Differences in malate dehydrogenases from the obligately piezophilic deep-sea bacteriumMoritellasp. strain 2D2 and the psychrophilic bacteriumMoritellasp. strain 5710

The gene encoding malate dehydrogenase (MDH) of the obligately piezophilic deep-sea bacterium Moritella sp. strain 2D2 was cloned and sequenced. There were two positions [close to the active site (Ala-180) and in the subunit interaction site (His-229)] with 2D2-specific substitutions. The MDH genes of strain 2D2 and a psychrophilic bacterium Moritella sp. strain 5710 exhibiting the highest sequence similarity were overexpressed in Escherichia coli. The 2D2 MDH was more heat-stable than the 5710 MDH. The apparent Km value at 62.1 MPa for NADH of the 2D2 MDH was higher than that of the 5710 MDH. The 2D2 MDH in which a His-Gln substitution was introduced at position 229 decreased the thermal stability and Km value at 62.1 MPa. The 5710 MDH that was substituted Gln-229 with His increased the thermal stability and Km value at 62.1 MPa. These results indicate that the His residue at position 229 of the 2D2 MDH may play a role in the thermal stability and the MDH function at high pressure.

Malic enzymes of Trichomonas vaginalis: two enzyme families, two distinct origins

The cytosolic malic enzyme of the amitochondriate protist Trichomonas vaginalis was purified to homogeneity and characterized. The corresponding gene was sequenced and compared with its hydrogenosomal homologue from the same organism. The enzymes were found to differ in coenzyme specificity, molecular mass and physiological role. The cytosolic malic enzyme is a dimer consisting of two 42-kDa subunits with strict specificity for nicotinamide adenine dinucleotide phosphate (NADP(+)), and has a presumed function of pyruvate and NADPH production. The hydrogenosomal malic enzyme is a tetramer of 60-kDa subunits that preferentially utilizes nicotinamide adenine dinucleotide (NAD(+)) to NADP(+). The hydrogenosomal enzyme supplies the hydrogenosome with pyruvate for further catabolic processes linked with substrate-level phosphorylation. Phylogenetic analysis of malic enzymes showed the existence of two distinct families of these enzymes in nature, which differ in subunit size. The trichomonad cytosolic malic enzyme belongs to the small subunit-type family that occurs almost exclusively in prokaryotes. In contrast, the hydrogenosomal malic enzyme displays a close relationship with the large subunit-type family of the enzyme, which is found in mitochondria, plastids and the cytosol of eukaryotes. The eubacterial origin of trichomonad cytosolic malic enzyme suggests an occurrence of horizontal gene transfer from a eubacterium to the ancestor of T. vaginalis. The presence of both prokaryotic and eukaryotic type of malic enzyme in different compartments of a single eukaryotic cell appears to be unique in nature.

Characterization of the interactions between Asp141 and Phe236 in the Mn2+-l-malate binding of pigeon liver malic enzyme

The cytosolic malic enzyme from pigeon liver is very sensitive to the metal-catalysed oxidation systems. Our previous studies using the Cu2+-ascorbate as the oxidation system showed that the enzyme was oxidized and cleaved at several positions, including Asp141. The recently resolved crystal structure of pigeon liver malic enzyme revealed that Asp141 was near to the metal-binding site, but was not a direct metal ligand. However, Asp141 is located next to Phe236, which directly follows the metal ligands Glu234 and Asp235. Mutation at Asp141 caused a drastic effect on the metal-binding affinity of the enzyme. Since Asp141 and Phe236 are highly conserved in most species of malic enzyme, we used a double-mutant cycle to study the possible interactions between these two residues. Four single mutants [D141A (Asp141-->Ala), D141N, F236A and F236L] and four double mutants (D141A/F236A, D141N/F236A, D141A/F236L and D141N/F236L), plus the wild-type enzyme were successfully cloned, expressed and purified to homogeneity. The secondary, tertiary and quaternary structures of these mutants, as assessed by CD, fluorescence and analytical ultracentrifuge techniques, were similar to that of the wild-type enzyme. Initial velocity experiments were performed to derive the various kinetic parameters, which were used to analyse further the free energy change and the coupling energy (DeltaDeltaG(int)) between any two residues. The dissociation constants for Mn2+ ( K (d,Mn)) of the D141A and F236A mutants were increased by approx. 6- and 65-fold respectively, compared with that of the wild-type enzyme. However, the K (d,Mn) for the double mutant D141A/F236A was only increased by 150-fold. A coupling energy of -2.12 kcal/mol was obtained for Asp141 and Phe236. We suggest that Asp141 is involved in the second sphere of the metal-binding network of the enzyme.

