Two forms of NADP-dependent malic enzyme in expanding maize leaves

Expression, purification, and characterization of two NADP-malic enzymes of rice (Oryza sativa L.) in Escherichia coli

NADP-malic enzymes (NADP-ME) are isozymes in plants. To clarify the diversity and function of NADP-ME isozymes in rice, we produced two active GST-fused NADP-ME proteins, NADP-ME2 and NADP-ME3 in Escherichia coli, and the fusion proteins were purified by affinity chromatography using a glutathione-Sepharose 4B column. After enzymatic cleavage of the GST tag, final yields were 1.4 mg/g wet cell weight (wcw) for NADP-ME2 and 3.5 mg/g wcw for NADP-ME3, respectively, and the molecular weights of NADP-ME2 and NADP-ME3 were about 65 and 62 kDa, respectively. The optimum pH is 7.3 for NADP-ME2 and 7.7 for NADP-ME3. The Km values for malate of NADP-ME2 and NADP-ME3 were 2.6 and 3.1 mM, whereas the Km values for NADP were 79 and 93 microM, respectively. The Kcat values of NADP-ME2 and NADP-ME3 for malate were about 91.7 and 96.7 s-1, respectively, and the Kcat values for NADP about 88.3 and 98.3 s-1, respectively. These results suggest that the two rice isozymes of NADP-ME in vitro have similar kinetic parameter.

Light activation of NADP malic enzyme in leaves of maize: marginal increase in activity, but marked change in regulatory properties of enzyme

This article reports the characteristics of light activation of NADP-malic enzyme (NADP-ME, EC 1.1.1.40) in leaf discs of maize (Zea mays cv. VMH 404) for the first time. The leaf discs were illuminated in the presence of 2 mmol/L bicarbonate, as light activation increases in the presence of bicarbonate. Upon illumination, the Vmax of NADP-ME increased by about 30%. Although small, the increase was consistent and significant. The changes in regulatory properties of NADP-ME were quite pronounced. The extent of light activation was similar when substrate (malate) concentration was either 4 mmol/L (saturating) or 0.01 mmol/L (limiting). There was only a marginal change in the Km for malate, but there was marked change in the response of NADP-ME to activators or inhibitors. The Ki for pyruvate and oxalate increased by 100 and 67% respectively, while the Ka for the citrate and succinate increased by 36 and 32% respectively. These results suggest that the NADP-ME becomes less sensitive to feedback inhibition on illumination. The light-induced change seems to be due, at least partially, to the reduction of dithiols, as incubation of leaf extracts with DTE dampened light activation of NADP-ME. We conclude that the properties of NADP-ME do change on illumination. Although there was only a marginal increase in the activity of the enzyme on illumination of leaf discs, the changes in regulatory properties of NADP-ME were marked.

An isozyme of the NADP-malic enzyme of a CAM plant, Aloe arborescens, with variation on conservative amino acid residues

In Aloe arborescens, an obligate CAM plant, Western analysis detected three major isoforms of NADP-malic enzyme (NADP-ME), 72kDa with a pI of 6.0, 65kDa with a pI of 5.6 and 65kDa with a pI of 5.5. Among them, the 65kDa protein with a pI of 5.5 was leaf-specific, and the 65kDa protein with a pI of 5.6 was found only in roots, whereas the 72kDa protein was uniformly detected in both organs. Activity staining indicated enzyme activity of both 65kDa NADP-MEs but little activity of the 72kDa protein. A cDNA clone encoding a leaf-abundant NADP-ME, AME1, was isolated. Deduced amino acid sequence of AME1 showed a high degree of homology to known NADP-MEs, but it was also found that AME1 contained substitutions on five conservative amino acid residues, some of which have been predicted to be important for their enzyme activity. Transgenic rice carrying the aloe AME1 gene efficiently produced an additional 65kDa protein with a pI of 5.5 as an active NADP-ME. These results indicate that AME1 corresponds to the leaf-specific 65kDa NADP-ME, which may be involved in CAM photosynthesis. It was also shown that substitutions of these conservative amino acid residues identified in AME1 still allowed it to give enzyme activity.

