Malate dehydrogenases of leaf tissue from Spinacia oleracea: properties of three isoenzymes

Gene number in species of Astereae that have different chromosome numbers

Differences in the gametic chromosome numbers (n = 4, 5, 9) of species in the Astereae tribe of the Compositae have been variously interpreted. One hypothesis proposes that n = 9 was the original base number of the group and that the lower numbers resulted from aneuploid reduction. The alternative hypothesis asserts that the ancestral base number was n = 4 or n = 5 and that species in which n = 9 are allotetraploids derived by hybridization between taxa with the lower numbers. Electrophoretic analysis of 17 enzyme systems in five species of Machaeranthera, in which n = 4, 5, and 9, and two species of Aster in which n = 5 and 9, demonstrates that all of these species have the same number of gene loci specifying the tested enzymes. The absence of isozyme multiplicity in the species in which n = 9 suggests that they did not arise by polyploidy.

Limited proteolysis of inactive tetrameric chloroplast NADP-malate dehydrogenase produces active dimers

Carboxy-terminal amino acids of NADP-dependent malate dehydrogenase (EC 1.1.1.82) from pea chloroplasts were removed by treatment with carboxypeptidase Y. This results in the activation of the inactive oxidized enzyme, while activation by light in vivo is thought to occur via reduction of an intrasubunit disulfide bridge. After proteolytic activation the oxidized enzyme had a specific activity of 100 U/mg protein, which is 50% of the maximal activity of the control enzyme in the reduced state. When the truncated enzyme was reduced with dithiothreitol (DTT), the specific activity was further increased to 1200 U/mg. While the native enzyme is composed of four identical subunits of 38,900 Da, the truncated malate dehydrogenase forms dimers composed of two subunits of 38,000 Da. No further change of molecular mass or activity was noticed subsequent to prolonged incubation of native NADP-malate dehydrogenase with carboxypeptidase Y for several days. When the enzyme is denatured by 2 M guanidine-HCl, the proteolytic activation proceeds more rapidly, but only transiently. The truncated enzyme is less accessible to activation by reduced thioredoxin, but the stimulation of activity by DTT alone is more rapid than that of the native enzyme. These results indicate that only a small carboxy-terminal peptide of native NADP-malate dehydrogenase from pea chloroplasts is accessible to proteolytic degradation and that this peptide is involved in the regulation of activity, tetramer formation, and thioredoxin binding. While the pH optimum for catalytic activity of the intact reduced enzyme is at pH 8.0-8.5, it is shifted to more acidic values upon proteolysis of NADP-malate dehydrogenase. At pH values below 8 the reduced truncated enzyme exhibits substrate inhibition by oxaloacetate.

Crystalline NAD/NADP-dependent malate dehydrogenase; the enzyme from the thermoacidophilic archaebacterium Sulfolobus acidocaldarius

Malate dehydrogenase from Sulfolobus acidocaldarius has been purified 240-fold to apparent electrophoretic homogeneity. The enzyme shows a specific activity of 277 U/mg and crystallizes readily. The relative molecular mass of the native enzyme is estimated as 128,500 by ultracentrifugation. After cross-linking a relative molecular mass of 134,000 is found by sodium dodecyl sulfate gel electrophoresis. Malate dehydrogenase from S. acidocaldarius is composed of four subunits of identical size with a relative molecular mass of 34,000. Active-enzyme sedimentation in the analytical ultracentrifuge indicates that the tetramer is the catalytically active species. Kinetic studies in the direction of oxaloacetate reduction showed a Km for NADH of 4.1 microM and a Km for oxaloacetate of 52 microM. Oxaloacetate exhibits substrate inhibition at higher concentrations, L-malate, NAD and NADP were found to be product inhibitors. The enzymatic activity is inhibited by 2-oxoglutarate but not by the adenosine nucleotides AMP, ADP and ATP. Only low activity is detected in the direction of malate oxidation. Malate dehydrogenase from S. acidocaldarius utilizes both NADH and NADPH to reduce oxaloacetate. The enzyme shows A-side stereospecificity for both nicotinamide dinucleotides.

