Human mitochondrial NADP-dependent isocitrate dehydrogenase in man-mouse somatic cell hybrids.

Human mitochondrial NADP-dependent isocitrate dehydrogenase (IDH-2) is expressed in man-mouse somatic cell hybrids as a dimeric molecule. The gene specifing this enzyme was observed to be syntenic with the mannose phosphate isomerase locus in the 56 primary man-mouse clones in this series. The human IDH-2 locus, therefore, may be assigned to chromosome 15.     

Purification and various properties of hyaloplasmic NADP-dependent isocitrate dehydrogenase from the rabbit adrenal gland

NADP-dependent isocitrate dehydrogenase was isolated from the hyaloplasmic fraction of rabbit adrenal glands and purified by ammonium sulfate and polyethylene glycol fractionation and chromatography on DEAE-Sephadex A-50 to a specific activity of 26.8 U/mg with a 53% yield. Polyacrylamide gel electrophoresis revealed one distinct protein band with mobility corresponding to Mr approximately 50 000 in the presence of SDS. Data from gel filtration suggest that the detergent-untreated isocitrate dehydrogenase has a twice as great molecular mass, which is indicative of its dimeric structure of identical subunits. The pH optimum for the adrenal isocitrate dehydrogenase-catalyzed reaction is 7.5-7.7; the apparent activation energy is 61.3 kJ X mol-1. Mn2+ activate the enzyme more effectively than Mg2+. The curve for the dependence of the isocitrate dehydrogenase reaction rate versus D-isocitrate and NADP concentrations is S-shaped. At low substrate or coenzyme concentrations the Hill coefficient is 2.0 and 1.9, respectively, which serves as a kinetic attribute of positive cooperativity of their interaction with isocitrate dehydrogenase. The concentrations of D-isocitrate and NADP providing for the half-maximal rate of the reaction are 3.8 and 6.6 microM, respectively.     

Regulation of ethanol-induced toxicity by mitochondrial NADP-dependent isocitrate dehydrogenase

Chronic alcohol administration has been known to increase peroxynitrite hepatotoxicity by enhancing concomitant production of nitric oxide and superoxide. We previously reported that control of the mitochondrial redox balance and the cellular defense against oxidative damage are primary functions of mitochondrial NADP⁺-dependent isocitrate dehydrogenase (IDPm) through to supply NADPH for antioxidant systems. In the present study, we demonstrate that modulation of IDPm expression in HepG2 cells regulates ethanol-induced toxicity. We observed the significantly enhanced protection to cell killing, lipid peroxidation, protein oxidation, oxidative DNA damage, and decrease in generation of intracellular reactive oxygen species and reactive nitrogen species in IDPm-overexpressed cells compared to control cells upon exposure to ethanol. In contrast, transfection of HepG2 cells with IDPm short interfering RNA markedly decreased the expression of IDPm, modulating cellular redox status and subsequently enhancing the susceptibility of ethanol-induced toxicity. These studies support the hypothesis that IDPm plays an important role in regulating the toxicity induced by ethanol presumably through maintaining the cellular redox status.     

Mitochondrial NADP-dependent isocitrate dehydrogenase knockdown inhibits tumorigenicity of melanoma cells

The potent cytotoxicity of reactive oxygen species (ROS) can cause various diseases but may also serve as a powerful weapon capable of destroying cancer cells. Although the balance between generation and elimination of ROS is maintained by the proper function of antioxidative systems, the severe disturbance of cellular redox status may cause various damages, leading to cell death. Mitochondrial NADP⁺-dependent isocitrate dehydrogenase (IDH2), an NADPH-generating enzyme, is one of the major antioxidant and redox regulators in mitochondria. To assess the effect of IDH2 knockdown in the malignancy process, we generated B16F10 melanoma cells stably transfected either with the cDNA for mouse IDH2 cloned in antisense orientation or with a control vector. Mice injected with B16F10 cells harboring IDH2 downregulation showed a dramatic reduction in tumor progression in comparison to mice administered control cells. This effect might be secondary to a shift from a reducing to an oxidative state in tumor cells. The tumor tissue of mice administered B16F10 cells transfected with the IDH2 cDNA exhibited induction of apoptosis and downregulation of angiogenesis markers. These observations demonstrate that reduction of IDH2 levels in malignant cells has anti-tumorigenic effects and suggest that IDH2 is a potential target for cancer therapy.     

