Expression, purification, and characterization of the recombinant NAD-malic enzyme from Ascaris suum

The cDNA encoding the 65-kDa subunit of malic enzyme from Ascaris suum was cloned into the bacterial expression vector pKK223-3 and overproduced in Escherichia coli. A protein with a subunit molecular mass of 65,000 was expressed at a level of up to 3% of the total soluble protein in JM109, as judged by SDS-PAGE. The enzyme was purified using column chromatography on phenyl-Sepharose followed by orange-A agarose. The purification procedure resulted in a 32-fold purification with an overall yield of 51%. The bacterially expressed enzyme exhibits kinetic constants identical to those measured for native A. suum NAD-malic enzyme.

Differences in isoenzyme patterns of axenically and monoxenically grown Acanthamoeba and Hartmannella

Axenically and monoxenically grown Acanthamoeba castellanii, Acanthamoeba polyphaga and different isolates of Hartmannella vermiformis strains were examined by polyacrylamide isoelectric focusing in the pH range 3-10. Isoenzyme patterns of acid phosphatase (AP), propionyl esterase (PE), malate dehydrogenase (MDH), alcohol dehydrogenase (ADH), glucose phosphate isomerase (GPI) and phosphoglucomutase (PGM) were compared. Zymograms were used to reveal differences in typical isoenzyme patterns between axenically and monoxenically grown amoebae and to compare axenically grown A. castellanii, A. polyphaga and H. vermiformis. Comparison of zymograms for AP, PE and MDH between axenically grown Acanthamoeba and Hartmannella strains revealed different isoenzyme patterns. Acanthamoeba showed strong bands for ADH and extremely weak bands for GPI and PGM, while Hartmannella lacked ADH but possessed bands for GPI and PGM. Comparison of zymograms from axenically and monoxenically grown amoebae revealed a lower intensity and even lack of typical isoenzyme bands in lysates from monoxenic cultures. The observed changes in typical isoenzyme patterns induced by the bacterial substrate can influence the correct isoenzymatic typing of different strains in clinical and phylogenetic studies.

Properties of NAD(+)- and NADP(+)-dependent malic enzymes of Rhizobium (Sinorhizobium) meliloti and differential expression of their genes in nitrogen-fixing bacteroids

The wild-type NAD(+)-dependent malic enzyme (dme) gene of Rhizobium (now Sinorhizobium) meliloti was cloned and localized to a 3.1 kb region isolated on the cosmid pTH69. This cosmid complemented the symbiotic nitrogen fixation (Fix-) phenotype of R. meliloti dme mutants. The dme gene was mapped by conjugation to between the cys-11 and leu-53 markers on the R. meliloti chromosome. beta-Galactosidase activities measured in bacterial strains carrying either dme-lacZ or tme-lacZ gene fusions (the tme gene encodes NADP(+)-dependent malic enzyme) indicated that the dme gene was expressed constitutively in free-living cells and in N2-fixing bacteroids whereas expression of the tme gene was repressed in bacteroids. The R. meliloti dme gene product (DME) was overexpressed in and partially purified from Escherichia coli. The properties of this enzyme, together with those of the NADP(+)-dependent malic enzyme (TME) partially purified from R. meliloti dme mutants, were determined. Acetyl-CoA inhibited DME but not TME activity. This result supports the hypothesis that DME, together with pyruvate dehydrogenase, forms a pathway in which malate is converted to acetyl-CoA.

Purification and characterization of the NADP-malic enzyme from Bradyrhizobium japonicum A1017

An NADP-malic enzyme [EC 1.1.1.40] was purified to homogeneity from Bradyrhizobium japonicum A1017, and the molecular and physiological characteristics were surveyed. The molecular mass of one subunit of the purified enzyme was evaluated to be 77,600 Da by SDS-PAGE, and the native enzyme was a tetramer in pH 7.0 and dimer in pH 8.0 conditions, showing complex oligomeric characteristics corresponding to pH value.

