Involvement of diminution of glutathione, produced by deficiency of methionine in the diet, in the elevation of malic enzyme level in rat liver

Rats fed on a low protein diet show an increase in the specific activity of malic enzyme and a concomitant decrease of glutathione concentration. We have studied the effect on malic enzyme activity of supplementing of low protein diet with essential amino acids. Only when methionine was excluded from the diet did the specific activity of malic enzyme increase to the same extent as found in rats fed with low protein diet. Immunoprecipitation of malic enzyme indicated that specific activity changes are the result of changes in the amounts of enzyme. Under all dietary conditions studied, the increase in malic enzyme activity is associated with a decrease in the concentration of GSH. To evaluate the possible causative role of GSH in malic enzyme induction, the specific activity of malic enzyme was measured in rats treated with BSO, an inhibitor of GSH biosynthesis. The results show that in BSO-treated rats the decrease of GSH levels is also accompanied by an increase in the activity of malic enzyme.     

Regulation of malic enzyme gene expression by nutrients,hormones, and growth factors in fetal hepatocyte primary cultures

The culture of fetal hepatocytes for 64 h in medium supplemented with 5 mM glucose, T3, insulin, and dexamethasone resulted in the coordinate precocious expression of malic enzyme mRNA, protein, and specific activity. T3 was the main inducer; meanwhile, insulin exerted a small synergistic effect when added with T3. Dexamethasone had a potentiation effect on the T3 response of malic enzyme mRNA expression regardless of the presence of insulin. This effect of dexamethasone on T3 response of malic enzyme mRNA expression was time (64 h) and glucose dependent. Glucagon, and to a greater degree dibutyryl-cAMP, repressed malic enzyme mRNA as well as protein expression by T3 and dexamethasone, in the absence of insulin. Glucose and other carbon sources such as lactate-pyruvate or dihydroxyacetone induced the abundance of malic enzyme mRNA in the absence of hormones. Insulin and T3 produced a high accumulation of malic enzyme mRNA in lactate-pyruvate medium, this effect being decreased by dexamethasone. EGF supressed the induction produced by T3 and dexamethasone on malic enzyme mRNA, while the expression of β-actin mRNA remained essentially unmodified. © 1993 Wiley-Liss, Inc.     

NADP+ -dependent malic enzyme of Rhizobium meliloti.

The bacterium Rhizobium meliloti, which forms N2-fixing root nodules on alfalfa, has two distinct malic enzymes; one is NADP+ dependent, while a second has maximal activity when NAD+ is the coenzyme. The diphosphopyridine nucleotide (NAD+)-dependent malic enzyme (DME) is required for symbiotic N2 fixation, likely as part of a pathway for the conversion of C4-dicarboxylic acids to acetyl coenzyme A in N2-fixing bacteroids. Here, we report the cloning and localization of the tme gene (encoding the triphosphopyridine nucleotide [NADP+]-dependent malic enzyme) to a 3.7-kb region. We constructed strains carrying insertions within the tme gene region and showed that the NADP+ -dependent malic enzyme activity peak was absent when extracts from these strains were eluted from a DEAE-cellulose chromatography column. We found that NADP+ -dependent malic enzyme activity was not required for N2 fixation, as tme mutants induced N2-fixing root nodules on alfalfa. Moreover, the apparent NADP+ -dependent malic enzyme activity detected in wild-type (N2-fixing) bacteroids was only 20% of the level detected in free-living cells. Much of that residual bacteroid activity appeared to be due to utilization of NADP+ by DME. The functions of DME and the NADP+ -dependent malic enzyme are discussed in light of the above results and the growth phenotypes of various tme and dme mutants.     

