DUT fosters thymidine generation and an increase of mtDNA. Second, fumarate-induced ME2 dimers abrogate ME2 monomer binding to mitochondrial ribosome protein L45, freeing it for mitoribosome assembly and mtDNA-encoded protein production. Methylation of the ME2-fumarate binding site by protein arginine methyltransferase-1 inhibits fumarate signaling to constrain mitobiogenesis. Notably, acute myeloid leukemia is highly dependent on mitochondrial function and is sensitive to targeting of the fumarate-ME2 axis. Therefore, mitobiogenesis can be manipulated in normal and malignant cells through ME2, an unanticipated governor of mitochondrial biomass production that senses nutrient availability through fumarate.

Malic Enzyme 1 Is Associated with Tumor Budding in Oral Squamous Cell Carcinomas

Budding at the tumor invasive front has been correlated with the malignant properties of many cancers. Malic enzyme 1 (ME1) promotes the Warburg effect in cancer cells and induces epithelial–mesenchymal transition (EMT) in oral squamous cell carcinoma (OSCC). Therefore, we investigated the role of ME1 in tumor budding in OSCC. Tumor budding was measured in 96 human OSCCs by immunostaining for an epithelial marker (AE1/AE3), and its expression was compared with that of ME1. A significant correlation was observed between tumor budding and ME1 expression. The correlation increased with the progression of cancer. In human OSCC cells, lactate secretion decreased when lactate fermentation was suppressed by knockdown of ME1 and lactate dehydrogenase A or inhibition of pyruvate dehydrogenase (PDH) kinase. Furthermore, the extracellular pH increased, and the EMT phenotype was suppressed. In contrast, when oxidative phosphorylation was suppressed by PDH knockdown, lactate secretion increased, extracellular pH decreased, and the EMT phenotype was promoted. Induction of chemical hypoxia in OSCC cells by CoCl₂ treatment resulted in increased ME1 expression along with HIF1α expression and promotion of the EMT phenotype. Hypoxic conditions also increased matrix metalloproteinases expression and decreased mitochondrial membrane potential, mitochondrial oxidative stress, and extracellular pH. Furthermore, the hypoxic treatment resulted in the activation of Yes-associated protein (YAP), which was abolished by ME1 knockdown. These findings suggest that cancer cells at the tumor front in hypoxic environments increase their lactate secretion by switching their energy metabolism from oxidative phosphorylation to glycolysis owing to ME1 overexpression, decrease in extracellular pH, and YAP activation. These alterations enhance EMT and the subsequent tumor budding. Tumor budding and ME1 expression are thus considered useful markers of OSCC malignancy, and ME1 is expected to be a relevant target for molecular therapy.

Inhibition of NADP-malic enzyme activity by H2 S and NO in sweet pepper (Capsicum annuum L.) fruits

NADPH is an essential cofactor in many physiological processes. Fruit ripening is caused by multiple biochemical pathways in which, reactive oxygen and nitrogen species (ROS/RNS) metabolism is involved. Previous studies have demonstrated the differential modulation of nitric oxide (NO) and hydrogen sulfide (H₂ S) content during sweet pepper (Capsicum annuum L.) fruit ripening, both of which regulate NADP-isocitrate dehydrogenase activity. To gain a deeper understanding of the potential functions of other NADPH-generating components, we analyzed glucose-6-phosphate dehydrogenase (G6PDH) and 6-phosphogluconate dehydrogenase (6PGDH), which are involved in the oxidative phase of the pentose phosphate pathway (OxPPP) and NADP-malic enzyme (NADP-ME). During fruit ripening, G6PDH activity diminished by 38%, while 6PGDH and NADP-ME activity increased 1.5- and 2.6-fold, respectively. To better understand the potential regulation of these NADP-dehydrogenases by H₂ S, we obtained a 50-75% ammonium-sulfate-enriched protein fraction containing these proteins. With the aid of in vitro assays, in the presence of H₂ S, we observed that, while NADP-ME activity was inhibited by up to 29-32% using 2 and 5 mM Na₂ S as H₂ S donor, G6PDH and 6PGDH activities were unaffected. On the other hand, NO donors, S-nitrosocyteine (CysNO) and DETA NONOate also inhibited NADP-ME activity by 35%. These findings suggest that both NADP-ME and 6PGDH play an important role in maintaining the supply of NADPH during pepper fruit ripening and that H₂ S and NO partially modulate the NADPH-generating system.

Interaction between Silver Nanoparticles and Two Dehydrogenases: Role of Thiol Groups

Widely used silver nanoparticles (AgNPs) are readily accessible to biological fluids and then surrounded by proteins. However, interactions between AgNPs and proteins are poorly understood. Two dehydrogenases, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and malate dehydrogenase (MDH), are chosen to investigate these interactions. Ag bound to thiol groups of these enzymes significantly decreases the number of free thiols available. Dose-dependent inhibition of enzyme activities is observed in both AgNPs and Ag⁺ treatments. Based on the concentration required to inhibit 50% activity, GAPDH and MDH are 24-30 fold more sensitive to Ag⁺ than to AgNPs suggesting that the measured 4.2% Ag⁺ containing AgNPs can be responsible for the enzymes inhibition. GAPDH, with a thiol group in its active site, is more sensitive to Ag than MDH, displaying many thiol groups but none in its active site, suggesting that thiol groups at the active site strongly determines the sensitivity of enzymes toward AgNPs. In contrast, the dramatic changes of circular dichroism spectra show that the global secondary structure of MDH under AgNPs treatment is more altered than that of GAPDH. In summary, this study shows that the thiol groups and their location on these dehydrogenases are crucial for the AgNPs effects.

MDH1 and MPP7 Regulate Autophagy in Pancreatic Ductal Adenocarcinoma

Pancreatic ductal adenocarcinoma (PDAC) is driven by metabolic changes in pancreatic cells caused by oncogenic mutations and dysregulation of p53. PDAC cell lines and PDAC-derived xenografts grow as a result of altered metabolic pathways, changes in stroma, and autophagy. Selective targeting and inhibition of one of these may open avenues for the development of new therapeutic strategies. In this study, we performed a genome-wide siRNA screen in a PDAC cell line using endogenous autophagy as a readout and identified several regulators of autophagy that were required for autophagy-dependent PDAC cell survival. Validation of two promising candidates, MPP7 (MAGUK p55 subfamily member 7, a scaffolding protein involved in cell-cell contacts) and MDH1 (cytosolic Malate dehydrogenase 1), revealed their role in early stages of autophagy during autophagosome formation. MPP7 was involved in the activation of YAP1 (a transcriptional coactivator in the Hippo pathway), which in turn promoted autophagy, whereas MDH1 was required for maintenance of the levels of the essential autophagy initiator serine-threonine kinase ULK1, and increased in the activity upon induction of autophagy. Our results provide a possible explanation for how autophagy is regulated by MPP7 and MDH1, which adds to our understanding of autophagy regulation in PDAC. SIGNIFICANCE: This study identifies and characterizes MPP7 and MDH1 as novel regulators of autophagy, which is thought to be responsible for pancreatic cancer cell survival.

