Differential characteristics of the cytosol and mitochondrial isozymes of malic enzyme from bovine brain. Effects of dicarboxylic acids and sulfhydryl reagents

Comparative studies on NADP(+)-linked malic enzyme in the central nervous system of ectothermic and endothermic animals

The maximum activity and intracellular distribution of NADP(+)-linked malic enzyme in brain of Mammalia, Aves, Reptilia, Amphibia and Pisces are reported. Malic enzyme activity was present in all animals brains investigated. Most of the enzyme activity was located in the mitochondrial fraction. In brain of endothermic animals the activity of malic enzyme was several-fold higher than in ectothermic animals. Other NADPH-producing enzymes (i.e. NADP(+)-linked isocitrate dehydrogenase and hexosemonophosphate shunt dehydrogenase) activities were essentially similar in all animals brains tested. However, the total potential capability of NADPH production was lower in ectothermic animals (due mainly to lower malic enzyme activity). It is suggested that the presence of NADP(+)-linked malic enzyme in the brain may be related mainly to mitochondrial metabolism, especially to maintain the mitochondrial pool of NADP+ in reduced form.

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

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.

Oxalacetate decarboxylase and pyruvate carboxylase activities, and effect of sulfhydryl reagents in malic enzyme from Sulfolobus solfataricus

Malic enzyme (S)-malate: NADP+ oxidoreductase (oxaloacetate-decarboxylating, EC 1.1.1.40) purified from the thermoacidophilic archaebacterium Sulfolobus solfataricus, strain MT-4, catalyzed the metal-dependent decarboxylation of oxaloacetate at optimum pH 7.6 at a rate comparable to the decarboxylation of L-malate. The oxaloacetate decarboxylase activity was stimulated about 50% by NADP but only in the presence of MgCl2, and was strongly inhibited by L-malate and NADPH which abolished the NADP activation. In the presence of MnCl2 and in the absence of NADP, the Michaelis constant and Vm for oxaloacetate were 1.7 mM and 2.3 mumol.min-1.mg-1, respectively. When MgCl2 replaced MnCl2, the kinetic parameters for oxaloacetate remained substantially unvaried, whereas the Km and Vm values for L-malate have been found to vary depending on the metal ion. The enzyme carried out the reverse reaction (malate synthesis) at about 70% of the forward reaction, at pH 7.2 and in the presence of relatively high concentrations of bicarbonate and pyruvate. Sulfhydryl residues (three cysteine residues per subunit) have been shown to be essential for the enzymatic activity of the Sulfolobus solfataricus malic enzyme. 5,5'-Dithiobis(2-nitrobenzoic acid), p-hydroxymercuribenzoate and N-ethylmaleimide caused the inactivation of the oxidative decarboxylase activity, but at different rates. The inactivation of the overall activity by p-hydroxymercuribenzoate was partially prevented by NADP singly or in combination with both L-malate and MnCl2, and strongly enhanced by the carboxylic acid substrates; NADP + malate + MnCl2 afforded total protection. The inactivation of the oxaloacetate decarboxylase activity by p-hydroxymercuribenzoate treatment was found to occur at a slower rate than that of the oxidative decarboxylase activity.

Changes of the NADP-linked malic enzyme in the developing rat skeletal muscle

The activities of NADP-linked malic enzyme, hexose monophosphate shunt dehydrogenases and NADP-linked isocitrate dehydrogenase were studied during development of skeletal muscle and compared with those in the liver. The variation patterns of malic enzyme activity in the liver and in the skeletal muscle were very similar, however the amplitude of the changes was different. The enzyme activity increased approx 16-fold in the liver and about 2-fold in skeletal muscle at the same stage of development. In skeletal muscle the increase of the malic enzyme activity was only slightly higher than of lactic dehydrogenase and citrate synthase. Studies on the intracellular distribution of malic enzyme in skeletal muscle showed that both mitochondrial and extramitochondrial enzymes increased between 20th and 37th day of life, the increase of the extramitochondrial enzyme being more pronounced. The hexose monophosphate shunt dehydrogenases activity showed an increase in the liver but no change was observed in the skeletal muscle at the weaning time. Changes in the activity of the liver and skeletal muscle isocitrate dehydrogenase were not significant between 10th and 80th day of life. The results suggest that the malic enzyme in the liver is playing a different physiological role than in the skeletal muscle.