Antisense inhibition of cytosolic NADP-dependent isocitrate dehydrogenase in transgenic potato plants

Cytosolic NADP-dependent isocitrate dehydrogenase (cyt-NADP-ICDH; EC 1.1.1.42) has been suggested to play a major role in the production of 2-oxoglutarate, an important precursor for amino acid synthesis. Using an antisense RNA approach under the control of the cauliflower mosaic virus 35S promoter, transgenic potato plants were created in which NADP-ICDH activity was reduced to 8% of the wild-type level in leaves. Residual activity was almost completely due to mitochondrial and chloroplastic NADP-ICDH isoforms. Activity staining after non-denaturing polyacrylamide gel electrophoresis revealed the complete absence of a major activity band in leaves of antisense plants. No differences in growth or development, including flower formation and tuber yield, were observed between transgenic and wild-type plants. Photosynthesis and respiration were also unchanged. Levels of amino acids were the same in wild-type and cyt-NADP-ICDH antisense plants, even when accumulation of amino acids was induced by incubation of detached leaves in tap water in the dark (`induced senescence'). Consistent with a reduction in NADP-ICDH activity, however, were slight increases in the levels of isocitrate (up to 2.5-fold) and citrate (up to 2-fold). 2-Oxoglutarate was not reduced. Our data indicate that potato plants can cope with a severe reduction in cyt-NADP-ICDH activity without major shifts in growth and metabolism. Received: 28 July 1997 / Accepted: 3 November 1997     

Expression of human mitochondrial NADP-dependent isocitrate dehydrogenase during lymphocyte activation

In the process of identifying genes involved in optimization of lymphocyte activation, we have cloned the human mitochondrial NADP-dependent isocitrate dehydrogenase (mNADP-IDH) cDNA. The cDNA and its deduced amino acid (AA) sequence had a high degree of homology with those of the porcine and bovine. The heart and muscle had the highest constitutive expression of the gene. The expression of steady-state mRNA in the resting T and B lymphocytes was low but was induced after mitogen stimulation. The mRNA levels peaked around 48 h and remained elevated at 72 h. At the protein level, the micothondrial but not cytosolic NADP-IDH activity was augmented after the mitogen stimulation. There was no cell cycle-dependent fluctuation of mNADP-IDH expression in synchronized Jurkat cells. In T and B cells, rapamycin (RAPA) could repress the mitogen-stimulated mNADP-IDH expression, although most of the early or late phase activation-related genes including a G-protein β subunit-related gene H12.3 were not affected by the drug. The restricted expression of the gene in certain tissues and the activation-related expression in lymphocytes suggest that this gene might be necessary for optimal functions in heart, muscle, and the activated lymphocytes. © 1996 Wiley-Liss, Inc.     

Purification and properties of NADP-dependent isocitrate dehydrogenase from the bovine adrenal cortex

NADP-dependent isocitrate dehydrogenase (EC 1.1.42) was isolated and 430 times purified from the hyaloplasm fraction of bull adrenal cortex using fractionation by ammonium sulphate and acetone, heat treatment, chromatography on DEAE-Sephadex A-50, gel-filtration on Sephadex G-200 and affinity chromatography on 2',5'-ADP-sepharose 4B. The specific activity of homogeneous enzyme is 60 units per 1 mg of protein at 30 degrees C, yield--34%, pH optimum--8.0, molecular weight, determined by gel filtration on Sephadex G-200, is 96 kDa. The preparation electrophoresis in PAAG in the presence of DS-Na reveals one protein fraction with the mobility corresponding to that of protein having molecular weight of 46 kDa. The data obtained evidence for a dimer structure of the isocitrate dehydrogenase molecule from bull adrenals.     