[Isolation, purification, and properties of malate dehydrogenases from sulfur bacteria Beggiatoa leptomitiformis]

Malate dehydrogenase (E.C. 1.1.1.37) from bacterium Beggiatoa leptomitiformis was isolated and purified 123 times using a five-step purification procedure including the enzyme extraction, ammonium sulfate protein fractionation, gel filtration, ion exchange chromatography, and gel chromatography. The enzyme was homogenous according to the electrophoresis data; its activity was 20.43 U/mg proteins. This malate dehydrogenase is a homotetramer (Mr = 172 kDa). The catalytic and thermodynamic properties, as well as the analysis of the published data suggest that the tetrameric structure of the enzyme allows it to participate in constructive metabolism supplying the cell with organic acids as a source of carbon.

Analysis of the Shewanella oneidensis proteome by two-dimensional gel electrophoresis under nondenaturing conditions

Proteomes are dynamic, i.e., the protein components of living cells change in response to various stimuli. Protein changes can involve shifts in the abundance of protein components, in the interactions of protein components, and in the activity of protein components. Two-dimensional gel electrophoresis (2-DE) coupled with peptide mass spectrometry is useful for the analysis of relative protein abundance, but the denaturing conditions of classical 2-DE do not allow analysis of protein interactions or protein function. We have developed a nondenaturing 2-DE method that allows analysis of protein interactions and protein functions, as demonstrated in our analysis of the cytosol and crude membrane fractions of the facultative anaerobe Shewanella oneidensis MR-1. Our experiments demonstrate that enzymatic activity is retained under the sample and protein separation methods described, as shown by positive malate dehydrogenase activity results. We have also found protein interactions within both the soluble and membrane fractions. The method described will be useful for the characterization of the functional proteomes of microbial systems.

Purification and physicochemical properties of malate dehydrogenase from bacteria of the genus Beggiatoa

Homogeneous malate dehydrogenase (MDH) with a specific activity of 20-24 units per mg protein was purified from the sulfur bacterium Beggiatoa leptomitiformis strain D-402 grown organotrophically and lithotrophically and from the organotrophic bacterium Beggiatoa alba. MDHs from the B. leptomitiformis strain D-402 grown under organotrophic conditions and from B. alba are homodimers with the subunit molecular weight of 40 kD. Tetrameric MDH is formed in B. leptomitiformis strain D-402 grown under lithotrophic conditions. The dimeric and tetrameric forms of MDH from B. leptomitiformis D-402 display some differences in kinetic properties.

Isoform expression in the multiple soluble malate dehydrogenase of Hoplias malabaricus (Erythrinidae, Characiformes)

Kinetic properties and thermal stabilities of Hoplias malabaricus liver and skeletal muscle unfractionated malate dehydrogenase (MDH, EC 1.1.1.37) and its isolated isoforms were analyzed to further study the possible sMDH-A* locus duplication evolved from a recent tandem duplication. Both A (A1 and A2) and B isoforms had similar optima pH (7.5-8.0). While Hoplias A isoform could not be characterized as thermostable, B could as thermolabile. A isoforms differed from B isoform in having higher Km values for oxaloacetate. The possibly duplicated A2 isoform showed higher substrate affinity than the A1. Hoplias duplicated A isoforms may influence the direction of carbon flow between glycolisis and gluconeogenesis.