Malic Enzyme 1 (ME1) is pro-oncogenic in ApcMin/+ mice

Cytosolic Malic Enzyme (ME1) provides reduced NADP for anabolism and maintenance of redox status. To examine the role of ME1 in tumor genesis of the gastrointestinal tract, we crossed mice having augmented intestinal epithelial expression of ME1 (ME1-Tg mice) with ApcMin/+ mice to obtain male ApcMin/+/ME1-Tg mice. ME1 protein levels were significantly greater within gut epithelium and adenomas of male ApcMin/+/ME1-Tg than ApcMin/+ mice. Male ApcMin/+/ME1-Tg mice had larger and greater numbers of adenomas in the small intestine (jejunum and ileum) than male ApcMin/+ mice. Male ApcMin/+/ME1-Tg mice exhibited greater small intestine crypt depth and villus length in non-adenoma regions, correspondent with increased KLF9 protein abundance in crypts and lamina propria. Small intestines of male ApcMin/+/ME1-Tg mice also had enhanced levels of Sp5 mRNA, suggesting Wnt/β-catenin pathway activation. A small molecule inhibitor of ME1 suppressed growth of human CRC cells in vitro, but had little effect on normal rat intestinal epithelial cells. Targeting of ME1 may add to the armentarium of therapies for cancers of the gastrointestinal tract.

Expression of cytosolic malic enzyme (ME1) is associated with disease progression in human oral squamous cell carcinoma

Malic enzyme 1 (ME1) is a multifunctional protein involved in glycolysis, the citric acid cycle, NADPH production, glutamine metabolism, and lipogenesis. It is overexpressed in various cancers. We examined the expression of ME1 in 119 oral squamous cell carcinomas (OSCCs) using immunohistochemistry. Malic enzyme 1 expression was moderate to strong in 57 (48%) OSCCs and correlated with pT, pN, clinical stage, and histological grade. In 37 cases with prognostic evaluation, moderate to strong ME1 expression indicated a worse prognosis than did weak ME1 expression. Malic enzyme 1 knockdown or inactivation by lanthanide inhibited cell proliferation and motility and suppressed the epithelial-mesenchymal transition in HSC3 human OSCC cells. Knockdown of ME1 also shifted energy metabolism from aerobic glycolysis and lactate fermentation to mitochondrial oxidative phosphorylation, and the redox status from reductive to oxidative. In a mouse tumor model, lanthanide suppressed tumor growth and increased survival time. These findings reveal that ME1 is a valid target for molecular therapy in OSCC.

Oligomeric interfaces as a tool in drug discovery: Specific interference with activity of malate dehydrogenase of Plasmodium falciparum in vitro

Malaria remains a major threat to human health, as strains resistant to current therapeutics are discovered. Efforts in finding new drug targets are hampered by the lack of sufficiently specific tools to provide target validation prior to initiating expensive drug discovery projects. Thus, new approaches that can rapidly enable drug target validation are of significant interest. In this manuscript we present the crystal structure of malate dehydrogenase from Plasmodium falciparum (PfMDH) at 2.4 Å resolution and structure-based mutagenic experiments interfering with the inter-oligomeric interactions of the enzyme. We report decreased thermal stability, significantly decreased specific activity and kinetic parameters of PfMDH mutants upon mutagenic disruption of either oligomeric interface. In contrast, stabilization of one of the interfaces resulted in increased thermal stability, increased substrate/cofactor affinity and hyperactivity of the enzyme towards malate production at sub-millimolar substrate concentrations. Furthermore, the presented data show that our designed PfMDH mutant could be used as specific inhibitor of the wild type PfMDH activity, as mutated PfMDH copies were shown to be able to self-incorporate into the native assembly upon introduction in vitro, yielding deactivated mutant:wild-type species. These data provide an insight into the role of oligomeric assembly in regulation of PfMDH activity and reveal that recombinant mutants could be used as probe tool for specific modification of the wild type PfMDH activity, thus offering the potential to validate its druggability in vivo without recourse to complex genetics or initial tool compounds. Such tool compounds often lack specificity between host or pathogen proteins (or are toxic in in vivo trials) and result in difficulties in assessing cause and effect-particularly in cases when the enzymes of interest possess close homologs within the human host. Furthermore, our oligomeric interference approach could be used in the future in order to assess druggability of other challenging human pathogen drug targets.

A Novel Malate Dehydrogenase 2 Inhibitor Suppresses Hypoxia-Inducible Factor-1 by Regulating Mitochondrial Respiration

We previously reported that hypoxia-inducible factor (HIF)-1 inhibitor LW6, an aryloxyacetylamino benzoic acid derivative, inhibits malate dehydrogenase 2 (MDH2) activity during the mitochondrial tricarboxylic acid (TCA) cycle. In this study, we present a novel MDH2 inhibitor compound 7 containing benzohydrazide moiety, which was identified through structure-based virtual screening of chemical library. Similar to LW6, compound 7 inhibited MDH2 activity in a competitive fashion, thereby reducing NADH level. Consequently, compound 7 reduced oxygen consumption and ATP production during the mitochondrial respiration cycle, resulting in increased intracellular oxygen concentration. Therefore, compound 7 suppressed the accumulation of HIF-1α and expression of its target genes, vascular endothelial growth factor (VEGF) and glucose transporter 1 (GLUT1). Moreover, reduction in ATP content activated AMPK, thereby inactivating ACC and mTOR the downstream pathways. As expected, compound 7 exhibited significant growth inhibition of human colorectal cancer HCT116 cells. Compound 7 demonstrated substantial anti-tumor efficacy in an in vivo xenograft assay using HCT116 mouse model. Taken together, a novel MDH2 inhibitor, compound 7, suppressed HIF-1α accumulation via reduction of oxygen consumption and ATP production, integrating metabolism into anti-cancer efficacy in cancer cells.

Fumarate analogs act as allosteric inhibitors of the human mitochondrial NAD(P)+-dependent malic enzyme

Human mitochondrial NAD(P)⁺-dependent malic enzyme (m-NAD(P)-ME) is allosterically activated by the four-carbon trans dicarboxylic acid, fumarate. Previous studies have suggested that the dicarboxylic acid in a trans conformation around the carbon-carbon double bond is required for the allosteric activation of the enzyme. In this paper, the allosteric effects of fumarate analogs on m-NAD(P)-ME are investigated. Two fumarate-insensitive mutants, m-NAD(P)-ME_R67A/R91A and m-NAD(P)-ME_K57S/E59N/K73E/D102S, as well as c-NADP-ME, were used as the negative controls. Among these analogs, mesaconate, trans-aconitate, monomethyl fumarate and monoethyl fumarate were allosteric activators of the enzyme, while oxaloacetate, diethyl oxalacetate, and dimethyl fumarate were found to be allosteric inhibitors of human m-NAD(P)-ME. The IC₅₀ value for diethyl oxalacetate was approximately 2.5 mM. This paper suggests that the allosteric inhibitors may impede the conformational change from open form to closed form and therefore inhibit m-NAD(P)-ME enzyme activity.

Hypolipidaemic effects of cyanidin 3-glucoside rich extract from black rice through regulating hepatic lipogenic enzyme activities

BACKGROUND

Black rice is rich in anthocyanins, especially cyanidin-3-glucoside (C3G). This study examined the effects of a C3G-rich extract from black rice on hyperlipidaemia induced by a high fat/cholesterol diet (HFCD) in rats.

RESULTS

Male Sprague-Dawley rats were fed either HFCD or HFCD containing 150 mg kg⁻¹ body weight C3G (HFCD+C3G) for 4 weeks. We found that C3G significantly decreased serum levels of total cholesterol, free cholesterol, triglycerides, and free fatty acids in rats fed a HFCD. Similarly, hepatic cholesterol and triglyceride levels and the activities of hepatic lipogenic enzymes (malic enzyme and glucose-6-phosphate dehydrogenase) were significantly reduced by C3G supplementation.

