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

Characterization of two members of a novel malic enzyme class

The Gram-negative bacterium Rhizobium meliloti contains two distinct malic enzymes. We report the purification of the two isozymes to homogeneity, and their in vitro characterization. Both enzymes exhibit unusually high subunit molecular weights of about 82 kDa. The NAD(P)(+) specific malic enzyme [EC 1.1.1.39] exhibits positive co-operativity with respect to malate, but Michaelis-Menten type behavior with respect to the co-factors NAD(+) or NADP(+). The enzyme is subject to substrate inhibition, and shows allosteric regulation by acetyl-CoA, an effect that has so far only been described for some NADP(+) dependent malic enzymes. Its activity is positively regulated by succinate and fumarate. In contrast to the NAD(P)(+) specific malic enzyme, the NADP(+) dependent malic enzyme [EC 1.1.1.40] shows Michaelis-Menten type behavior with respect to malate and NADP(+). Apart from product inhibition, the enzyme is not subjected to any regulatory mechanism. Neither reductive carboxylation of pyruvate, nor decarboxylation of oxaloacetate, could be detected for either malic enzyme. Our characterization of the two R. meliloti malic enzymes therefore suggests a number of features uncharacteristic for malic enzymes described so far.

ATP depletion, purine riboside triphosphate accumulation and rat thymocyte death induced by purine riboside

Purine riboside (purine-1-D-ribofuranoside, nebularine), an adenosine analog, exerts cytotoxic effect both in vivo and in vitro. However, exact biochemical mechanism for its toxicity and sensitivity of lymphoid cells remains unknown. The present experiments have examined the sequential metabolic changes leading to cell death, induced in cultured rat thymocytes during incubation with purine riboside. Among 22 analogs tested, purine-riboside and tubercidin were most toxic as determined by trypan blue exclusion and lactate dehydrogenase leakage from the cells. 2-Chloroadenosine and 2'-deoxyadenosine were only moderately toxic, whereas other analogs tested were without effect on cell viability. In the presence of purine riboside, more than 90% of ATP was lost after 2 h of incubation. Hypoxanthine accumulated in the medium and the formation of purine-riboside triphosphate exceeded 4-fold the physiological concentration of ATP in the cell. Inhibition of adenosine kinase by 5-iodotubercidin reversed the cytotoxic effect of purine riboside. Interestingly, cells virtually deprived of ATP after 2 h of incubation with purine riboside maintained high nucleotide energy charge value and high viability. Purine riboside triphosphate was capable to replace ATP in stimulation of glycolysis in cell-free thymus extract. We conclude that for a short time (a few hours) purine riboside triphosphate formed in the cell may serve in the absence of ATP as an intermediate of cellular energy metabolism in rat thymocytes. However, possibly due to toxic effects of purine-riboside triphosphate, cells were finally dying. Thus, ATP depletion and adenosine kinase mediated purine riboside phosphates formation are the principle causes of rat thymocytes death exposed to purine riboside.

Chimeric structure of the NAD(P)+- and NADP+-dependent malic enzymes of Rhizobium (Sinorhizobium) meliloti

Malic enzymes catalyze the oxidative decarboxylation of malate to pyruvate in conjunction with the reduction of a nicotinamide cofactor. We determined the DNA sequence and transcriptional start sites of the genes encoding the diphosphopyridine nucleotide-dependent malic enzyme (DME, EC 1.1.1.39) and the triphosphopyridine nucleotide-dependent malic enzyme (TME, EC 1.1.1. 40) of Rhizobium (Sinorhizobium) meliloti. The predicted DME and TME proteins contain 770 and 764 amino acids, respectively, and are approximately 320 amino acids larger than previously characterized prokaryotic malic enzymes. The increased size of DME and TME resides in the C-terminal extensions which are similar in sequence to phosphotransacetylase enzymes (EC 2.3.1.8). Modified DME and TME proteins which lack this C-terminal region retain malic enzyme activity but are unable to oligomerize into the native state. Data base searches have revealed that similar chimeric malic enzymes were uniquely present in Gram-negative bacteria. Thus DME and TME appear to be members of a new class of malic enzyme characterized by the presence of a phosphotransacetylase-like domain at the C terminus of the protein.

