Pigeon liver malic enzyme

Summary Malic enzyme of pigeon liver is a tetrameric molecule with identical, or nearly-identical subunits. It catalyzes, in addition to oxidative decarboxylation of L-malate, the following metal activated component reactions: Oxalacetate decarboxylase; reductase with broad specificity on -ketocarboxylic acids; a NADP⁺-dependent dismutation of L-malate to L-lactate; and proton exchange between pyruvate and medium water. The kinetic mechanism of oxidative decarboxylase is sequential and ordered, with NADP⁺ adding first to the metal enzyme, followed by L-malate, and by the release of products CO₂, pyruvate, and NADPH. NADPH release, or a conformation change preceeding it, is rate-limiting in the overall reaction.Chemical modification studies indicate the presence of histidyl and lysyl residues at the nucleotide site, and tyrosyl residues at the carboxylic acid site. The involvement of protonated histidine(s) in NADPH binding is implicated by results of direct titration experiments, which also suggest a role of this residue as a proton sink in the catalytic reaction.A cysteinyl SH group is located near (but not at) each of the substrate-sites on the enzyme tetramer. Reaction of these groups with SH reagents causes selective loss of activities involving decarboxylation (i.e., oxidative decarboxylase, reductive carboxylase, and oxalacetate decarboxylase), owing to blockage of the reversible carbon-carbon cleavage step by the bulky substituent. All-of-the-sites reactivity is observed for non-specific thiol reagents such as 5,5 dithiobis-(2-nitrobenzoic acid), N-ethylmaleimide, iodoacetate, and iodoacetamide. While bromopyruvate, which is reduced by the enzyme to L-bromolactate under catalytic conditions, alkylates these groups in an active-site directed manner with half-of-the-sites stoichiometry. The remaining two SH groups are reactive toward non-specific reagents, but at rates 2.4 - 3.6 fold lower than do the same groups on the unalkylated enzyme. This behavior is interpreted in terms of the ligand-induced negative cooperativity concept of Koshland, et al. (Biochemistry 5: 365–385, 1966): Reaction of bromopyruvate induces a conformation change on the alkylated subunit which is transmitted to the unoccupied subunit neighbor, turning off its catalytic site for reaction with L-malate, as well as converting the initial fast SH groups into slow, or unreactive SH groups.In equilibrium binding experiments, all-of-the-sites reactivity is seen with nucleotide cofactors NADP⁺ and NADPH. Binding of Mn²⁺, or L-malate in the presence of Mn²⁺ and NADPH is biphasic, showing two tight sites with dissociation constants in the micromolar range, and two weak sites with 10–100 fold lower affinities. The presence of tight and weak L-malate sites is confirmed by fluorescence titration experiments which also yields similar affinities for the substrate molecule. In kinetic studies, two types of non-equivalent, and functionally distinct sites are detected. At saturating NADP⁺, and Mn²⁺ and L-malate levels corresponding to binding at tight sites, typical Michaelian behavior is observed. The reaction is inhibited uncompentitively by L-malate at higher concentrations corresponding to occupancy at all of the L-malate sites. Occupancy of Mn²⁺ at weak metal sites as well has no effect at low L-malate, but prevents substrate inhibition at high L-malate.A tentative half-of-the-sites model consistent with results of chemical modification, binding, and kinetic experiments is proposed for this enzyme. This model implicates involvement of subunit cooperativity in the catalytic process. Malic enzyme is depicted as a tetramer composed of inititally identical subunits, each containing an active-site capable of binding all reactants. Mn²⁺ and L-malate bind anticooperatively to the tight and weak sites, in contrast to NADP⁺ which binds equivalently to all sites. On the fully active enzymes, only half (or the tight) of the subunits are simultaneously undergoing catalysis. Binding of L-malate (but not Mn²⁺) at the adjacent weak subunits causes a slow isomerization of the enzyme, and inhibition of NADPH dissociation from the catalytic subunits. Binding of Mn²⁺ at the same sites prevents this change and thereby relieving substrate inhibition. This model is further supported by results of active-site titration experiments, such as the half-size burst of enzyme-bound NADPH in the transient state, and half-of-the-sites reactivity of oxalate, an analog for the transition state intermediate of the reaction.Abbreviations DTNB 5,5 dithiobis-(2-nitrobenzoic acid) - NEM N-ethylmaleimide - BP bromopyruvate - DTT dithiothreitol     

High malic enzyme activity in tumor cells and its cross-reaction with antipigeon liver malic enzyme serum

Summary Rabbit antibodies against pigeon liver malic enzyme (EC 1.1.1.40) were prepared. The antiserum gave single precipitation line with crude pigeon liver extract. Cross reaction was observed with partially purified malic enzyme or crude extract from chicken liver. Positive cross reaction was also observed with the concentrated cytosolic fraction of two human carcinoma cell lines which were demonstrated to contain high malic enzyme activity. All other proteins examined did not react with the antibodies. When purified pigeon liver malic enzyme was mixed with the antiserumin vitro, a time-dependent inactivation of the enzyme activity was observed. Protection of the enzyme activity against antiserum inactivation was afforded by NADP⁺ orL-malate. Metal Mn²⁺ gave little protection.     

