Malate valves: old shuttles with new perspectives

Malate valves act as powerful systems for balancing the ATP/NAD(P)H ratio required in various subcellular compartments in plant cells. As components of malate valves, isoforms of malate dehydrogenases (MDHs) and dicarboxylate translocators catalyse the reversible interconversion of malate and oxaloacetate and their transport. Depending on the co-enzyme specificity of the MDH isoforms, either NADH or NADPH can be transported indirectly. Arabidopsis thaliana possesses nine genes encoding MDH isoenzymes. Activities of NAD-dependent MDHs have been detected in mitochondria, peroxisomes, cytosol and plastids. In addition, chloroplasts possess a NADP-dependent MDH isoform. The NADP-MDH as part of the 'light malate valve' plays an important role as a poising mechanism to adjust the ATP/NADPH ratio in the stroma. Its activity is strictly regulated by post-translational redox-modification mediated via the ferredoxin-thioredoxin system and fine control via the NADP⁺ /NADP(H) ratio, thereby maintaining redox homeostasis under changing conditions. In contrast, the plastid NAD-MDH ('dark malate valve') is constitutively active and its lack leads to failure in early embryo development. While redox regulation of the main cytosolic MDH isoform has been shown, knowledge about regulation of the other two cytosolic MDHs as well as NAD-MDH isoforms from peroxisomes and mitochondria is still lacking. Knockout mutants lacking the isoforms from chloroplasts, mitochondria and peroxisomes have been characterised, but not much is known about cytosolic NAD-MDH isoforms and their role in planta. This review updates the current knowledge on MDH isoforms and the shuttle systems for intercompartmental dicarboxylate exchange, focusing on the various metabolic functions of these valves.

Emerging concept for the role of photorespiration as an important part of abiotic stress response

When plants are exposed to stress, generation of reactive oxygen species (ROS) is often one of the first responses. In order to survive, cells attempt to down-regulate the production of ROS, while at the same time scavenging ROS. Photorespiration is now appreciated as an important part of stress responses in green tissues for preventing ROS accumulation. Photorespiratory reactions can dissipate excess reducing equivalents and energy either directly (using ATP, NAD(P)H and reduced ferredoxin) or indirectly (e.g., via alternative oxidase (AOX) and providing an internal CO2 pool). Photorespiration, however, is also a source of H2 O2 that is possibly involved in signal transduction, resulting in modulation of gene expression. We propose that photorespiration can assume a major role in the readjustment of redox homeostasis. Protection of photosynthesis from photoinhibition through photorespiration is well known. Photorespiration can mitigate oxidative stress under conditions of drought/water stress, salinity, low CO2 and chilling. Adjustments to even mild disturbances in redox status, caused by a deficiency in ascorbate, AOX or chloroplastic NADP-malate dehydrogenase, comprise increases in photorespiratory components such as catalase, P-protein of glycine decarboxylase complex (GDC) and glycine content. The accumulation of excess reducing equivalents or ROS in plant cells also affects mitochondria. Therefore, a strong interaction between the chloroplast redox status and photorespiration is not surprising, but highlights interesting properties evident in plant cells. We draw attention to the fact that a complex network of multiple and dynamic systems, including photorespiration, prevents oxidative damage while optimising photosynthesis. Further experiments are necessary to identify and validate the direct targets of redox signals among photorespiratory components.

Glutamine and alanyl-glutamine dipeptide reduce mesenteric plasma extravasation, leukocyte adhesion and tumor necrosis factor-α (TNF-α) release during experimental endotoxemia

Glutamine (GLN) appears to be an essential nutrient during organism development and critical illness. The aim of our study was to evaluate the effects of GLN and its generic preparation alanyl-glutamine-dipeptide (DIP) on the microcirculation in endotoxemia in rats and its effects on tonus or aortal rings in vitro. Male Lewis rats (n=40) were separated in 4 groups. Group 1 (CON) served as healthy control group while the other groups received an endotoxin bolus i.v. (5 mg/kg lipopolysaccharide, LPS i.v.). In group 3 (LPS+GLN) 0.75 g/kg-1 GLN i.v. before LPS challenge was administered. In group 4 (LPS+DIP) DIP containing 0.75 g/kg GLN was given. Leukocyte-endothelial interactions and mesenteric plasma extravasation were determined at 0, 1 and 2 hours during the experiment by intravital fluorescence microscopy (IVM). Cytokine release (TNF-alpha, IL-1 beta, IL-6, IL-10) was measured by ELISA. GLN treatment reduced leukocyte adherence (-49.7% vs. LPS group, p<0.05) and plasma extravasation (-12.3% vs. LPS group, p<0.05) significantly during endotoxemia compared to untreated LPS animals. In group 4 (DIP+LPS), a decrease of leukocyte adherence (-56.0%) and mesenteric plasma extravasation (-18.8% vs. LPS group, p<0.05) was also found. TNF-alpha levels were reduced in both GLN and DIP (p<0.05). In vitro experiments demonstrated that glutamine agents could attenuate the response to contracting agents in presence of the vascular endothelium, implying nitric oxide pathway. In vivo, GLN as well as DIP pre-treatment diminish the detrimental impact of endotoxemia on the mesenteric microcirculation and the TNF-alpha release, the effects whose clinical importance should be further examined.

