Malate dehydrogenase (decarboxylating) (NADP) isoenzymes of Opuntia stem tissue. Mitochondrial, chloroplast, and soluble forms

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-linked malic enzyme. Purification from maize leaves, Mr and subunit composition

1. The isolation of NADP-linked malic enzyme (EC 1.1.1.40) from maize leaves is described, together with studies of its Mr and subunit composition. 2. The enzyme was purified to apparent homogeneity by affinity chromatography on N6-aminohexyl-2',5'-bisphosphoadenosine-agarose, gel filtration with Sephadex G-100 and ion-exchange chromatography on DEAE-Sephadex A-50. A purification of 140-fold with a 30% yield was obtained. 3. A detailed study of the Mr by several methods revealed the existence of different Mr forms in solution. 4. In the presence of dithiothreitol the enzyme appears to be present in triethanolamine buffer, pH 7.5, as a tetramer with a subunit Mr of 60,000 and an S20,w of 10.75 S. 5. In phosphate buffer, pH 7.0, it seems to be a dimer of Mr 120,000 with an S20,w of 7.95 S. 6. In the absence of dithiothreitol, lower-Mr forms were detected by sedimentation-equilibrium and sedimentation-velocity studies in triethanolamine buffer. 7. Results from gel filtration gave Mr values of about 340,000 in both buffers.

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.

Cell organelles from crassulacean acid metabolism (CAM) plants : II. Compartmentation of enzymes of the crassulacean acid metabolism

The intracellular distribution of enzymes involved in the Crassulacean acid metabolism (CAM) has been studied in Bryophyllum calycinum Salisb. and Crassula lycopodioides Lam. After separation of cell organelles by isopycnic centrifugation, enzymes of the Crassulacean acid metabolism were found in the following cell fractions: Phosphoenolpyruvate carboxylase in the chloroplasts; NAD-dependent malate dehydrogenase in the mitochondria and in the supernatant; NADP-dependent malate dehydrogenase and phosphoenolpyruvate carboxykinase in the chloroplasts; NADP-dependent malic enzyme in the supernatant and to a minor extent in the chloroplasts; NAD-dependent malic enzyme in the supernatant and to some degree in the mitochondria; and pyruvate; orthophosphate dikinase in the chloroplasts. The activity of the NAD-dependent malate dehydrogenase was due to three isoenzymes separated by (NH4)2SO4 gradient solubilization. These isoenzymes represented 17, 78, and 5% of the activity recovered, respectively, in the order of elution. The isoenzyme eluting first was associated with the mitochondria and the second isoenzyme was of cytosolic origin, while the intracellular location of the third isoenzyme was probably the peroxisome. Based on these findings, the metabolic path of Crassulacean acid metabolism within cells of CAM plants is discussed.

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

Etiolated maize leaves (Zea mays L.) contain a major isozyme of NADP-dependent malic enzyme (L-malate dehydrogenase, decarboxylating, EC 1.1.1.40) having an isoelectric point of 5.28±0.03, a Km (L-malate) 0.3-0.6 mM at pH 7.45; a broad pH optimum around pH 6.9 under the conditions of assay; a molecular weight of 280,000 (sometimes accompanied by a minor component of 150,000); and an NAD-dependent activity about 1/50 the NADP-dependent activity. This isozyme, resembling the NADP-malic enzyme of vertebrates, is labeled type 1. The dominant isozyme of young green leaves (type 2) has, however, a pI 4.90±0.03, a Km (L-malate) 0.10-0.15 mM, a pH optimum of 8, and a molecular weight of 280,000. It is also more stable and exhibits an appreciable NAD-dependent activity (1/5-1/7 the NADP activity). Both isozymes show linear kinetics, dependence on Mn or Mg ions, similar Km (NADP(+)), and the typical increase of Km for L-malate with increasing pH values. Type 1 isozyme of maize is assumed to be cytosolic. Type 2 corresponds in each property to the chloroplast enzyme of bundle-sheath cells. It is present at a low level in etiolated leaves and develops to a high specific activity (up to 100 nmol min(-1) mg protein(-1) by 150 h illumination) during photosynthetic differentiation, replacing the type 1 form.