CONCLUSION

These results suggest that C3G can ameliorate HFCD-induced hyperlipidaemia in part by modulating the activities of hepatic lipogenic enzymes.

The mechanism behind the antileishmanial effect of zinc sulphate. II. Effects on the enzymes of the parasites

When used in vitro, zinc sulphate has a direct antileishmanial effect. To see if this effect involved the inhibition of the parasites' enzymes, extracts of the promastigotes and axenic amastigotes of Leishmania major (MHOM/IQ/93/MRC6) and L. tropica (MHOM/IQ/93/MRC2) were prepared. Zinc sulphate, at various concentrations, was then added to samples of these extracts before the activities, in the samples, of certain key enzymes of the Embden-Meyerhof pathway, hexose-monophosphate shunt and citric-acid cycle, and of two enzymes associated with virulence (protease and acid phosphatase), were determined. The zinc was found to inhibit every enzyme investigated, usually in a dose-dependent manner. Thus the direct antileishmanial effect of zinc may result, partially or entirely, from the inhibition of enzymes that are necessary for the parasites' carbohydrate metabolism and virulence.

[Comparative characterization of catalytic properties of mitochondrial and cytoplasmic forms of NAD-dependent malate dehydrogenase from the rat liver at norm and in toxic hepatitis]

The purification and comparative characterization of some catalytic properties of liver mitochondrial and cytosolic NAD-dependent malate dehydrogenase (NAD-MDH; EC 1.1.1.37) from normal rats and rats with experimental toxic hepatitis (ETH) have been carried out. It has been found that there are some differences in catalytic and regulatory properties of liver NAD-MDH from control animals and rats with ETH. It has been shown that Fe2+ and Cu2+ ions inhibit the enzyme, and the inhibition degree is different at norm and under toxic hepatitis. Ca2+ ions insignificantly activate cytosolic NAD-MDH under pathology and do not influence the mitochondrial isoform.

Effects of structural analogues of the substrate and allosteric regulator of the human mitochondrial NAD(P)+-dependent malic enzyme

Fumarate, a four-carbon trans dicarboxylic acid, is the allosteric activator of the human mitochondrial NAD(P)(+)-dependent malic enzyme (m-NAD(P)-ME). In this paper, we discuss the effects of the structural analogues of fumarate on human m-NAD(P)-ME. Succinate, a dicarboxylic acid with a carbon-carbon single bond, can also activate the enzyme, but the activating effect of succinate is less than that of fumarate. Succinamide, a diamide of succinate, cannot activate the enzyme and is a poor active-site inhibitor. The cis isomer of fumarate, maleic acid, significantly inhibits the ME activity, suggesting that the trans configuration of fumarate is crucial for operating the allosteric regulation of the enzyme. Other dicarboxylic acids, including glutaconic acid, malonic acid and alpha-ketoglutarate, cannot activate the enzyme and inversely inhibit enzyme activity. Our data suggest that these structural analogues are mainly active-site inhibitors, although they may enter the allosteric site to inhibit the enzyme. Furthermore, these data also suggest that the dicarboxylic acid must be in a trans conformation for allosteric activation of the enzyme.

Design, Synthesis, and Biological Evaluation of Plasmodium falciparum Lactate Dehydrogenase Inhibitors

Plasmodium falciparum lactate dehydrogenase (pfLDH) is a key enzyme for energy generation of malarial parasites and is a potential antimalarial chemotherapeutic target. It is known that the oxamate moiety, a pyruvate analog, alone shows higher inhibition against pfLDH than human LDHs, suggesting that it can be used for the development of selective inhibitors. Oxamic acid derivatives were designed and synthesized. Derivatives 5 and 7 demonstrated activities against pfLDH with IC50 values of 3.13 and 1.75 muM, respectively, and have 59- and 7-fold selectivity over mammalian LDH, respectively. They also have micromolar range activities against Plasmodium falciparum malate dehydrogenase (pfMDH), which may fill the role of pfLDH when the activity of pfLDH is reduced. Thus, certain members of these oxamic acid derivatives may have dual inhibitory activities against both pfLDH and pfMDH. It is presumed that dual LDH/MDH inhibitors would have enhanced potential as antimalarial drugs.

Conformational changes of maize and wheat NADP-malic enzyme studied by quenching of protein native fluorescence

Quenching of tryptophan fluorescence of maize and wheat NADP-malic enzyme by KI and acrylamide was studied after denaturating proteins with guanidine hydrochloride, and subjecting them to different pH values or temperatures. Protein unfolding by guanidine hydrochloride resulted in a red shift of the fluorescence spectrum, providing further support for the motion that several of the tryptophan residues evolved from an apolar to a polar environment. Protein denaturation was accompanied by an increase in the effective dynamic quenching constant values and by loss of the enzyme's activities. Thermal denaturation gave results consistent with the ones observed for chemical denaturation suggesting that a putative intermediate is involved in the denaturation process. Finally, exposure of both enzymes at various pH values allowed us to infer the number of accessible tryptophan residues in the different oligomeric conformations. The results suggest that the aggregation process seems to be different for each enzyme. Thus, as the maize enzyme associated from monomer to tetramer, one tryptophan residue would change from a polar to an apolar environment, while the association of the wheat enzyme would cause that two tryptophan residues to be excluded from quenching. Hitherto, quenching of the tryptophan fluorescence provides a good tool for studying conformational changes of proteins. The future availability of the crystal structures of plant NADP-malic enzymes will offer a good validation point for our model and the technology used.

Identification of domains involved in tetramerization and malate inhibition of maize C4-NADP-malic enzyme

C(4) photosynthetic NADP-malic enzyme (ME) has evolved from non-C(4) isoforms and gained unique kinetic and structural properties during this process. To identify the domains responsible for the structural and kinetic differences between maize C(4) and non-C(4)-NADP-ME several chimeras between these isoforms were constructed and analyzed. By using this approach, we found that the region flanked by amino acid residues 102 and 247 is critical for the tetrameric state of C(4)-NADP-ME. In this way, the oligomerization strategy of these NADP-ME isoforms differs markedly from the one that present non-plant NADP-ME with known crystal structures. On the other hand, the region from residue 248 to the C-terminal end of the C(4) isoform is involved in the inhibition by high malate concentrations at pH 7.0. The inhibition pattern of the C(4)-NADP-ME and some of the chimeras suggested an allosteric site responsible for such behavior. This pH-dependent inhibition could be important for regulation of the C(4) isoform in vivo, with the enzyme presenting maximum activity while photosynthesis is in progress.

Enzymatic and physico-chemical characteristics of recombinant cMDH and mMDH of Clonorchis sinensis

The cytosol and mitochondrial malate dehydrogenases (MDHs, EC 1.1.1.37) of Clonorchis sinensis were expressed in Escherichia coli as a fusion protein with a 6xHis and GST tag, respectively. The cytosol MDH of Clonorchis sinensis (Cs-cMDH) has higher resistibility to acid than mitochondrial MDH (Cs-mMDH). The Cs-cMDH also has higher heat resistibility and thermal stability than Cs-mMDH. Although there is only 22.8% identity between the amino acid sequences of Cs-cMDH and Cs-mMDH, they share several conserved residues. There are some differences between the circular dichroism spectra of Cs-cMDH and Cs-mMDH, but they have approximate percentages of helix. 4,4'-Bisdimethylamino diphenylcarbinol can decrease the Cs-mMDH activity but not the Cs-cMDH activity. Paraziquantel, metronidazole and albendazole did not inhibit the enzymes' activity, but adenosine 5'-monophosphate showed competitive inhibition to enzyme, with the Ki for Cs-cMDH and Cs-mMDH being 2.81 and 0.49 mM, respectively.

