Glutathione reductase fromSaccharomyces cerevisiae undergoes redox interconversionin situ andin vivo

Redox interconversion of glutathione reductase was studiedin situ withS. cerevisiae. The enzyme was more sensitive to redox inactivation in 24 hour-starved cells than in freshly-grown ones. While 5 μM NADPH or 100 μM NADH caused 50% inactivation in normal cells in 30 min, 0.75 μM NADPH or 50 μM NADH promoted a similar effect in starved cells. GSSG reactivated the enzyme previously inactivated by NADPH, ascertaining that the enzyme was subjected to redox interconversion. Low EDTA concentrations fully protected the enzyme from NADPH inactivation, thus confirming the participation of metals in such a process. Extensive inactivation was obtained in permeabilized cells incubated with glucose-6-phosphate or 6-phosphogluconate, in agreement with the very high specific activities of the corresponding dehydrogenases. Some inactivation was also observed with malate, L-lactate, gluconate or isocitrate in the presence of low NADP⁺ concentrations. The inactivation of yeast glutathione reductase has also been studiedin vivo. The activity decreased to 75% after 2 hours of growth with glucono-δ-lactone as carbon source, while NADPH rose to 144% and NADP⁺ fell to 86% of their initial values. Greater changes were observed in the presence of 1.5 μM rotenone: enzymatic activity descended to 23% of the control value, while the NADH/NAD⁺ and NADPH/NADP⁺ ratios rose to 171% and 262% of their initial values, respectively. Such results indicate that the lowered redox potential of the pyridine nucleotide pool existing when glucono-δ-lactone is oxidized promotesin vivo inactivation of glutathione reductase.     

Correlation between manganese-deficiency,loss of respiratory chain complex I activity and citric acid production in Aspergillus niger

The correlation between manganese deficiency, loss of mitochondrial respiratory chain NADH: ubiquinone oxidoreductase (complex I) activity and citric acid overproduction in the Aspergillus niger strain B 60 was analysed. With increasing manganese-supplementation of the production medium the loss of complex I activity and the production of citric acid was reduced. Addition of manganese during growth stopped further loss of complex I activity and further increase of citric acid production. A possible causality between complex I deficiency and citric acid overproduction is discussed.     

Isolation and analysis of a cDNA clone that encodes an alfalfa (Medicago sativa) aspartate aminotransferase

Summary Nitrate reductase (NR) assays revealed a bi-specific NAD(P)H-NR (EC 1.6.6.2.) to be the only nitrate-reducing enzyme in leaves of hydroponically grown birches. To obtain the primary structure of the NAD(P)H-NR, leaf poly(A)⁺ mRNA was used to construct a cDNA library in the lambda gt11 phage. Recombinant clones were screened with heterologous gene probes encoding NADH-NR from tobacco and squash. A 3.0 kb cDNA was isolated which hybridized to a 3.2 kb mRNA whose level was significantly higher in plants grown on nitrate than in those grown on ammonia. The nucleotide sequence of the cDNA comprises a reading frame encoding a protein of 898 amino acids which reveals 67%–77% identity with NADH-nitrate reductase sequences from higher plants. To identify conserved and variable regions of the multicentre electron-transfer protein a graphical evaluation of identities found in NR sequence alignments was carried out. Thirteen well-conserved sections exceeding a size of 10 amino acids were found in higher plant nitrate reductases. Sequence comparisons with related redox proteins indicate that about half of the conserved NR regions are involved in cofactor binding. The most striking difference in the birch NAD(P)H-NR sequence in comparison to NADH-NR sequences was found at the putative pyridine nucleotide binding site. Southern analysis indicates that the bi-specific NR is encoded by a single copy gene in birch. These sequence data appeared in the EMBL/GenBank/DDBJ nucleotide sequence data bases under the accession number X54097     

Nitrate nutrition ofDeschampsia flexuosa (L.) Trin. in relation to nitrogen deposition in Sweden