L-Malate dehydrogenase from Pseudomonas stutzeri: purification and characterization

L-Malate dehydrogenase (MDH) from Pseudomonas stutzeri was purified to homogeneity by a two-step procedure comprising anion-exchange chromatography and affinity chromatography on immobilized anthraquinone alpha-ketocarboxyl biomimetic dye. The enzyme has molecular mass of 66,500 Da and consists of two identical subunits of molecular mass of approximately 34,000 Da. Initial velocity, product inhibition, and binding studies were consistent with an ordered Bi-Bi mechanism for the enzyme action and the formation of a ternary complex. The enzyme is susceptible to activation and inhibition by its substrates. Thermodynamic analysis and kinetic inhibition studies were performed for determining basic equilibrium and kinetic constants. Malate dehydrogenase was covalently inactivated by a dichlorotriazine dye, Vilmafix Blue A-R (VBAR). The inactivation process follows first-order kinetics, and the results from kinetic analysis suggested the formation of a noncovalent enzyme-dye complex prior to the covalent reaction, with Kd 84.6 microM and a maximum rate constant 0.16 min(-1). The enzyme inactivation process was partially inhibited by substrates and inhibitors. Quantitatively inactivated MDH contained approximately 1 mole of dye per mole of enzyme subunit. The denatured enzyme contains 10 sulfhydryl groups per subunit, as shown after reaction with 5,5'-dithio-bis-(2-nitrobenzoic acid), of which 5 can be titrated also in the native enzyme, exhibiting time-dependent reactivity. One sulfhydryl group is located in the coenzyme binding site. This study shows that the physical and catalytic properties of P. stutzeri MDH strongly resemble those of the mitochondrial eukariotic enzyme. This finding strengthens the existing view that, in the evolution process, the mitochondrial MDH might have appeared before the cytoplasmic.

Isolation and characterization of cytosolic malate dehydrogenase from Trichomonas vaginalis

Malate dehydrogenase (EC 1.1.1.37.) (MDH) was purified to apparent homogeneity from the cytosolic fraction of the protozoan Trichomonas vaginalis Donné. The four step purification included ion-exchange chromatography (DEAE-Sephacel and Q-Sepharose, elution with NaCl) and affinity chromatography (Reactive Red Agarose, elution with NADH and NaCl). The enzyme was purified about 132-fold (30.6% yield) to a specific activity of 352 units mg-1. The Km values determined at pH 7.8 (pH optimum from 7.5 to 8.3) for oxaloacetate and NADH were 16.2 microM and 10.6 microM, respectively. The MDH activity was inhibited by the substrate, decreasing to 50% at about 1 microM concentration of oxaloacetate. The reverse reaction from malate to oxaloacetate showed a pH optimum around pH 9.5. The Km for malate and NAD+ (determined at pH 7.8) were 1220 microM and 69.9 microM, respectively. SDS-PAGE analysis of the purified MDH revealed a single band with an apparent size of 34.5 kDa. The native molecular weight was estimated by HPLC gel filtration to be 60 kDa, which indicates that the T. vaginalis MDH exists as a dimer.

L-Malate dehydrogenase from Pseudomonas stutzeri: purification and characterization

L-Malate dehydrogenase (MDH) from Pseudomonas stutzeri was purified to homogeneity by a two-step procedure comprising anion-exchange chromatography and affinity chromatography on immobilized anthraquinone alpha-ketocarboxyl biomimetic dye. The enzyme has molecular mass of 66,500 Da and consists of two identical subunits of molecular mass of approximately 34,000 Da. Initial velocity, product inhibition, and binding studies were consistent with an ordered Bi-Bi mechanism for the enzyme action and the formation of a ternary complex. The enzyme is susceptible to activation and inhibition by its substrates. Thermodynamic analysis and kinetic inhibition studies were performed for determining basic equilibrium and kinetic constants. Malate dehydrogenase was covalently inactivated by a dichlorotriazine dye, Vilmafix Blue A-R (VBAR). The inactivation process follows first-order kinetics, and the results from kinetic analysis suggested the formation of a noncovalent enzyme-dye complex prior to the covalent reaction, with Kd 84.6 microM and a maximum rate constant 0.16 min(-1). The enzyme inactivation process was partially inhibited by substrates and inhibitors. Quantitatively inactivated MDH contained approximately 1 mole of dye per mole of enzyme subunit. The denatured enzyme contains 10 sulfhydryl groups per subunit, as shown after reaction with 5,5'-dithio-bis-(2-nitrobenzoic acid), of which 5 can be titrated also in the native enzyme, exhibiting time-dependent reactivity. One sulfhydryl group is located in the coenzyme binding site. This study shows that the physical and catalytic properties of P. stutzeri MDH strongly resemble those of the mitochondrial eukariotic enzyme. This finding strengthens the existing view that, in the evolution process, the mitochondrial MDH might have appeared before the cytoplasmic.