Leishmania major and Leishmania tropica: II. Effect of an immunomodulator, S(2) complex on the enzymes of the parasites

S(2) complex has been reported to have a direct antileishmanial effect. The possibility that the direct antileishmanial effect may be due to inhibition of key enzymes involved in glucose metabolism and/ or enzymes associated with virulence was investigated. Cell pellets were prepared from cultures of both axenic amastigotes and promastigotes of Leishmania major (MHOM/IQ/93/MRC6) and L. tropica (MHOM/IQ/93/MRC2). S(2) complex, at various concentrations, was added to the enzyme extracts prepared from the pellets. Results show that in the Embden-Meyerhof pathway, both hexokinase and glucose-phosphate isomerase but not fructophosphokinase were dose dependently inhibited. In the hexose-monophosphate shunt both glucose-6-phosphate dehydrogenase and ribose-5-phosphate isomerase were dose dependently inhibited. Malic dehydrogenase and malic enzyme from the citric-acid cycle were both dose dependently inhibited but succinate dehydrogenase from the same pathway was not inhibited. Both enzymes associated with virulence (protease and acid phosphatase), showed activation rather than inhibition at higher doses of S(2) complex. Thus, the direct antileishmanial effect of S(2) complex may result, partially or entirely, from the inhibition of enzymes that are necessary for the parasites' carbohydrate metabolism.

A rapid screening method to detect specific inhibitors of pyruvate orthophosphate dikinase as leads for C4 plant-selective herbicides

Plants using the C(4) photosynthetic pathway are highly represented among the world's worst weeds, with only 4 C(4) species being agriculturally productive (maize, sorghum, millet, and sugar cane). With the C(4) acid cycle operating as a biochemical appendage of C(3) photosynthesis, the additional enzymes involved in C(4) photosynthesis represent an attractive target for the development of weed-specific herbicides. The rate-limiting enzyme of this metabolic pathway is pyruvate orthophosphate dikinase (PPDK). PPDK, coupled with phosphoenolpyruvate carboxylase and nicotinamide adenine dinucleotide-malate dehydrogenase, was used to develop a microplate-based assay to detect inhibitors of enzymes of the C(4) acid cycle. The resulting assay had a Z' factor of 0.61, making it a high-quality assay able to reliably identify active test samples. Organic extracts of 6679 marine macroscopic organisms were tested within the assay, and 343 were identified that inhibited the 3 enzyme-coupled reaction. A high confirmation rate was achieved, with 95% of these hit extracts proving active again upon retesting. Sequential addition of phosphoenolpyruvate and oxaloacetate to the assay facilitated identification of 83 extracts that specifically inhibited PPDK.

[Free-radical oxidation and regulation of cytoplasmic NADP-dependent malate dehydrogenase in rat cardiomyocytes at norm and under ischemia]

Experimental ishemia of rat myocardium was accompanied by increase of light sum (S) and maximal intensity (Imax) of chemiluminescence, amount of a malonic dialdehyde and conjugated dienes in cytoplasmic fraction. The activity of NADP-dependent malate dehydrogenase (EC 1.1.1.82; NADP-MDH) was 1.6 times higher in rat heart under ischemia. NADP-MDH was purified from normal and ischemia-exposed rat myocardium. Using NADP-MDH purified enzyme preparations the values of Hill coefficient for oxaloacetate (1.83 +/- 0.07 and 1.50 +/- 0.10) and Km for NADPH (0.058 +/- 0.003 and 0.096 +/- 0.004 mM) were determined for the enzyme at norm and under ischemia respectively. Effects of Fe2+, Ca2+, Cu2+ ions, H2O2, oxidized and reduced glutathione, adenine nucleotides influence on functioning of NADP-MDH from rat heart at norm and under ischemic conditions have been investigated.

Basic residues play key roles in catalysis and NADP(+)-specificity in maize (Zea mays L.) photosynthetic NADP(+)-dependent malic enzyme

C(4)-specific (photosynthetic) NADP(+)-dependent malic enzyme (NADP(+)-ME) has evolved from C(3)-malic enzymes and represents a unique and specialized form, as indicated by its particular kinetic and regulatory properties. In the present paper, we have characterized maize (Zea mays L.) photosynthetic NADP(+)-ME mutants in which conserved basic residues (lysine and arginine) were changed by site-directed mutagenesis. Kinetic characterization and oxaloacetate partition ratio of the NADP(+)-ME K255I (Lys-255-->Ile) mutant suggest that the mutated lysine residue is implicated in catalysis and substrate binding. Moreover, this residue could be acting as a base, accepting a proton in the malate oxidation step. At the same time, further characterization of the NADP(+)-ME R237L mutant indicates that Arg-237 is also a candidate for such role. These results suggest that both residues may play 'back-up' roles as proton acceptors. On the other hand, Lys-435 and/or Lys-436 are implicated in the coenzyme specificity (NADP(+) versus NAD(+)) of maize NADP(+)-ME by interacting with the 2'-phosphate group of the ribose ring. This is indicated by both the catalytic efficiency with NADP(+) or NAD(+), as well as by the reciprocal inhibition constants of the competitive inhibitors 2'-AMP and 5'-AMP, obtained when comparing the double mutant K435/6L (Lys-435/436-->Ile) with wild-type NADP(+)-ME. The results obtained in the present work indicate that the role of basic residues in maize photosynthetic NADP(+)-ME differs significantly with respect to its role in non-plant MEs, for which crystal structures have been resolved. Such differences are discussed on the basis of a predicted three-dimensional model of the enzyme.

Inhibition of lactate dehydrogenase activity by polymeric NAD derivatives with different NAD densities

Polymeric NAD derivatives with different NAD densities were prepared by the copolymerization of NAD N6-[N-(N-acryloyl-1-methoxycarbonyl-5-aminopentyl)-propionamide] with acrylamide in different molar ratios, from 4 x 10(-4) to 1 x 10(-2). The cofactor activities of the polymeric NAD derivatives for lactate, yeast alcohol and horse liver alcohol dehydrogenases decreased with the increase in the NAD density of the polymer, and the lactate dehydrogenase showed no activity for the polymeric NAD derivatives with NAD densities of 2 x 10(-3) and 1 x 10(-2) mol/mol acrylamide polymerized. The polymeric NAD derivatives inhibited the activities of lactate and yeast alcohol dehydrogenases but did not inhibit those of horse liver alcohol and malate dehydrogenases. The polymeric NAD derivatives were competitive inhibitors with respect to NAD for the reaction of lactate dehydrogenase: the inhibition constant decreased from 2.2 microM to 0.02 microM with the increase in the NAD density from 4 x 10(-4) to 1 x 10(-2) mol/mol acrylamide polymerized.

Protective effect of curcumin during selenium induced toxicity on dehydrogenases in hepatic tissue

Selenium administration resulted in a marked decrease in the activity levels of the liver succinate dehydrogenase, malate dehydrogenase, and lactate dehydrogenase while pyruvate dehydrogenase increased significantly (P<0.001) in the wistar rat. The degree of decrease of these enzymes was significantly less (P<0.001) when rats were treated with curcumin, a natural constituent Curcuma longa. Curcumin seems to prevent oxidative damage mediated through selenium and protect the dehydrogenases possibly through its anti-oxidative property.