Conversion of serine to glycerate in intact spinach leaf peroxisomes: role of malate dehydrogenase

In photorespiration, leaf peroxisomes convert serine to glycerate via serine-glyoxylate aminotransferase and NADH-hydroxypyruvate reductase. We isolated intact spinach leaf peroxisomes in 0.25 M sucrose, and characterized their enzymatic conversion of serine to glycerate using physiological concentrations of substrates and coenzymes. In the presence of glycolate (glyoxylate), and NADH and NAD alone or together in physiological proportions, the rate of serine-to-glycerate conversion was enhanced and sustained by the addition of malate. The rate was similar at 1 and 5 mM serine, but was two to three times higher in 50 mM than 5 mM malate. In the presence of NAD and malate, there was 1:1 stoichiometric formation of glycerate and oxaloacetate. Addition of 1 or 5 mM glutamate resulted in a negligible enhancement of the conversion of hydroxypyruvate to glycerate. Intact peroxisomes produced glycerate from either serine or hydroxypyruvate at a rate two times higher than osmotically lysed peroxisomes. These results suggest that under physiological conditions, the peroxisomal malate dehydrogenase operates independent of aspartate-alpha-ketoglutarate aminotransferase in supplying NADH for hydroxypyruvate reduction. This supply of NADH is the rate-limiting step in the conversion of serine to glycerate. The compartmentation of hydroxypyruvate reductase and malate dehydrogenase in the peroxisomes confers a higher efficiency in the supply of NADH for hydroxypyruvate reduction under a normal, high NAD/NADH ratio in the cytosol.

Purification of particulate malate dehydrogenase and phosphoenolpyruvate carboxykinase from Leishmania mexicana mexicana

The particulate activities of Leishmania mexicana mexicana amastigote malate dehydrogenase (L-malate:NAD+ oxidoreductase, EC 1.1.1.37) and phosphoenolpyruvate carboxykinase (ATP:oxaloacetate carboxy-lyase (transphosphorylating) EC 4.1.1.49) have been purified to apparent electrophoretic homogeneity by hydrophobic interaction chromatography using Phenyl-Sepharose CL-4B, affinity chromatography using 5'AMP-Sepharose 4B, and gel filtration using Sephadex G-100. Malate dehydrogenase was purified 150-fold overall with a final specific activity of 1230 units/mg protein and a recovery of 63%. Phosphoenolpyruvate carboxykinase was purified 132-fold with a final specific activity of 30.3 units/mg protein and a recovery of 20%. Molecular weights determined by gel filtration and SDS-gel electrophoresis were 39 800 and 33 300 for malate dehydrogenase and 63 100 and 65 100 for phosphoenolpyruvate carboxykinase, respectively. Kinetic studies with malate dehydrogenase assayed in the direction of oxaloacetic acid reduction showed a Km(NADH) of 41 microM and a Km(oxaloacetic acid) of 39 microM. For malate oxidation there was a Km(malate) of 3.6 mM and a Km(NAD) of 0.79 mM. Oxaloacetic acid exhibited substrate inhibition at concentrations greater than 0.83 mM and malate was found to be a product inhibitor at high concentrations. However, there was no modification of enzyme activity by a number of glycolytic intermediates and cofactors, suggesting that malate dehydrogenase is not a major regulatory enzyme in L. m. mexicana. The results show that these L. m. mexicana amastigote enzymes are in several ways similar to their mammalian counterparts; nevertheless, their apparent importance and unique subcellular organization in the parasite make them potential targets for chemotherapeutic attack.