Iron-ascorbate cleavable malic enzyme from hydrogenosomes of Trichomonas vaginalis: purification and characterization

Two isoforms of NAD(P)(+)-dependent malic enzyme (EC 1.1.1.39) were isolated from hydrogenosomes of Trichomonas vaginalis. A positively charged isoform at pH 7 was obtained in a single purification step using cation-exchange chromatography. The second isoform, negatively charged at pH 7.5, was partially purified using a combination of anion-exchange and affinity chromatography. Both isoforms displayed similar physical and kinetic properties. Molecular weight determination of the native enzyme suggested a homotetrameric arrangement of the 60 kDa subunits. The enzyme utilized NAD+ (Km, 6-6.3 microM) preferentially to NADP+ (Km, 125-145 microM). The NAD(+)-dependent activity showed a broad pH optimum with maximum under alkaline conditions (pH 9) likely to be present inside hydrogenosomes. Immunocytochemical studies using a polyclonal rabbit antibody raised against purified T. vaginalis malic enzyme proved hydrogenosomal localization of the enzyme. Subfractionation of hydrogenosomes suggested an association of the malic enzyme with the hydrogenosomal membranes. The 60 kDa malic enzyme subunit was highly sensitive to non-enzymatic cleavage by an iron-ascorbate system resulting in two enzymatically inactive fragments of about 31 kDa. Microsequencing of the fragments revealed that the 60 kDa subunit was cleaved at the metal-binding site between Asp279-Ile280. The enzyme inactivation was inhibited by an excess of manganese. Iron-dependent posttranslational modification might contribute to the regulation of malic enzyme activity in vivo.

Purification and characterization of malate dehydrogenase from the syntrophic propionate-oxidizing bacterium strain MPOB

Malate dehydrogenase from the syntrophic propionate-oxidizing bacterium strain MPOB was purified 42-fold. The native enzyme had an apparent molecular mass of 68 kDa and consisted of two subunits of 35 kDa. The enzyme exhibited maximum activity with oxaloacetate at pH 8.5 and 60 degrees C. The Ka for oxaloacetate was 50 microM and for NADH 30 microM. The Km values for L-malate and NAD were 4 and 1.1 mM, respectively. Substrate inhibition was found at oxaloacetate concentrations higher than 250 microM. The N-terminal amino acid sequence of the enzyme was similar to the sequences of a variety of other malate dehydrogenases from plants, animals and micro-organisms.

Further studies on the bioaffinity chromatography of NAD(+)-dependent dehydrogenases using the locking-on effect

Previous studies have capitalized on ordered kinetic mechanisms in the design of biospecific affinity chromatographic methods for highly efficient purifications and mechanistic studies of enzymes. The most direct tactic has been the use of immobilised analogues of the following, usually enzyme-specific substrates, e.g., lactate/pyruvate in the case of lactate dehydrogenase for which NAD+ is the leading substrate. Such immobilised specific substrates are, however, often difficult or impossible to synthesise. The locking-on strategy reverses the tactic by using the more accessible immobilised leading substrate, immobilised NAD+, as adsorbent with soluble analogues of the enzyme-specific ligands (e.g., lactate in the case of lactate dehydrogenase) providing a substantial reinforcement of biospecific adsorption sufficient to effect adsorptive selection of an enzyme from a group of enzymes such as the NAD(+)-specific enzymes. The value of this approach is demonstrated using model studies with lactate dehydrogenase (LDH, EC 1.1.1.27), alcohol dehydrogenase (ADH, EC 1.1.1.1), glutamate dehydrogenase (GDH, EC 1.4.1.3) and malate dehydrogenase (MDH, EC 1.1.1.37). Purification of bovine liver GDH in high yield from crude extracts is described using the tactic.