Metal-Induced reversible structural interconversion of human mitochondrial NAD(P)+-dependent malic enzyme

Human mitochondrial NAD(P)+-dependent malic enzyme was strongly inhibited by Lu3+. The X-ray crystal structures indicated a structural change between the metal-free and Lu3+-containing enzymes (Yang Z, Batra R, Floyd DL, Hung HC, Chang GG, Tong L. Biochem Biophys Res Commun 2000;274:440-444). We characterized the reversible slow-binding mechanism and the structural interconversion between Mn2+- and Lu3+-containing human mitochondrial malic enzymes. When Lu3+ was added, the activity of the human enzyme showed a downward curve over time, similar to that of the pigeon enzyme. The rate of the transformation (k(obs)) from the initial rate to the steady-state rate increased hyperbolically with the concentration of Lu3+, suggesting the involvement of an isomerization step. Lu3+ had a much higher affinity for the isomerized form (K*(i,Lu (app)) = 4.8 microM) than that of the native form (K(i,Lu (app)) = 148 microM). When an excess of Mn2+ was added to the Lu3+-inhibited enzyme, assays of the kinetic activity showed an upward trend, indicating reactivation. This result also indicated that the reactivation was a slow process. Fluorescence quenching experiments confirmed that the Lu3+-induced isomerization was completely reversible. The dynamic quenching constants for the metal-free, Mn2+-containing, and Lu3+-containing enzyme were 3.08, 3.07, and 3.8 M(-1), respectively. When the Lu3+-containing enzyme was treated with excess Mn2+, the dynamic quenching constant returned to the original value (3.09 M(-1)). These results indicated that binding of Mn2+ did not induce any conformational change in the enzyme. The open form transformed to the closed form only after substrate binding. Lu3+, on the other hand, transformed the open form into a catalytically inactive form. Excess Mn2+ could replace Lu3+ in the metal binding site and convert the inactive form back into the open form. This reversible process was slow in both directions because of the same but opposite structural change involved.

Identification and prediction of promiscuous aggregating inhibitors among known drugs

Some small molecules, often hits from screening, form aggregates in solution that inhibit many enzymes. In contrast, drugs are thought to act specifically. To investigate this assumption, 50 unrelated drugs were tested for promiscuous inhibition via aggregation. Each drug was tested against three unrelated model enzymes: beta-lactamase, chymotrypsin, and malate dehydrogenase, none of which are considered targets of these drugs. To be judged promiscuous, the drugs had to inhibit all three enzymes, do so in a time-dependent manner, be sensitive to detergent and to enzyme concentration, and form particles detectable by light scattering. Of the 50 drugs tested, 43 were nonpromiscuous by these criteria. Surprisingly, four of the drugs showed promiscuous, aggregation-based inhibition at concentrations below 100 microM: clotrimazole, benzyl benzoate, nicardipine, and delavirdine. Three other drugs also behaved as aggregation-based inhibitors, but only at high concentrations (about 400 microM). To investigate possible structure-activity relationships among promiscuous drugs, five analogues of the antifungal clotrimazole were studied. Three of these, miconazole, econazole, and sulconazole, were promiscuous but the other two, fluconazole and ketoconazole, were not. Using recursive partitioning, these experimental results were used to develop a model for predicting aggregate-based promiscuity. This model correctly classified 94% of 111 compounds-47 aggregators and 64 nonaggregators-that have been studied for this effect. To evaluate the model, it was used to predict the behavior of 75 drugs not previously investigated for aggregation. Several preliminary points emerge. Most drugs are not promiscuous, even at high concentrations. Nevertheless, at high enough concentrations (20-400 microM), some drugs can aggregate and act promiscuously, suggesting that aggregation may be common among small molecules at micromolar concentrations, at least in biochemical buffers.

Kinase inhibitors: not just for kinases anymore

Kinase inhibitors are widely employed as biological reagents and as leads for drug design. Their use is often complicated by their lack of specificity. Although binding conserved ATP sites accounts for some of their nonspecificity, some compounds inhibit proteins not known to bind ATP. It has been found that promiscuous hits from high-throughput screening may act as aggregates. To explore whether this mechanism might explain the action of widely used nonspecific kinase inhibitors, 15 such compounds were studied. Eight of these, rottlerin, quercetin, K-252c, bisindolylmaleimide I, bisindolylmaleimide IX, U0126, indirubin, and indigo, inhibited three diverse non-kinase enzymes. Inhibition was time-dependent and sensitive to enzyme concentration; by light scattering, the compounds formed particles of 100-1000 nm diameter. These observations suggest that these eight kinase inhibitors, at least at micromolar concentrations, are promiscuous and act as aggregates. Results obtained from the use of these compounds at micromolar or higher concentrations against individual enzymes should be interpreted cautiously.

Kinetic and molecular modeling of nucleoside and nucleotide inhibition of malate dehydrogenase

We studied the inhibition of mitochondrial malate dehydrogenase (mMDH) by the nucleotides cAMP, AMP, ADP, ATP. The experimental kinetic studies showed that the nucleotides were competitive inhibitors and that cAMP was probably the most potent inhibitor. To explain these observations, we used molecular modeling to determine the location, orientation, and relative binding energy of the nucleotides to mMDH. The order of the calculated binding energies, from lowest (most favorable) to highest, was cAMP, AMP, ADP, and ATP, which corresponded somewhat to the order of the experimentally determined inhibition constants.

Molecular mechanism for the regulation of human mitochondrial NAD(P)+-dependent malic enzyme by ATP and fumarate

The regulation of human mitochondrial NAD(P)+-dependent malic enzyme (m-NAD-ME) by ATP and fumarate may be crucial for the metabolism of glutamine for energy production in rapidly proliferating tissues and tumors. Here we report the crystal structure at 2.2 A resolution of m-NAD-ME in complex with ATP, Mn2+, tartronate, and fumarate. Our structural, kinetic, and mutagenesis studies reveal unexpectedly that ATP is an active-site inhibitor of the enzyme, despite the presence of an exo binding site. The structure also reveals the allosteric binding site for fumarate in the dimer interface. Mutations in this binding site abolished the activating effects of fumarate. Comparison to the structure in the absence of fumarate indicates a possible molecular mechanism for the allosteric function of this compound.

Inhibition of endogenous thyroid hormone receptor-beta and peroxisome proliferator-activated receptor-alpha activities by humic acid in a human-derived liver cell line

Humic acid (HA), know to be ubiquitous in the natural environment, is present in almost all soil, surface water, and plants. Earlier studies indicate that HA can affect thyroid economy via binding with iodide, inhibiting both thyroid peroxidase and hepatic 5'-deiodinase in rodents. However, the effect of HA, a peroxisome proliferator in rodents, on thyroid hormone receptor (TR) and peroxisome proliferator-activated receptor (PPAR) in human cells has not yet been examined. In this study, we demonstrate that the malic enzyme activity and the transcriptional activities of endogenous TR and PPAR were inhibited after treatment with HA in human hepatocyte Chang liver cell line. Although the protein expression levels of TR-beta, PPAR-alpha and retinoid X receptor-alpha (RXRalpha) were not changed significantly by HA treatment, both the binding abilities of endogenous TR-beta on thyroid hormone response element (TRE) and PPAR-alpha on the PPAR response element (PPRE) were inhibited by HA treatment. The study of the subcellular distribution of HA, relying on the inherent HA fluorescence, showed that HA distributed in the intracellular compartments including cytoplasm and nucleus. The 50% binding inhibition values (CI(50)) of HA on ME-TRE (malic enzyme gene-TRE) and ACOX-PPRE (acylCoA oxidase gene-PPRE) were 19.31 and 19.94 microg/mL, respectively. These results suggest that HA-induced endemic goiter may link in part to the disruption of TRbeta and PPARalpha function in human Chang liver cells. This model may be useful in the investigation of environmental goitrogens.