Summary Current and maximally induced nitrate reductase activity (NRA), total-N, nitrate, K, P, Ca, Mg, Mo and sucrose in leaves ofDeschampsia flexuosa was measured three times during the vegetation period in forests along a deposition gradient (150 km) in south Sweden, in north Sweden where the nitrogen deposition is considerably lower, and at heavily N-fertilized plots. In addition, the interaction between nitrogen nutrition and light was studied along transects from clearings into forest in both south and north Sweden. Plants from sites with high nitrogen deposition had elevated current NRA compared to plants from less polluted sites, indicating high levels of available soil nitrate at the former. Current NRA and total N concentration in grass from sites with high deposition resembled those found at heavily N-fertilized plots. Under such circumstances, the ratio current NRA: maximally induced NRA as well as the concentration of nitrate was high, while the concentration of sucrose was low. This suggests that the grass at these sites was already utilizing a large portion of its capacity to assimilate nitrate. Light was found to play an important role in the assimilation of nitrate; leaf concentration of sucrose was found to be negatively correlated with both nitrate and total N. Consequently, grass growing under dense canopies in south Sweden is not able to dilute N by increasing growth. The diminished capacity of the grass to assimilate nitrate will increase leaching losses of N from forests approaching N saturation.     

Stress studies in lentil (Lens esculenta Moench)

The response of soil exchangeable sodium percentage levels to nitrate reductase activity, nitrite reductase activity, free proline, DNA, RNA, chlorophyll a and b contents and yield components in lentil (Lens esculenta Moench)cv. PL 406 was studied in a replicated pot experiment. All the biochemical observations were recorded at four growth stages i.e. 30, 60, 90 and 120 days after sowing (DAS). Germination occurred up to exhangeable sodium percentage of 30, but plants survived only up to 25. With increasing exchangeable sodium percentage, there was a continuous decrease in chlorophyll a and b content, nitrate and nitrite reductase enzyme activities and DNA and RNA content. Increasing level of sodicity enhanced the free proline content up to 60 DAS, after which values fell.Number of pods per plant, 1000 grain weight and grain yield were significantly reduced with increasing level of sodicity, but the number of grains per pod was not affected.     

Control of nitrogenase inAzospirillum sp.

Many N₂-fixing organisms can turn off nitrogenase activity in the presence of NH₄ ⁺ and turn it on again when the NH₄ ⁺ is exhausted. One of the most interesting systems for accomplishing this is by covalent modification of one subunit of dinitrogenase reductase by dinitrogenase reductase ADP-ribosyltransferase (DRAT). The system can be reactivated when NH₄ ⁺ is exhausted, by dinitrogenase reductase activating glycohydrolase (DRAG) which removes the inactivating group. It is fascinating that some species of the genusAzospirillum possess the DRAT and DRAG systems (A. lipoferum andA. brasilense), whereasA. amazonense in the same genus lacks DRAT and DRAG.A. amazonense responds to NH₄ ⁺ but does not exhibit modification of dinitrogenase reductase characteristic of the action of DRAT. However, it has been possible to clone DRAT and DRAG and to introduce them intoA. amazonense, whereupon they become functional in this organism. The DRAT and DRAG system does not appear to function inAcetobacter diazotrophicus, an organism isolated from sugar cane, that fixes N₂ at a pH as low as 3.0.A. diazotrophicus does show a rather sluggish response to NH₄ ⁺. A level of about 10 M NH₄ ⁺ is required to switch off the system. The response to NH₄ ⁺ is influenced by the dissolved oxygen concentration (DOC) as has been reported forAzospirillum sp. A DOC in equilibrium with 0.1 to 0.2 kPa O₂ seems optimal for the response inA. diazotrophicus.     

