Interaction between NAD-dependent isocitrate dehydrogenase, alpha-ketoglutarate dehydrogenase complex, and NADH:ubiquinone oxidoreductase

Interaction between the alpha-ketoglutarate dehydrogenase complex and NAD+-dependent isocitrate dehydrogenase was detected with a variety of techniques including polyethylene glycol precipitation, ultracentrifugation, and centrifugal gel filtration on a Sepharose 6B column. The interaction was specific in that citrate synthase, cytosolic malate dehydrogenase, and NADP-dependent isocitrate dehydrogenase did not interact with alpha-ketoglutarate dehydrogenase complex. The interaction was not inhibited by either 0.1 M KCl or 0.4 M (NH4)2SO4, but was completely prevented by 5% glycerol. A new method for the preparation of NADH: ubiquinone oxidoreductase resulted in an enzyme having a protein subunit composition similar to that of classical complex I preparation. Evidence is given for the existence of ternary complexes containing NADH:ubiquinone oxidoreductase-alpha-ketoglutarate dehydrogenase complex-NAD-dependent isocitrate dehydrogenase and NADH: ubiquinone oxidoreductase-alpha-ketoglutarate dehydrogenase complex-succinate thiokinase. These data suggest that a part of the citric acid cycle may be located in the vicinity of NADH: ubiquinone oxidoreductase. These complexes may facilitate the transport of metabolites among these enzymes without their equilibrating with the whole compartment.

Evidence for a multiple subunit composition of plant NAD malic enzyme

Malate dehydrogenase (decarboxylating) (EC 1.1.1.39) was purified to near homogeneity from both a C3 plant, Solanum tuberosum, and a CAM plant, Crassula argentea. Sodium dodecyl sulfate-gel electrophoresis of both enzymes revealed an alpha,beta subunit composition with corresponding molecular mass assignments of 61,000 and 55,000 daltons. Isoelectric focusing under native conditions showed only one constituent malic enzyme form with an isoelectric point of 5.1. No evidence of additional isoenzymes was found. Urea isoelectric focusing showed the alpha subunit to be more acidic than the beta subunit. Peptide mapping by limited proteolysis with Staphylococcus aureus V-8 protease, trypsin, and endoproteinase Arg-C eliminated the possibility that a precursor-product relationship may have existed between the two subunits and demonstrated that they each possess unique primary sequences. Further support for this conclusion was obtained when significant differences in the contents of glutamic acid, isoleucine, and arginine were revealed by amino acid analysis of the isolated subunits. There was no apparent activity associated with the separated subunits (as resolved by urea-DEAE chromatography), but activity could be found in a reconstituted system, thereby indicating an (alpha,beta)n protomeric configuration. This is the first case where malic enzyme has been conclusively shown to be constructed from nonidentical subunits. This phenomenon has been observed only for the NAD malic enzyme isolated from plants.

Malic enzyme from archaebacterium Sulfolobus solfataricus. Purification, structure, and kinetic properties

An NADP-preferring malic enzyme ((S)-malate:NADP oxidoreductase (oxalacetate-decarboxylating) EC 1.1.1.40) with a specific activity of 36.6 units per mg of protein at 60 degrees C and an isoelectric point of 5.1 was purified to homogeneity from the thermoacidophilic archaebacterium Sulfolobus solfataricus, strain MT-4. The purification procedure employed ion exchange chromatography, ammonium sulfate fractionation, affinity chromatography, and gel filtration. Molecular weight determinations demonstrated that the enzyme was a dimer of Mr 105,000 +/- 2,000 with apparently identical Mr 49,000 +/- 1,500 subunits. Amino acid composition of S. solfataricus enzyme was determined and found to be significantly higher in tryptophan content than the malic enzyme from Escherichia coli. In addition to the NAD(P)-dependent oxidative decarboxylation of L-malate, S. solfataricus malic enzyme was able to catalyze the decarboxylation of oxalacetate. The enzyme absolutely required divalent metal cations and it displayed maximal activity at 85 degrees C and pH 8.0 with a turnover number of 376 s-1. The enzyme showed classical saturation kinetics and no sigmoidicity was detected at different pH values and temperatures. At 60 degrees C and in the presence of 0.1 mM MnCl2, the Michaelis constants for malate, NADP, and NAD were 18, 3, and 250 microM, respectively. The S. solfataricus malic enzyme was shown to be very thermostable.