Glyconeogenic pathway in isolated skeletal muscles of rats

Although the conversion of lactate to glycogen (glyconeogenesis) in muscle was demonstrated a long time ago, the biochemical reactions responsible for this process are still a controversial matter. In the present study, advantage was taken from the specific inhibition induced by phenylalanine on muscle pyruvate kinase (PK) to investigate the role of reverse PK activity in muscle glyconeogenesis. Addition of phenylalanine to the incubation medium of a preparation of isolated, intact skeletal muscles that maintain metabolic activity for several hours reduced by 50% the rate of incorporation of [14C]lactate or [14C]bicarbonate into muscle glycogen. Muscle extracts presented high levels of maximal activity of PK in the reverse direction, which was completely blocked in the presence of phenylalanine. In contrast, mercaptopicolinic acid, an inhibitor of phosphoenolpyruvate carboxykinase (PEPCK), did not affect the incorporation of 14C from either lactate or bicarbonate into muscle glycogen. Maximal PEPCK activity was much lower in muscle extracts than in gluconeogenic or glyceroneogenic tissues and was suppressed in the presence of mercaptopicolinic acid. The data suggest that a reversal of the metabolic flux through the reaction catalyzed by PK contributes to the accumulation of lactate-derived glycogen that occurs in skeletal muscle under certain physiological conditions.

Dietary protein concentration regulates the mRNA expression of chicken hepatic malic enzyme

Chicken hepatic malic enzyme activity varies with dietary protein content. The mechanisms responsible for this alteration in activity are unclear. In a series of four experiments, broiler chicks were allowed free access for 1.5, 3, 6 or 24 h to a low (13 g/100 g diet), basal (22 g/100 g diet) or high (40 g/100 g diet) protein diet. The diets were isocaloric and had equal concentrations of dietary fat. Hepatic malic enzyme mRNA expression and enzyme activity as well as total liver lipid concentration were examined for each experimental duration. There were no differences in the expression of the mRNA for malic enzyme at 1.5 h, but at 3, 6 and 24 h, malic enzyme mRNA expression was significantly (P < 0.05) reduced in chicks fed the high protein diet and significantly enhanced in chicks fed the low protein diet compared with chicks fed the basal diet. Hepatic malic enzyme activities and total lipid concentration were not different among the chicks fed the different diets at 1.5 and 3 h. At 6 and 24 h, malic enzyme activity and total liver lipid concentration were both significantly greater in birds fed the low protein diet compared with levels in the birds fed the other two diets. In birds fed the high protein diet, malic enzyme activity and total liver lipid concentration were significantly reduced at 24 h compared with birds fed the basal diet. In a final experiment, the observed differences in malic enzyme mRNA expression at 6 h were confirmed when chicks were given access to isocaloric diets with the same protein levels as the initial 4 experiments, but with the dietary concentration of carbohydrate held constant. The results suggest that previously observed alterations in the activity of malic enzyme, which were correlated with dietary protein intake, are due to rapid changes in the mRNA expression of this enzyme.

Relationships between inhibition constants, inhibitor concentrations for 50% inhibition and types of inhibition: new ways of analysing data

The concentration of an inhibitor that decreases the rate of an enzyme-catalysed reaction by 50%, symbolized i(0.5), is often used in pharmacological studies to characterize inhibitors. It can be estimated from the common inhibition plots used in biochemistry by means of the fact that the extrapolated inhibitor concentration at which the rate becomes infinite is equal to -i(0.5). This method is, in principle, more accurate than comparing the rates at various different inhibitor concentrations, and inferring the value of i(0.5) by interpolation. Its reciprocal, 1/i(0.5), is linearly dependent on v(0)/V, the uninhibited rate divided by the limiting rate, and the extrapolated value of v(0)/V at which 1/i(0.5) is zero allows the type of inhibition to be characterized: this value is 1 if the inhibition is strictly competitive; greater than 1 if the inhibition is mixed with a predominantly competitive component; infinite (i.e. 1/i(0.5) does not vary with v(0)/V) if the inhibition is pure non-competitive (i.e. mixed with competitive and uncompetitive components equal); negative if the inhibition is mixed with a predominantly uncompetitive component; and zero if it is strictly uncompetitive. The type of analysis proposed has been tested experimentally by examining inhibition of lactate dehydrogenase by oxalate (an uncompetitive inhibitor with respect to pyruvate) and oxamate (a competitive inhibitor with respect to pyruvate), and of cytosolic malate dehydrogenase by hydroxymalonate (a mixed inhibitor with respect to oxaloacetate). In all cases there is excellent agreement between theory and experiment.

In vivo and in vitro effects of prolactin and growth hormone on lipid metabolism in a teleost, Anabas testudineus (Bloch)

Prolactin (PRL) has an important role in the regulation of water and electrolyte homeostasis in teleosts. The present study was designed to evaluate the role of PRL and GH on malic enzyme (ME), glucose-6-phosphate dehydrogenase (G6PDH) and isocitrate dehydrogenase (ICDH) in Anabas testudineus. Ovine prolactin significantly inhibited ME, G6PDH and ICDH activities when administered in vivo compared to vehicle treated controls. In vivo administration of PRL reversed the action of bromocryptine on enzyme activities. Ovine growth hormone in vivo also modified the effect of bromocryptine but not to the level of prolactin. Combined action of PRL+GH in vivo was most effective in keeping the enzyme activities at normal level after bromocryptine treatment. Prolactin in vitro also reversed the action of bromocryptine on enzyme activities, while GH in vitro failed to do so. Hence, prolactin seems to have an inhibitory effect on lipid metabolism in this teleost. Combined action of PRL+GH is more prominent in in vivo conditions at low PRL levels. Dopaminergic pathways may be involved in the control of prolactin and to some extent on growth hormone secretion.

Substrate and cofactor binding to fluorescently labeled cytoplasmic malate dehydrogenase

Cytoplasmic malate dehydrogenase (cMDH) is a key enzyme in several metabolic pathways. Though its activity has been examined extensively, there are lingering mechanistic uncertainties involving substrate and cofactor binding. To more completely understand this enzyme's interactions with cofactor and substrate ligands, a fluorescent reporter group was introduced into the enzyme's structure. This was accomplished by selective modification of Cys 110. The reaction placed an aminonaphthaline sulfonic acid group near the enzyme's active site. Substrate, inhibitor, and NAD binding activities were characterized using changes in this label's fluorescence. Results demonstrated that both substrate and cofactor molecules bound to the enzyme in the absence of their companion ligands. This is in contrast to strictly ordered cofactor then substrate binding as has been suggested by kinetic analyses of closely related enzymes. Binding results also indicated that the cofactor, NAD, bound to cMDH in a negatively cooperative manner, but substrates and the inhibitor, hydroxymalonate, bound non-cooperatively. Multiple substrate binding modes were identified and interactions between substrate and cofactor binding were found.