Different regulatory properties of the cytosolic and mitochondrial forms of malic enzyme isolated from human brain

The human brain contains a cytosolic and mitochondrial form of NADP(+)-dependent malic enzyme. To investigate their possible metabolic roles we compared the regulatory properties of these two iso-enzymes. The mitochondrial malic enzyme exhibited a sigmoid substrate saturation curve at low malate concentration which was shifted to the right at both higher pH values and in the presence of low concentration of Mn2+ or Mg2+. Succinate or fumarate increased the activity of the mitochondrial malic enzyme at low malate concentration. Both activators shifted the plot of reaction velocity versus malate concentration to the left, and removed sigmoidicity, but the maximum velocity was unaffected. The activation was associated with a decrease in Hill coefficient from 2.3 to 1.1. The human brain cytosolic malic enzyme displayed a hyperbolic substrate saturation kinetics and no sigmoidicity was detected even at high pH and low malate concentrations. Succinate or fumarate exerted no effect on the enzyme activity. Excess of malate inhibited the oxidative decarboxylation catalysed by cytosolic enzyme at pH 7.0 and below. In contrast, decarboxylation catalysed by mitochondrial malic enzyme, was unaffected by the substrate. These results suggest that under in vivo conditions, cytosolic malic enzyme catalyses both oxidative decarboxylation of malate and reductive carboxylation of pyruvate, whereas the role of mitochondrial enzyme is limited to decarboxylation of malate. One may speculate that in vivo the reaction catalysed by cytosolic malic enzyme supplies dicarboxylic acids (anaplerotic function) for the formation of neurotransmitters, while the mitochondrial enzyme regulates the flux rate via Krebs cycle by disposition of the tricarboxylic acid cycle intermediates (cataplerotic function).

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.

Significant amounts of glycogen are synthesized from 3-carbon compounds in astroglial primary cultures from mice with participation of the mitochondrial phosphoenolpyruvate carboxykinase isoenzyme

The incorporation was studied of the gluconeogenic substrates lactate, alanine, aspartate and glutamate into glycogen of astroglial primary cultures derived from mouse brain. The incorporation was inhibited by 3-mercaptopicolinate, an inhibitor of one of the characteristic gluconeogenic enzymes, phosphoenolpyruvate carboxykinase. Only the mitochondrial isoenzyme of phosphoenolpyruvate carboxykinase was detectable in the astroglial primary cultures. After the incubation of glucose-starved cells with medium containing a mixture of [6-3H]glucose and [U-14C]glucose, the newly synthesized glycogen showed a 3H/14C ratio which was approximately 15% less than the isotope ratio for the medium. The decrease of the isotope ratio was not significantly inhibited by 3-mercaptopicolinate, indicating a cycling of approximately 15% of the glucose to the level of the triose phosphates before its incorporation into astroglial glycogen. During the initial phase of glycogen resynthesis, the contribution of the gluconeogenic substrates appeared to be higher. This was in agreement with the accumulation of fructose 2,6-bisphosphate during refeeding. A participation of gluconeogenic substrates in glycogen metabolism was also detectable when the glycogen content was not changing significantly.

Purification and properties of cytosolic and mitochondrial malic enzyme isolated from human brain

Three isoforms of malic enzyme have been described in mammalian tissues: a cytosolic NADP(+)-dependent enzyme, a NADP(+)-dependent mitochondrial isoform and a mitochondrial isozyme which can use both NAD+ and NADP+ but is more effective with NAD+. We purified mitochondrial and cytosolic malic enzyme from human brain extract to apparent homogeneity in order to compare properties of these isozymes and to verify whether mitochondria contain one or two malic enzyme. Specific activities of both isoforms are approx. 90 mumol/min/mg of protein, which corresponds to about 1900-fold purification. The two isozymes have identical native molecular mass (257 kDa) and are presumably tetramers composed of four identical subunits (M(r) = 64 kDa). The isoelectric point of cytosolic isozyme is 5.65, and that of mitochondrial one is 7.0. The isozymes show a substantial difference in their capability to catalyse the reductive carboxylation of pyruvate to malate: the maximal carboxylation rate approaches 80% that of decarboxylation velocity for the cytosolic enzyme, and only 17% for the mitochondrial isozyme. The coenzyme specificity of both isozymes is not stringent; NADP+ is the preferred and NAD+ can substitute it, although with much lower efficiency. The homogenous cytosolic malic enzyme catalysed decarboxylation of oxaloacetate and NADPH-dependent reduction of pyruvate at about 24 and 0.5% of the maximum rate of NADP-dependent oxidative decarboxylation of malate respectively. Decarboxylation of oxaloacetate catalysed by mitochondrial malic enzyme has not been detectable, while NADP-linked reduction of pyruvate approaches only 0.15% of the maximum rate of NADP-linked oxidative decarboxylation of malate.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of lipogenesis in rat brown adipose tissue by clofibrate

