Purification and characterization of malate dehydrogenase from Cryptococcus neoformans

The NAD-dependent malate dehydrogenase (EC 1.1.1.37) was purified from Cryptococcus neoformans, a basidiomycetious yeast that is an opportunistic pathogen of AIDS patients. The purified enzyme was a dimer of 35 kDa subunits that exhibited uncompetitive substrate inhibition by oxalacetate, typical for mitochondrial malate dehydrogenases from other sources. Product inhibition studies indicated an ordered sequential kinetic mechanism, with pyridine dinucleotide being the substrate that binds to the free enzyme form. Unique aspects of this malate dehydrogenase were inhibition by zinc ion, competitive versus malate with Ki of 30 microM, and inhibition by heparin. Heparin inhibition was competitive versus either NAD or malate, with Ki of 0.35 microM. Heparin molecules of nominal molecular weight of 30,000 or 3000 were equally effective inhibitors. A model is presented to explain the high affinity of the enzyme for heparin.

NADP-malic enzyme from maize leaves: a fluorescence study

NADP-malic enzyme from maize leaves is covalently labeled with a fluorescent-SH reactive probe eosin-5-maleimide (EMA), which reacts with groups that are totally protected by NADP against inactivation. The comparison of the emission fluorescence spectra of the native and the modified enzyme suggests the proximity of the fluorescent groups of the native enzyme (probably tryptophanyl groups) and the EMA modified residues. Intrinsic fluorescence quenching studies shows that NADP is the only substrate capable to interact with the fluorescent excited groups of the enzyme, while Mg2+ is able to increase this interaction. Quenching studies of EMA-bound fluorescence shows that the NADP-binding site was modified and thus uncapable of further interaction with the nucleotide. When the results of protection studies are combined with those of extrinsic quenching experiments, we must conclude that EMA reacts with sulfhydryl groups that are involved in the NADP-binding site of the enzyme.

Identification of carboxylation enzymes and characterization of a novel four-subunit pyruvate:flavodoxin oxidoreductase from Helicobacter pylori

The enzyme activities responsible for carboxylation reactions in cell extracts of the gastric pathogen Helicobacter pylori have been studied by H14CO3- fixation and spectrophotometric assays. Acetyl coenzyme A carboxylase (EC 6.4.1.2) and malic enzyme (EC 1.1.1.40) activities were detected, whereas pyruvate carboxylase (EC 6.4.1.1), phosphoenolpyruvate carboxylase (EC 4.1.3.1) and phosphoenolpyruvate carboxykinase (EC 4.1.1.49) activities were absent. However, a pyruvate-dependent, ATP-independent, and avidin-insensitive H14CO3- fixation activity, which was shown to be due to the isotope exchange reaction of pyruvate:flavodoxin oxidoreductase (EC 1.2.7.1), was present. The purified enzyme is composed of four subunits of 47, 36, 24, and 14 kDa. N-terminal sequence analysis showed that this enzyme is related to a recently recognized group of four-subunit pyruvate:ferredoxin oxidoreductases previously known only from hyperthermophiles. This enzyme from H. pylori was found to mediate the reduction of a number of artificial electron acceptors in addition to a flavodoxin isolated from H. pylori extracts, which is likely to be the in vivo electron acceptor. Indirect evidence that the enzyme is capable of in vitro reduction of the anti-H. pylori drug metronidazole was also obtained.

Identification of Asp258 as the metal coordinate of pigeon liver malic enzyme by site-specific mutagenesis