Purification procedure and N-terminal amino acid sequence of yeast malate dehydrogenase isoenzymes

A method has been devised for the rapid isolation of malate dehydrogenase isoenzymes. First, anionic proteins were precipitated with polyethyleneimine, whilst hydrophobic malate dehydrogenase remained in the supernatant fluid. Secondly, the supernatant was 30% saturated with ammonium sulfate and the two isoenzymes were separated by hydrophobic phenyl-Sepharose CL-4B chromatography. For further purification the enzymes were chromatofocused, and polybuffer was removed by hydrophobic chromatography. Affinity chromatography with blue Sepharose CL-6B [1] was used as final purification step. The purified isoenzymes were homogeneous as shown by isoelectric focusing and could be used for N-terminal sequencing. 34 amino acid residues could be identified for the cytoplasmic isoenzyme and 56 amino acid residues for the mitochondrial isoenzyme. Although there are regions of strong homology between both isoenzymes, the sequence differences clearly showed support that both isoenzymes are coded by different genes. Sequence comparison clearly indicated that the N-terminus of the cytoplasmic enzyme extended that of the mitochondrial enzyme by 12 amino acid residues. The amino acid sequence of the extending sequence resembled that of leading sequences known for enzymes which are transported into the mitochondria. The assumed leading sequence is discussed with respect to its possible role in glucose inactivation.

Affinity chromatography on 2',5'-ADP-Sepharose 4B for purification of malic enzyme from crustacean muscle

Shrimp abdomenal muscle NADP-dependent malic enzyme (E.C.1.1.1.40) was purified about 1500-fold to a specific activity of 48 units (mumol/min)/mg at 30 degrees C with good quantitative recovery in three chromatographic steps, including affinity chromatography on 2',5'-ADP-Sepharose 4B, a "substrate activation" method using malate substrate plus manganese chloride. In addition to the malate-manganese chloride substrate pair, succinate or glutamate plus manganese chloride or magnesium chloride could be used in this "substrate activation" method for crustacean NADP-malic enzyme purification on 2',5'-ADP-Sepharose 4B. Affinity chromatography alone purified malic enzyme almost 43 fold, and the overall method resulted in homogeneous enzyme since polyacrylamide gel electrophoresis of the native purified enzyme revealed only a single band staining for protein and enzyme activity.

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.

Isolation, properties and role in progesterone biosynthesis of cytosolic malic enzyme from human term placenta

Cytosolic malic enzyme (L-malate: NADP oxidoreductase decarboxylating, EC I. I. I.40) has been isolated and purified from postmitochondrial supernatant of human term placenta by ammonium sulphate fractionation, chromatography on diethylaminoethyl- (DEAE-)cellulose, Sepharose 6B, ADP-Sepharose 4B and Ultrogel AcA-34 to apparent homogeneity as judged from polyacrylamide gel electrophoresis (PAGE). The specific activity of the purified enzyme was 24.0 mumol X min-1 X mg-1 protein, which corresponds to about 7500-fold purification. The molecular weight of the native enzyme was determined by gel filtration to be about 250,000. Sodium dodecyl sulphate- (SDS-)PAGE showed one polypeptide band of molecular weight 63,000. It appears that the native protein is a tetramer composed of subunits of identical molecular weight. The isoelectric point of the purified malic enzyme was pH 5.55. The enzyme was shown to carboxylate pyruvate in the presence of high concentrations of pyruvate and bicarbonate at about 80 per cent of the rate of the forward reaction. The optimum pH for the carboxylation reaction was pH 7.3, and that for the decarboxylation reaction varied with malate concentration. The Km values, determined at pH 7.2, for malate, NADP+, Mn2+, and Mg2+ were 81 microM, 10 microM, 2.5 microM and 0.6 mM, respectively. The Km values for pyruvate, NADPH and bicarbonate were 4 mM, 25 microM and 20 mM, respectively. The enzyme converted malate to pyruvate (at pH 6.3) in the presence of 5 mM NAD+ at approximately 80 per cent of the maximum rate with NADP+. It exhibited oxaloacetate decarboxylase activity at about 10 per cent of the rate of oxidative decarboxylation of malate (with NADP+ as coenzyme) and pyruvate reductase activity at about 4 per cent of the rate of oxidative decarboxylation of malate with NADP+. The oxidative decarboxylation of malate was inhibited by malate at lower values of pH of incubation medium. This inhibition gradually decreased as the pH of the incubation medium increased. No inhibition was observed at pH 8.2. The addition of purified cytoplasmic malic enzyme, pyruvate, bicarbonate and NADPH generating system (consisting of NADP+, glucose 6-phosphate, glucose 6-phosphate dehydrogenase) stimulated about twofold progesterone biosynthesis by the isolated human placental mitochondria. This stimulation was abolished by arsenite and fluorocitrate. A possible role for the cytosolic malic enzyme in the regulation of progesterone biosynthesis in human placenta is discussed.