The effect of clofibrate (Atromid S, ethyl-2-(4-chlorophenoxy)-2-methylpropionate) administration for 7 days to rats on lipogenesis and on some lipogenic enzyme activities in brown adipose tissue (BAT), liver and white adipose tissue (WAT) was examined. As compared to control rats the rate of lipogenesis in BAT in the clofibrate-treated animals was significantly decreased. The rate of liver lipogenesis increased slightly, whereas lipogenesis in the WAT was not affected by clofibrate. In BAT, the drug treatment resulted in depression of fatty acid synthase, ATP-citrate lyase, malic enzyme, glucose 6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase activities. The activity of liver fatty acid synthase did not change, ATP-citrate lyase activity slightly decreased, whereas the activity of malic enzyme significantly increased in this organ after clofibrate feeding. The ATP-citrate lyase activity in WAT decreased, while fatty acid synthase and other lipogenic enzymes were not changed after clofibrate feeding. Clofibrate treatment did not influence the activity of NADP-linked isocitrate dehydrogenase and malate dehydrogenase (enzymes not linked directly to lipogenesis), either in BAT, liver or WAT. The data presented suggest that the hypolipidaemic effect of clofibrate in the rat may be due (possibly among other mechanisms) to reduction of the rate of fatty acid synthesis in BAT but not in the liver and WAT.

Developmental changes of lipogenic enzyme activities and lipogenesis in brown adipose tissue and liver of the rat

1. Brown adipose tissue (BAT) and liver lipogenesis in vivo estimated by using 3H2O as tracer was very low and did not change significantly between 10 and 20 days after birth. Lipogenesis increased dramatically in both tissues by weaning at 20 days, peaking between 25 and 30 days of age. Since that time the rate of fatty acid synthesis in BAT decreased gradually to reach adult level after 2 months, whereas in the liver there was a sharp decrease of lipogenesis. 2. The activities of fatty acid synthase, citrate cleavage enzyme, malic enzyme and glucose 6-phosphate dehydrogenase essentially followed a similar course of developmental changes as lipogenesis. 3. In contrast to the enzymes listed above NADP-linked isocitrate dehydrogenase remained unaltered over the period studied, whereas lactate and malate dehydrogenases exhibited very high activity at 10 days after birth and from then decreased to reach adult level at the age of about 20 days. 4. The data obtained indicate that no substantial differences could be detected in the developmental pattern of lipogenesis and lipogenic enzyme activities between BAT and liver up to 30 days of age but after this time these processes were not co-ordinated in both tissues. Beyond this time the BAT was characterized by a much higher rate of lipogenesis than the liver. 5. The results are discussed in terms of the nutrient changes and the relationship between thermogenesis and lipogenesis in BAT.

Clofibrate feeding increases cytoplasmic but not mitochondrial malic enzyme activity in rat kidney cortex

Administration of clofibrate for 21 days to rats increased the malic enzyme activity in the kidney cortex by about 80 per cent. This effect seems to be specific since the drug did not alter significantly the activity either of lactate dehydrogenase, citrate synthase or total mitochondrial protein content in this organ. The increase in activity of malic enzyme in the 13,000 g supernatant (extramitochondrial) fraction in rats treated with the drug was about 80 per cent, whereas in the pellet (mitochondrial fraction) it was about 40 per cent. The specific activity of malic enzyme in the kidney cortex cytosol from clofibrate-treated rats was about twice that in controls. In contrast clofibrate treatment did not affect its specific activity in isolated mitochondria. Calculations showed that 0.57 and 0.53 mumoles min-1 g-1 wet tissue of mitochondrial malic enzyme was obtained in control and clofibrate-treated rats respectively. Thus, clofibrate feeding increases the amount of cytoplasmic but not mitochondrial malic enzyme activity.