Pigeon liver malic enzyme was inactivated by ferrous sulfate in the presence of ascorbate. Manganese and some other divalent metal ions provided complete protection of the enzyme against the Fe(2+)-induced inactivation. The inactivated enzyme was subsequently cleaved by the Fe(2+)-ascorbate system at Asp258-Ile259, which was presumably the Mn(2+)-binding site of the enzyme [Wei, C. H., Chou, W. Y., Huang, S. M., Lin, C. C., & Chang, G. G. (1994) Biochemistry 33, 7793-7936]. For identification of Asp258 as the putative metal-binding site of the enzyme, we prepared four mutant enzymes substituted at Asp258 with glutamate (D258E), asparagine (D258N), lysine (D258K), or alanine (D258A), respectively. These mutant proteins were recombinantly expressed in a bacterial expression system (pET-15b) with a stretch of histidine residues attached at the N-terminus and were successfully purified to apparent homogeneity by a single Ni-chelated affinity column. Among the four mutants, only D258E possessed 0.8% residual activity after purification; all other purified mutants had < 0.0001% residual activity in catalyzing the oxidative decarboxylation of L-malate. The D258E mutant was susceptible to inactivation by the Fe(2+)-ascorbate system, albeit with much slower inactivation rate, and was protected by the Mn2+ to a lesser extent as compared to the wild-type enzyme. None of the mutants were cleaved by the Fe(2+)-ascorbate system under conditions that cleaved the natural or wild-type enzyme at Asp258.(ABSTRACT TRUNCATED AT 250 WORDS)

Engineering a domain-locking disulfide into a bacterial malate dehydrogenase produces a redox-sensitive enzyme

Light-dependent reduction of cystine disulfide bonds results in activation of several of the enzymes of photosynthetic carbon metabolism within the chloroplast. We have modeled the tertiary structure of four of these light-activated enzymes, namely NADP-linked malate dehydrogenase, glyceraldehyde-3-P dehydrogenase, fructosebisphosphatase, and sedoheptulosebisphosphatase, and identified cysteines in each enzyme that be expected to form inactivating disulfide bonds (Li, D., F. J. Stevens, M. Schiffer, and L. E. Anderson, 1994. Biophys. J. 67:29-35). We have now converted two residues in the Escherichia coli NAD-linked malate dehydrogenase to cysteines and produced a redox-sensitive enzyme. Oxidation of domain-locking cysteine residues in the mutant enzyme clearly mimics dark inactivation of the redox-sensitive chloroplast dehydrogenase. This result is completely consistent with our proposed mechanism.

Glyoxysomal malate dehydrogenase and malate synthase from soybean cotyledons (Glycine max L.): enzyme association, antibody production and cDNA cloning

In order to investigate a possible association between soybean malate synthase (MS; L-malate glyoxylate-lyase, CoA-acetylating, EC 4.1.3.2) and glyoxysomal malate dehydrogenase (gMDH; (S)-malate: NAD+ oxidoreductase, EC 1.1.1.37), two consecutive enzymes in the glyoxylate cycle, their elution profiles were analyzed on Superdex 200 HR fast protein liquid chromatography columns equilibrated in low- and high-ionic-strength buffers. Starting with soluble proteins extracted from the cotyledons of 5-d-old soybean seedlings and a 45% ammonium sulfate precipitation, MS and gMDH coeluted on Superdex 200 HR (low-ionic-strength buffer) as a complex with an approximate relative molecular mass (Mr) of 670,000. Dissociation was achieved in the presence of 50 mM KCl and 5 mM MgCl2, with the elution of MS as an octamer of M(r) 510,000 and of gMDH as a dimer of M(r) 73,000. Polyclonal antibodies raised to the native copurified enzymes recognized both denatured MS and gMDH on immunoblots, and their native forms after gel filtration. When these antibodies were used to screen a lambda ZAP II expression library containing cDNA from 3-d-old soybean cotyledons, they identified seven clones encoding gMDH, whereas ten clones encoding MS were identified using an antibody to SDS-PAGE-purified MS. Of these cDNA clones a 1.8 kb clone for MS and a 1.3-kb clone for gMDH were fully sequenced. While 88% identity was found between mature soybean gMDH and watermelon gMDH, the N-terminal transit peptides showed only 37% identity. Despite this low identity, the soybean gMDH transit peptide conserves the consensus R(X6)HL motif also found in plant and mammalian thiolases.