COX-1 and COX-2 contribute differentially to the LPS-induced release of PGE2 and TxA2 in liver macrophages

LPS induces an immediate release of thromboxane TxA2 and a delayed release of PGE2. Dexamethasone suppresses the LPS-induced release of TxA2 and PGE2. In the first 8 h after LPS addition, the specific COX-2 inhibitor SC236 inhibits the PGE2 and TxA2 release by about 80% and 20%, whereas the release of PGE2 and TxA2 between 8 and 24 h is inhibited by about 40% and 35%, respectively. Resident liver macrophages express substantial amounts of COX-1, TxAS, cPGES and mPGES-2, small amounts of COX-2 but almost no detectable amounts of mPGES-1. LPS induces an increase of COX-2 and mPGES-1, but does not change COX-1, cPGES, mPGES-2 and TxAS at protein level. Dexamethasone suppresses almost completely the LPS-induced effects on COX-2 and mPGES-1. It is concluded that (1) COX-1 and COX-2 are involved in the LPS-induced synthesis of TxA2 and PGE2; (2) TxA2 release is catalyzed at early time-points by the combined action of COX-1 and TxAs, whereas at later time points the newly expressed COX-2 couples to TxAS and contributes to the TxA2 release; (3) PGE2 release within the first 8 h is predominantly catalyzed by COX-2, whereas at later time-points COX-1 couples to the newly expressed mPGES-1 and contributes to the PGE2 release.

PGE2 exerts its effect on the LPS-induced release of TNF-alpha, ET-1, IL-1alpha, IL-6 and IL-10 via the EP2 and EP4 receptor in rat liver macrophages

Lipopolysaccharide (LPS) induces a release of tumor necrosis factor (TNF)-alpha, endothelin (ET)-1, interleukin (IL)-1alpha, IL-6 and IL-10 in rat liver macrophages (Kupffer cells). Prostaglandin (PG)E2 inhibits the release of the fibrogenic mediators TNF-alpha, ET-1 and IL-1alpha, and enhances the release of the anti-fibrogenic mediators IL-6 and IL-10. This effect of PGE2 is mimicked by specific agonists for the PGE2 receptors EP2 and EP4; whereas, agonists for the PGE2 receptors EP1 and EP3 are inactive. Rat liver macrophages express mRNA encoding the PGE2 receptors EP2 and EP4 but not the PGE2 receptors EP1 and EP3. These data suggest that PGE2 exerts its anti-fibrogenic effect through the EP2 and EP4 receptor by inhibiting the release of the fibrogenic mediators TNF-alpha, ET-1 and IL-1alpha, and by enhancing the release of the anti-fibrogenic mediators IL-6 and IL-10 in liver macrophages.

Diverse functional coupling of cyclooxygenase 1 and 2 with final prostanoid synthases in liver macrophages

Lipopolysaccharide (LPS) treatment of resident liver macrophages resulted in a coordinated enhanced expression of cytosolic phospholipase A(2) (cPLA(2)), cyclooxygenase (COX)-2 and prostaglandin E(2)-synthase. LPS-pretreated liver macrophages showed a higher release of PGE(2) after zymosan, phorbol ester and A23187, of PGF(2alpha) after zymosan and A23187, whereas the release of thromboxane B(2) and PGD(2) was unchanged. Inhibition of COX-1 and -2 by specific inhibitors (SC560, SC236) inhibited the prostanoid release between 50-80% and 20-40%, respectively, indicating a predominant role for COX-1. In detail (1) the zymosan-induced release of all prostanoids was inhibited to a similar degree by the COX-1 inhibitor (about 70%) and the COX-2 inhibitor (20-30%), (2) PGE(2) release after all stimuli was inhibited to a greater extent by SC560 (70-90%) compared to SC236 (5-30%), (3) the phorbol ester- and A23187-induced release of PGF(2alpha) and PGD(2) was inhibited equally (40-50%) by both inhibitors, (3) TxB(2) release after phorbol ester and A23187 was inhibited by SC560 by 50 and 30%, and by SC236 by 50 and 70%, respectively. cPLA(2), COX-1 and -2, and the final prostanoid synthases were found in different subcellular fractions. These results indicate, that the functional coupling of COX-1 and -2 to final prostanoid synthases depends on the stimulation of the cells.

