[Compartmentation and properties of the MDH-isoenzymes from watermelon cotyledons (Citrullus vulgaris Schrad.)]

Characterization of a cytosolic malate dehydrogenase cDNA which encodes an isozyme toward oxaloacetate reduction in wheat

Malate dehydrogenase (MDH), which is ubiquitous in nature, catalyzes the interconversion of oxaloacetate and malate. Higher plants contain multiple forms of MDH that differ in co-enzyme specificity, subcellular localization and physiological function. Cytosolic NAD-dependent MDH (cyMDH) is one class of MDH that has not been extensively characterized in plants. Here we report the cloning of a cDNA from wheat by RT-PCR and cDNA library screening, which is designated as TaMDH. Sequence analysis indicated that TaMDH exhibits a highly similarity to other plant cyMDHs. Immunological analysis confirmed that TaMDH encoded a cytosolic NAD-dependent MDH. The secondary and three-dimensional structures of TaMDH were analyzed by molecular modeling. DNA gel-blot analyses demonstrated that TaMDH gene exists as two copies in the wheat genome. RNA and protein gel-blot hybridization indicated that both TaMDH mRNA and protein were constitutively expressed in vegetative tissues of wheat, with slightly lower levels in roots than in leaves and stems. In silico analysis indicated that TaMDH was also expressed in various reproductive tissues and tissues under many different stress conditions. Kinetic analysis of bacterially expressed and purified protein confirmed that TaMDH catalyzed a reaction driven towards malate synthesis, which is consistent with other cyMDHs. Evolutionary analysis showed that this class of genes evolved from a very ancestral gene. The cyMDH represents an ancestral form of MDH, which is highly conserved in plants, animals and bacteria. This implies that cyMDHs are housekeeping genes and may have very essential functions in plant metabolism.

The plant PTS1 receptor: similarities and differences to its human and yeast counterparts

Two targeting signals, PTS1 and PTS2, mediate import of proteins into the peroxisomal matrix. We have cloned and sequenced the watermelon (Citrullus vulgaris) cDNA homologue to the PTS1 receptor gene (PEX5). Its gene product, CvPex5p, belongs to the family of tetratricopeptide repeat (TPR) containing proteins like the human and yeast counterparts, and exhibits 11 repeats of the sequence W-X2-(E/S)-(Y/F/Q) in its N-terminal half. According to fractionation studies the plant Pex5p is located mainly in the cytosolic fraction and therefore could function as a cycling receptor between the cytosol and glyoxysomes, as has been proposed for the Pex5p of human and some yeast peroxisomes. Transformation of the Hansenula polymorpha peroxisome deficient pex5 mutant with watermelon PEX5 resulted in restoration of peroxisome formation and the synthesis of additional membranes surrounding the peroxisomes. These structures are labeled in immunogold experiments using antibodies against the Hansenula polymorpha integral membrane protein Pex3p, confirming their peroxisomal nature. The plant Pex5p was localized by immunogold labelling mainly in the cytosol of the yeast, but also inside the newly formed peroxisomes. However, import of the PTS1 protein alcohol oxidase is only partially restored by CvPex5p.

Alfalfa malate dehydrogenase (MDH): molecular cloning and characterization of five different forms reveals a unique nodule-enhanced MDH

Malate dehydrogenase (MDH) catalyzes the readily reversible reaction of oxaloacetate reversible malate using either NADH or NADPH as a reductant. In plants, the enzyme is important in providing malate for C4 metabolism, pH balance, stomatal and pulvinal movement, respiration, beta-oxidation of fatty acids, and legume root nodule functioning. Due to its diverse roles the enzyme occurs as numerous isozymes in various organelles. While antibodies have been produced and cDNAs characterized for plant mitochondrial, glyoxysomal, and chloroplast forms of MDH, little is known of other forms. Here we report the cloning and characterization of cDNAs encoding five different forms of alfalfa MDH, including a plant cytosolic MDH (cMDH) and a unique novel nodule-enhanced MDH (neMDH). Phylogenetic analyses show that neMDH is related to mitochondrial and glyoxysomal MDHs, but diverge from these forms early in land plant evolution. Four of the five forms could effectively complement an E. coli Mdh- mutant. RNA and protein blots show that neMDH is most highly expressed in effective root nodules. Immunoprecipitation experiments show that antibodies produced to cMDH and neMDH are immunologically distinct and that the neMDH form comprises the major form of total MDH activity and protein in root nodules. Kinetic analysis showed that neMDH has a turnover rate and specificity constant that can account for the extraordinarily high synthesis of malate in nodules.