Purification of proteins susceptible to oxidation at cysteine residues: identification of malate dehydrogenase as a target for S-glutathiolation

The cysteine residues of proteins are susceptible to a range of oxidative modifications, including S-glutathiolation. Here we present a study in which we used glutathione-agarose to purify cardiac proteins that undergo S-glutathiolation during oxidative stress. We then identified one of the oxidized proteins as malate dehydrogenase.

[Dependence of the structure of malate dehydrogenase on the type of metabolism in fresh water filamentous colorless sulfur bacteria of the Beggiatoa species]

Major pathways of carbon metabolism were studied in strains D-402 and D-405 of freshwater colorless sulfur bacteria of the genus Beggiatoa grown organotrophically and mixotrophically. The bacteria were found to possess all the enzymes of the tricarboxylic acid (TCA) and glyoxylate cycles. When organotrophic growth changed to mixotrophic one, the activity of the TCA cycle enzymes decreased 2- to 3-fold, but the activity of enzymes of the glyoxylate cycle increased threefold. It follows that, in the oxidation of thiosulfate, organic compounds no longer play the leading part in the energy metabolism, and most of electrons that enter the electron transport chain (ETC) derive from inorganic sulfur compounds. A connection was established between the structure and kinetic characteristics of malate dehydrogenase--an enzyme of the TCA and glyoxylate cycles--and the type of carbon metabolism in the strains studied. Malate dehydrogenase in organotrophically grown cells of strains D-402 and D-405 is dimeric, whereas in strain D-402 grown mixotrophically it is tetrameric.

Enzymes of malate metabolism in Mesorhizobium ciceri CC 1192

Electrophoretic studies were performed on enzymes concerned with the oxidation of malate in free-living and bacteroid cells of Mesorhizobium ciceri CC 1192, which forms nitrogen-fixing symbioses with chickpea (Cicer arietinum L.) plants. Two malate dehydrogenases were detected in extracts from both types of cells in native polyacrylamide electrophoresis gels that were stained for enzyme activity. One band of malate dehydrogenase activity was stained only in the presence of NADP+, whereas the other band was revealed with NAD+ but not NADP+. Further evidence for the occurrence of separate NAD- and NADP-dependent malate dehydrogenases was obtained from preliminary enzyme kinetic studies with crude extracts from free-living M. ciceri CC 1192 cells. Activity staining of electrophoretic gels also indicated the presence of two malic enzymes in free-living and bacteroid cells of M. ciceri CC 1192. One malic enzyme was active with both NAD+ and NADP+, whereas the other was specific for NADP+. Possible roles of the multiple forms of malate dehydrogenase and malic enzyme in nitrogen-fixing symbioses are discussed.

Structural studies of the pigeon cytosolic NADP(+)-dependent malic enzyme

Malic enzymes are widely distributed in nature, and have important biological functions. They catalyze the oxidative decarboxylation of malate to produce pyruvate and CO(2) in the presence of divalent cations (Mg(2+), Mn(2+)). Most malic enzymes have a clear selectivity for the dinucleotide cofactor, being able to use either NAD(+) or NADP(+), but not both. Structural studies of the human mitochondrial NAD(+)-dependent malic enzyme established that malic enzymes belong to a new class of oxidative decarboxylases. Here we report the crystal structure of the pigeon cytosolic NADP(+)-dependent malic enzyme, in a closed form, in a quaternary complex with NADP(+), Mn(2+), and oxalate. This represents the first structural information on an NADP(+)-dependent malic enzyme. Despite the sequence conservation, there are large differences in several regions of the pigeon enzyme structure compared to the human enzyme. One region of such differences is at the binding site for the 2'-phosphate group of the NADP(+) cofactor, which helps define the cofactor selectivity of the enzymes. Specifically, the structural information suggests Lys362 may have an important role in the NADP(+) selectivity of the pigeon enzyme, confirming our earlier kinetic observations on the K362A mutant. Our structural studies also revealed differences in the organization of the tetramer between the pigeon and the human enzymes, although the pigeon enzyme still obeys 222 symmetry.