Regulation of chloroplast metabolism in leaves: Evidence that NADP-dependent glyceraldehydephosphate dehydrogenase,but not ferredoxin-NADP reductase,controls electron flow to phosphoglycerate in the dark-light transition

P₇₀₀ is rapidly, but only transiently photooxidized upon illuminating dark-adapted leaves. Initial oxidation is followed by a reductive phase even under far-red illumination which excites predominantly photosystem (PS) I. In this phase, oxidized P₇₀₀ is reduced by electrons coming from PSII. Charge separation in the reaction center of PSI is prevented by the unavailability of electron acceptors on the reducing side of PSI. It is subsequently made possible by the opening of an electron gate which is situated between PSI and the electron acceptor phosphoglycerate. Electron acceptors immediately available for reduction while the gate is closed corresponded to 10 nmol · (mg chlorophyll)^–1 electrons in geranium leaves, 16 nmol · (mg chlorophyll)^–1 in sunflower and 22 nmol · (mg chlorophyll)^–1 in oleander. Reduction of NADP during the initial phase of P₇₀₀ oxidation showed that the electron gate was not represented by ferredoxin-NADP reductase. Availability of ATP indicated that electron flow was not hindered by deactivation of the thylakoid ATP synthetase. It is concluded that NADP-dependent glyceraldehydephosphate dehydrogenase is completely deactivated in the dark and activated in the light. The rate of activation depends on the length of the preceding dark period. As chloroplasts contain both NAD- and NADP-dependent glyceraldehydephosphate dehydrogenases, deactivation of the NADP-dependent enzyme disconnects chloroplast NAD and NADP systems and prevents phosphoglycerate reduction in the dark at the expense of NADPH and ATP which are generated by glucose-6-phosphate oxidation and glycolytic starch breakdown, respectively.Abbreviations Chl chlorophyll - P₇₀₀ electron donor pigment in the reaction center of photosystem I Cooperation of the Institute of Botany of the University of Würzburg with the Institute of Astrophysics and Atmospheric Physics of the Estonian Academy of Sciences in Tartu was supported by the Deutsche Forschungsgemeinschaft and the Estonian Academy of Sciences. This work was performed within the Sonderforschungsbereich 251 of the University of Würzburg.     

Stimulation of insulin release from pancreatic islets by quinones

Coenzyme Q (CoQ₀) and other quinones were shown to be potent insulin secretagogues in the isolated pancreatic islet. The order of potency was CoQ₀benzoquinonehydroquinonemenadione. CoQ₆ and CoQ₁₀ (ubiquinone), duroquinone and durohydroquinone did not stimulate insulin release. CoQ₀'s insulinotropism was enhanced in calcium-free medium and CoQ₀ appeared to stimulate only the second phase of insulin release. CoQ₀ inhibited inositol mono-, bis- and trisphosphate formation. Inhibitors of mitochondrial respiration (rotenone, antimycin A, FCCP and cyanide) and the calcium channel blocker verapamil, did not inhibit CoQ₀-induced insulin release. Dicumarol, an inhibitor of quinone reductase, did not inhibit CoQ₀-induced insulin release, but it did inhibit glucose-induced insulin release suggesting that the enzyme and quinones play a role in glucose-induced insulin release. Quinones may stimulate insulin release by mimicking physiologically-occuring quinones, such as CoQ₁₀, by acting on the plasma membrane or in the cytosol. Exogenous quinones may bypass the quinone reductase reaction, as well as many reactions important for exocytosis.     

Molecular cloning,characterization and expression of a nitrofuran reductase gene ofescherichia coli

Mini-mu derivatives carrying plasmid replicons can be used to clone genesin vivo. This method was adopted to generate phasmid clones which were later screened for their ability of restore nitrofurantoin sensitivity of a nitrofuran-resistant host by eliciting nitroreductase activity. One phasmid-derived clone (pAJ101) resulted in considerable increase in nitroreductase activity when introduced into a nitrofurantoin-resistant mutant ofEscherichia coli with reduced nitroreductase activity. Subsequently, a 1.8 kb fragment obtained from pAJ101 by partial digestion with 5au3A, was subcloned into pUC18 to yield pAJ102. The nitroreductase activity attributable to pAJ102 was capable of reducing both nitrofurantoin and nitrofurazone. The polypeptides encoded by pAJ102 were identified by the minicell method. A large, well-defined band corresponding to 37 kDa and a smaller, less-defined band corresponding to 35 kDa were detected. Tnl000 mutagenesis was used to delineate the coding segment of the 1.8 kb insert of pAJ102. A 0.8 kb stretch of DNA was shown to be part of the nitroreductase gene. The gene was mapped at 19 min on theEscherichia coli linkage map.