Malate dehydrogenase isoenzymes: cellular locations and role in the flow of metabolites between the cytoplasm and cell organelles

Malate dehydrogenases belong to the most active enzymes in glyoxysomes, mitochondria, peroxisomes, chloroplasts and the cytosol. In this review, the properties and the role of the isoenzymes in different compartments of the cell are compared, with emphasis on molecular biological aspects. Structure and function of malate dehydrogenase isoenzymes from plants, mammalian cells and ascomycetes (yeast, Neurospora) are considered. Significant information on evolutionary aspects and characterisation of functional domains of the enzymes emanates from bacterial malate and lactate dehydrogenases modified by protein engineering. The review endeavours to give up-to-date information on the biogenesis and intracellular targeting of malate dehydrogenase isoenzymes as well as enzymes cooperating with them in the flow of metabolites of a given pathway and organelle.

Import of glyoxysomal malate dehydrogenase precursor into glyoxysomes: A heterologous in-vitro system

A heterologous in-vitro system is described for the import of the precursor to glyoxysomal malate dehydrogenase from watermelon (Citrullus vulgaris Schrad., cv. Kleckey's Sweet No. 6) cotyledons into glyoxysomes from castor-bean (Ricinus communis L.) endosperm. The 41-kDa precursor is posttranslationally sequestered and correctly processed to the mature 33-kDa subunit by a crude glyoxysomal fraction or by glyoxysomes purified on a sucrose gradient. The import and the cleavage of the extrasequence is not inhibited by metal chelators such as 1,10-phenanthroline and ethylenediaminetetraacetic acid. Uncouplers (carbonylcyanide m-chlorophenylhydrazone), ionophores (valinomycin), or inhibitors of oxidative phosphorylation (oligomycin) and ATP-ADP translocation (carboxyatractyloside) do not interfere, thus indicating the independence of the process of import by the organelle from the energization of the glyoxysomal membrane.

Import of in-vitro-synthesized glyoxysomal malate dehydrogenase into isolated watermelon glyoxysomes

Glyoxysomal malate dehydrogenase (gMDH; EC 1.1.1.37) is synthesized by a reticulocyte system in the presence of watermelon mRNA (Citrullus vulgaris Schrad., var. Kleckey's Sweet No 6) as a cytosolic, higher-molecular-weight precursor (41 kdalton). We now show that this precursor is posttranslationally sequestered by a crude glyoxysomal fraction or by glyoxysomes purified on a Percoll(R) gradient to a proteolytically protected form (60 min proteinase-K treatment at 4° C) with the size of the gMDH subunit (33 kdalton). In the presence of buffer instead of organelles a complete degradation of the precursor is obtained. The in-vitro organelle import, however, depends upon the presence of proteases such as proteinase K or trypsin. After short proteolytic treatments (e.g. 10 min proteinase K at 4° C), the correct processing of the MDH precursor is obtained even in the absence of organelles. This product, however, is not sequestered in vitro to a protease-resistant form by glyoxysomes. The possibility is discussed that under in-vivo conditions pre-gMDH is processed on the outside of the glyoxysomal membrane and transferred immediately after processing into the organelle presumably as a gMDH monomer followed by refolding and dimerization.