Purification and characterisation of isocitrate dehydrogenase and malate dehydrogenase from Mycobacterium tuberculosis and evaluation of their potential as suitable antigens for the serodiagnosis of tuberculosis

SETTING

Enzymes from Mycobacterium tuberculosis are potent antigens and might thus be of interest in the serodiagnosis of tuberculosis.

OBJECTIVE

The purpose of the study was to purify and characterize the two enzymes isocitrate dehydrogenase (IDH) and malate dehydrogenase (MDH) from, M. tuberculosis and to evaluate their potential in the serodiagnosis of tuberculosis.

DESIGN

The two enzymes were analysed for specificity by electrophoresis and then purified by means of affinity chromatography using reactive dyes and ion exchange chromatography. The two isolated enzyme fractions were analysed by ELISA, using antisera against related organisms. They were then tested as antigens in ELISA together with sera from tuberculous patients and controls.

RESULTS

The electrophoretical analyses showed that the two enzymes each differed markedly from the corresponding enzymes of other mycobacteria. The serological analyses, however, could not distinguish between either IDH or MDH from other mycobacteria, but organisms of other genera, such as Nocardia, gave much weaker responses. When IDH and MDH were tested with sera from tuberculous patients and controls the former gave clearly higher optical density values than the latter.

CONCLUSION

The enzymes/antigens IDH and MDH may be of value in developing a serological test for tuberculosis. The latter fraction seemed particularly capable of discriminating patients from controls.

Purification and characterization of a malic enzyme from the ruminal bacterium Streptococcus bovis ATCC 15352 and cloning and sequencing of its gene

Malic enzyme (EC 1.1.1.39), which catalyzes L-malate oxidative decarboxylation and pyruvate reductive carboxylation, was purified to homogeneity from Streptococcus bovis ATCC 15352, and properties of this enzyme were determined. The 2.9-kb fragment containing the malic enzyme gene was cloned, and the sequence was determined and analyzed. The enzymatic properties of the S. bovis malic enzyme were almost identical to those of other malic enzymes previously reported. However, we found that the S. bovis malic enzyme catalyzed unknown enzymatic reactions, including reduction of 2-oxoisovalerate, reduction of 2-oxoisocaproate, oxidation of D-2-hydroxyisovalerate, and oxidation of D-2-hydroxyisocaproate. The requirement for cations and the optimum pH of these unique activities were different from the requirement for cations and the optimum pH of the L-malate oxidative decarboxylating activity. A sequence analysis of the cloned fragment revealed the presence of two open reading frames that were 1,299 and 1,170 nucleotides long. The 389-amino-acid polypeptide deduced from the 1,170-nucleotide open reading frame was identified as the malic enzyme; this enzyme exhibited high levels of similarity to malic enzymes of Bacillus stearothermophilus and Haemophilus influenzae and was also similar to other malic enzymes and the malolactic enzyme of Lactococcus lactis.

Involvement of Phe19 in the Mn(2+)-L-malate binding and the subunit interactions of pigeon liver malic enzyme