Cardiac malic enzyme in Friedreich's disease

We measured the activity of cytosolic and of mitochondrial malic enzyme in the hearts from 4 patients with Friedreich's disease and from two non-ataxic control subjects. There was a wide variability in the results and the slight overall decreases in both enzyme activities were not considered to be statistically significant. From these and other results, we conclude that deficient mitochondrial malic enzyme activity is not a constant or primary feature of Friedreich's disease.

Friedreich's disease 1982: etiologic hypotheses a personal analysis

The author reviews the arguments for and against the four etiologic hypotheses in Friedreich's disease that have been proposed since 1974: the "pyruvate hypothesis", the "lipid-membrane hypothesis", the "energy-defect hypothesis" and finally the "taurine hypothesis". While none of these hypotheses are mutually exclusive, the author shows that all of these mechanisms play some role in the pathophysiology of the symptoms, but that only the "taurine hypothesis" appears to be compatible with all the known facts and the biochemical abnormalities reported. The author proposed that the taurine retention defect (possibly due to a block in the high affinity-low capacity transport of taurine - The TH System) is a primary event in Friedreich's disease. Whether it is the primary genetic event still has to be determined.

Thermostable, ammonium-activated malic enzyme of Clostridium thermocellum

"Malic" enzyme (L-malate:NADP+ oxidoreductase (oxaloacetate-decarboxylating, EC 1.1.1.40) was purified from Clostridium thermocellum by DEAE-cellulose, agarose-NADP and Sephadex G-200 column chromatography. The 117-fold purified "malic" enzyme displayed a maximum activity of 135 units/mg at 40 degrees C and represented 0.8% of the total cell protein. Sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis analysis of the protein suggested 90% purity and an approximate tetrameric subunit molecular weight of 40 000. The enzyme absolutely required both bivalent and monovalent cations for catalysis. Mn2+ and NH4+ were the most effective cationic activators examined. Increasing NH4+ concentration increased both enzyme activity and affinity toward L-malate. The apparent Km for L-malate was 3 X 10(-4) M at 0.4 mM NH4Cl. Enzyme activity increased linearly when temperature was raised between 22-60 degrees C and a Q10 of 2.1 was calculated from an Arrhenius plot. The enzyme was stable at heating at 60 degrees C but was denatured at higher temperatures. The enzyme half-life was 10 min at 72 degrees C. The enzyme displayed a broad pH optimum (7.2-87.2 for Tris-HCl buffer) but was inactivated by p-chloromercuribenzoate. The high thermal stability, low apparent molecular weight and NH4+ activation are properties not common to all previously described "malic" enzymes.

A survey on the formation and localization of secondary isozymes in mammalia

A survey on the formation of secondary isozymes (= multiple molecular forms of enzymes) is given by means of well-documented enzyme systems. Further examples of a certain type of formation are summarized in tabular form. Eight different classes of enzyme variants deriving from translational processes are discussed. These are: aggregation, polymerization, oxidation and reduction of free SH groups, limited proteolysis, cleavage of carbohydrate residues, deamidation, noncovalent binding of coenzymes, and conformational isomerism. In addition, the intracellular distribution of secondary isozymes is discussed, as are the formation of artificial enzyme variants and the recognition of multiple enzyme forms caused by an exchange of neutral amino acids. About 200 original papers are cited. The reference list was completed in early 1979.

Isolation and regulatory properties of mitochondrial malic enzyme from rat skeletal muscle

Mitochondrial malic enzyme (L-malate:NADP+ oxidoreductase (oxalo-acetate-decarboxylating), EC 1.1.1.40) has been isolated from rat skeletal muscle by (NH4)2SO4 fractionation, chromatography on DEAE-cellulose and Ultrogel AcA 34. Specific activity of the purified enzyme was 25 micromol/min per mg of protein which corresponds to about 840-folf purification. The enzyme was shown to carboxylate pyruvate in the presence of high concentrations of KHCO3 and pyruvate at about 15% of the rate of the forward reaction. The Km values determined at pH 7.2 for malate, NADP and Mn2+ were 0.33 mM, 6.8 microM and 7.1 microM, respectively. The Km values for pyruvate, NADPH and KHCO3 were 8.3 mM, 19.6 microM, and 24.4 mM, respectively. Purified enzyme showed allosteric properties at low concentration of malate and this characteristic can be modified by succinate and fumarate which do not affect the maximum velocity of the reaction. The pH optimum for decarboxylation reaction was between 7.2 and 8.4. Possible metabolic role of mitochondrial malic enzyme in skeletal muscle is discussed.