Cell-free synthesis of watermelon glyoxysomal malate dehydrogenase: a comparison with the mitochondrial isoenzyme

Cotyledons of dark-grown watermelon seedlings contain during the first period of germination, 4 and later 5 MDH isoenzymes. Whereas isoenzymes I, II, and IV belong to the cytosol, isoenzyme III is confined to the mitochondria (mMDH) and isoenzyme V to the glyoxysomes (gMDH). The organelle-bound MDHs were purified to homogeneity and characterized. They are remarkably similar with respect to their kinetic properties, but differ widely in other characteristics, e.g. MW, IEP, thermostability, and serological properties. Both isoenzymes are synthesized de novo during germination by cytoplasmic ribosomes. In a reticulocyte cell-free system programmed by watermelon poly (A+) RNA, gMDH and mMDH are synthesized as larger precursors as compared to the authentic polypeptide chains, with an extra sequence of ca. 8.0 kilodaltons in the case of gMDH and ca. 3.3 kilodaltons with mMDH. The processing of the gMDH precursor by an enriched organelle fraction in not complete. If the post-translational incorporation of the in vitro product into the organelles is followed by proteinase treatment that completely hydrolyzes noncompartmentalized gMDH, the authentic product is obtained. A model for the transport mechanism is given.

Glyoxysomal and mitochondrial malate dehydrogenase of watermelon (Citrullus vulgaris) cotyledons : II. Kinetic properties of the purified isoenzymes

Kinetic parameters of the glyoxysomal and mitochondrial malate dehydrogenase (EC 1.1.1.37) of watermelon (Citrullus vulgaris Schrad.) cotyledons were compared. The data were obtained by initial rate experiments at pH 8.5 in both directions of the reaction using homogeneous enzyme preparations. Substrate inhibition at physiologically significant concentrations was observed with reduced nicotinamide adenine dinucleotide (NADH) (50% inhibition at 0.65 mmol·l(-1) NADH), but not with oxaloacetate, L-malate or oxidized nicotinamide adenine dinucleotide. The inhibition of both isoenzymes by 5'adenosine monophosphate was studied. Inhibition was found to be competitive with respect to NADH and non-competitive with respect to oxaloacetate. The apparent inhibitor constants at 200 μmol·l(-1) of the fixed substrates were 3.2 and 1.6 mmol·1(-1) for NADH, and 3.2 and 5.2 mmol·l(-1) for oxaloacetate with the glyoxysomal and mitochondrial isoenzymes, respectively. The energy of activation was determined for oxaloacetate reduction by glyoxysomal (E a =3.14×10(4)J×mol(-1)) and mitochondrial (E a =4.10×10(4) J x mol(-1)) MDH from Arrhenius plots, which exhibited constant slopes throughout the range of thermal stability.Despite considerable structural differences, the results indicate very similar kinetic behaviour of the glyoxysomal and mitochondrial isoenzymes. The physiological significance of the data are discussed in relation to the gluconeogenic processes occuring in cotyledons during germination.

Glyoxysomal malate dehydrogenase of watermelon cotyledons: De novo synthesis on cytoplasmic ribosomes

The development of glyoxysomal malate dehydrogenase (gMDH, EC 1.1.1.37) during early germination of watermelon seedlings (Citrullus vulgaris Schrad.) was determined in the cotyledons by means of radial immunodiffusion. The active isoenzyme was found to be absent in dry seeds. By density labelling with deuterium oxide and incorporation of [(14)C] amino acids it was shown that the marked increase of gMDH activity in the cotyledons during the first 4 days of germination was due to de novo synthesis of the isoenzyme. The effects of protein synthesis inhibitors (cycloheximide and chloramphenicol) on the synthesis of gMDH indicated that the glyoxysomal isoenzyme was synthesized on cytoplasmic ribosomes. Possible mechanisms by which the glyoxysomal malate dehydrogenase isoenzyme reaches its final location in the cell are discussed.