Mitochondrial Malic Enzyme: Purification from Bovine Brain, Generation of an Antiserum, and Immunocytochemical Localization in Neurons of Rat Brain

Abstract: To elucidate the cellular location of mitochondrial malic enzyme in brain, immunocytochemical studies were performed. For this purpose, mitochondrial malic enzyme was purified to apparent homogeneity from bovine brain and used for the immunization of rabbits. Subjecting the antiserum to affinity purification on immobilized antigen as an absorbent yielded a purified immunoreactive antibody preparation, which was characterized by probing cytosolic and mitochondrial fractions of bovine and rat brain in western blotting. As neither crossreactivity with cytosolic malic enzyme nor immunoreactivity against other proteins could be observed, the antibody preparation was found suitable for immunocytochemistry. By using sections of perfusion-fixed rat brain, considerable resolution was achieved at the light-microscopic level. Distinct and specific staining of neurons was observed; in contrast, no staining of astrocytes and possibly unspecific staining within the nuclei of oligodendrocytes were obtained. From these data, it is concluded that mitochondrial malic enzyme is located in neurons; however, in astrocytes, the enzyme appears to be either lacking or present at a much lower level. A protective role against oxidative stress in neurons is proposed for mitochondrial malic enzyme.     

Human NAD(+)-dependent mitochondrial malic enzyme. cDNA cloning, primary structure, and expression in Escherichia coli

Mitochondrial NAD(+)-dependent malic enzyme (EC 1.1.1.40) is expressed in rapidly proliferating cells and tumor cells, where it is probably linked to the conversion of amino acid carbon to pyruvate. In this paper, we report the cDNA cloning, amino acid sequence, and expression in Escherichia coli of functional human NAD(+)-dependent mitochondrial malic enzyme. The cDNA is 1,923 base pairs long and contains an open reading frame coding for a 584-amino acid protein. The molecular mass is 65.4 kDa for the unprocessed precursor protein. Comparison of the amino acid sequence of the human protein with the published NADP(+)-dependent mammalian cytosolic or plant chloroplast malic enzymes reveals highly conserved regions interrupted with long stretches of amino acids without significant homology. Expression of the processed protein in E. coli yielded an enzyme with the same kinetic and allosteric properties as malic enzyme purified from human cells.     

Study of properties of NADP malate dehydrogenase from corn leaves]

Kinetic properties of purified chloroplast isoenzyme of the "malic" enzyme from corn leaves were studied. The enzyme had optimum activity at pH 8.0 and 36 degrees C. Under standart conditions the Michaelis constants for the "malic" enzyme with Mn2+ as cofactor are 0.091 mM for malate and 0.04 mM for NADP. In case of Mg2+ as cofactor they are 0.66 and 0.02 mM respectively. Respective Km values for the cofactors Mn2+ and Mg2+ are 0.018 and 0.091 mM. The activity of the "malic" enzyme was inhibited by reduced NADP and NAD, ATP, ADP, fructose-1,6-diphosphate, oxaloacetic, oxalic, glyoxylic, glycolic and alpha-ketoglutaric acids, as well as by phosphate anions and pyrophosphate. The inhibitory effect of all metabolites and ions is more pronounced in case of Mn, rather than Mg, used as cofactors for the reaction. A possibility of metabolic regulation of NADP-"malic" enzyme activity in the leaves of C4-plants, is discussed.     

Nicotinamide adenine dinucleotide phosphate-malic enzyme of rat liver. Purification, properties, and immunochemical studies.

Rat liver malic enzyme (EC 1.1.1.40) was purified from livers of rats fasted and refed a high sucrose diet containing 1% desiccated thyroid powder. The purification was accomplished by a six-step procedure. The specific activity of the purified enzyme was increased 181-fold above that of the initial high speed supernatant of liver extracts. Slight additional purification of malic enzyme was achieved with preparative disc electrophoresis. The specific activities of the purified rat liver malic enzyme from the least two steps were between 28.0 and 30.5 units per mg of protein. Homogeneity of the purified enzyme was determined by disc and starch gel electrophoresis as well as sedimentation velocity and sedimentation equilibrium studies. The molecular weight and S20, w values of rat liver malic enzyme are 268,000 and 10.2, respectively. Amino acid analysis based on milligram of protein hydrolyzed yielded higher amounts of leucine and glutamic acid but lower quantities of alanine and voline per subunit than the corresponding Escherichia coli enzyme...     