Human mitochondrial malic enzyme variants: properties of the different polymorphic forms

1. The human mitochondrial malic enzyme polymorphism was found to exist in the Scottish population with similar allele frequencies to those reported previously for Caucasian populations. 2. The mitochondrial malic enzyme variants MEM1, MEM2-1 and MEM2 which form the polymorphism have been separated from the cytoplasmic malic enzyme and partially purified by DEAE Sephadex chromatography. 3. The properties of the three mitochondrial malic enzyme variants were examined. No differences were found between the variants in Km for NADP, Km for pyruvate, Mn2+ and Mg2+ activation, Ki for dicumarol, heat stability, pH or ionic strength optimum.

High activity of NADP-dependent malic enzyme in mitochondria from abdomen muscle of the crayfish Orconectes limosus

1. Mitochondria isolated from abdomen muscle of crayfish Orconectes limosus exhibit malic enzyme activity in the presence of L-malate, NADP and Mn2+ ions after addition of Triton X-100. Under optimal conditions about 230 nmole of reduced NADP and an equivalent amount of pyruvate are produced per min per mg of mitochondrial protein. 2. The pH optimum for decarboxylation of L-malate is about 7.5. 3. The apparent Km for L-malate, NADP and Mn2+ ions was found to be 0.66, 0.012, and 0.0025 mM, respectively. 4. The requirement for Mn2+ can be replaced by Mg2+, Co2+ and Ni2+ ions; however, higher concentrations of these ions than Mn2+ are required for a full stimulation of malic enzyme activity. 5. Oxaloacetate and pyruvate inhibited the enzyme activity in a competitive manner with apparent Ki values of 0.05 mM and 5.4 mM, respectively.

Enzymatic organization of the subcommissural organ

In the subcommissural organ (SCO) of the guinea pig, rat, golden hamster, and mouse the activity and distribution of enzymes related to the energy-supplying metabolism and of some marker enzymes of different cell organelles have been investigated by means of mostly modified histochemical methods. The results were compared with findings in the ciliated ependyma of the ventricular wall and with those in the ependyma of the choroid plexus of the third ventricle. In the ependymal part of the SCO only a moderate activity of hexokinase is observed in its specialized columnar cells whereas a high activity is present both in the ciliated ependyma and the choroid plexus. - The staining pattern of glucose-6-phosphatase is similar to that of hexokinase but this enzyme is found is the SCO only. - Likewise hexokinase, glycogen granules and enzymes related to glycogen metabolism (phosphoglucomutase, uridine-diphosphoglucose pyrophosphorylase, glycogen synthetase and phosphorylase) are regularly found most numerous and active in the nuclear and supra-nuclear area of the ependymal part. These enzymes are less active in both the other ependymal regions. - Uridine-diphosphoglucose dehydrogenase could not be demonstrated in the SCO. The NADP-linked enzymes of the pentose phosphate shunt, glucose-6-phosphate and 6-phosphogluconate dehydrogenase, show a moderate activity which decreases also from the nuclear towards the apical area of the ependymal cells of the SCO. Enzymes of the glycolytic pathway, such as glucosephosphate isomerase, fructose-6-phosphate kinase, fructose-I,6-diphosphate aldolase, glyceraldehyde-3-phosphate and lactate dehydrogenase, are highly active in the SCO and are located mainly in the supranuclear area, too. Fructose-1,6-diphosphatase could not be demonstrated thus indicating that in the SCO the pathway is most probably only glycolytic but not gluconeogenetic. Compared to the ependyma of the ventricular wall and of the choroid plexus, in the SCO the M type subunits of lactate dehydrogenase predominate. Glycolytic enzymes are also very active in the choroid plexus but less in the ciliated ependyma. Compared to the ciliated ependyma and especially to the ependyma of the choroid plexus, the activities of enzymes which are only present in mitochondria (NAD-linked isocitrate dehydrogenase, succinate dehydrogenase, NAD-linked malate dehydrogenase after preextraction, cytochrome oxidase, 3-hydroxybutyrate and glycerolphosphate and glutamate dehydrogenase) are relatively low. Mitochondria are accumulated near the superior pole of the nuclei as well as in the most apical part of the ependymal cells. - The staining pattern of NADP-linked isocitrate and malate dehydrogenase as well as of NADH dehydrogenase suggests that these enzymes are localized both in and out of mitochondria. The extramitochondrial activity of the first two enzymes might be localized in the cytosol. The extramitochondrial activity of NADH dehydrogenase might be localized in the endoplasmic reticulum...