Conversion of glycerate to serine in intact spinach leaf peroxisomes

Intact spinach (Spinacia oleracea L.) leaf peroxisomes converted glycerate to serine in the presence of NAD and alanine. The reaction proceeded optimally at pH9. Addition of oxaloacetate or alpha-ketoglutarate plus aspartate enhanced the conversion about three-fold. Alteration of the concentration of one of the reaction components, consisting of 2 mM glycerate, 0.2 mM NAD, 0.5 mM oxaloacetate, and 2 mM alanine, revealed half-saturation constants of 0.45 mM for glycerate, 0.06 mM for NAD, 0.02 mM for oxaloacetate, and 0.33 mM for alanine. The conversion proceeded with the formation of hydroxypyruvate followed by serine; hydroxypyruvate did not accumulate to a high amount in the presence or absence of alanine. The amino group donor could be alanine (half-saturation constant, 0.33 mM), glycine (0.45 mM), or asparagine (0.67 mM); the three amino acids produced roughly similar Vmax values. The results indicate that, in the conversion of glycerate to serine, the transamination is catalyzed by a hydroxypyruvate aminotransferase with characteristics unknown among all other studied leaf peroxisomal aminotransferases. The peroxisomal membrane is sparsely permeable to NAD/NADH, and the participation of the peroxisomal malate dehydrogenase in an electron shuttle system across the membrane in the regeneration of NAD/NADH is suggested.

Cell organelles from crassulacean acid metabolism (CAM) plants : II. Compartmentation of enzymes of the crassulacean acid metabolism

The intracellular distribution of enzymes involved in the Crassulacean acid metabolism (CAM) has been studied in Bryophyllum calycinum Salisb. and Crassula lycopodioides Lam. After separation of cell organelles by isopycnic centrifugation, enzymes of the Crassulacean acid metabolism were found in the following cell fractions: Phosphoenolpyruvate carboxylase in the chloroplasts; NAD-dependent malate dehydrogenase in the mitochondria and in the supernatant; NADP-dependent malate dehydrogenase and phosphoenolpyruvate carboxykinase in the chloroplasts; NADP-dependent malic enzyme in the supernatant and to a minor extent in the chloroplasts; NAD-dependent malic enzyme in the supernatant and to some degree in the mitochondria; and pyruvate; orthophosphate dikinase in the chloroplasts. The activity of the NAD-dependent malate dehydrogenase was due to three isoenzymes separated by (NH4)2SO4 gradient solubilization. These isoenzymes represented 17, 78, and 5% of the activity recovered, respectively, in the order of elution. The isoenzyme eluting first was associated with the mitochondria and the second isoenzyme was of cytosolic origin, while the intracellular location of the third isoenzyme was probably the peroxisome. Based on these findings, the metabolic path of Crassulacean acid metabolism within cells of CAM plants is discussed.

Cell organelles from crassulacean-acid-metabolism (CAM) plants : I. Enzymes in isolated peroxisomes

Cell organelles were isolated from the CAM plants Crassula lycopodioides Lam., Bryophyllum calycinum Salisb. and Sedum rubrotinctum R.T. Clausen by isopycnic centrifugation in sucrose gradients. The inclusion of 2.5% Ficoll in the grinding medium proved to be essential for a satisfactory separation of cell organelles during the subsequent centrifugation. Peroxisomes, mitochondria, and whole and broken chloroplasts were at least partially resolved as judged by marker-enzyme-activity profiles. The isolated peroxisomes contained activities of glycollate oxidase, catalase, hydroxypyruvate reductase, glycine aminotransferase, serine-glyoxylate aminotransferase, and aspartate aminotransferase, comparable to activities found in spinach (Spinacia oleracea L.) leaf peroxisomes. In contrast to spinach, however, only little, if any, particulate malate dehydrogenase activity could be attributed to isolated peroxisomes of the three CAM plants.

A comparison of the malate dehydrogenase of the propionic acid bacteria with the mammalian soluble and mitochondrial isoenzymes

1. Like the malate dehydrogenases of eucaryotic cells, the Propionibacterium shermanii enzyme is a dimer consisting of two 35,000 molecular weight subunits. 2. In electrophoretic behavior, resistance to substrate inhibition and stability to heating and dilution the P. shermanii MDH is more similar to the s-MDH than to the m-MDH of pig heart. 3. The P. shermanii MDH has a high turnover number (ca. 140,000) as well as Km values for both L-malate and oxalacetate which are four times higher than the mammalian isoenzymes. 4. A coupled assay for MDH using the malate-lactate transhydrogenase and diaphorase is described in which both substrates, L-malate and NAD, are regenerated.