Separation of malate dehydrogenase isoenzymes by affinity chromatography on 5'-AMP-Sepharose

The mitochondrial and glyoxysomal isoenzymes of malate dehydrogenase (EC 1.1.1.27) from watermelon cotyledons and the mitochondrial isoenzyme from pig heart adsorbed reversibly to 5'-AMP-Sepharose. They were specifically eluted with low concentrations of NADH rather than by NAD. In contrast, the cytoplasmic isoenzymes showed no affinity to the matrix-bound ligand. These binding properties are discussed in terms of structural and regulatory differences of the particulate and soluble malate dehydrogenase isoenzymes. Affinity chromatography on 5'-AMP-Sepharose significantly improved the purification of the particulate malate dehydrogenase isoenzymes with respect to homogeneity, yield, and the number of purification steps. In the case of the glyoxysomal isoenzyme it was the essential procedure to obtain complete purification of the enzyme.

Mitochondrial malate dehydrogenase of watermelon cotyledons: Time course and mode of enzyme activity changes during germination

Specific antibodies were prepared against the purified mitochondrial malate dehydrogenase (EC 1.1.1.37) from cotyledons of watermelon seedlings (Citrullus vulgaris Schrad.). The isoenzyme was assayed by means of quantitative radial immunodiffusion. Cotyledons of ungerminated seeds were found to contain mitochondrial MDH. During the first 4 days of germination the enzyme activity increased threefold finally contributing 16% to the total MDH activity extracted from cotyledon tissue. Isopycnic CsCl density centrifugation was used to investigate the mode of activity increase. After a four-day period of labelling with deuterium oxide and purification of the mitochondrial isoenzyme, a density shift of 0.021kgx1(-1), accompanied by considerable band broadening of the enzyme profile was observed. These findings are evidence for the de novo synthesis of mitochondrial MDH and its relatively slow turnover in germinating seeds.

[Antibodies against glyoxysomal membranes]

Antibodies against glyoxysomes were prepared by injection of purified glyoxysomal membranes from watermelon cotyledons (day 3) into rabbits and isolation of the gammaglobulin fraction from the antiserum by ammonium sulfate fractionation. Double gel diffusion as well as two-dimensional immunoelectrophoresis (tandem technique, Fig. 4) tests using the purified antiserum and glyoxysomes or crude cotyledon extracts gave a single converging immunoprecipitation band. The antigen concentrations at different developmental stages were determined by one-dimensional immunoelectrophoresis (Fig. 5). Cotyledons of dry seeds contain low but measurable amounts of antigen. After day 2 there is a fast increase and after day 3 a subsequent slow decline of antigen. Since mitochondria and endoplasmic reticulum also formed single identical bands with antiglyoxysoma antibodies (Fig. 6) the results of Fig. 5 reflect the synthesis and destruction of membrane material belonging to several organelles.The reported experiments support the hypothesis of Kagawa, Lord and Beevers (1973) that components of glyoxysomal and mitochondrial membranes originate in a light membrane fraction derived from the endoplasmic reticulum. The consequences of this hypothesis in terms of organelle morphogenesis are discussed.

[Distribution of microbody enzymes from Chlamydomonas on sucrose gradients]

The crude homogenate of cells from Chlamydomonas reinhardii was placed on a linear gradient from 30% to 60% sucrose and centrifuged for 4 hours at 60 000 g. After fractionation of the gradient the distribution of enzymes was determined. Hydroxypyruvate reductase and glycolate dehydrogenase, two markers for peroxisomes, appeared with one sharp peak at density 1.185 g/cm(3) within the gradient. Twenty-five percent of the hydroxypyruvate reductase was particulate and 75% was found as solubles in the top fractions. The peaks of both enzymes matched exactly the peak of the cytochrome oxidase which is a marker for mitochondria. The profile for malate dehydrogenase was the same as that for hydroxypyruvate reductase. Catalase, however, showed two peaks. One coincided with the peak of cytochrome oxidase and the other appeared at density 1.22 g/cm(3). More than 50% of the catalase moved into the gradient during centrifugation.