A triple mutant, F19S/N250S/L353Q, of pigeon liver malic enzyme was found to have no detectable enzymatic activity [Chou, W.-Y., Huang, S.-M., & Chang, G.-G. (1994) Arch. Biochem. Biophys. 310, 158-166]. In the present study, point mutants at these positions (F19S, N250S, and L353Q) were prepared by site-directed mutagenesis. Both N250S and L353Q have kinetic properties similar to those of the wild-type. On the other hand, the K(m)(app) values for both Mn2+ and L-malate of F19S were increased by approximately 10-fold, while the kcat value was decreased by 5-fold, which results in a decrease of the apparent catalytic efficiency (kcat/K(mNADP)K(mMal)K(mMn) by approximately 300-fold. These results clearly indicate that the F19S mutation is mainly responsible for the undetectable enzyme activity of the triple mutant. Three more Phe19 mutants (F19Y, F19G, and F19A) were then prepared. There is a direct correlation between the size of the substitutes and the affinities for Mn2+ and L-malate. The kinetic parameters for F19Y were similar to those for wild-type. Both F19A and F19G reveal a 5-fold decrease of kcat values. Two K(dMn) values for the high- and low-affinity sites, respectively, were detectable for the wild-type. On the contrary, only one K(dMn) value was detected for the F19 mutants, which was increased in the order of F19G > F19A > F19S > F19Y, with F19G being the most affected mutant. The K(mMal) values of F19G and F19A were increased 100- and 6-fold, respectively. The catalytic efficiency (kcat/K(mNADP)K(dMal)K(dMn)) of F19G was decreased to only 0.01% of that of the wild-type. The above results clearly indicate that the hydrophobic aromatic ring at position 19 plays a critical role in L-malate and Mn2+ binding. Furthermore, all mutants that have a small residue at position 19 exist as monomers. Therefore, Phe19 may locate in or near the regions for Mn(2+)-L-malate binding as well as for the subunit contact. These results are compatible with the asymmetric model for the quaternary structure of malic enzyme we proposed previously [Chang, G.-G., Huang, T.-M., Huang, S.-M., & Chou, W.-Y. (1994) Eur. J. Biochem. 225, 1021-1027]. The possible roles of the N-terminus of malic enzyme were also addressed.

Cloning, sequencing and functional expression of a DNA encoding pig cytosolic malate dehydrogenase: purification and characterization of the recombinant enzyme

Using the polymerase chain reaction, DNA encoding cytosolic malate dehydrogenase (cMDH) has been cloned from a pig heart cDNA library. Large amounts of the enzyme (30 mg per litre of original culture) have been produced in Escherichia coli using an inducible expression vector (pKK223-3) in which the 5'-non-coding region of the gene was replaced with the tac promoter. The complete nucleotide sequence of the DNA is reported for the first time. The recombinant cMDH purified was shown to be identical to the native enzyme according to: chromatographic behaviour, isoelectric point, N-terminal amino acid sequence, and physiochemical and catalytic properties.

Kinetic studies of purified malate dehydrogenase in liver of streptozotocin-diabetic rats and the effect of leaf extract of Aegle marmelose (L.) Correa ex Roxb

The functional basis of diabetes-mellitus to a certain extent, can be elucidated by studying diabetes-induced changes in metabolic enzymes. Malate dehydrogenase (MDH), is an enzyme directly involved in glucose metabolism. The kinetic parameters of MDH and its purified cytosolic isozyme, S-MDH, have been studied in the liver of streptozotocin-diabetic rats; also the potential of the leaf extract of A. marmelose as an anti-diabetic agent was investigated. The Km of the liver enzyme increased significantly, in both crude and purified preparations in the diabetic state when compared to the respective controls. Insulin as well as leaf-extract treatment of the diabetic rats brought about a reversal of Km values to near normal. Vmax of purified S-MDH was significantly higher in the diabetic state when compared to the control. Insulin and leaf extract treatment did not reverse this change. Since MDH is an important enzyme in glucose metabolism, the variation in its quantitative and qualitative nature may contribute to the pathological status of diabetes. The fact that leaf extract of A. marmelose was found to be as effective as insulin in restoration of blood glucose and body weight to normal levels, the use of A. marmelose as potential hypoglycemic agent is suggested.

Plant glyoxysomal but not mitochondrial malate dehydrogenase can fold without chaperone assistance

Glyoxysomal (gMDH) and mitochondrial malate dehydrogenase (mMDH) from watermelon are synthesized as higher molecular weight precursor proteins. By overexpressing the precursor forms as well as the mature subunits with a histidine arm at the carboxy-terminus, it has been possible to purify relatively large amounts especially of the glyoxysomal precursor protein for studies of their refolding capacities after denaturation with guanidinium hydrochloride, heat or low pH. Glyoxysomal MDH and its precursor is capable of its spontaneous folding over a wide range of temperature conditions. Refolding can be enhanced by inclusion of BSA and ATP as stabilisers in the folding buffer. The N-terminal transit peptide of gMDH facilitates folding, but does not function as an intramolecular chaperon. Chemically denatured mitochondrial MDH requires chaperones for refolding. GroEL/GroES/ATP increase the yield and rate of watermelon mMDH folding dramatically while GroEL and Mg-ATP alone are not sufficient to provide folding assistance similar to the results with hydrophobic mammalian mMDH. The watermelon glyoxysomal MDH interacts with GroEL-like hydrophilic mammalian cytoplasmic MDH, a binding which has to be released by Mg-ATP before spontaneous folding can ensue. Interestingly, watermelon mMDH exhibited a much higher heat stability than gMDH or mammalian mMDH in the presence of BSA/ATP as well as GroEL/GroES/ATP. The differences between glyoxysomal and chaperone-assisted mitochondrial folding patterns are discussed.