Purification and characterization of membrane-bound malate dehydrogenase from Acetobacter sp. SKU 14

Membrane-bound NAD(P)-independent malate dehydrogenase (EC 1.1.99.16) was purified to homogeneity from the membrane of thermotolerant Acetobacter sp. SKU 14, an isolate from Thailand. The enzyme was solubilized from the membrane fraction of glycerol-grown cells with 1% Triton X-100 in the presence of 0.1 M KCl, and purified to homogeneity through steps of column chromatographies on DEAE-Sephadex A-50 and DEAE-Toyopearl in the presence of 0.1% Triton X-100. The purified enzyme showed a single protein band in both native-PAGE and SDS-PAGE. The enzyme was a homodimer with a molecular mass of 60 kDa subunit and had noncovalently bound FAD as the cofactor. The enzyme was stable over pH 5 and had its maximum activity at pH 11.0 when ferricyanide was used as an electron acceptor. The enzyme activity was elevated by the addition of ammonium ions. The substrate specificity was very strict to only L-malate, of which the apparent Km was 10 mM and over 20 compounds involving D-malate were not oxidized by the enzyme.

Distinguishing clonal apple rootstocks by isozymes banding patterns

Molecular characterisation of clonal apple rootstocks using isozymes was carried out to identify isozyme polymorphism in seven clonal apple rootstocks and to identify the most characteristic and stable enzyme markers for each individual rootstock. Five enzyme systems were studied out of which polyphenol oxidase, malate dehydrogenase, acid phosphatase and peroxidase were useful in discriminating among the rootstocks. The peroxidase enzyme system showed maximum variation and esterase showed the least variation among the rootstocks. Out of seven rootstocks, three were distinguished on the basis of one enzyme system only (M.3 with MDH or PER, M.7 with PPO or PER and MM. 111 with MDH). Out of the sixteen loci studied seven were found to be polymorphic. Genetic variation among the rootstocks was explained on the basis of various parameters. The percentage of polymorphic loci varied from 13.33 to 35.71 per cent.

Non-ideality by sedimentation velocity of halophilic malate dehydrogenase in complex solvents

We have investigated the potential of sedimentation velocity analytical ultracentrifugation for the measurement of the second virial coefficients of proteins, with the goal of developing a method that allows efficient screening of different solvent conditions. This may be useful for the study of protein crystallization. Macromolecular concentration distributions were modeled using the Lamm equation with the approximation of linear concentration dependencies of the diffusion constant, D = D(o) (1 + k(D)c), and the reciprocal sedimentation coefficient s = s(o)/(1 + k(s)c). We have studied model distributions for their information content with respect to the particle and its non-ideal behavior, developed a strategy for their analysis by direct boundary modeling, and applied it to data from sedimentation velocity experiments on halophilic malate dehydrogenase in complex aqueous solvents containing sodium chloride and 2-methyl-2,4-pentanediol, including conditions near phase separation. Using global modeling for three sets of data obtained at three different protein concentrations, very good estimates for k(s) and s degrees and also for D degrees and the buoyant molar mass were obtained. It was also possible to obtain good estimates for k(D) and the second virial coefficients. Modeling of sedimentation velocity profiles with the non-ideal Lamm equation appears as a good technique to investigate weak inter-particle interactions in complex solvents and also to extrapolate the ideal behavior of the particle.