Purification of Cytosolic Malic Enzyme from Bovine Brain, Generation of Monoclonal Antibodies, and Immunocytochemical Localization of the Enzyme in Glial Cells of Neural Primary Cultures

Abstract: Cytosolic malic enzyme (EC 1.1.1.40) was purified from bovine brain 5,600-fold to a specific activity of 47 U/mg. The enzyme is a homotetramer with a subunit molecular mass of 60 kDa and an isoelectric point of 6.2. Mouse monoclonal antibodies raised against this enzyme were purified and shown to be monospecific, as indicated by immunoblotting. Immunocytochemical examination of rat astroglia-rich primary cultures at the light microscopic level revealed colocalization of cytosolic malic enzyme with the astroglial marker glial fibrillary acidic protein. Also, a colocalization with the oligodendroglial marker myelin basic protein was found. Neurons in rat neuron-rich primary cultures did not show positive staining. The data suggest that cytosolic malic enzyme is a glial enzyme and is lacking in neurons.     

Subregional and Intracellular Distribution of NADP-Linked Malic Enzyme in Human Brain

High total activity (expressed as μmol/min/g of wet tissue or per milligram of DNA) and differential subregional distribution of NADP-linked malic enzyme was found in autopsy specimens of human brain. Striatum showed the highest activity of malic enzyme, which was two to five-fold higher than that in other human organs tested. High activity was also found in frontal cortex, while the lowest activity of the enzyme in the central nervous system was found in cerebellum, substantia alba, and corpus callosum. In striatum, frontal cortex, pens, and cerebellum more than 80% of total malic enzyme activity was localized in the mitochondrial fraction, while in substantia alba and corpus callosum approximately 60% of the enzyme activity was present in the mitochondrial fraction. Relatively high specific activity of malic enzyme was found in a crude mitochondrial fraction isolated from various regions of human brain. The highest specific activity was found in the mitochondria isolated from striatum (more than 100 nmol/min/mg of mitochondrial protein); the lowest, but still high (approximately 32 nmol/min/mg of mitochondrial protein) was present in corpus callosum. These data and the different ratios of citrate synthase to mitochondrial malic enzyme activities found in different regions of brain suggest that human brain mitochondria, like the mitochondria isolated from other mammalian brains, are extremely heterogenous. A possible role of mitochondrial malic enzyme in human brain metabolism is discussed.     

Light-stimulated synthesis of NADP malic enzyme in leaves of maize

Illumination of etiolated maize plants for 80 h brings about a 15-20-fold increase in activity of NADP malic enzyme (EC 1.1.1.40). Increases in NADP malic enzyme protein and in the level of translatable mRNA for this protein occur simultaneously with the activity increase. Radiolabeled amino acids are also incorporated into NADP malic enzyme during this time. These results are consistent with the conclusion that an increase in NADP malic enzyme activity during greening results from de novo synthesis of NADP malic enzyme protein. Polyadenylated RNA extracted from greening maize leaves directs the synthesis in vitro of a protein 12,000 daltons larger than NADP malic enzyme purified from corn leaves. This protein is a precursor of NADP malic enzyme because 1) both the precursor and mature NADP malic enzyme are immunoprecipitated by antibody made against NADP malic enzyme purified from corn leaves, 2) both NADP malic enzyme protein and the level of mRNA for the precursor increase during greening, and 3) peptide maps of the precursor and of mature NADP malic enzyme are very similar. Mature NADP malic enzyme and its precursor (synthesized in vitro) both migrate on sodium dodecyl sulfate-polyacrylamide gradient gels as doublet bands. Peptide analyses show all bands to be structurally related.     