Cytoplasmic malic enzyme from mouse kidneys

NADP⁺-dependent cytoplasmic malic enzyme was purified to homogeneity from mouse kidneys by a two-step procedure involving 8-(6-aminohexyl)-amino-2, 5-ADP-Sepharose affinity chromatography and DEAE-Sephadex ion exchange chromatography. The biochemical properties of the purified enzyme from DBA/2J mice were characterized. These include the determination of molecular weight and amino acid compositions, steady-state kinetics, thermal stability and inactivations by iodoacetate and urea. The native enzyme is a tetramer with a molecular weight of 270,000.K_m's for NADP⁺, l-malate, NADPH and pyruvate were determined to be 3.3 µm,, 50 µm, 10.5 gm respectively. Similar to the pigeon liver enzyme, the mouse enzyme exhibits an ordered kinetic mechanism proceeding with the binding of coenzyme first. The enzyme is only weakly inhibited by ATP and other cellular metabolites. A remarkable similarity in amino acid compositions was found between the mouse and rat liver malic enzymes.Abbreviations DTNB 5,5-dithio, bis-nitrobenzoic acid     

Mitochondrial malic enzyme in human leukocytes

Leukocyte samples from 316 unrelated blood donors were screened for malic enzyme (MEM). The frequency of the common allele in this investigation was MEM1 = 0.63. There is evidence for the existence of a rare fourth allele MEM4.     

Nad malic enzyme from citrus fruit

NAD malic enzyme activity was found in the 15 000 g precipitate of citrus leaf and fruit tissues. The enzyme activity in juice vesicle tissue did not change during the fruit growing period, but doubled following ripening. Partially purified enzyme was activated by CoA or FDP. Affinity for malate changed depending on enzyme concentration. The dependency was lost by addition of tricarboxylic acids but not dicarboxylic acids.     

Determinants of nucleotide-binding selectivity of malic enzyme

Malic enzymes have high cofactor selectivity. An isoform-specific distribution of residues 314, 346, 347 and 362 implies that they may play key roles in determining the cofactor specificity. Currently, Glu314, Ser346, Lys347 and Lys362 in human c-NADP-ME were changed to the corresponding residues of human m-NAD(P)-ME (Glu, Lys, Tyr and Gln, respectively) or Ascaris suum m-NAD-ME (Ala, Ile, Asp and His, respectively). Kinetic data demonstrated that the S346K/K347Y/K362Q c-NADP-ME was transformed into a debilitated NAD⁺-utilizing enzyme, as shown by a severe decrease in catalytic efficiency using NADP⁺ as the cofactor without a significant increase in catalysis using NAD⁺ as the cofactor. However, the S346K/K347Y/K362H enzyme displayed an enhanced value for k _cat,NAD, suggesting that His at residue 362 may be more beneficial than Gln for NAD⁺ binding. Furthermore, the S346I/K347D/K362H mutant had a very large K _m,NADP value compared to other mutants, suggesting that this mutant exclusively utilizes NAD⁺ as its cofactor. Since the S346K/K347Y/K362Q, S346K/K347Y/K362H and S346I/K347D/K362H c-NADP-ME mutants did not show significant reductions in their K _m,NAD values, the E314A mutation was then introduced into these triple mutants. Comparison of the kinetic parameters of each triple-quadruple mutant pair (for example, S346K/K347Y/K362Q versus E314A/S346K/K347Y/K362Q) revealed that all of the K _m values for NAD⁺ and NADP⁺ of the quadruple mutants were significantly decreased, while either k _cat,NAD or k _cat,NADP was substantially increased. By adding the E314A mutation to these triple mutant enzymes, the E314A/S346K/K347Y/K362Q, E314A/S346K/K347Y/K362H and E314A/S346I/K347D/K362H c-NADP-ME variants are no longer debilitated but become mainly NAD⁺-utilizing enzymes by a considerable increase in catalysis using NAD⁺ as the cofactor. These results suggest that abolishing the repulsive effect of Glu314 in these quadruple mutants increases the binding affinity of NAD⁺. Here, we demonstrate that a series of E314A-containing c-NADP-ME quadruple mutants have been changed to NAD⁺-utilizing enzymes by abrogating NADP⁺ binding and increasing NAD⁺ binding.     

Quaternary structure of pigeon liver malic enzyme

Pigeon liver malic enzyme (EC 1.1.1.40) has a double dimer quaternary structure. The NADP+ analogs, aminopyridine adenine dinucleotide phosphate and nicotinamide-1,N6-ethenoadenosine dinucleotide phosphate, bind to the enzyme anti-cooperatively. In the presence of non-cooperative competing ligand NADP+, the binding parameter Hill coefficients of these analogues changed very little. Binding of L-malate with enzyme-AADP+ complex first enhanced then reduced the nucleotide fluorescence. Two L-malate binding sites, with Kd values of 23-30 and 270-400 microM, respectively. for the tight and weak binding sites were postulated. A hybrid model between the sequential and pre-existing asymmetrical models was proposed for the pigeon liver malic enzyme.     

