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

S-Nitrosylated proteins in pea (Pisum sativum L.) leaf peroxisomes: changes under abiotic stress

Peroxisomes, single-membrane-bounded organelles with essentially oxidative metabolism, are key in plant responses to abiotic and biotic stresses. Recently, the presence of nitric oxide (NO) described in peroxisomes opened the possibility of new cellular functions, as NO regulates diverse biological processes by directly modifying proteins. However, this mechanism has not yet been analysed in peroxisomes. This study assessed the presence of S-nitrosylation in pea-leaf peroxisomes, purified S-nitrosylated peroxisome proteins by immunoprecipitation, and identified the purified proteins by two different mass-spectrometry techniques (matrix-assisted laser desorption/ionization tandem time-of-flight and two-dimensional nano-liquid chromatography coupled to ion-trap tandem mass spectrometry). Six peroxisomal proteins were identified as putative targets of S-nitrosylation involved in photorespiration, β-oxidation, and reactive oxygen species detoxification. The activity of three of these proteins (catalase, glycolate oxidase, and malate dehydrogenase) is inhibited by NO donors. NO metabolism/S-nitrosylation and peroxisomes were analysed under two different types of abiotic stress, i.e. cadmium and 2,4-dichlorophenoxy acetic acid (2,4-D). Both types of stress reduced NO production in pea plants, and an increase in S-nitrosylation was observed in pea extracts under 2,4-D treatment while no total changes were observed in peroxisomes. However, the S-nitrosylation levels of catalase and glycolate oxidase changed under cadmium and 2,4-D treatments, suggesting that this post-translational modification could be involved in the regulation of H(2)O(2) level under abiotic stress.

L-Malate dehydrogenase from Pseudomonas stutzeri: purification and characterization

L-Malate dehydrogenase (MDH) from Pseudomonas stutzeri was purified to homogeneity by a two-step procedure comprising anion-exchange chromatography and affinity chromatography on immobilized anthraquinone alpha-ketocarboxyl biomimetic dye. The enzyme has molecular mass of 66,500 Da and consists of two identical subunits of molecular mass of approximately 34,000 Da. Initial velocity, product inhibition, and binding studies were consistent with an ordered Bi-Bi mechanism for the enzyme action and the formation of a ternary complex. The enzyme is susceptible to activation and inhibition by its substrates. Thermodynamic analysis and kinetic inhibition studies were performed for determining basic equilibrium and kinetic constants. Malate dehydrogenase was covalently inactivated by a dichlorotriazine dye, Vilmafix Blue A-R (VBAR). The inactivation process follows first-order kinetics, and the results from kinetic analysis suggested the formation of a noncovalent enzyme-dye complex prior to the covalent reaction, with Kd 84.6 microM and a maximum rate constant 0.16 min(-1). The enzyme inactivation process was partially inhibited by substrates and inhibitors. Quantitatively inactivated MDH contained approximately 1 mole of dye per mole of enzyme subunit. The denatured enzyme contains 10 sulfhydryl groups per subunit, as shown after reaction with 5,5'-dithio-bis-(2-nitrobenzoic acid), of which 5 can be titrated also in the native enzyme, exhibiting time-dependent reactivity. One sulfhydryl group is located in the coenzyme binding site. This study shows that the physical and catalytic properties of P. stutzeri MDH strongly resemble those of the mitochondrial eukariotic enzyme. This finding strengthens the existing view that, in the evolution process, the mitochondrial MDH might have appeared before the cytoplasmic.

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.

Sequence homologies between glyoxysomal and mitochondrial malate dehydrogenase

The comparison of mitochondrial and glyoxysomal malate dehydrogenase (EC 1.1.1.37) from cotyledons of germinating watermelon (Citrullus vulgaris Schrad., cv. Kleckey's Sweet No. 6) by means of serological methods and peptide patterns revealed a high degree of homology. The N-terminal sequence analysis yielded a distinct presequence of eight or nine amino-acid residues, respectively, which is followed by an almost identical stretch of at least 20 amino-acid residues. A very similar domain has been recognized for mitochondrial malate dehydrogenase from porcine heart and yeast, and for Escherichia coli malate dehydrogenase.

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 : I. Molecular properties of the purified isoenzymes

Molecular properties of the glyoxysomal and mitochondrial isoenzyme of malate dehydrogenase (EC 1.1.1.37; L-malate: NAD(+) oxidoreductase) from watermelon cotyledons (Citrullus vulgaris Schrad.) were investigated, using completely purified enzyme preparations. The apparent molecular weights of the glyoxysomal and mitochondrial isoenzymes were found to be 67,000 and 74,000 respectively. Aggregation at high enzyme concentrations was observed with the glyoxysomal but not with the mitochondrial isoenzyme. Using sodium dodecyl sulfate electrophoresis each isoenzyme was found to be composed of two polypeptide chains of identical size (33,500 and 37,000, respectively). The isoenzymes differed in their isoelectric points (gMDH: 8,92, mMDH: 5.39), rate of heat inactivation (gMDH: τ1/2 at 40°C=3.0 min; mMDH: stable at 40°C; τ1/2 at 60°C=4.5 min), adsorption to dextran gels at low ionic strenght, stability against alkaline conditions and their pH optima for oxaloacetate reduction (gMDH: pH 6.6, mMDH: pH 7.5). Very similar pH optima, however, were observed for L-malate oxidation (pH 9.3-9.5). The results indicate that the glyoxysomal and mitochondrial MDH of watermelon cotyledons are distinct proteins of different structural composition.