Pyruvate carboxylation in the rat heart. Role of biotin-dependent enzymes

Pyruvate carboxylation in the isolated perfused rat heart was studied under steady-state conditions. A biotin deficiency resulting in a 90% decrease in myocardial pyruvate carboxylase left the pyruvate carboxylation rate unchanged. Pyruvate carboxylation in heart muscle must therefore take place by means of an enzyme which does not contain biotin. The kinetic properties and mass-action ratio of the NADP-linked malic enzyme in heart muscle can be taken as circumstantial evidence in favour of the role of malic enzyme in pyruvate carboxylation in myocardium.

Purification and properties of cytosolic malic enzyme from human skeletal muscle

1. An NADP+-dependent malic enzyme was purified 7940-fold from the cytosolic fraction of human skeletal muscle with a final yield of 55.8% and a specific activity of 38.91 units/mg of protein. 2. The purification to homogeneity was achieved by ammonium sulfate fractionation, DEAE-Sepharose chromatography, affinity chromatography on NADP+-Agarose, gel filtration on Sephacryl S-300 and rechromatography on the affinity column. 3. Either Mn2+ or Mg2+ was required for activity: the pH optima with Mn2+ and Mg2+ were 8.1 and 7.5, respectively. The enzyme showed Michaelis-Menten kinetics. At pH 7.5 the apparent Km values with Mn2+ and Mg2+ for L-malate and NADP+ were 0.246 mM and 5.8 microM, and 0.304 mM and 5.8 microM, respectively. The Km values with Mn2+ for pyruvate, NADPH and bicarbonate were 8.6 mM, 6.1 microM and 22.2 mM, respectively. 4. The enzyme was also able to decarboxylate malate in the presence of NAD+. At pH 7.5 the reaction rate was approximately 10% of the rate in the presence of NADP+, with a Km value for NAD+ of 13.9 mM. 5. The following physical parameters were established: s0(20.w) = 10.48, Stokes' radius = 5.61 nm, pI = 5.72 Mr of the dissociated enzyme = 61,800. The estimates of the native apparent Mr yielded a value of 313,000 upon gel filtration, and 255,400 with f/fo = 1.33 by combining the chromatographic data with the sedimentation measurements. 6. The electron microscopy analysis of the uranyl acetate-stained enzyme revealed a tetrameric structure. 7. Investigations to detect sugar moieties indicated that the enzyme contains carbohydrate side chains, a property not previously reported for any other malic enzyme.

Mitochondrial malic enzyme from crustacean and fish muscle

1. In contrast to mammalian skeletal muscle mitochondria, the only substrate that crustacean and fish mitochondria oxidize at a high rate is malate. 2. The mitochondria isolated from muscles of fish and crayfish exhibit a high activity of malic enzyme. 3. Assuming that malic enzyme is responsible for the conversion of malate to pyruvate in animal muscle, it could be expected that the mitochondria which possess high activity of this enzyme should oxidize malate very rapidly when oxygen is available. 4. Some properties of different molecular forms of malic enzyme are reviewed.

Evidence for two distinct mitochondrial malic enzymes in human skeletal muscle: purification and properties of the NAD(P)+-dependent enzyme

Human muscle mitochondria reduced either NADP+ or NAD+ in the presence of L-malate and Mn2+ or Mg2+. After polyacrylamide slab gel electrophoresis and agarose gel isoelectrofocusing, two bands were seen in mitochondrial extract, one strictly NADP+-dependent and the other reacting with either NAD+ or NADP+. The two mitochondrial malic enzymes were separated by DEAE-Sepharose chromatography. The NAD+/NADP+-dependent enzyme was purified 1600-fold with a final yield of 34% and a final specific activity of 32.9 units/mg of protein by employing affinity chromatography on Agarose-ATP. SDS electrophoresis revealed a single band having an apparent Mr = 64,000. Estimates of the native apparent molecular weight upon gel filtration yielded a value of 140,300. Kinetic characterization showed that succinate and ATP were activator and inhibitor, respectively. In the absence of succinate the Km values for malate, NAD+ and NADP+ were 3.7, 0.13 and 0.78 mM, respectively; in the presence of succinate the Km value for malate was 1.9 mM. ATP was found to be an inhibitor competitive with malate, with a Ki (ATP) of 0.2 mM. This is the first report to show that mammalian skeletal muscle mitochondria contains two distinct malic enzymes, one active with either NAD+ or NADP+ and the other active only with NADP+.

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.