Purification and properties of the cytosolic and mitochondrial forms of aspartate aminotransferase and malate dehydrogenase from rat heart

The present work describes the purification from rat heart of the mitochondrial and cytosolic forms of the enzymes of the malate--aspartate shuttle, aspartate aminotransferase (EC 2.6.1.1) and malate dehydrogenase (EC 1.1.1.37), by a single procedure after the preparation of the original crude extract. In 10 purification steps, the four enzymes were obtained electrophoretically pure in yields ranging from 6 to 54% of their respective isoenzyme levels in the crude extract. Apoenzymes were formed from the aminotransferases by reacting them with cysteine sulfinate and dialyzing. Complete reconstitution was obtained after a brief incubation with pyridoxal phosphate. All four enzymes are dimers. The mitochondrial isoenzymes are of slightly lower molecular weight than their respective cytosolic forms. Michaelis constants and maximal velocities were derived by the use of primary and secondary plots. In general, the properties of the enzymes from rat heart are similar to the properties of the enzymes from other animal sources.

Comparison of the precursor and mature forms of rat heart mitochondrial malate dehydrogenase

The complete amino acid sequence of mitochondrial malate dehydrogenase from rat heart has been determined by chemical methods. Peptides used in this study were purified after digestions with cyanogen bromide, trypsin, endoproteinase Lys C, and staphylococcal protease V-8. The amino acid sequence of this mature enzyme is compared with that of the precursor form, which includes the primary structure of the transit peptide. The transit peptide is required for incorporation into mitochondria and appears to be homologous to the NH2-terminal arm of a related cytoplasmic enzyme, pig heart lactate dehydrogenase. The amino acid differences between the rat heart and pig heart mitochondrial malate dehydrogenases are analyzed in terms of the three-dimensional structure of the latter. Only 12/314 differences are found; most are conservative changes, and all are on or near the surface of the enzyme. We propose that the transit peptide is located on the surface of the mitochondrial malate dehydrogenase precursor.

Comparative analysis of the malate dehydrogenases isolated from the cytosolic fraction of several tissues of guinea-pig Cavia porcellus

The specific activities of the malate dehydrogenase and lactate dehydrogenase present in the soluble fraction of several guinea-pig tissues are reported. The electrophoretic patterns showed always two forms (A and B) with malate dehydrogenase activity and the five isoenzymes of lactate dehydrogenase. Chromatography of the different soluble fractions through 5' AMP-Sepharose allowed both molecular forms of malate dehydrogenase to be separated and obtained free from lactate dehydrogenase. Comparative studies of the two forms of malate dehydrogenase evidenced that the A and B forms exhibited cytosolic and mitochondrial characteristics, respectively.