Purification and properties of two malate dehydrogenases from Candida sp. N-16 grown on methanol

Two malate dehydrogenases (MDH-M1 and MDH-M2) were found in a methanol-using yeast, Candida sp. N-16. MDH-M2 was induced with methanol. These enzymes were purified as electrophoretically and isoelectrophoretically homogeneous proteins. The molecular weights of MDH-M1 and MDH-M2 were estimated to be about 78,000 (homodimer) and 160,000 (homotetramer). Several kinetic properties were significantly different between the two enzymes. The value (2.07) of Vmax(oxaloacetate)/Vmax(malate) and Kcats (555 s(-1) for oxaloacetate, 481 s(-1) for NADH) of MDH-M2 were higher than the ratio (1.37) of Vmax and Kcats (241 s(-1) for oxaloacetate, 271 s(-1) for NADH) of MDH-M1, respectively. The activity of MDH-M2 was inhibited by a high concentration of NAD+ and the activity of MDH-M1 by oxaloacetate.

Characterization of alanine and malate dehydrogenases from a marine psychrophile strain PA-43

Alanine dehydrogenase (AlaDH: EC 1.4.1.1), malate dehydrogenase (MDH: EC 1.1.1.37), and glutamate dehydrogenase (EC 1.4.1.2), all NAD+ dependent, were detected in extracts from a psychrophilic bacterium, strain PA-43, isolated from a sea urchin off the Icelandic coast. Characterization tests suggested that the strain had a close relationship to Vibrio, but sequencing of part of the 16S rDNA gene placed the bacterium among Shewanella species in a constructed phylogenetic tree. The bacterium had an optimum growth temperature of 16.5 degrees C, and maximum dehydrogenase expression was obtained in a rich medium supplemented with NaCl. Both AlaDH and MDH were purified to homogeneity. AlaDH is a hexamer, with an approximate relative molecular mass of 260,000, whereas MDH is dimeric, with an apparent relative molecular mass of approximately 70,000. Both enzymes were thermolabile, and the optimum temperatures for activity were shifted toward lower temperatures than those found in the same enzymes from mesophiles, 37 degrees C for MDH and approximately 47 degrees C for AlaDH. The pH optima for AlaDH in the forward and reverse reactions were 10.5 and 9, respectively, whereas those for MDH were 10-10.2 and 8.8, respectively. Partial amino acid sequences, comprising approximately 30% of the total sequences from each enzyme, were determined for N-terminal, tryptic, and chymotryptic fragments of the enzymes. The AlaDH showed the highest similarity to AlaDHs from the psychrotroph Shewanella Ac10 and the mesophile Vibrio proteolyticus, whereas MDH was most similar to the MDHs from the mesophiles Escherichia coli and Haemophilus influenzae, with lower identity to the psychrophilic malate dehydrogenases from Vibrio 5710 and Photobacterium SS9.

A pre-genetic study of the isoforms of malic enzyme associated with lipid accumulation in Mucor circinelloides

The oleaginous fungus Mucor circinelloides possesses at least six isoforms of malic enzyme (EC 1.1.1.40), a key lipogenic enzyme in filamentous fungi. These isoforms were detected using a specific stain for activity after native PAGE of cell extracts. Only one isoform (isoform IV) was associated with lipid accumulation, appearing only after N-exhaustion from the medium (which is a pre-requisite for lipid accumulation) in glucose-growing cells. Isoforms I, II, V and VI were involved in anaerobic growth and only appeared under O(2)-limited conditions. Isoform III appeared to be constitutive and was formed under conditions of active (balanced) growth and is therefore thought to play a crucial role in basic metabolism. Growth on acetate increased the amount of cell lipid (from 25-27% in glucose-grown cells to 37-38% in acetate-grown cells) accumulated by M. circinelloides and this was associated with the appearance of isoform IV of malic enzyme prior to N-exhaustion in these cultures. Amino acid sequence analysis of isoforms III and IV suggests that these two malic enzymes may be encoded by a single gene and that isoform IV is formed from isoform III by post-translational modification initiated by either N-limitation (when glucose was the carbon source) or growth on acetate as the sole carbon source.