Isolation and properties of glycerol-3-phosphate oxidoreductase from human placenta

Glycerol-3-phosphate oxidoreductase (sn-glycerol 3-phosphate: NAD+ 2-oxidoreductase, EC 1.1.1.8) from human placenta has been purified by chromatography on 2,4,6-trinitrobenzenehexamethylenediamine-Sepharose, DEAE-Sephadex A-50 and 5'-AMP-Sepharose 4B approximately 15800-fold with an overall yield of about 19%. The final purified material displayed a specific activity of about 88 mumol NADH min-1 mg protein-1 and a single protein band on polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulphate. The native molecular mass, determined by Ultrogel AcA 44 filtration, was 62000 +/- 2000 whereas the subunit molecular mass, established on polyacrylamide gel in the presence of 0.1% sodium dodecyl sulphate, was 38000 +/- 500. The isoelectric point of the enzyme protein, determined by column isoelectric focusing, was found to be 5.29 +/- 0.09. The pH optimum of the placental enzyme was in the range 7.4-8.1 for dihydroxyacetone phosphate reduction and 8.7-9.2 for sn-glycerol 3-phosphate oxidation. The apparent Michaelis constants (Km) for dihydroxyacetone phosphate, NADH, sn-glycerol 3-phosphate and NAD+ were 26 microM, 5 microM, 143 microM and 36 microM respectively. The activity ratio of cytoplasmic glycerol-3-phosphate oxidoreductase to mitochondrial glycerol-3-phosphate dehydrogenase in human placental tissue was 1:2. The consumption of oxygen by human placental mitochondria incubated with the purified glycerol-3-phosphate oxidoreductase, NADH and dihydroxyacetone phosphate was similar to that observed in the presence of sn-glycerol 3-phosphate. The possible physiological role of glycerol-3-phosphate oxidoreductase in placental metabolism is discussed.

Properties and function of mitochondrial malate enzyme from bass liver

Malate enzyme (L-malate: NADP+ oxidoreductase oxaloacetate decarboxylating, EC 1.1.1.40) from bass liver mitochondria was purified to over 90% of homogeneity by gel filtration, affinity and ion exchange chromatographies. The apparent molecular weight estimated by gel filtration was 316,000. Analysis of the enzyme on sodium dodecylsulphate-polyacrylamide disc gel electrophoresis was shown to be a tetramere protein. The enzyme required bivalent cations for catalysis, (Mn2+ or Mg2+) and displayed a narrow pH optimum (8.4-8.6 for Tris-HCl buffer) and was inactivated by p-chloromercuribenzoate. The double reciprocal initial velocity plots of both of the substrates, NADP and malate, were linear and intercepting at a point that suggests a sequential mechanism. Product inhibition studies with NADP and malate as variable substrate are consistent with an ordered Bi-Ter mechanism.

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.

Malic enzymes of salmon trout heart mitochondria: separation and some physicochemical properties of NAD-preferring and NADP-specific enzymes

Mitochondria isolated from the heart of the Baltic salmon trout Salmo trutta contain two distinct malic enzymes. One of these enzymes (NAD-preferring malic enzyme) catalyses the oxidative decarboxylation of malate in the presence of either NAD or NADP. The specific activity with NAD was six times that with NADP as coenzyme. The second enzyme is specific for NADP. These two malic enzymes have been separated by: ion exchange chromatography of DEAE-Sephacel, affinity chromatography on 2',5'ADP-Sepharose 4B, gel filtration on Sephacryl S-300 and polyacrylamide gel electrophoresis. The mol. wts of the two native malic enzymes determined by gel filtration were found to be 280,000 and 190,000 for NAD-preferring and NADP-specific malic enzyme, respectively. Chromatofocusing revealed the isoelectric points of the two enzymes at pH 5.45 and 5.85 for NAD-preferring and NADP-specific malic enzyme, respectively.