Mitochondrial NADP-dependent malic enzyme of cod heart. Rate of forward and reverse reaction

The mitochondrial NADP-dependent malic enzyme (EC 1.1.1.40) was purified about 300-fold from cod Gadus morhua heart to a specific activity of 48 units (mumol/min)/mg at 30 degrees C. The possibility of the reductive carboxylation of pyruvate to malate was studied by determination of the respective enzyme properties. The reverse reaction was found to proceed at about five times the velocity of the forward rate at a pH 6.5. The Km values determined at pH 7.0 for pyruvate, NADPH and bicarbonate in the carboxylation reaction were 4.1 mM, 15 microM and 13.5 mM, respectively. The Km values for malate, NADP and Mn2+ in the decarboxylation reaction were 0.1 mM, 25 microM and 5 microM, respectively. The enzyme showed substrate inhibition at high malate concentrations for the oxidative decarboxylation reaction at pH 7.0. Malate inhibition suggests a possible modulation of cod heart mitochondrial NADP-malic enzyme by its own substrate. High NADP-dependent malic enzyme activity found in mitochondria from cod heart supports the possibility of malate formation under conditions facilitating carboxylation of pyruvate.

Design of a new automatic chromatography system for efficient enzyme purification equipped with a time-shared multiple activity analyzer

A new system equipped with a computer-controlled multiple activity analyzer has been developed for the efficient purification of multiple enzymes. The system consists of the following units: conventional enzyme fractionation system with a peristaltic pump, liquid chromatographic column, fraction collector, and uv monitor; computer-operated uv-vis spectrophotometer equipped with a thermo-regulated metal block and a flow-through type silica cuvette; personal computer; dot matrix printer; cooling facility; and automatic sampling-mixing system. The whole system is operated by a newly designed time-sharing computer program for periodic and repetitive sampling of the column eluants containing multiple kinds of enzymes and of designated assay mixtures for each enzyme and for measurement of the initial velocity of spectrophotometric signals. For example, a mixture of aspartase (EC 4.3.1.1) and malate dehydrogenase (EC 1.1.1.39) and also a mixture of these two enzymes and glutamate dehydrogenase (EC 1.4.1.3 or EC 1.4.1.4) were analyzed by the above system using gel permeation chromatography, and the two or three enzyme activities were repeatedly monitored within 4 min. Based on the above results further possibilities for the application of the system for a variety of purposes are discussed.

Isoenzyme patterns of Entamoeba histolytica isolates from asymptomatic carriers: use of gradient acrylamide gels

A vertical polyacrylamide gradient gel (3% to 7%) was designed to facilitate the electrophoretic resolution and classification of isoenzyme patterns of Entamoeba histolytica isolates. The following enzyme systems were used: phosphoglucomutase (PGM), hexokinase (HX), glucosephosphate isomerase (GPI), and malate dehydrogenase (ME). The modifications in the electrophoretic procedure and sample preparation allowed the reproducible comparison of enzyme patterns of axenic, monoxenic, and mixed cultures of E. histolytica isolated from humans. The clear distinction obtained in gradient polyacrylamide gels, between amebic isoenzyme bands and those from bacteria, renders this technique adequate for application to epidemiological studies where mixed cultures are used. The isoenzyme patterns of eight isolates from asymptomatic carriers, rigorously characterized by the absence of clinical, endoscopic, and serological findings were studied and compared with three well characterized pathogenic strains, cultured under axenic conditions. Our observations confirm the existence of distinct isoenzyme patterns for PGM, HX, and GPI in pathogenic and nonpathogenic strains, and reveal the consistent presence of more than one band for GPI. In addition, a previously undescribed band for GPI with an Rf of 0.64 in a carrier strain was found. The results suggest that while carriers usually harbor amebas with nonpathogenic isoenzyme patterns, pathogenic patterns also may be found in carriers.

Malate dehydrogenase species in the cytosolic fraction of chicken liver

The malate dehydrogenase activity in the cytosolic fraction isolated from chicken hepatocytes is resolved by DEAE-Sephacel chromatography in three active, electrophoretically distinct, species obtained in homogeneous form by affinity chromatography on 5'-AMP-Sepharose and Blue-Sepharose. Two of those species, according to the results obtained, might represent different conformational isomers of the enzyme molecule. Their purified preparations show identical amino-acid compositions and physico-chemical properties very similar to those of the cytosolic isoenzyme of other sources. The third one corresponds to a slight contamination of the mitochondrial isoenzyme.