Some effects of Ca2+, Mg2+, and Mn2+ on the ultrastructure,light-scattering properties,and malic enzyme activity of adrenal cortex mitochondria

The ultrastructure and 90 ° light-scattering capacity of adrenal cortex mitochondria have been examined under conditions which lead to an activation of malic enzyme activity in these mitochondria. After isolation, the mitochondria display an aggregate ultrastructure which does not resemble the vesicular (orthodox) form normally seen in vivo. Under conditions of malic enzyme activation (presence of malate, NADP⁺, Mg²⁺ and 1 mm Ca²⁺), the ultrastructure reverts to a vesicular form as seen in vivo. Of these required components, only Ca²⁺ affects the ultrastructure. The ultrastructural transformation from the aggregate to the orthodox form is always accompanied by a decrease in the 90 ° light-scattering capacity. When produced by Ca²⁺, transformation requires energy-dependent Ca²⁺ uptake if an oxidizable substrate is present. In the absence of substrate, the transformation occurs as an apparent energy-independent effect. Mn²⁺ can substitute for Ca²⁺ only in the presence of substrate. In de-energized mitochondria, Mn²⁺ prevents the effects of Ca²⁺. The activation of malic enzyme is always preceded by a decrease in light scattering and transformation to the orthodox ultrastructure; however, the presence of the orthodox form is not a sufficient condition since subsequent chelation of free Ca²⁺ fails to reverse either the decrease in light scattering or ultrastructural transformation but does reverse the enzyme activation. In addition, levels of Mn²⁺ which effectively depress light-scattering capacity and produce the orthodox form, fail to activate malic enzyme significantly. The data are discussed as they relate to Ca²⁺-induced damage in mitochondria.     

The pigeon heart 5''-nucleotidase responsible for ischaemia-induced adenosine formation.

The pH-induced reversible dissociation of pigeon liver malic enzyme (EC 1.1.1.40) was studied by combined use of chemical cross-linking and SDS/polyacrylamide-gel electrophoresis. The tetrameric enzyme showed a pH-dependent dissociation in an acidic environment. At pH values above 8.0 most molecules existed as tetramers. The enzyme was gradually dissociated at lower pH. When the pH was below 5.0 most of the enzyme was present as the monomeric forms. Reassociation of the subunits was accomplished by adjusting the pH to neutrality. The dissociation and reassociation were almost instantaneous. No trimer was detected. The pigeon liver malic enzyme was thus shown to have a double-dimer quaternary structure with D2 symmetry. In the presence of substrates, the monomer-dimer-tetramer equilibrium favours the direction of dissociation. Tartronate, an L-malate analogue, was found to be more effective than L-malate in this process. When the monomeric forms were immobilized, the enzyme subunits were found to be fully active in catalysis. A possible arrangement of the four identical subunits of the enzyme molecule is proposed to account for the results obtained in this investigation. The origin of the half-of-the-sites reactivity of pigeon liver malic enzyme is also discussed.     

The NADPH consumption regulates the NADPH-producing pathways (pentose phosphate cycle and malic enzyme) in rat adipocytes

The changes in the activity of the pentose phosphate cycle and the malic enzyme produced by the activation or inhibition of different NADPH-consuming pathways have been studied. The inhibition of the fatty acid synthesis by kynurenate produced a decrease in the flux through the pentose phosphate cycle and a diminution in the malic enzyme pathway. The incubation of the adipocytes in the presence of ter-butyl-hydroperoxide, a compound which is metabolized via a NADPH-consuming pathway, produced a big increase in the pentose phosphate cycle and the malic enzyme activities. The regulation of these NADPH-producing pathways by the NADPH/NADP ratio is discussed.     