Effect of acetate and octanoate on tricarboxylic acid cycle metabolite disposal during propionate oxidation in the perfused rat heart

Tricarboxylic acid cycle pool size is determined by anaplerosis and metabolite disposal. The regulation of the latter during propionate metabolism was studied in isolated perfused rat hearts in the light of the characteristics of NADP-linked malic enzyme, which is inhibited by acetyl-CoA. The acetyl-CoA concentration was varied by infusions of acetate and octanoate, and the rate of metabolite disposal was calculated from a metabolic balance sheet compiled from the relevant metabolic fluxes. Propionate addition increased the tricarboxylic acid cycle pool size 4-fold and co-infusion of acetate or octanoate did not change it further. Propionate caused a decrease in the CoA-SH concentration and a 10-fold increase in the propionyl-CoA concentration. A paradoxical increase in the CoA-SH concentration was observed upon co-infusion of acetate in the presence of propionate, an effect probably caused by competitive inhibition of propionate activation. A more pronounced decline in the propionyl-CoA concentration was observed upon the co-infusion of octanoate. In a metabolic steady state, acetate and octanoate reduced propionate disposal only slightly, but did not increase the tricarboxylic acid cycle pool size. The results are in accord with the notion that the tricarboxylic acid pool size is mainly regulated by the anaplerotic mechanisms.

Evidence for the role of malic enzyme in the rapid oxidation of malate by cod heart mitochondria

Mitochondria isolated from the heart of cod (Gadus morrhua callarias) oxidized malate as the only exogenous substrate very rapidly. Pyruvate only slightly increased malate oxidation by these mitochondria. This is in contrast with the mitochondria isolated from rat and rabbit heart which oxidized malate very slowly unless pyruvate was added. Arsenite and hydroxymalonate (an inhibitor of malic enzyme) inhibited the respiration rate of mitochondria isolated from cod heart, when malate was the only exogenous substrate. Inhibition caused by hydroxymalonate was reversed by the addition of pyruvate. In the presence of arsenite, malate was converted to pyruvate by cod heart mitochondria. Cod heart mitochondria incubated in the medium containing Triton X-100 catalyzed the reduction of NADP+ in the presence of L-malate and Mn2+ at relatively high rate (about 160 nmoles NADPH formed/min/mg mitochondrial protein). The oxidative decarboxylation of malate was also taking place when NADP+ was replaced by NAD+ (about 25 nmol NADH formed per min per mg mitochondrial protein). These results suggest that the mitochondria contain both NAD+- and NADP+-linked malic enzymes. These two activities were eluted from DEAE-Sephacel as two independent peaks. It is concluded that malic enzyme activity (presumably both NAD+- and NADP+-linked) is responsible for the rapid oxidation of malate (as the only external substrate) by cod heart mitochondria.

The effect of clofibrate feeding on the NADP-linked dehydrogenases activity in rat tissue

Administration of clofibrate to the rat increased several fold the activity of malic enzyme in the liver. Clofibrate treatment resulted also in an increased activity of the hepatic hexose monophosphate shunt dehydrogenases but was without effect on NADP-linked isocitrate dehydrogenase. The increased activity of malic enzyme in the liver resulting from the administration of clofibrate was inhibited by ethionine and puromycin, which suggests that de novo synthesis of the enzyme protein did occur as the result of the drug action. In contrast to the liver malic enzyme, the enzyme activity in kidney cortex increased only two-fold, whereas in the heart and skeletal muscle the activity was not affected by clofibrate administration.

Rat brain malic enzyme: subcellular distribution and kinetic studies from two brain regions

1. The subcellular distribution of malic enzyme in two different brain regions (frontal cortex and striatum) is studied in adult rats. 2. A bimodal distribution is found in both regions: 75% being localized in the mitochondrial fraction and the remaining 25% in the cytosol. 3. In the frontal cortex, free mitochondria is enriched with the enzyme, while, in striatum, free as well as synaptic mitochondria, presented the same activity. 4. Kinetic studies of the malic enzymes show two Km values when malate is used as substrate. A higher Km value for free mitochondria as compared with a lower one found for the cytosolic and synaptosomal mitochondria suggests the presence of two enzyme populations. 5. The following are common characteristics for the two enzyme populations: NADP dependence, use of either Mg2+ or Mn2+ as cofactor and hyperbolic malate saturation curves not affected by dicarboxylic acids.