Polymorphism of erythrocyte malic enzyme in the goat

Three electrophoretic variants of erythrocyte malic enzyme (ME) in goats were reported. Inheritance data indicate that they are controlled by codominant alleles. The allele frequencies in four Mediterranean populations are given.     

Regulation of hepatic malic enzyme by perfluorodecanoic acid

Perfluorodecanoic acid (PFDA) administration to adult male rats increased both the activity of hepatic malic enzyme and liver weight in a dose-dependent manner. Hepatomegaly and augmented activity of malic enzyme in liver were apparent within one day following PFDA administration and reached a plateau by three days posttreatment. Malic enzyme quantity per liver in PFDA-treated rats was elevated within one day following dosing and increased continually throughout five days posttreatment. Administration of PFDA to rats in the fed state also led to an increase in the specific activity of hepatic malic enzyme that peaked at three days following dosing. When compared to the fed condition, rats fasted for 48 hours had a decrease in both relative liver weight and the quantity of supernatant protein per liver. The total activity (U/liver) and specific activity of malic enzyme in the liver were also reduced in the fasted state. During the 24 hours after treatment in rats fasted for 48 hours, the body weight as well as the absolute and relative liver weight of animals receiving vehicle declined continuously in the absence of feed. Following the administration of PFDA to fasted rats, body weight was maintained until eight hours posttreatment but then declined at a rate similar to that found with the vehicle-treated group. Absolute and relative liver weight in PFDA-treated rats were increased significantly at eight hours posttreatment when compared to those receiving vehicle, and this increment was maintained throughout the rest of the 24 hours following dosing. While the activity and enzyme content of hepatic malic enzyme decreased in the vehicle-treated group, administration of PFDA to rats fasted for 48 hours prevented their decline. The specific activity of hepatic malic enzyme in 48 hours fasted rats receiving PFDA was also elevated significantly at 16 hours posttreatment. Thus, the administration of PFDA to the adult male rat in both the fed and fasted nutritional states was found to regulate hepatic malic enzyme by not only increasing enzyme quantity but also by augmenting the specific activity, (ie, catalytic state) of the enzyme.     

Rapid purification and radioimmunoassay of cytosolic malic enzyme

A very rapid and highly effective procedure has been devised for the isolation of homogeneous malic enzyme from rat liver cytosol. A combination of precipitation with 10 to 20% polyethylene glycol, ion-exchange chromatography on DEAE-cellulose, and affinity chromatography on Procion Red HE-3B Agarose was used to prepare 3 to 4 mg of homogeneous malic enzyme from the livers of two rats in 18 h. In addition to introducing the advantages of simplicity, speed, and high yield (31%) the new method eliminates potentially denaturing steps (heat treatment, ethanol fractionation) and prolonged dialysis procedures used in other purification schemes. Malic enzyme purified by this new method was use to immunize rabbits. The resulting antibodies bound purified rat liver and mouse liver malic enzymes with very similar affinities and also avidly complexed cytosolic malic enzyme from two murine cell lines, 3T3-L1 preadipocytes and 3T3-C2 fibroblasts. When purified malic enzyme was incubated with lactoperoxidase, glucose oxidase and Na 125I 1.8 atoms of 125I were incorporated per molecule of enzyme with full retention of catalytic activity, subunit size, and immunoreactivity. The antiserum, the purified enzyme, and enzymatically iodinated 125I-malic enzyme were used to construct a sensitive, competitive binding radioimmunoassay for the measurement of malic enzyme mass in the range of 1 to 100 ng.     

Primary structure of the maize NADP-dependent malic enzyme

Chloroplast-localized NADP-dependent malic enzyme (EC 1.1.1.40) (NADP-ME) provides a key activity for the carbon 4 fixation pathway. In maize, nuclear encoded NADP-ME is synthesized in the cytoplasm as a precursor with a transit peptide that is removed upon transport into the chloroplast stroma. We present here the complete nucleotide sequence for a 2184-base pair full-length maize NADP-ME cDNA. The predicted precursor protein is 636 amino acids long with a Mr of 69,800. There is a strong codon bias found in the amino-terminal portion of NADP-ME that is present in genes for the other enzymes of the C-4 photosynthetic pathway. The NADP-ME transit peptide has general features common to other known chloroplast stroma transit peptides. Comparison of mature maize NADP-ME to the amino acid sequences of known malic enzymes shows two conserved dinucleotide-binding sites. There is a third highly conserved region of unknown function. On the basis of amino acid sequence similarity, the maize chloroplastic enzyme is more closely related to eukaryotic cytosolic isoforms of malic enzyme than to prokaryotic isoforms. We discuss the functional and evolutionary relationship between the chloroplastic and cytosolic forms of NADP-ME.