Concomitant purification and characterization of malate dehydrogenase, aspartate transaminase, nucleoside diphosphate kinase and enolase from rabbit liver cytosol

A procedure was developed for purifying the cytosolic isoforms of malate dehydrogenase, aspartate transaminase, enolase and nucleoside diphosphate kinase from a single preparation of rabbit liver. The procedure includes chromatography on reactive-dye, radial-flow columns, and elution of enzymes from columns by substrates, to obtain high yields in a minimal amount of time. The scheme avoids steps used in previous methods that are either difficult to execute in large-scale preparations, or alter the native forms of the enzymes. Examination of the purified enzymes by SDS-PAGE indicated that nearly homogeneous preparations had been obtained. The native molecular weight, subunit molecular weight, and isoelectric point(s) were determined for each enzyme. Although preparations of nucleoside diphosphate kinase purified from cytosol showed only a single band on SDS-PAGE, isoelectric focusing revealed the presence of multiple isoforms.

Partial purification and kinetic properties of cytoplasmic malate dehydrogenase from ovine liver Echinococcus granulosus protoscolices

Cytoplasmic malate dehydrogenase from ovine liver Echinococcus granulosus protoscolices was purified 22-fold by QAE- and SP-Sephadex chromatography. The pH optimum of the enzyme was 8.0 in either Tris-HCI or barbital buffer. The Km values of oxaloacetate and NADH were 0.400 +/- 0.018 and 0.410 +/- 0.038 mM, respectively. The enzyme lost about 90% of its activity when heated for 2 min at 65 degrees C. A 61.4% inhibition of the enzyme was noted at 4 mM concentration of diethyl pyrocarbonate. A 3 mM concentration of fructose 1,6-diphosphate inhibited the enzyme by 76.5%. The inhibition was non-competitive with respect to NADH with a Ki value of 0.85 mM. A 75% inhibition of the enzyme was noted at 1 mM concentration of mebendazole that inhibited the enzyme upon competing with NADH with a Ki value of 0.176 mM. A 2-mM concentration of citrate almost doubled the enzyme activity. The enzyme was inhibited at high concentrations of either substrate. The enzyme was not inhibited by p-hydroxymercuribenzoate or fumarate. The enzyme was absolutely specific for NADH as a cofactor. The properties of this enzyme are compared with those of the enzyme from the host liver, the cyst fluid and some other animal sources. The results are discussed in terms of the differences among the properties of the host liver, the cyst fluid and the protoscolices enzymes. The biochemical basis for the use of mebendazole in the treatment of echinococcosis is also elucidated.

Nonidentity of the cDNA sequence of human breast cancer cell malic enzyme to that from the normal human cell