Purification and properties of cytosolic and mitochondrial malic enzyme isolated from human brain

Three isoforms of malic enzyme have been described in mammalian tissues: a cytosolic NADP(+)-dependent enzyme, a NADP(+)-dependent mitochondrial isoform and a mitochondrial isozyme which can use both NAD+ and NADP+ but is more effective with NAD+. We purified mitochondrial and cytosolic malic enzyme from human brain extract to apparent homogeneity in order to compare properties of these isozymes and to verify whether mitochondria contain one or two malic enzyme. Specific activities of both isoforms are approx. 90 mumol/min/mg of protein, which corresponds to about 1900-fold purification. The two isozymes have identical native molecular mass (257 kDa) and are presumably tetramers composed of four identical subunits (M(r) = 64 kDa). The isoelectric point of cytosolic isozyme is 5.65, and that of mitochondrial one is 7.0. The isozymes show a substantial difference in their capability to catalyse the reductive carboxylation of pyruvate to malate: the maximal carboxylation rate approaches 80% that of decarboxylation velocity for the cytosolic enzyme, and only 17% for the mitochondrial isozyme. The coenzyme specificity of both isozymes is not stringent; NADP+ is the preferred and NAD+ can substitute it, although with much lower efficiency. The homogenous cytosolic malic enzyme catalysed decarboxylation of oxaloacetate and NADPH-dependent reduction of pyruvate at about 24 and 0.5% of the maximum rate of NADP-dependent oxidative decarboxylation of malate respectively. Decarboxylation of oxaloacetate catalysed by mitochondrial malic enzyme has not been detectable, while NADP-linked reduction of pyruvate approaches only 0.15% of the maximum rate of NADP-linked oxidative decarboxylation of malate.(ABSTRACT TRUNCATED AT 250 WORDS)

Purification, cDNA cloning and heterologous expression of the human mitochondrial NADP(+)-dependent malic enzyme

Mitochondrial NADP(+)-dependent malic enzyme (ME; EC 1.1.1.39) has been purified to homogeneity and characterized kinetically from bovine heart. Partial amino acid sequence information allowed amplification of a specific bovine cDNA, which was used to isolate a full-length human cDNA of this isoform of ME. The cDNA is 1930 bp long and codes for a protein of 604 amino acids. Comparison of the amino acid sequence of this isoform with published sequences of other human ME isoforms shows stretches of homology interrupted by larger regions with significant differences. The human protein has been expressed in Escherichia coli, and the recombinant human protein has the same kinetic properties as the corresponding protein purified from bovine heart. Northern blot analysis showed a strong tissue-specific transcription with a predominantly high expression-rate in organs with a low division-rate.

Dissociation of pigeon-liver malic enzyme in reverse micelles

Pigeon-liver malic enzyme has a tendency to aggregate at a large concentration of protein. The larger aggregates (hexamer and octamer) were demonstrated to be enzymically active with specific activity similar to that of the tetramer. When the enzyme was embedded in a reverse micellar system prepared by dissolving the surfactant sodium bis(2-ethylhexyl)-sulfosuccinate (AOT) in isooctane, the tetrameric enzyme dissociated into monomers. The dissociated monomers were also enzymically active but with diminished specific activity relative to the activity in aqueous media. The decreased enzyme activity in reverse micelles was due to interactions of surfactant with the enzyme molecules, suggesting that the cytosolic malic enzyme is located near the plasma membrane. When the dissociation was monitored by altering the degree of hydration of the system (represented by the ratio [H2O]/[AOT]), the detergent and organic solvent slightly affected KTD, the dissociation constant of tetramer to dimers (T <--> 2 D), but increased KDM, the dissociation constant of dimer to monomers (D <--> 2 M), by 1-2 orders of magnitude; this change caused a 2-3 orders of magnitude increase in the overall dissociation constant KTM (T <--> 4 M). The dissociation of the tetrameric malic enzyme to monomers was favored by approximately 16 kJ/mol in AOT/isooctane reverse micelles versus aqueous media. We propose water-shell and induced-fit models for the enzyme in AOT/isooctane reverse micelles at large and small [H2O]/[AOT] ratios to explain this data, respectively. The asymmetric quaternary structure of the enzyme [Lee, H. J. & Chang, G. G. (1990) FEBS Lett. 277, 175-179] was re-evaluated in terms of the subunit interactions and various interconvertible enzyme forms.