[NADP-dependent malate dehydrogenase from bovine adrenal cortex cytoplasm. Isolation and properties]

The NADP-dependent decarboxylating malate dehydrogenase was isolated from the cytoplasmic fraction of bovine adrenal cortex and purified 3530-fold by 3-fold ammonium sulfate fractionation, ion-exchange chromatography on DEAE-Toyopearl 650 M and DEAE-Sephadex A-50 with subsequent two-fold gel filtration through Toyopearl HW-55. The specific activity of homogeneous enzyme preparations was equal to 60 U/mg protein with a 30% yield. The enzyme molecular weight as determined by gel filtration on Sephadex G-20 was 155000. Upon polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate malate dehydrogenase dissociated into two subunits with Mr 77000. The Arrhenius plot for the reaction rate showed a break at 30 degrees C. The values of activation energy and temperature coefficient above and below the breakpoint were equal to 45049 and 147188 J X mol-1; 1.68 and 2.63, respectively. Within the temperature range of 26-40 degrees C, malate dehydrogenase exhibited hyperbolic kinetics with respect to the substrate. At 30 degrees C, Km for malate was equal to 250 microM, whereas at 40 degrees C it was 130 microM. The curve for the dependence of the initial reaction velocity versus NADP concentration was S-shaped. The Hill coefficient was 1.4, which testifies to positive cooperativity of NADP interaction with malate dehydrogenase.

Purification and properties of malate dehydrogenase from the thermoacidophilic archaebacterium Thermoplasma acidophilum

Malate dehydrogenase from the thermoacidophilic archaebacterium Thermoplasma acidophilum is purified 50-fold to electrophoretic homogeneity. The purified enzyme crystallizes readily. Native malate dehydrogenase shows a relative molecular mass of 144 000. It is a tetramer of identical subunits with a relative molecular mass of 36 600. Malate dehydrogenase from Thermoplasma uses both NADH and NADPH as coenzyme to reduce oxaloacetate. The enzyme shows A-side (pro-R) stereospecificity for both coenzymes. The pH optimum for the reduction of oxaloacetate in the presence of NADH is found to be at pH 8.1. At pH 7.4 the Km value for oxaloacetate is found to be 5.6 microM while for NADH a value of 11.7 microM is found. The homogeneous enzyme shows a turnover number of kcat = 108 s-1.

Purification and properties of mitochondrial malate dehydrogenase of Toxocara canis muscle

Mitochondrial malate dehydrogenase was purified from muscle extracts of Toxocara canis by means of Sephadex G-100 gel filtration, DEAE-Sephadex ion-exchange chromatography and 5'AMP-Sepharose 4B affinity chromatography. The purified enzyme showed an optimum pH for the reduction of oxaloacetate of 7.3 in Tris-HCl buffer and of pH 7.5-7.8 in phosphate buffer. The m-MDH showed values of 3.2 kcal/mol and 10.5 kcal/mol for the energy of activation, calculated from the Arrhenius equation. The mitochondrial enzyme was found to be more susceptible to thermal inactivation as compared with the cytosolic isoenzyme. Kinetic experiments showed that the m-MDH of Toxocara canis is inhibited by excess oxaloacetate but not by excess NADH. The apparent Km for oxaloacetate reduction was 53 microM and 0.54 mM for L-malate oxidation.

Simultaneous purification by affinity chromatography of rat liver mitochondrial aspartate aminotransferase and malate dehydrogenase and electrophoretic properties

Mitochondrial aspartate aminotransferase and malate dehydrogenase were purified to homogeneity from rat liver by the use of aspartate-coupled Sepharose, ion exchange, and Blue Sepharose chromatography. This procedure permits rapid preparation of these enzymes. The pI of each enzyme was determined and anomalous electrophoretic properties of aspartate aminotransferase were described.