Non-photosynthetic 'malic enzyme' from maize: a constituvely expressed enzyme that responds to plant defence inducers

The characterization of a non-photosynthetic isoform of NADP-malic enzyme (NADP-ME) from maize roots, which represents nearly 7% of the total soluble protein of this tissue, was performed. The molecular properties of the purified protein, as well as the kinetic parameters determined, indicate that the NADP-ME isoform present in maize roots differs from the photosynthetic enzyme implicated in the C4 cycle, but is similar, or identical, to the enzyme previously characterized from etiolated maize leaves (Maurino, Drincovich and Andreo, Biochem. Mol. Biol. Int. 38 (1996) 239-250). A full-length ORF encoding a plastidic NADP-ME (almost identical to the maize root NADP-ME, GenBank accession number U39958) was cloned from a root cDNA library as well as isolated by reverse transcription (RT)-PCR using green leaves mRNA as template. These results indicate that root NADP-ME does not constitute a root-specific isoform, but represents a protein with a constitutive pattern of expression in plastids of the C4 plant maize. The amount of NADP-ME measured by activity, western and northern blot was modified when different stress conditions (including treatments with cellulase, fungal elicitors, jasmonate and hypoxic treatment) were applied to maize roots, indicating that the enzyme from maize roots is under transcriptional or post-transcriptional regulation by effectors related to plant defence responses. It is deduced that the induction of housekeeping genes, like non-photosynthetic NADP-ME, whose constitutive role may be the provision of reductive power in non-photosynthetic plastids, is likely to accompany the defence response.

A family of highly conserved glycosomal 2-hydroxyacid dehydrogenases from Phytomonas sp

Phytomonas sp. contains two malate dehydrogenase isoforms, a mitochondrial isoenzyme with a high specificity for oxaloacetate and a glycosomal isozyme that acts on a broad range of substrates (Uttaro, A. D., and Opperdoes, F.R. (1997) Mol. Biochem. Parasitol. 89, 51-59). Here, we show that the low specificity of the latter isoenzyme is the result of a number of recent gene duplications that gave rise to a family of glycosomal 2-hydroxyacid dehydrogenase genes. Two of these genes were cloned, sequenced, and overexpressed in Escherichia coli. Although both gene products have 322 amino acids, share 90.4% identical residues, and have a similar hydrophobicity profile and net charge, their kinetic properties were strikingly different. One isoform behaved as a real malate dehydrogenase with a high specificity for oxaloacetate, whereas the other showed no activity with oxaloacetate but was able to reduce other oxoacids, such as phenyl pyruvate, 2-oxoisocaproate, 2-oxovalerate, 2-oxobutyrate, 2-oxo-4-methiolbutyrate, and pyruvate.

Lysine 199 is the general acid in the NAD-malic enzyme reaction

Site-directed mutagenesis was used to change K199 in the Ascaris suum NAD-malic enzyme to A and R and Y126 to F. The K199A mutant enzyme gives a 10(5)-fold decrease in V and a 10(6)-fold decrease in V/K(malate) compared to the WT enzyme. In addition, the ratio for partitioning of the oxalacetate intermediate toward pyruvate and malate changes from a value of 0.4 for the WT enzyme to 1.6 for K199A, and repeating the experiment with A-side NADD gives isotope effects of 3 and 1 for the WT and K199A mutant enzymes, respectively. The K199R mutant enzyme gives only a factor of 10 decrease in V, and the pK for the general acid in this mutant enzyme has increased from 9 for the WT enzyme to >10 for the K199R mutant enzyme. Tritium exchange from solvent into pyruvate is catalyzed by the WT enzyme, but not by the K199A mutant enzyme. The Y126F mutant enzyme gives a 10(3)-fold decrease in V. The oxalacetate partition ratio and isotope effect on oxalacetate reduction for the Y126F mutant enzyme are identical, within error, to those measured for the WT enzyme. Thus, Y126 is important to the overall reaction, but its role at present is unclear. Data are consistent with K199 functioning as the general acid that protonates C3 of enolpyruvate to generate the pyruvate product in the malic enzyme reaction.