Engineering the quaternary structure of an enzyme: construction and analysis of a monomeric form of malate dehydrogenase from Escherichia coli

The citric acid cycle enzyme, malate dehydrogenase (MDH), is a dimer of identical subunits. In the crystal structures of 2 prokaryotic and 2 eukaryotic forms, the subunit interface is conformationally homologous. To determine whether or not the quaternary structure of MDH is linked to the catalytic activity, mutant forms of the enzyme from Escherichia coli have been constructed. Utilizing the high-resolution structure of E. coli MDH, the dimer interface was analyzed critically for side chains that were spatially constricted and needed for electrostatic interactions. Two such residues were found, D45 and S226. At their nearest point in the homodimer, they are in different subunits, hydrogen bond across the interface, and do not interact with any catalytic residues. Each residue was mutated to a tyrosine, which should disrupt the interface because of its large size. All mutants were cloned and purified to homogeneity from an mdh- E. coli strain (BHB111). Gel filtration of the mutants show that D45Y and D45Y/S226Y are both monomers, whereas the S226Y mutant remains a dimer. The monomeric D45Y and D45Y/S226Y mutants have 14,000- and 17,500-fold less specific activity, respectively, than the native enzyme. The dimeric S226Y has only 1.4-fold less specific activity. All forms crystallized, indicating they were not random coils. Data have been collected to 2.8 A resolution for the D45Y mutant. The mutant is not isomorphous with the native protein and work is underway to solve the structure by molecular replacement.

Preparation and kinetic characterization of a fusion protein of yeast mitochondrial citrate synthase and malate dehydrogenase

We have expressed the DNA of the fusion of CS1 to MDH1 in Escherichia coli gltA-. The fusion protein (CS1/MDH1) is the C-terminus of CS1 linked in-frame to the N-terminus of MDH1 with a short linker of glycyl-seryl-glycyl. The fusion protein produced was isolated and purified. Gel filtration studies indicated that CS1/MDH1 had a M(r) of approximately 170,000. Western blotting analysis with SDS gel indicated a M(r) of approximately 90,000-95,000 (theoretical M(r) = 87,000). This is the expected M(r) for the fusion protein subunit. The kinetics of CS1 and MDH1 activities of the fusion protein were compared to those of the free enzymes. In addition, the effect of AAT reaction, as a competitor for the intermediate OAA of the coupled MDH-CS reaction, was examined. It was observed that AAT was a less effective competitor for OAA when the CS1/MDH1 fusion protein is used than when the separate enzymes are employed. In addition, the transient time for the coupled reaction sequence was less for the fusion protein than for the free enzymes.

Malate dehydrogenase and glucose-6-phosphate dehydrogenase, key markers for studying the genetic diversity of Actinobacillus actinomycetemcomitans

Cell-free extracts of strains belonging to the 5 serotypes of A. actinomycetemcomitans were screened for several enzymes. Enzymes representative of the pentose phosphate pathway/hexose monophosphate shunt and the TCA cycle were present. Of these glucose-6-phosphate dehydrogenase (G6PDH) and malate dehydrogenase (MDH) were the most readily detected and stable. MDH and G6PDH retained more than 50% of their activities at alkaline pHs (10-11) for up to 6 h and 3 h at 25 degrees C, respectively, while at pH 6.5, 50% of their activities were lost within 2-3 h. The Km for malate oxidation catalysed by MDH was 5.8 x 10(-4) M while that for glucose-6-phosphate oxidation was 2.0 x 10(-4) M. The pH optima for MDH and G6PDH oxidation activities were 10 and 9.5, respectively. Among the 5 designated serotypes of A. actinomycetemcomitans three groups were delineated by multilocus enzyme electrophoresis using MDH and G6PDH.