A cDNA coding for human breast cancer cell cytosolic NADP(+)-dependent malic enzyme was obtained. This cDNA is composed of a length of 2084 base pairs, with 1698 base pairs coding for 565 amino acid residues and a length of 386 base pairs representing a 3'-noncoding region. Comparing this nucleotide sequence with that from the normal human tissue [Loeber, G., Dworkin, M. B., Infante, A., and Ahorn, H. (1994), FEBS Lett. 344, 181-186] reveals that three nucleotides in the open reading frame and the length of 3'-noncoding region of the cDNA are different. One of the changes results in a substitution of serine at position 438 for proline, which, however, may not cause significant changes in the predicted secondary structure. A partial cDNA lacking the first 84 nucleotides in the open reading frame was successfully cloned and expressed functionally in Escherichia coli cells. Its Km value for L-malate (1.21 +/- 0.11 mM) is four times higher than that for the natural human breast cancer cell malic enzyme (0.29 +/- 0.04 mM) but similar to that for the full-length recombinant enzyme (1.06 +/- 0.07 mM). The Km values for Mn2+ and NADP+ (0.26 +/- 0.03 and 0.97 +/- 0.4 microM, respectively) are similar to those for the natural enzyme (0.12 +/- 0.02 and 1.9 +/- 0.3 microM, respectively) or the recombinant wild-type enzyme (0.56 +/- 0.04 and 0.44 +/- 0.02 microM, respectively). A recombinant pigeon liver malic enzyme without the first 13 amino acid residues was used for comparison. The Km values for L-malate and Mn2+ of the truncated enzyme (11.2 +/- 0.9 mM and 61.2 +/- 4.6 microM, respectively) are over 40 times larger than those for the natural pigeon liver malic enzyme (0.21 +/- 0.02 mM and 1.06 +/- 0.08 microM, respectively) or the recombinant wild-type enzyme (0.25 +/- 0.01 mM and 1.48 +/- 0.05 microM, respectively). We suggest that the N-terminus of malic enzyme may be required for the substrate binding during the catalytic cycle.

The catalytic site of chloroplastic NADP-dependent malate dehydrogenase contains a His/Asp pair

Plant chloroplastic NADP-malate dehydrogenase is unique among malate dehydrogenases because of its reductive activation in the light and cofactor specificity. In this paper, the role of His229 in sorghum leaf protein has been investigated by site-directed mutagenesis. His229 was replaced by Asn and Gln, both mutations yielding an inactive protein. The role of a conserved Asp (Asp201) as a possible partner of His229 in catalysis has been studied by the same approach. Both Asp mutants (D201A, D201N) were only slightly active and were essentially characterized by a dramatically increased Km for oxaloacetate (45-80-fold). pH dependence of catalytic rates revealed differences between the two Asp mutants. These results demonstrate that, in sorghum leaf NADP-dependent malate dehydrogenase, His229 is involved in catalysis in interaction with Asp201.

Biomimetic-dye affinity chromatography for the purification of mitochondrial L-malate dehydrogenase from bovine heart

Seven biomimetic anthraquinone triazinyl dye-ligands, bearing as triazine-linked terminal moiety (keto)carboxylated structures mimicking substrates and inhibitors of malate dehydrogenase (MDH), were immobilised on cross-linked agarose Ultrogel A6R. These biomimetic ligands are terminal-ring analogues of commercial nonbiomimetic Cibacron blue 3GA (CB3GA) and parent Vilmafix blue A-R (VBAR). The biomimetic-dye adsorbents, along with nonbiomimetic adsorbents bearing immobilised CB3GA and VBAR, were evaluated for their ability to purify mitochondrial malate dehydrogenase (mMDH) from bovine heart. All but two biomimetic-dye adsorbents displayed higher purifying ability for MDH, compared to nonbiomimetic-dye adsorbents. Furthermore, immobilised anthraquinone-dyes were able to discriminate between the mitochondrial and the cytoplasmic MDH isoenzymes, binding only to the former. One immobilised biomimetic-dye (BM5), bearing as biomimetic terminal moiety 4-aminophenyloxanylic acid, showed the highest purifying ability. This affinity adsorbent was exploited in the purification of mMDH from unpretreated bovine heart extract in one-step. The procedure afforded mMDH at 54% overall yield and of specific activity approx. 1300 U mg-1 (25 degrees C), using step-elution with a mixture containing 0.1 mM beta-nicotinamide adenine dinucleotide (NAD+) and 1.5 mM sulphite. Commercial analytical-grade bovine heart mitochondrial MDH, when assayed under identical conditions, gave a specific activity not exceeding 950 U mg-1. The well-known adsorbent Cibacron blue 3GA-agarose exhibited 8% lower recovery and 25% lower purification for mMDH. The product obtained from the procedure based on the BM5-adsorbent was free of cytoplasmic MDH, glutamic-oxaloacetic transaminase (GOT) and fumarase, and since it has also shown high specific activity, it should be suitable for analytical applications.