The effect of dietary fat on the activity,content, rates of synthesis,and degradation and translation of messenger RNA coding for malic enzyme in mouse liver

The specific activity (units activity/mg cytosolic protein) of malic enzyme was found to be three-fold higher in the livers of mice fed a semipurified diet containing 50% ($\text{w}\text{w}$) glucose and 15% ($\text{w}\text{w}$) saturated and monounsaturated but no polyunsaturated fat (hydrogenated cottonseed oil) over an 11-day period than in the livers of mice fed a standard laboratory mouse chow (Purina) diet. In contrast, when other lab chow-fed mice were fed an isocaloric diet containing 15% ($\text{w}\text{w}$) polyunsaturated fat (corn oil), no change in the specific activity of malic enzyme occurred over a similar period of time. Rocket immunoelectrophoresis performed on cytosols from both dietary groups demonstrated that the livers of mice consuming the hydrogenated cottonseed oil diet contained approximately three times more malic enzyme protein than did the livers from the corn oil-fed animals. In mice pulse-labeled with l-[4,5-³H]leucine, the rate of hepatic malic enzyme synthesis (relative to that for total protein) was approximately twofold greater in the hydrogenated cottonseed oil-fed mice than in their corn oil-fed counterparts whereas the rate of hepatic malic enzyme degradation was similar for both groups. Immunotitration of liver malic enzyme from hydrogenated cottonseed oil-fed and corn oil-fed mice revealed identical equivalence points, demonstrating that the catalytic efficiency of mouse liver malic enzyme had not been affected by the type of dietary fat administered. When total liver RNA, isolated from the hydrogenated cottonseed oil- and the corn oil-fed animals, was translated in cell-free translation systems (wheat germ extract and reticulocyte lysate) we found that both dietary treatments had resulted in an increase in the activity of malic enzyme messenger RNA. Furthermore, there were no significant differences between the two dietary groups in this regard. These results suggest that hepatic malic enzyme specific activity in high-carbohydrate polyunsaturated fat-fed mice is regulated principally by dietary-induced changes in the rate of enzyme synthesis and not by the activity of messenger RNA coding for the enzyme.     

Regulation of the dicarboxylic acid part of the citric acid cycle in Bacillus subtilis.

The regulation of alpha-ketogluterate dehydrogenase, succinate dehydrogenase, fumarase, malate dehydrogenase, and malic enzyme has been studied in Bacillus subitilis. The levels of these enzymes increase rapidly during late exponential phase in a complex medium and are maximal 1 to 2 h after the onset of sporulation. Regulation of enzyme synthesis has been studied in the wild type and different citric acid cycle mutants by adding various metabolites to the growth medium. Alpha-ketoglutarate dehydrogenase is induced by glutamate or alpha-ketoglutarate; succinate dehydrogenase is repressed by malate; and fumarase and malic enzyme are induced by fumarate and malate, respectively. The addition of glucose leads to repression of the citric acid cycle enzymes whereas the level of malic enzyme is unaffected. Studies on the control of enzyme activities in vitro have shown that alpha-ketoglutarate dehydrogenase and succinate dehydrogenase are inhibited by oxalacetate. Enzyme activities are also influenced by the energy level, expressed as the energy charge of the adenylate pool. Isocitrate dehydrogenase, alpha-ketoglutarate dehydrogenase, succinate dehydrogenase, and malic enzyme are inhibited at high energy charge values, whereas malate dehydrogenase is inhibited at low energy charge. A survey of the regulation of the citric acid cycle in B.subtilis, based on the present work and previously reported results, is presented and discussed.     

Kinetics and regulation of hepatoma mitochondrial NAD(P) malic enzyme.

Kinetic studies of Morris 7777 hepatoma mitochondrial NAD(P) malic enzyme were consistent with an ordered mechanism where NAD adds to the enzyme before malate and dissociation of NADH from the enzyme is rate-limiting. In addition to its active site, malate apparently also associates with a lower affinity with an activator site. The activator fumarate competes with malate at the activator site and facilitates dissociation of NADH from the enzyme. The ratio of NAD(P) malic enzyme to malate dehydrogenase activity in the hepatoma mitochondrial extract was found to be too low, even in the presence of known inhibitors of malate dehydrogenase, to account for the known ability of NAD(P) malic enzyme to intercept exogenous malate from malate dehydrogenase in intact tumor mitochondria (Moreadith, R.W., and Lehninger, A.L. (1984) J. Biol. Chem. 259, 6215-6221). However, NAD(P) malic enzyme may be able to intercept exogenous malate because according to the present results, it can associate with the pyruvate dehydrogenase complex, which could localize NAD(P) malic enzyme in the vicinity of the inner mitochondrial membrane. The activity levels of some key metabolic enzymes were found to be different in Morris 7777 mitochondria than in liver or mitochondria of other rapidly dividing tumors. These results are discussed in terms of differences among tumors in their ability to utilize malate, glutamate, and citrate as respiratory fuels.     