Characterization of a bean (Phaseolus vulgaris L.) malic-enzyme gene

We have isolated a genomic clone encoding a plant NADP(+)-dependent malic enzyme (NADP-ME). This clone, isolated from bean (Phaseolus vulgaris L.), covers the entire gene (exons, introns) and 5'-flanking regions. DNA sequencing defines 20 exons spanning approximately 4.5 kb, which range over 48-235 bp in size. All 19 introns are fairly small (79-391). The first intron resides in the 5'-untranslated leader sequence. Introns 10, 11 and 16 are located at positions identical to a rat malic-enzyme gene. In the promoter region, a TATA box (TATATATA) is easily recognized 41 bp upstream of a single transcription-initiation site. Two potential cis-acting elements with homology to elements from plant genes, activated by UV light and fungal elicitors, were identified at positions -153 and -312, respectively. Southern-blot analysis suggests a single gene copy, but also other distantly related genes, in the bean genome. The deduced NADP-ME protein of 589 amino acids exhibits features consistent with a cytoplasmic location. We describe the organization of the NADP-ME protein into functional domains located on separate exons. The evolution of malic-enzyme genes coding for isoforms in different cellular compartments of plants and animals is discussed.

Characterization and expression of a NADP-malic enzyme cDNA induced by salt stress from the facultative crassulacean acid metabolism plant, Mesembryanthemum crystallinum

The facultative halophyte and crassulacean acid-metabolism plant, Mesembryanthemum crystallium shifts from C3 photosynthesis to crassulacean acid metabolism when exposed to high-salt or drought conditions. To study the molecular basis of this metabolic transition, the expression of NADP(+)-dependent malic enzyme (NADP-ME), which catalyzes the decarboxylation of malate to release pyruvate and CO2, has been investigated. The complete nucleotide sequence of a full-length cDNA clone was determined and found to contain a single open reading frame encoding a 585-amino-acid polypeptide of 64284 Da. The ice plant (M. crystallinum) NADP-ME shares amino acid identities in the range 72.5-79.0% when compared to other higher-plant enzymes and is more closely related to C3 rather than C4 forms of the enzyme. Genomic Southern-blot analysis of ice-plant DNA indicates that NADP-ME is encoded by a small gene family. Steady-state transcript levels increase 8-10-fold in response to salt stress in the leaves. Transcript levels in roots are extremely low and are unaffected by salt-stress treatment. Nuclear run-on experiments, using isolated nuclei from leaf tissue, confirm that the accumulation of NADP-ME transcripts is, in part, the result of increased transcription of this gene during salt stress.

NADP(+)-malic enzyme from sugarcane leaves: structural properties studied by thermal inactivation

The irreversible thermal inactivation of the sugarcane leaf NADP(+)-malic enzyme was studied at 50 degrees C and pH 7.0 and 8.0. Depending on the preincubation conditions, thermal inactivation followed mono- or biphasic first-order kinetics. A two-step behavior in the irreversible denaturation process was found when protein concentration was sufficiently low. The protein concentration necessary to obtain monlphasic thermal inactivation kinetics was lower at pH 8.0 than at pH 7.0. The results suggest that biphasic inactivation kinetics are the consequence of the existence of two different oligomeric forms of the enzyme (dimer and tetramer), with the dimer being more stable in regards to thermal inactivation. The effects of the substrate and essential cofactors on the thermostability and equilibrium between the dimeric and tetrameric enzyme forms were also studied. Depending on the pH, NADP+, L-malate, and Mg2+ all had a protective effect on the stability of the dimeric and tetrameric species during thermal treatment. However, these ligands showed different effects on the aggregation state of the enzyme. NADP+ and L-malate induced dissociation, especially at pH 8.0, whereas Mg2+ induced aggregation of the protein. By studying the thermal inactivation kinetics at 50 degrees C and different pH values it was observed that the equilibrium between dimers and tetramers was dramatically affected in the range of pH 7.0-8.0. These results suggest that an amino acid residue(s) in the protein with an apparent pKa value of 7.7 needs to be deprotonated to stabilize aggregation of the enzyme to the tetrameric form.

Primary structure of NADP-dependent malic enzyme in the dicotyledonous C4 plant Flaveria trinervia

The primary structure of NADP-dependent malic enzyme (NADP-ME) of the dicotyledonous C4 plant Flaveria trinervia was determined from sequence analysis of a cDNA clone containing the complete coding region. Comparison of the mature F. trinervia NADP-ME with the maize enzyme reveals extensive sequence similarity. In contrast, no significant similarity can be detected between the putative transit peptides of the two enzymes. This suggests that the corresponding parts of the genes arose independently from each other during evolution of mono- and dicotyledonous C4 plants.