Glyoxysomal and mitochondrial malate dehydrogenase of watermelon (Citrullus vulgaris) cotyledons : II. Kinetic properties of the purified isoenzymes

Kinetic parameters of the glyoxysomal and mitochondrial malate dehydrogenase (EC 1.1.1.37) of watermelon (Citrullus vulgaris Schrad.) cotyledons were compared. The data were obtained by initial rate experiments at pH 8.5 in both directions of the reaction using homogeneous enzyme preparations. Substrate inhibition at physiologically significant concentrations was observed with reduced nicotinamide adenine dinucleotide (NADH) (50% inhibition at 0.65 mmol·l(-1) NADH), but not with oxaloacetate, L-malate or oxidized nicotinamide adenine dinucleotide. The inhibition of both isoenzymes by 5'adenosine monophosphate was studied. Inhibition was found to be competitive with respect to NADH and non-competitive with respect to oxaloacetate. The apparent inhibitor constants at 200 μmol·l(-1) of the fixed substrates were 3.2 and 1.6 mmol·1(-1) for NADH, and 3.2 and 5.2 mmol·l(-1) for oxaloacetate with the glyoxysomal and mitochondrial isoenzymes, respectively. The energy of activation was determined for oxaloacetate reduction by glyoxysomal (E a =3.14×10(4)J×mol(-1)) and mitochondrial (E a =4.10×10(4) J x mol(-1)) MDH from Arrhenius plots, which exhibited constant slopes throughout the range of thermal stability.Despite considerable structural differences, the results indicate very similar kinetic behaviour of the glyoxysomal and mitochondrial isoenzymes. The physiological significance of the data are discussed in relation to the gluconeogenic processes occuring in cotyledons during germination.

Purification and properties of cytoplasmic and mitochondrial malate dehydrogenases of Physarum polycephalum

Two isoenzymes of malate dehydrogenase (MDH) were demonstrated in plasmodia of Physarum polycephalum by polyacrylamide-gel electrophoresis. The more "cathodal" form was uniquely associated with mitochondria (M-MDH) and the other form was found in the soluble cytoplasm (S-MDH). The isoenzymes were separated by acetone fractionation of soluble plasmodial homogenates acidified to pH 5.0. The M-MDH was purified 201-fold by cetylpyridinium chloride treatment, fractionation with ammonium sulfate, gradient elution from sulfoethyl cellulose at pH 6.0, and Sephadex G-100 chromatography. The S-MDH was purified 155-fold by ammonium sulfate fractionation, diethylaminoethyl cellulose chromatography, gradient elution from sulfoethyl cellulose at pH 5.5, and Sephadex G-100 chromatography. The optimal cis-oxalacetate concentrations were 0.35 mM for M-MDH and 0.25 mM for S-MDH, and the optimal pH for both isoenzymes was 7.6 for oxalacetate reduction. The optimal l-malate concentrations were 5 mM for S-MDH and 6 mM for M-MDH, and both isoenzymes exhibited an optimal pH of 10.0 for L-malate oxidation. The Michaelis constants of S-MDH and M-MDH served to discriminate between the isoenzymes. The S-MDH was more heat-stable than the M-MDH. High concentrations of oxalacetate and malate inhibited S-MDH more than M-MDH. The isoenzymes were further distinguished by their utilization of analogues of nicotinamide adenine dinucleotide. Many properties of the Physarum isoenzymes were similar to those of more complex organisms, especially vertebrates.

[Compartmentation and properties of the MDH-isoenzymes from watermelon cotyledons (Citrullus vulgaris Schrad.)]

Five MDH-isoenzymes (I-V) from cotyledons of dark-grown water-melon seedlings older than 2 days can be identified by disc-electrophoresis. By isolating and fractionating cell organelles (10000 g fraction) by density gradient centrifugation (Fig. 1) the following compartmentation of the MDH-isoenzymes can be shown: the mitochondria contain isoenzyme III and the glyoxysomes preponderantly (if not exclusive) isoenzyme V (Fig. 2), whereas the isoenzymes I, II, and IV belong to the cytosol.The 5 MDH-isoenzymes differ in several properties, e.g. differential precipitation by ammonium sulfate (Fig. 4) or isoelectric point (Fig. 5). The glyoxysomal MDH is a relatively basic protein (isoelectric point at pH 8.7), whereas the isoelectric points of the other isoenzymes lie between pH 6.4 (IV) and pH 4.7 (I).