Slow binding of metal ions to pigeon liver malic enzyme: a general case

Pigeon liver malic enzyme was inhibited by lutetium ion through a slow-binding process, which resulted in a concave down tracing of the enzyme activity assay. The fast initial rates were independent of lutetium ion concentration, while the slow steady-state rates decreased with increasing Lu(3+) concentration. The observed rate constant for the transition from initial rate to steady-state rate, k(obs), exhibited saturation kinetics as a function of Lu(3+) concentration, suggesting the involvement of an isomerization process between two enzyme forms (R-form and T-form). The binding affinity of Lu(3+) to the R-form is weaker (K(d,Lu) = 14 microM) than that of Mn(2+) (K(m,Mn) = 1.89 microM); however, Lu(3+) has much tighter binding affinity with the T-form ( = 0.83 microM). Lu(3+) was shown to be a competitive inhibitor with respect to Mn(2+), which suggests that Lu(3+) and Mn(2+) are competing for the same metal binding site of the enzyme. These observations are in accordance with the available crystal structure information, which shows a distorted active site region of the Lu(3+)-containing enzyme. Other divalent cations, i.e., Fe(2+), Cu(2+), or Zn(2+), also act as time-dependent slow inhibitors for malic enzyme. The dynamic quenching constants of the intrinsic fluorescence for the metal-free and Lu(3+)-containing enzymes are quite different, indicating the conformational differences between the two enzyme forms. The secondary structure of these two enzyme forms, on the other hand, was not changed. The above results indicated that replacement of the catalytically essential Mn(2+) by other metal ions leads to a slow conformational change of the enzyme and consequently alters the geometry of the active site. The transformed enzyme conformation, however, is unfavorable for catalysis. Both the chemical nature of the metal ion and its correct coordination in the active site are essential for catalysis.

The role of active site arginines of sorghum NADP-malate dehydrogenase in thioredoxin-dependent activation and activity

The activation of sorghum NADP-malate dehydrogenase is initiated by thiol/disulfide interchanges with reduced thioredoxin followed by the release of the C-terminal autoinhibitory extension and a structural modification shaping the active site into a high efficiency and high affinity for oxaloacetate conformation. In the present study, the role of the active site arginines in the activation and catalysis was investigated by site-directed mutagenesis and arginyl-specific chemical derivatization using butanedione. Sequence and mass spectrometry analysis were used to identify the chemically modified groups. Taken together, our data reveal the involvement of Arg-134 and Arg-204 in oxaloacetate coordination, suggest an indirect role for Arg-140 in substrate binding and catalysis, and clearly confirm that Arg-87 is implicated in cofactor binding. In contrast with NAD-malate dehydrogenase, no lactate dehydrogenase activity could be promoted by the R134Q mutation. The decreased susceptibility of the activation of the R204K mutant to NADP and its increased sensitivity to the histidine-specific reagent diethylpyrocarbonate indicated that Arg-204 is involved in the locking of the active site. These results are discussed in relation with the recently published NADP-MDH three-dimensional structures and the previously established three-dimensional structures of NAD-malate dehydrogenase and lactate dehydrogenase.

The cytotoxic lipid peroxidation product, 4-hydroxy-2-nonenal, specifically inhibits decarboxylating dehydrogenases in the matrix of plant mitochondria

4-Hydroxy-2-nonenal (HNE), a cytotoxic product of lipid peroxidation, inhibits O(2) consumption by potato tuber mitochondria. 2-Oxoglutarate dehydrogenase (OGDC), pyruvate dehydrogenase complex (PDC) (both 80% inhibited) and NAD-malic enzyme (50% inhibited) are its major targets. Mitochondrial proteins identified by reaction with antibodies raised to lipoic acid lost this antigenicity following HNE treatment. These proteins were identified as acetyltransferases of PDC (78 kDa and 55 kDa), succinyltransferases of OGDC (50 kDa and 48 kDa) and glycine decarboxylase H protein (17 kDa). The significance of the effect of these inhibitions on the impact of lipid peroxidation and plant respiratory functions is discussed.

Potent and competitive inhibition of malic enzymes by lanthanide ions

The catalytic activity of malic enzyme (ME), a member of a new class of oxidative decarboxylases, requires the presence of divalent cations (Mn(2+), Mg(2+), and others). The crystal structure at 2.9 A resolution of human mitochondrial NAD(+)-dependent malic enzyme in a ternary complex with NAD(+) and the lanthanide ion Lu(3+), which has similar radius as Mn(2+), reveals a new conformation of the enzyme. The active site in this ternary complex is in an open form, while the organization of the tetramer of the enzyme actually resembles that with a closed active site. The Lu(3+) ion is bound to the enzyme at the same site as Mn(2+). Kinetic studies showed that Lu(3+) is a potent inhibitor of both the human NAD(P)(+)-dependent ME and the NADP(+)-dependent ME from pigeon liver, and is competitive with respect to the divalent cation, consistent with the structural information.

Kinetics of the coupled reaction catalysed by a fusion protein of yeast mitochondrial malate dehydrogenase and citrate synthase

The mechanistic implications of the kinetic behaviour of a fusion protein of mitochondrial malate dehydrogenase and citrate synthase have been reanalysed in view of predictions based on experimentally determined kinetic parameter values for the dehydrogenase and synthase activities of the protein. The results show that the time-course of citrate formation from malate in the coupled reaction catalysed by the fusion protein can be most satisfactorily accounted for in terms of a free-diffusion mechanism when consideration is taken to the inhibitory effects of NADH and oxaloacetate on the malate dehydrogenase activity. The effect of aspartate aminotransferase on the coupled reaction is likewise fully consistent with that expected for a free-diffusion mechanism. It is concluded that no tenable kinetic evidence is available to support the proposal that the fusion protein catalyses citrate formation from malate by a mechanism involving channelling of the intermediate oxaloacetate.

Structure of a closed form of human malic enzyme and implications for catalytic mechanism

Malic enzymes are widely distributed in nature and have many biological functions. The crystal structure of human mitochondrial NAD(P)+-dependent malic enzyme in a quaternary complex with NAD+, Mn++ and oxalate has been determined at 2.2 A resolution. The structures of the quaternary complex with NAD+, Mg++, tartronate or ketomalonate have been determined at 2.6 A resolution. The structures show the enzyme in a closed form in these complexes and reveal the binding modes of the cation and the inhibitors. The divalent cation is coordinated in an octahedral fashion by six ligating oxygens, two from the substrate/inhibitor, three from Glu 255, Asp 256 and Asp 279 of the enzyme, and one from a water molecule. The structural information has significant implications for the catalytic mechanism of malic enzymes and identifies Tyr 112 and Lys 183 as possible catalytic residues. Changes in tetramer organization of the enzyme are also observed in these complexes, which might be relevant for its cooperative behavior and allosteric control.

Purification and characterization of the 16-kDa heat-shock-responsive protein from the thermophilic cyanobacterium Synechococcus vulcanus, which is an alpha-crystallin-related, small heat shock protein

A 16-kDa protein, one of the major proteins that accumulates upon heat-shock treatment in the thermophilic cyanobacterium Synechococcus vulcanus, was purified to apparent homogeneity. The N-terminal and internal amino acid sequences of the protein exhibited a homology to the alpha-crystallin-related, small heat shock proteins from other organisms. The protein was designated HspA. Size-exclusion chromatography and nondenaturing gel electrophoresis demonstrated that HspA formed a large homo-oligomer consisting of 24 subunits. It prevented the aggregation of porcine malic dehydrogenase at 45 degrees C and 50 degrees C and citrate synthase at 50 degrees C. The activity of the malic dehydrogenase, however, was not protected under these heat-shock conditions or reactivated after a shift in temperature from 45 or 50 degrees C to 21 degrees C. HspA was able to enhance the refolding of chemically denatured rabbit muscle lactate dehydrogenase in an ATP-independent manner. A homologue to the 16-kDa protein was also found to be induced upon heat-shock treatment in the mesophilic cyanobacterium Synechocystis sp. PCC 6803.