The putative L-lactate dehydrogenase from Methanococcus jannaschii is an NADPH-dependent L-malate dehydrogenase

The enzyme encoded by Methanococcus jannaschii open reading frame (ORF) 0490 was purified and characterized. It was shown to be an NADPH-dependent [lactate dehydrogenase (LDH)-like] L-malate dehydrogenase (MalDH) and not an L-lactate dehydrogenase, as had been suggested previously on the basis of amino acid sequence similarity. The results show the importance of biochemical data in the assignment of ORF function in genomic sequences and have implications for the phylogenetic distribution of members of the MalDH/LDH enzyme superfamilies within the prokaryotic kingdom.

Cloning of the malic enzyme gene from Corynebacterium glutamicum and role of the enzyme in lactate metabolism

Malic enzyme is one of at least five enzymes, known to be present in Corynebacterium glutamicum, capable of carboxylation and decarboxylation reactions coupling glycolysis and the tricarboxylic acid cycle. To date, no information is available concerning the physiological role of the malic enzyme in this bacterium. The malE gene from C. glutamicum has been cloned and sequenced. The protein encoded by this gene has been purified to homogeneity, and the biochemical properties have been established. Biochemical characteristics indicate a decarboxylation role linked to NADPH generation. Strains of C. glutamicum in which the malE gene had been disrupted or overexpressed showed no detectable phenotype during growth on either acetate or glucose, but showed a significant modification of growth behavior during lactate metabolism. The wild type showed a characteristic brief period of exponential growth on lactate followed by a linear growth period. This growth pattern was further accentuated in a malE-disrupted strain (Delta malE). However, the strain overexpressing malE maintained exponential growth until all lactate had been consumed. This strain accumulated significantly larger amounts of pyruvate in the medium than the other strains.

Biochemical, electrophoretic and immunohistochemical aspects of malate dehydrogenase in truffles (Ascomycotina)

The malate dehydrogenase (MDH; EC 1.1.1.37; L-malate-NAD(+)-oxidoreductase) activities of truffles of the genus Tuber (Tuber melanosporum Vittad., Tuber brumale Vittad., Tuber aestivum Vittad., Tuber magnatum Pico, Tuber rufum Pico) have been characterized with regard to the K(m) and V(max) values in the direct and reverse reactions. The isoelectrofocusing has revealed bands showing pI values ranging from pH 5.85 to 7.8. The MDH of T. melanosporum has been partially purified by hydroxyapatite treatment, DEAE-cellulose and Sephadex G-75 columns. With the partially purified T. melanosporum MDH activity polyclonal anti-T. melanosporum MDH antibodies have been prepared and used to localize MDH in the mycorrhizae and ascocarps of T. melanosporum. These antibodies inhibit T. melanosporum MDH activity as well as that of T. magnatum but not that of rabbit liver; this supports the specificity of the MDH antibodies used to localize MDH in truffle tissues.

The dimer contact area of sorghum NADP-malate dehydrogenase: role of aspartate 101 in dimer stability and catalytic activity

During thioredoxin-mediated activation of chloroplastic NADP-malate dehydrogenase, a homodimeric enzyme, the interaction between subunits is known to be loosened but maintained. A modeling of the 3D structure of the protein identified Asp-101 as being potentially involved in the association between subunits through an electrostatic interaction. Indeed, upon site-directed substitution of Asp-101 by an asparagine, the mutated enzyme behaved mainly as a monomer. The mutation strongly affected the catalytical efficiency of the enzyme. The now available 3D structure of the enzyme shows that Asp-101 is protruding at the dimer interface, interacting with Arg-268 of the neighbouring subunit.