Immobilized metal-ion affinity partitioning of NAD(+)-dependent dehydrogenases in poly(ethylene glycol)-dextran two-phase systems

Affinity partitioning of yeast alcohol dehydrogenase (YADH), lactate dehydrogenase from rabbit muscle (MLDH) and lactate and malate dehydrogenases from pig heart (HLDH and HMDH, respectively) were studied in aqueous two-phase systems containing metal ions (Cu2+, Ni2+, Zn2+ and Cd2+) chelated by iminodiacetate-poly(ethylene glycol) (IDA-PEG). The partitioning behaviour of the enzymes in the presence of Cu(II)-IDA-PEG was studied as a function of the concentration of NaCl, the pH of the medium and the concentration of added selected agents. It was demonstrated that the partition effect (delta log K) of dehydrogenases in the presence of Cu(II)-IDA-PEG and the affinity of enzymes for immobilized Cu2+ ions increases in the order MLDH > YADH > HMDH > or = HLDH. It was shown that the determined variations in the enzyme affinities for Cu(II)-IDA-PEG might be related to the differences in the content of histidine residues accessible to the solvent.

A single amino acid mutation enhances the thermal stability of Escherichia coli malate dehydrogenase

The stability of wild-type Escherichia coli malate dehydrogenase was compared with a mutant form of the enzyme with the amino acid residue at position 102 changed from arginine to glutamine. The mutation occurs on the underside of a mobile loop which closes over the active-site cleft on formation of the enzyme/cofactor/substrate ternary complex. The mutant enzyme is kinetically compromised while the wild-type enzyme is highly specific for oxaloacetate. The mutant enzyme was shown to be more resistant to irreversible thermal denaturation by thermal inactivation experiments and high-sensitivity differential scanning calorimetry than the wild-type enzyme. In contrast, resistance of both enzymes to reversible unfolding in guanidinium chloride was similar. Circular dichroic spectropolarimetry shows the secondary structures of the enzymes are similar but there is a demonstrable difference in tertiary structure. From the position of the mutation, it is conjectured that the substitution on a mobile surface loop results in partial closure of the loop and greater resistance to thermal inactivation of the mutant enzyme. However, molecular modelling combined with circular dichroic spectropolarimetry indicate that the mutation may have a more widespread effect on the structure than simply partial closure of the mobile surface loop as the environment of distant tyrosine residues is altered. Resistance of the wild-type enzyme to thermal inactivation can be increased by cofactor addition, which may have the effect of partial closure of the mobile surface loop, but has little effect on the mutant enzyme.

Molecular weight of cytoplasmic malate dehydrogenase, mitochondrial malate dehydrogenase and lactate dehydrogenase of a freshwater catfish

The multiple molecular forms of cytoplasmic malate dehydrogenase (cMDH), mitochondrial malate dehydrogenase (mMDH) and lactate dehydrogenase (LDH) were studied in the liver and skeletal muscle of the freshwater catfish, Clarias batrachus. There were two electrophoretically distinguishable bands (AA and BB) of cMDH and mMDH which suggests that they are apparently encoded at two gene loci (A and B) in both the tissues. However, the presence of a single band (LDH-1) of LDH in liver and double bands (LDH-1 and LDH-2) in skeletal muscle in which LDH-2 was predominant reflects the differential expression of LDH genes in different metabolic tissues to meet the requirement of energy production. The AA isoform (74 kd) of liver cMDH was smaller than those of the AA form (110 kd) of skeletal muscle. In contrast, the BB isoform of liver (42 kd) and skeletal muscle (54 kd) were more or less similar in size. Unlike the case of cMDH, the molecular weight of AA isoform (115 kd) of liver mMDH was higher than those of the AA form (87 kd) of skeletal muscle. Whereas the molecular weight of BB isoform (58 kd) of liver was in proximity to the weight of BB form (44 kd) of skeletal muscle mMDH. The size of AA isoform (74 kd) of liver cMDH was smaller, while the AA isoform (110 kd) of skeletal muscle was larger as compared to AA form of mMDH in the liver (115 kd) and skeletal muscle (87 kd). But the size of BB isoform of both the isozymes was almost equal in these metabolic tissues.(ABSTRACT TRUNCATED AT 250 WORDS)