Critical role of Lys212 and Tyr140 in porcine NADP-dependent isocitrate dehydrogenase

Lys212 and Tyr140 are close to the enzyme-bound isocitrate in the recently determined crystal structure of porcine NADP-specific isocitrate dehydrogenase (Ceccarelli, C., Grodsky, N. B., Ariyaratne, N., Colman, R. F., and Bahnson, B. J. (2002) J. Biol. Chem. 277, 43454-43462). We have constructed mutant enzymes in which Lys212 is replaced by Gln, Tyr, and Arg, and Tyr140 is replaced by Phe, Thr, Glu, and Lys. Wild type and mutant enzymes were each expressed in Escherichia coli and purified to homogeneity. At pH 7.4, the specific activity is decreased in K212Q, K212Y, and K212R, respectively, to 0.01-9% of wild type. The most striking change is in the pH-V(max) curves. Wild type depends on the deprotonated form of a group of pKaes 5.7, whereas this pKaes is increased to 7.4 in neutral K212Q and to 8.3 in K212Y. In contrast, the positive K212R has a pKaes of 5.9. These results indicate that (by electrostatic repulsion) a positively charged residue at position 212 lowers the pK of the nearby ionizable group in the enzyme-substrate complex. Lys212 may also stabilize the carbanion formed initially on substrate decarboxylation. The Tyr140 mutants have specific activities at pH 7.4 that are reduced to 0.2-0.5% of those of wild type, whereas their Km values for isocitrate and NADP are not increased. Most notable are the altered pH-V(max) profiles. V(max) is constant from pH 5.3 to 8 for Y140F and Y140T and increases as pH is decreased for Y140E and Y140K. These results suggest that in wild type enzyme, Tyr140 is the general acid that protonates the substrate after decarboxylation and that the carboxyl and ammonium forms of Y140E and Y140K provide partial substitutes. Relative to wild type, the Y140T enzyme is specifically activated 106-fold by exogenous addition of acetic acid and 88-fold by added phenol; and the K212Q enzyme is activated 4-fold by added ethylamine. These chemical rescue experiments support the conclusion that Tyr140 and Lys212 are required for the catalytic activity of porcine NADP-dependent isocitrate dehydrogenase.     

The purification and properties of NADP-dependent isocitrate dehydrogenase from ox-heart mitochondria.

The purification of NADP-linked isocitrate dehydrogenase from ox heart mitochondria is described. The molecular weight from gel filtration, sedimentation equilibrium and gel electrophoresis is 90000+/-4000, and there are two subunits in the molecule each of which binds NADPH with enhancement of the coenzyme fluorescence. The amino-acid composition is reported, and the absorption coefficient, A1/280%, estimated from dry weight measurements is 11.8 cm-1.     

Location of the coenzyme binding site in the porcine mitochondrial NADP-dependent isocitrate dehydrogenase

The structure of crystalline porcine mitochondrial NADP-dependent isocitrate dehydrogenase (IDH) has been determined in complex with Mn2+-isocitrate. Based on structural alignment between this porcine enzyme and seven determined crystal structures of complexes of NADP with bacterial IDHs, Arg83, Thr311, and Asn328 were chosen as targets for site-directed mutagenesis of porcine IDH. The circular dichroism spectra of purified wild-type and mutant enzymes are similar. The mutant enzymes exhibit little change in Km for isocitrate or Mn2+, showing that these residues are not involved in substrate binding. In contrast, the Arg83 mutants, Asn328 mutants, and T311A exhibit 3-20-fold increase in the Km(NADP). We propose that Arg83 enhances NADP affinity by hydrogen bonding with the 3'-OH of the nicotinamide ribose, whereas Asn328 hydrogen bonds with N1 of adenine. The pH dependence of Vmax for Arg83 and Asn328 mutants is similar to that of wild-type enzyme, but for all the Thr311 mutants, pK(es) is increased from 5.2 in the wild type to approximately 6.0. We have previously attributed the pH dependence of Vmax to the deprotonation of the metal-bound hydroxyl of isocitrate in the enzyme-substrate complex, prior to the transfer of a hydride from isocitrate to NADP's nicotinamide moiety. Thr311 interacts with the nicotinamide ribose and is the closest of the target amino acids to the nicotinamide ring. Distortion of the nicotinamide by Thr311 mutation will likely be transmitted to Mn2+-isocitrate resulting in an altered pK(es). Because porcine and human mitochondrial NADP-IDH have 95% sequence identity, these results should be applicable to the human enzyme.     