Structural basis for light activation of a chloroplast enzyme: the structure of sorghum NADP-malate dehydrogenase in its oxidized form

Some key chloroplast enzymes are activated by light via a ferredoxin-thioredoxin reduction system which reduces disulfide bridges in the enzymes. We describe for the first time the structural basis for the redox activation of a chloroplast enzyme, the NADP-dependent malate dehydrogenase (MDH) from Sorghum vulgare whose structure has been determined and refined at 2.4 A resolution. In addition to the normal structural components of MDHs, the enzyme exhibits extensions at both the N- and C-termini, each of which contains a regulatory disulfide bridge which must be reduced for activation. The N-terminal disulfide motif is inserted in a cleft between the two subunits of the dimer, thereby locking the domains in each subunit. The C-terminal disulfide keeps the C-terminal residues tight to the enzyme surface and blocks access to the active site. Reduction of the N-terminal disulfide would release the stopper between the domains and give the enzyme the necessary flexibility. Simultaneous reduction of the C-terminal disulfide would free the C-terminal residues from binding to the enzyme and make the active site accessible.

Determination of the kinetic and chemical mechanism of malic enzyme using (2R,3R)-erythro-fluoromalate as a slow alternate substrate

(2R,3R)-erythro-Fluoromalate, but not the threo isomer, is a slow substrate for chicken liver malic enzyme with either NADP or 3-acetylpyridine-NADP (APADP) as the other substrate. The Km for erythro-fluoromalate is similar to that of malate, but the turnover number with NADP is 3300-fold slower, although 5.5-fold faster with APADP than with NADP. Deuteration of fluoromalate at C-2 gave an isotope effect on V/K of 1.39 with NADP and 3.32 with APADP. With NADP, the 13C isotope effects at C-4 were 1.0490 with unlabeled and 1.0364 with deuterated fluoromalate. With APADP, the corresponding values were 1.0138 and 1.0087. These data show that the mechanism is stepwise with both nucleotide substrates, in contrast to the reaction of malate and APADP, which was postulated to be concerted by Karsten et al. [Karsten, W. E., and Cook, P. F. (1994) Biochemistry 33, 2096-2103], a conclusion recently shown to be correct by Edens et al. [Edens, W. A., Urbauer, J. L., and Cleland, W. W. (1997) Biochemistry 36, 1141-1147]. To explain the effect of deuteration on the 13C isotope effect with APADP, it is necessary to assume a secondary 13C isotope effect at C-4 on the hydride transfer step of approximately 1.0064 (assuming 5.7 as the intrinsic primary deuterium isotope effect and 1.054 as the product of the 13C equilibrium isotope effect on hydride transfer and the intrinsic 13C isotope effect on decarboxylation). The secondary 13C isotope effect on hydride transfer is thought to result from hyperconjugation between the carbonyl group and C-4 of the enzyme-bound fluorooxaloacetate intermediate.

The autoinhibition of sorghum NADP malate dehydrogenase is mediated by a C-terminal negative charge

The chloroplastic NADP malate dehydrogenase is completely inactive in its oxidized form and is activated by thiol/disulfide interchange with reduced thioredoxin. To elucidate the molecular mechanism underlying the absence of activity of the oxidized enzyme, we used site-directed mutagenesis to delete or substitute the two most C-terminal residues (C-terminal Val, penultimate Glu, both bearing negative charges). We also combined these mutations with the elimination of one or both of the possible regulatory N-terminal disulfides by mutating the corresponding cysteines. Proteins mutated at the C-terminal residues had no activity in the oxidized form but were partially inhibited when pretreated with the histidine-specific reagent diethyl pyrocarbonate before activation, showing that the active site was partially accessible. Proteins missing both N-terminal regulatory disulfides reached almost full activity without activation upon elimination of the negative charge of the penultimate Glu. These results strongly support a model where the C-terminal extension is docked into the active site through a negatively charged residue, acting as an internal inhibitor. They show also that the reduction of both N-terminal bridges is necessary to release the C-terminal extension from the active site. This is the first report for a thiol-activated enzyme of a regulatory mechanism resembling the well known intrasteric inhibition of protein kinases.

Proteins of newly isolated mutants and the amino-terminal proline are essential for ubiquitin-proteasome-catalyzed catabolite degradation of fructose-1,6-bisphosphatase of Saccharomyces cerevisiae

Addition of glucose to cells of the yeast Saccharomyces cerevisiae growing on a non-fermentable carbon source leads to selective and rapid degradation of fructose-1,6-bisphosphatase. This so called catabolite inactivation of the enzyme is brought about by the ubiquitin-proteasome system. To identify additional components of the catabolite inactivation machinery, we isolated three mutant strains, gid1, gid2, and gid3, defective in glucose-induced degradation of fructose-1,6-bisphospha-tase. All mutant strains show in addition a defect in catabolite inactivation of three other gluconeogenic enzymes: cytosolic malate dehydrogenase, isocitrate lyase, and phosphoenolpyruvate carboxykinase. These findings indicate a common mechanism for the inactivation of all four enzymes. The mutants were also impaired in degradation of short-lived N-end rule substrates, which are degraded via the ubiquitin-proteasome system. Site-directed mutagenesis of the amino-terminal proline residue yielded fructose-1,6-bisphosphatase forms that were no longer degraded via the ubiquitin-proteasome pathway. All amino termini other than proline made fructose-1,6-bisphosphatase inaccessible to degradation. However, the exchange of the amino-terminal proline had no effect on the phosphorylation of the mutated enzyme. Our findings suggest an essential function of the amino-terminal proline residue for the degradation process of fructose-1,6-bisphosphatase. Phosphorylation of the enzyme was not necessary for degradation to occur.

Epoxyethane-/ethynesulfonamides with antifilarial activities. Degradation kinetics and inhibitory effect on filarial malate dehydrogenase and lactate dehydrogenase

Some epoxyethane-/ethynesulfonamides had shown strong filaricidal activity with inconstant reproducibility as a result of a lack of stability in aqueous solution. The degradation in hydroxylic and aprotic solutions of two epoxyethanesulfonamides and one ethynesulfonamide was investigated using TLC, HPLC, GC and mass spectrometry. For both epoxydes, the degradation rate followed first-order kinetics and was more rapid in hydroxylic than in aprotic solutions. The degradation increased with the temperature whereas it was not modified with and without light exposure. Four kinds of degradation products were found: the first one involved the oxidation of the epoxyde bond, the second the breaking of the N-S bond, the third a desulfonation product and the fourth was not identified. In contrast, the stability of ethynesulfonamide was better than those of epoxyethanesulfonamide. These results suggest that epoxyethanesulfonamides should be kept at +4 degrees C before being injected to animals during the study of biological activity. Since epoxyde compounds are known to have inhibitory effects on parasite energy metabolism enzymes, the compunds were evaluated on two major filarial enzymes: lactate dehydrogenase (LDH) and cytoplasmic malate dehydrogenase (MDH). Both epoxyethanesulfonamides showed only a slight inhibitory effect on filarial LDH and MDH confirming the evidence that the main mode of action of these compounds remains to discover. Moreover, ethynesulfonamide and the degradation products of both epoxyethane-sulfonamides had no effect on LDH and MDH.