Hysteresis of cytosolic NADP-malic enzyme II from Trypanosoma cruzi

NADP-malic enzyme II, one of two isoenzymes of NADP-malic enzyme (EC 1.1.1.40) in Trypanosoma cruzi epimastigotes, presents hysteretic behavior that results in a kinetic lag in the reaction progress curve. The lag is affected by the malate, aspartate and oxaloacetate concentrations in the assay mixture. This dependence suggests that hysteresis is due to an association-dissociation process influenced by the binding of these ligands to the enzyme. The enzyme was separated from NADP-malic enzyme I and purified 43-fold from a cell homogenate by a procedure involving column chromatography on DEAE-Sephacel and Cibacron-blue Sepharose. The molecular mass of the highly purified enzyme was determined as 126 kDa.

Demonstration of enzyme associations by countermigration electrophoresis in agarose gel

We propose a method to study multienzyme complex formation in vitro based on nondenaturing agarose gel electrophoresis. The enzymes with different isoelectric points (pI) were loaded at the opposite ends of the same lane of agarose gel and electrophoresis was performed at a pH value intermediate between their pI's. In cases where a complex of the enzymes was formed, an additional protein band of low electrophoretic mobility was found corresponding to the point where they crossed on the gel. This band contained both enzyme activities. The method was used to demonstrate association between two enzymes of the mitochondrial citric acid cycle, malate dehydrogenase and citrate synthase, and between the lysosomal hydrolases, beta-galactosidase and cathepsin A. Relative proportions of free and bound enzymes after electrophoresis suggest that interaction between the mitochondrial enzymes is relatively weak compared to that of lysosomal hydrolases. Microdensitometric scanning of countermigration electrophoresis gels was used to determine the stoichiometry of components in the complex.

Stepwise versus concerted oxidative decarboxylation catalyzed by malic enzyme: a reinvestigation

The NAD-malic enzyme catalyzes the divalent metal-ion-dependent oxidative decarboxylation of L-malate to yield CO2, pyruvate, and the reduced dinucleotide. With Mg2+ as the divalent metal ion activator, primary deuterium and tritium isotope effects have been obtained with several different alternative dinucleotide substrates. The partitioning ratio of oxalacetate to malate and pyruvate has also been determined with either NAD or 3-acetylpyridine adenine dinucleotide (3-APAD). These data have been used to calculate estimates of commitment factors and intrinsic isotope effects for the NAD-malic enzyme reaction. The calculated values of the intrinsic 13C and deuterium isotope effects with NADP are similar to the previously determined values for the chicken liver malic enzyme (Grissom, C.B., & Cleland, W. W. (1988) Biochemistry 27, 2927) and suggest that the transition-state structures are similar for the Ascaris NAD- and chicken liver NADP-malic enzymes. With NAD or NADP as the dinucleotide substrate, the data are all consistent with a stepwise chemical mechanism with oxidation of L-malate at C2 preceding decarboxylation of the bound oxalacetate intermediate. However, none of the data with the alternative dinucleotide substrates, 3-acetylpyridine adenine dinucleotide and 3-pyridine aldehyde adenine dinucleotide (PAAD), can be fit with satisfaction to the various criteria that support a stepwise mechanism with NAD(P). The mechanism with 3-APAD and PAAD is likely concerted. The most likely explanation for a change in the mechanism for oxidative decarboxylation from stepwise with NAD(P) to concerted with alternative dinucleotide substrates such as 3-APAD and PAAD is a difference in the configuration of bound malate when the different dinucleotide substrates are used.

Partial purification and comparison of the kinetic properties of ovine liver Echinococcus granulosus hydatid cyst fluid malate dehydrogenase and the cytoplasmic enzyme from the host liver

Ovine liver Echinococcus granulosus hydatid cyst fluid and cytoplasmic healthy ovine liver malate dehydrogenases were purified 24- and 30-fold by Sephadex G-200 and DEAE-Sephadex chromatography. Both enzymes were eluted with the same elution volume and the same salt concentration from the respective columns. The pH optimum of the enzymes from both sources was 8.4 in either Tris-HCl or barbital buffer. The Km values for oxaloacetate were 0.211 and 0.200 mM for hydatid cyst fluid and healthy ovine liver enzymes, respectively. The Km values for NADH were 0.220 and 0.213 mM for hydatid cyst fluid and healthy ovine liver enzymes, respectively. Enzyme from both sources demonstrated similar heat denaturation patterns. Both enzyme preparations were inhibited at high concentrations of either substrate. Neither enzyme was inhibited by para-hydroxymercuribenzoate or fumarate, and both enzyme preparations were specific for NADH as a cofactor. The results are discussed in terms of the possible infiltration of the host enzyme into the cyst fluid.