NADP-dependent malate dehydrogenase (decarboxylating) from sugar cane leaves. Kinetic properties of different oligomeric structures

NADP-dependent malate dehydrogenase (decarboxylating) from sugar cane leaves was inhibited by increasing the ionic strength in the assay medium. The inhibitory effect was higher at pH 7.0 than 8.0, with median inhibitory concentrations (IC50) of 89 mM and 160 mM respectively, for inhibition by NaCl. Gel-filtration experiments indicated that the enzyme dissociated into dimers and monomers when exposed to high ionic strength (0.3 M NaCl). By using the enzyme-dilution approach in the absence and presence of 0.3 M NaCl, the kinetic properties of each oligomeric species of the protein was determined at pH 7.0 and 8.0. Tetrameric, dimeric and monomeric structures were shown to be active but with different V and Km values. The catalytic efficiency of the oligomers was tetramer greater than dimer greater than monomer, and each quaternary structure exhibited higher activity at pH 8.0 than 7.0. Dissociation constants for the equilibria between the different oligomeric forms of the enzyme were determined. It was established that Kd values were affected by pH and Mg2+ levels in the medium. Results suggest that the distinct catalytic properties of the different oligomeric forms of NADP-dependent malate dehydrogenase and changes in their equilibrium could be the molecular basis for an efficient physiological regulation of the decarboxylation step of C4 metabolism.

CO2 is the inorganic carbon substrate of NADP malic enzymes from Zea mays and from wheat germ

NADP malic enzyme (EC 1.1.1.40) was extracted and partially purified from the green leaves of Zea mays var. Felix and from wheat germ. The active inorganic carbon species for both enzymes was, in contrast to an earlier report, CO2 not HCO3-. The apparent Km, CO2 for the maize enzyme was 1.2 mM and the apparent Km, CO2 for the wheat germ preparation was 4.2 mM under conditions of substrate saturation, pH 7.3 and 17 degrees C. These observations support the views that CO2, rather than HCO3- as has been suggested, is produced in bundle-sheath chloroplasts and that the reaction mechanism catalysed by plant cytosolic and chloroplastic NADP malic enzymes is similar to that proposed for avian NADP malic enzymes.

Isolation and biochemical characterization of grape malic enzyme

Malic enzyme (ME=L-malate: NADP oxidoreductase; E.C. 1.1.1.40) was extracted by Triton X-100-induced resolubilization of enzyme proteins which denaturize spontaneously upon homogenization of grape berry material. The purification procedure included fractionating with (NH4)2SO4, preparative IEF, and Sephadex G-100 chromatography. ME was identified by TLC of the radioactive product after supplementing the assay mixture with [(14)C]malate. Cofactor dependence, pH-optimum and affinities for substrates and cosubstrates were determined. Enzymic pI was found to be 5.8, the Hill coefficients range from 1 to 3. In malate decarboxylating direction at pH 7.4, grape ME displayed positive cooperativity toward the substrate, the curve approaching normal Michaelis-Menten-kinetics at pH 7.0. Substituting Mn(2+) for Mg(2+) not only increased maximal turnover rates, but also enzymic affinity for malate. These features were considered indicative of the regulatory properties of the enzyme. Their relevance for grape malate metabolism and fruit ripening is discussed.

A possible plasma membrane particle containing malic enzyme activity

A definite membrane fraction from Cucurbita hypocotyls, maize coleoptiles, and other plant tissues contains a NADP-dependent malic enzyme activity, up to 10% of overall tissue activity, and probably other soluble proteins. This "malic enzyme particle" is identified as plasmalemma on the basis of sedimentation behavior, density distribution in sucrose gradients, in comparison with enzyme markers, and sluggish penetration by the sugar Metrizamide. Enzyme binding to the plasma membrane is stable and scarcely sensitive to salts and EDTA, although all activity is released to the supernatant in the presence of Triton-X-100 or under hypotonic conditions. The properties of bound enzyme are similar to those of free enzyme in cell extracts. It is proposed that osmotically sensitive plasma membrane vesicles, containing cytoplasm fragments, are formed during homogenization. Low malic enzyme activities are also associated with Cucurbita proplastids.