Entamoeba histolytica: electrophoretic isoenzyme patterns of strains and their virulence in the cecum of gerbils (Meriones unguiculatus)

Six strains of Entamoeba histolytica isolated from humans were characterized by their electrophoretic isoenzyme patterns and by virulence in the cecum of Mongolian gerbils (Meriones unguiculatus). Amebae from symptomatic cases of amebiasis were found to belong to zymodeme group II, whereas strains isolated from asymptomatic infections belonged to zymodeme groups I or V. Strains recently isolated from symptomatic cases of amebiasis were highly invasive in the cecum of gerbils. The most prominent pathologic changes present in these infections were the destruction of the interglandular epithelium, crypt hyperplasia and trophozoite invasion of the crypts and submucosa. Strains from asymptomatic carriers were of low or moderate invasiveness in the animal model. Two of the carrier strains expressing a zymodeme group I pattern caused diffuse or transient focal erosions of the interglandular epithelium in some animals. Trophozoites of carrier strains did not invade the crypts nor cause sloughing of the surface epithelium. Presence of colonic lesions was correlated with passage of motile trophozoites in the feces. The severity of histologically demonstrable lesions in the cecum of gerbils was positively correlated with the isoenzyme patterns characteristic of the more virulent amebic strains.

Isolation and biochemical characterization of maleic-acid hydratase, an iron-requiring hydro-lyase

A procedure for the isolation of maleic acid hydratase (D-malate hydro-lyase, EC 4.2.1.31) of about 95% purity from rabbit kidneys is described. The enzyme consists of a single polypeptide chain of 582 amino-acid residues with an approximate molecular mass of 68 kDa. The enzyme is very unstable and has an absolute requirement for chloride ions. Addition of sodium sulphide during the purification process was essential to maintain the enzyme in an activatable state. The pure preparation has low activity but responds to activation with Fe2+ ions, Na2S and a thiol. The sequence of adding the activating reagents is critical to achieve optimal activity. Ni2+ and to a lesser extent Co2+ can replace iron in the activation process. The enzyme incorporates 4-5 mol iron/mol and 4.5-6 mol sulphide/mol during activation. In this process an [Fe-S] cluster appears to be built up, as indicated by optical and electron paramagnetic resonance (EPR) spectroscopy. In activated samples exposed to air the [Fe-S] cluster is EPR-detectable through an axial signal with g = 2.01 and g = 2.029 whose temperature and power saturation characteristics were similar to those of other [3Fe-xS] clusters. The activated enzyme, however, is readily inactivated even upon minor manipulation with destruction of the iron-sulfur core.

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.

The purification and steady-state kinetic behaviour of rabbit heart mitochondrial NAD(P)+ malic enzyme

The mitochondrial NAD(P)+ malic enzyme [EC 1.1.1.39, L-malate:NAD+ oxidoreductase (decarboxylating)] was purified from rabbit heart to a specific activity of 7 units (mumol/min)/mg at 23 degrees C. A study of the reductive carboxylation reaction indicates that this enzymic reaction is reversible. The rate of the reductive carboxylation reaction appears to be completely inhibited at an NADH concentration of 0.92 mM. A substrate saturation curve of this reaction with NADH as the varied substrate describes this inhibition. The apparent kinetic parameters for this reaction are Ka(NADH) = 239 microM and Vr = 1.1 mumol/min per mg at 23 degrees C. The steady-state product-inhibition patterns for pyruvate and NADH indicate a sequential binding of the substrates: NAD+ followed by L-malate. These data also indicate that NADH is the last product released. A steady-state kinetic model is proposed that incorporates NADH-enzyme dead-end complexes.

Properties and function of malate enzyme from Dicentrarchus labrax L. liver

Malate enzyme (L-malate:NADP+ oxidoreductase (oxaloacetate decarboxylating, EC 1.1.1.40) has been purified from Dicentrarchus labrax liver to 99% homogeneity by gel filtration, anion exchange and affinity chromatographies. The apparent molecular weight was estimated by gel filtration chromatography to be 148,000. Analysis of the enzyme on sodium dodecyl sulphate polyacrylamide disc gel electrophoresis was shown to be a tetrameric protein. The purified enzyme showed a pH optimum 8.5 (Tris-HCl buffer) and required bivalent cations for catalysis. The temperature-activity relationship for the enzyme showed broken Arrhenius plots with inflexions at 15 and 40 degrees C. Kinetic properties and the effects of some metabolites related to L-malate are studied.