Plant mitochondrial NAD+-dependent malic enzyme. cDNA cloning, deduced primary structure of the 59- and 62-kDa subunits, import, gene complexity and expression analysis

The 59- and 62-kDa subunits of the mitochondrial NAD+-dependent malic enzyme (EC 1.1.1.39) were purified from Solanum tuberosum L. (potato). NH2-terminal and internal amino acid sequence information was used to identify cDNAs encoding the two subunits. Comparison of the nucleotide sequences revealed that the subunits have 60% identity at the DNA level and 65% identity at the deduced amino acid level, implying that they are derived from a common ancestral gene. The plant NAD+-dependent malic enzymes belong to a family of related enzymes, including cytosolic and chloroplastic NADP+-dependent malic enzymes (EC 1.1.1.40) and bacterial NAD+-dependent malic enzymes (EC 1.1.1.38). The cDNAs were transcribed and translated in vitro and the resultant polypeptides imported into isolated mitochondria and shown to be processed. Southern blot analysis of potato genomic DNA revealed a simple pattern of hybridization for both subunits, indicating a simple gene structure or small number of genes encoding the two subunits. Northern blot analysis of RNA from a range of potato tissues has shown that the steady state levels for the two subunits are equivalent, suggesting that they are coordinately expressed.

Cloning and analysis of the C4 photosynthetic NAD-dependent malic enzyme of amaranth mitochondria

In some C4 plant species, a mitochondrial NAD-dependent malic enzyme (EC 1.1.1.39) (NAD-ME) catalyzes the decarboxylation of 4 carbon malate in the bundle sheath cells, releasing CO2 for the Calvin cycle of photosynthesis. In amaranth, a dicotyledonous NAD-ME-type C4 plant, the photosynthetic NAD-ME purified as two subunits of 65 and 60 kDa, designated alpha and beta, respectively. Antiserum raised against the alpha subunit reacted only with the 65-kDa protein in immunoblot analysis. Immunogold electron microscopy using the alpha subunit antiserum demonstrated that this protein was localized specifically to the mitochondrial matrix of bundle sheath cells. The complete nucleotide sequence of a 2300-base pair alpha subunit cDNA clone showed that this gene encodes a protein that contains all of the motifs required for a complete and functional malic enzyme. The alpha subunit has significant similarity along its entire length to other known NAD- and NADP-dependent malic enzymes from plants, animals, and bacteria. The findings presented here provide new insights about the C4 photosynthetic NAD-ME and its evolutionary relationship to other forms of malic enzyme present in eukaryotic and prokaryotic organisms.

Purification and partial characterization of malate dehydrogenase (decarboxylating) from Tritrichomonas foetus hydrogenosomes

Malate dehydrogenase (decarboxylating) from Tritrichomonas foetus hydrogenosomes was purified close to homogeneity by a combination of differential centrifugation, zwitterionic detergent solubilization, Red-Sepharose chromatography and anion-exchange chromatography. The enzyme with apparent subunit size of 59 kDa and native molecular mass of 308 kDa utilized NAD+ preferentially to NADP+ as a cofactor and required Mn2+ or Mg2+ for its activity. Affinity curves for malate and coenzymes were hyperbolic. Km for malate was 100 microM and 458 microM in the presence of NAD+ and NADP+, respectively. Km for NAD+ and for NADP+ in the presence of malate was 18 microM and 207 microM, respectively. The enzyme is proposed to be a tetramer with a possible physiological role in the maintenance of an appropriate NAD+/NADH ratio in hydrogenosomes.