Accumulation and disposal of tricarboxylic acid cycle intermediates during propionate oxidation in the isolated perfused rat heart

The role of the metabolite disposal mechanisms in the regulation of the tricarboxylic acid cycle pool size was studied in isolated perfused rat hearts oxidizing 2 mM propionate. Malate and succinate accumulated during the propionate metabolism. A further 118% increase in the malate concentration and 600% increase in the succinate concentration and a slight inhibition of the propionate uptake were observed during a subsequent KCl-induced arrest of the heart metabolizing propionate. When the mechanical activity of the heart was restored, the malate and succinate concentrations returned to the same levels as before the arrest of the heart, but the propionate uptake did not rise significantly. The mean disposal rates of the tricarboxylic acid cycle metabolites during the cardiac arrest and subsequent restoration of the activity were 1.4 and 2.4 mumol/min per g dry weight, respectively during cardiac arrest the malate carbon disposed was almost totally recovered as C3 compounds, whereas after the increase in the ATP-consumption most of it was oxidized. The result show that propionate is oxidized by heart muscle at an appreciable rate but the disposal rate of the tricarboxylic acid cycle intermediates is not tightly regulated by the cellular energy state. Although the metabolite pool size of the tricarboxylic acid cycle responds to change in the ATP consumption, the energy state appears to have a greater effect on the fate of the C3 compounds formed than on the actual rate of C4 compound disposition.

A new genetic variation of the malate dehydrogenase-like enzyme (MDL-1) in inbred rats and its possible linkage

A new polymorphism in the mitochondrial fraction of kidney homogenates was found by using discontinuous polyacrylamide gel electrophoresis. The polymorphism is tentatively designated MDL-1, since the enzyme was visualized with the staining solution for NADP-malate dehydrogenase (MOD) but differs from MOD. MDL-1 expresses three phenotypes: MDL-1A (fast), MDL-1AB (intermediate), and MDL-1B (slow). Progeny testing from genetic crosses indicates that its expression is determined by two codominant alleles, Mdl-1a and Mdl-1b, which segregate in a simple Mendelian fashion. Preliminary linkage data suggest that the locus for MDL-1 is probably linked to the nonagouti-agouti locus in rat linkage group IV.

Unusual behaviour of NADP-linked malic enzyme from crustacean tissues on 2',5' ADP-Sepharose 4B

1. Essential differences in the binding of NADP-dependent malic enzymes from mammals, fish and crustacea to 2',5' ADP-Sepharose 4B have been shown. 2. The enzymes from mammalian and fish tissues interact with 2',5' ADP-Sepharose, whereas the enzymes from crayfish Orconectes limosus and shrimp Crangon crangon tissues do not. 3. These results provide evidence that 2',5' ADP-Sepharose cannot be used for the purification of NADP-dependent malic enzyme from crayfish and shrimp and indicate presumably a structural difference in the NADP-binding site of crustacean enzyme.

Purification and some properties of extramitochondrial malic enzyme from rat skeletal muscle

Extramitochondrial malic enzyme (L-malate:NADP+ oxidoreductase (oxaloacetate-decarboxylating), EC 1.1.1.40) has been isolated from postmitochondrial supernatant of rat skeletal muscle, by (NH4)2SO4 fractionation, chromatography on DEAE-cellulose, Sepharose 6B, ADP-Sepharose and Ultrogel AcA-34 to apparent homogeneity as judged from polyacrylamide gel electrophoresis. Specific activity of purified enzyme was 20 mumol . min-1 per mg protein, which corresponds to about 3000-fold purification. The molecular weight of the native enzyme was determined by gel filtration to be 264 000. Sodium dodecyl sulphate (SDS)-polyacrylamide gel electrophoresis showed one polypeptide band of molecular weight 63 000. Thus, it appears that the native protein is a tetramer composed of identical molecular weight subunits. The isoelectric point of the isolated enzyme was at pH 6.15. The enzyme was shown to carboxylate pyruvate in the presence of high concentrations of bicarbonate and pyruvate at about 80% of the rate of the forward reaction. The Km values, determined at pH 7.2 for malate and NADP, were 0.125 mM and 11 microM, respectively. The Km values for pyruvate, NADPH and bicarbonate were 4.0 mM, 6.6 microM and 24 mM, respectively. The optimum pH for carboxylation reaction was at pH 7.1. The optimum pH for decarboxylation reaction varied with the malate concentration. The purified malic enzyme catalyzed the decarboxylation of oxaloacetate at pH 4.5. In a system consisting of isolated rat skeletal muscle mitochondria, pyruvate, bicarbonate and NADPH, cytoplasmic malic enzyme is able to replace added malate in stimulating oxidation of acetyl-CoA formed by oxidative decarboxylation of pyruvate. It is suggested that extramitochondrial malic enzyme might be one of the enzymes involved in the anaplerotic supply of Krebs cycle intermediates in skeletal muscle.