Nutritional and hormonal regulation of the translatable levels of malic enzyme and albumin mRNAs in avian liver cells in vivo and in culture

Relative synthesis of malic enzyme is stimulated 25-to 100-fold by feeding neonatal ducklings or by incubating embryonic chick hepatocytes in culture with triiodothyronine. Synthesis of the enzyme is almost completely blocked when fed birds are starved or when triiodothyronine-treated hepatocytes in culture are also treated with glucagon. Cytoplasmic poly(A)+ RNA was isolated from livers of intact ducklings or hepatocytes in culture treated as described above and translated in an mRNA-dependent rabbit reticulocyte lysate. The identity of malic enzyme synthesized in the cell-free system was confirmed by virtue of its antigenicity, subunit molecular weight, and proteolytic peptide pattern. Translatable levels of malic enzyme mRNA paralleled changes in relative synthesis of malic enzyme in vivo and in hepatocytes in culture. Translatable levels od albumin mRNA were either unaffected or changed in a direction opposite to that of malic enzyme mRNA. Thus, both nutritional and hormonal regulation of malic enzyme synthesis involves regulation of cytoplasmic translatable malic enzyme mRNA levels. The hepatocyte culture system is ideally suited for future studies on the regulation of malic enzyme mRNA synthesis and/or degradation by thyroid hormone and glucagon.     

Chromosome assignments of genes in man using mouse-human somatic cell hybrids: Mitochondrial superoxide dismutase (indophenol oxidase-B,tetrameric) to chromosome 6

Summary Evidence from mouse/human somatic cell hybrids is presented for the synteny of the genes for indophenol oxidase-B (tetrameric) and cytoplasmic malic enzyme (EC 1.1.1.40). Assignment of these two genes to chromosome 6 is further confirmed. The identification of indophenol oxidase-B (tetrameric) as mitochondrial superoxide dismutase is discussed.

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Zusammenfassung Befunde an somatischen Zellhybriden Maus/Mensch werden dargestellt, die für die Syntenie der Gene für Indophenol-Oxidase-B (tetramer) sowie für cytoplasmatisches malic enzyme (EC 1.1.1.40) sprechen. Außerdem wird die Zuordnung dieser beiden Gene zu Chromosom 6 bestätigt. Die Identifizierung der Indophenol-Oxidase-B (tetramer) als mitochondriale Superoxid-Dismutase wird diskutiert.

     

The effect of phenobarbitone on cytoplasmic NADP-linked dehydrogenase activities in rat liver

Phenobarbitone administered in drinking water (0.5 g/l) or by daily intraperitoneal injection (100 mg/kg body weight) consistently caused an elevation of hepatic NADP-linked malic enzyme in rats maintained on a pellet diet. Three to four days appeared to be required for maximum response. The effect was also observed in animals maintained on a protein rich diet, in which the basic hepatic malic enzyme activity was low, but not in animals maintained on a sucrose rich diet, in which the basic enzyme activity was almost twice normal. Methyl cholanthrene, administered by daily intraperitoneal injection (40 mg/kg body weight), resulted in elevated hepatic levels not only of malic enzyme but also of the pentose phosphate pathway dehydrogenases. The timing of the "starve-refeed" response of the hepatic NADP-linked dehydrogenases in phenobarbitone-treated rats was similar to that in controls, and similar maximum enzyme activities were reached. The role of cytoplasmic NADP-linked dehydrogenases in the provision of reducing equivalent is discussed, particularly in relation to hepatic microsomal drug metabolism.