Identification of a multienzyme complex of the tricarboxylic acid cycle enzymes containing citrate synthase isoenzymes from Pseudomonas aeruginosa

A multienzyme complex of tricarboxylic acid cycle enzymes, catalysing the consecutive reactions from fumarate to 2-oxoglutarate, has been identified in extracts of Pseudomonas aeruginosa prepared by gentle osmotic lysis of the cells. The individual enzyme activities of fumarase, malate dehydrogenase, citrate synthase, aconitase and isocitrate dehydrogenase can be used to reconstitute the complex. The citrate synthase isoenzymes, CSI and CSII, from this organism can be used either together or as the individual activities to reconstitute the complex. No complex can be reformed in the absence of CSI or CSII. Which CS isoenzyme predominates in the complex depends on the phase of growth at which the cells were harvested and the extract prepared. More CSI was found in the complex during exponential growth, whereas CSII predominated during the stationary phase. The results support the idea of a 'metabolon' in this organism, with the composition of the CS component varying during the growth cycle.

NADP-malic enzyme isoforms in maize leaves

Two isoforms of NADP-malic enzyme have been characterized in maize leaves. The 72 kDa-form of the protein, present mainly in etiolated maize leaves, has lower specific activity than the 62 kDa-form, which is implicated in C4 metabolism and predominates in green leaves. The larger form of the enzyme has higher Km values for NADP and malate and lower PH optimum. The antibodies raised against the 62 kDa-form of the protein react with the 72 kDa-form. Steady state levels of NADP-malic enzyme, as measured by the amount of protein and activity, increase several-fold when dark-grown maize seedlings are illuminated. This increase in protein is about 13-fold for the 62 kDa-form of the enzyme, while the 72 kDa-form remains practically constant after a transient increase. Northern blot analysis using a specific probe against the 62 kDa-form of the enzyme, reveals the increase of a 2.2 kb mRNA during greening. Southern hybridization analysis with genomic DNA suggests the presence of more than one gene encoding NADP-malic enzyme in maize. In this paper we provide biochemical and inmunological evidence suggesting that both isoforms are closely related and that the 72 kDa-form is also present in low levels in mature green leaves.

Primary structure of a cytosolic malate dehydrogenase of the amitochondriate eukaryote, Trichomonas vaginalis

The nucleotide sequence of a gene coding for a 37 kDa subunit of a cytosolic malate dehydrogenase of Trichomonas vaginalis was established. The sequence of a gDNA clone and a cDNA clone, which lacked seven amino-terminal codons, were identical, indicating an absence of introns from the gene. Cell fractionation combined with sequencing of peptide fragments of the purified enzyme showed that the gene codes for an expressed cytosolic enzyme. The derived amino acid sequence was closely related to cytosolic malate dehydrogenases from animals and plants and from the eubacteria Thermus aquaticus and Mycobacterium leprae and was more distant from the enzyme of mitochondria and from Escherichia coli and certain other eubacteria. In phylogenetic reconstructions this enzyme shared a most recent common ancestor with other cytosolic enzymes.

NAD+/NADP+-dependent malic enzyme: evidence of a NADP+ preferring activity in human skeletal muscle

The L-malate NAD(P)+ oxidoreductase (decarboxylating) E.C.1.1.1.39 was purified from human skeletal muscle; the specific activity estimated in the presence of NAPD+ as coenzyme was approximately 15 mumol/min/mg. The apparent Vmax values for NAD+ (approximately 8 mumol/min/mg) and NADP+ (approximately 16 mumol/min/mg) show that the enzyme (in this tissue) is more active in the presence of NAPD+. This observation was confirmed by the estimation of enzymatic activity in competition experiments where both NAD+ and NADP+ were used together as coenzymes. The absence of pyruvate carboxylation and of oxalacetate decarboxylation activities demonstrates that the enzyme studied is E.C.1.1.1.39. In addition, the apparent Km values for NAD+ and NADP+ were calculated (15 and 0.05 mM, respectively). This paper provides the first demonstration of a NADP+ preferring activity of the enzyme in human skeletal muscle.