Distribution of two isoforms of NADP-dependent isocitrate dehydrogenase in soybean (Glycine max)

Two different cDNAs that encode NADP-specific isocitrate dehydrogenase (NADP-IDH) isozymes of soybean (Glycine max) were characterized. The nucleotide sequences of the coding regions of these cDNAs have 74% identity to each other and give predicted amino acid sequences that have 83% identity to each other. Using PCR techniques, their coding regions were subcloned into a protein overexpression vector, pQE32, to yield pIDH4 and pIDH1, respectively. Both IDH4 and IDH1 enzymes were expressed in Escherichia coli as catalytically active His6 tagged proteins, purified to homogeneity by affinity chromatography on nickel chelate resin and rabbit polyclonal antibodies to each were generated. Surprisingly, antiserum to IDH4 did not react with IDH1 protein and IDH1 antiserum reacted only very weakly with IDH4 protein. IDH4 antibody reacts with a protein of expected molecular weight in cotyledon, young leaf, young root, mature root and nodules but the reaction with mature leaf tissue was low compared to other tissues. Western blot results show that IDH1 was not expressed in young roots but a protein that reacts with the IDH1 antibody was highly expressed in leaves, showing that there was tissue-specific accumulation of NADP-IDH isozymes in soybean.     

Molecular cloning of the cDNA of mouse mitochondrial NADP-dependent isocitrate dehydrogenase and the expression of the gene during lymphocyte activation

The current report documents the molecular cloning of the mouse mitochondrial NADP-dependent isocitrate dehydronegase (mNADP-IDH) cDNA. The cDNA was 1,863 bp in length and contained one open reading frame encoding a 523-residue polypeptide with a predicted molecular weight of 58 kDa. The cDNA and the deduced amino acid (AA) sequence of the mouse mNADP-IDH had a high degree of homology with those of porcine, bovine, alfalfa, and yeast. The recombinant mNADP-IDH expressed in Escherichia coli had active enzymatic function, as well as an expected molecular weight. The heart had the highest constitutive expression of the steady-state mNADP-IDH mRNA, followed by the kidney, while the expression of the gene in other tissues was low. The enzymatic activity of different tissues was in agreement with their mNADP-IDH mRNA levels. The resting lymphocytes had low constitutive expression of the gene, but the steady-state mRNA could be induced 48 h after mitogen stimulation. At the protein level, the resting lymphocytes had low enzymatic activity of mNADP-IDH, but the activity was augmented fivefold after mitogen stimulation. The cytosolic NADP-IDH, on the contrary, remained low or undetectable before and after the mitogen stimulation. Based on our current findings as well as the known roles of the mNADP-IDH in anabolism and in the isocitrate shuttle, it is conceivable that the mNADP-IDH is necessary for optimizing proliferation in lymphocytes. © 1996 Wiley-Liss, Inc.     

Proviruses of mouse mammary tumor virus in normal and neoplastic tissues from GR and C3Hf mouse strains.

We analyzed two experimental situations to assess the role of endogenous mouse mammary tumor virus (MMTV) DNA in the genesis of mammary carcinomas. (i) GR mice carry in their germ line one or more proviruses indistinguishable by limited restriction mapping from the proviruses introduced into cells by experimental infection with the highly tumorigenic virus isolated from GR mouse milk, MMTV(GR). Most tumors arising in GR mice contain one or more proviruses at various sites in tumor DNA in addition to those present endogenously. Detection of these new proviruses is possible as a consequence of the clonal or quasiclonal character of the tumors. (ii) C3H/He mice carry three units of endogenous viral DNA, none of which resembles the DNA of the commonly encountered strains of milk-borne MMTV. Nevertheless, MMTV-associated tumors arise late in life when these animals are removed from the influence of milk-borne virus; the responsible agent, MMTV(C3Hf), can also produce tumors in BALB/c mice. We found that tumors arising in both C3Hf/He mice and BALB/c mice infected with MMTV(C3Hf) were clonal or quasiclonal and contained one or more new copies of proviral DNA at various sites in the host genome. These new proviruses were readily distinguished from the proviruses of the common milk-borne virus strains and closely resembled unit II of endogenous MMTV DNA (Cohen et al., J. Virol., 32:483-496). Thus, in both experimental systems, we found evidence for new proviruses in mammary tumors, despite the preexistence of similar or identical proviruses in the germ line. The results suggest that the repositioning of MMTV proviruses may be required for the full expression of the oncogenic potential of endogenous MMTV DNA.