A soybean plastid-targeted NADH-malate dehydrogenase: cloning and expression analyses

A typical soybean (Glycine max) plant assimilates nitrogen rapidly both in active root nodules and in developing seeds and pods. Oxaloacetate and 2-ketoglutarate are major acceptors of ammonia during rapid nitrogen assimilation. Oxaloacetate can be derived from the tricarboxylic acid (TCA) cycle, and it also can be synthesized from phosphoenolpyruvate and carbon dioxide by phosphoenolpyruvate carboxylase. An active malate dehydrogenase is required to facilitate carbon flow from phosphoenolpyruvate to oxaloacetate. We report the cloning and sequence analyses of a complete and novel malate dehydrogenase gene in soybean. The derived amino acid sequence was highly similar to the nodule-enhanced malate dehydrogenases from Medicago sativa and Pisum sativum in terms of the transit peptide and the mature subunit (i.e., the functional enzyme). Furthermore, the mature subunit exhibited a very high homology to the plastid-localized NAD-dependent malate dehydrogenase from Arabidopsis thaliana, which has a completely different transit peptide. In addition, the soybean nodule-enhanced malate dehydrogenase was abundant in both immature soybean seeds and pods. Only trace amounts of the enzyme were found in leaves and nonnodulated roots. In vitro synthesized labeled precursor protein was imported into the stroma of spinach chloroplasts and processed to the mature subunit, which has a molecular mass of ∼34 kDa. We propose that this new malate dehydrogenase facilitates rapid nitrogen assimilation both in soybean root nodules and in developing soybean seeds, which are rich in protein. In addition, the complete coding region of a geranylgeranyl hydrogenase gene, which is essential for chlorophyll synthesis, was found immediately upstream from the new malate dehydrogenase gene.

Light-modulated NADP-malate dehydrogenases from mossfern and green algae: insights into evolution of the enzyme's regulation

Chloroplast NADP-dependent malate dehydrogenase is one of the best-studied light-regulated enzymes. In C3 plants, NADP-MDH is a part of the 'malate valve' that controls the export of reducing equivalents in the form of malate to the cytosol. NADP-MDH is completely inactive in the dark and is activated in the light with reduced thioredoxin. Compared with its permanently active NAD-linked counterparts, NADP-MDH exhibits N- and C-terminal sequence extensions, each bearing one regulatory disulphide. Upon reduction of the C-terminal disulphide, the enzyme active site becomes accessible for the substrate. Reduction of the N-terminal disulphide promotes a conformational change advantageous for catalysis. To trace the evolutionary development of this intricate regulation mechanism, we isolated cDNA clones for NADP-MDH from the mossfern Selaginella and from two unicellular green algae. While the NADP-MDH sequence from Selaginella demonstrates the classic cysteine pattern of the higher plant enzyme, the sequences from the green algae are devoid of the N-terminal regulatory disulphide. Phylogenetic analysis of new sequences and of those available in the databases led to the conclusion that the chloroplast NADP-MDH and the cytosolic NAD-dependent form arose via duplication of an ancestral eubacterial gene, which preceded the separation of plant and animal lineages. Redox-sensitive NADP-MDH activity was detected only in the 'green' plant lineage starting from the primitive prasinophytic algae but not in cyanobacteria, Cyanophora paradoxa, red algae and diatoms. The latter organisms therefore appear to utilize mechanisms other than the light-regulated 'malate valve' to remove from plastids excessive electrons produced by photosynthesis.

Functional coupling of cyclooxygenase 1 and 2 to discrete prostanoid synthases in liver macrophages

The profile of released prostanoids after addition of exogenous arachidonic acid to resident liver macrophages is different from the profile obtained in lipopolysaccharide-pretreated cells. In resident and lipopolysaccharide-pretreated cells, AA leads to a release of thromboxane B(2), prostaglandin F(2alpha), E(2), and D(2). A specifically enhanced formation of prostaglandin E(2) is obtained in lipopolysaccharide-pretreated cells. Resident liver macrophages express cyclooxygenase 1, and thromboxane A(2)-, prostaglandin F(2alpha)-, E(2)-, and D(2)-synthase. Treatment with lipopolysaccharide induces-in addition to cyclooxygenase 2-an enhanced expression of the prostaglandin E(2) synthase. In resident liver macrophages, the formation of prostanoids from exogenous arachidonic acid is completely inhibited by SC560 (a specific inhibitor of cyclooxygenase 1), but remains unchanged with SC236 (a specific inhibitor of cyclooxygenase 2). In lipopolysaccharide-pretreated liver macrophages, the formation of thromboxane B(2), prostaglandin F(2alpha) and D(2) is equally inhibited by SC560 and SC236 by about 50%. In contrast, the formation of prostaglandin E(2) is inhibited to a greater extent by SC560 (75%) compared to SC236 (26%). We conclude from these data, that in lipopolysaccharide-pretreated liver macrophages (i) cyclooxygenase 1 and 2 couple both to discrete prostanoid synthases, (ii) the functional coupling of cyclooxygenase 1 and 2 to the thromboxane A(2)-, prostaglandin F(2alpha)-, and D(2)-synthase is almost identical, and (iii) the enhanced prostaglandin E(2) synthesis is due to an enhanced expression of the prostaglandin E(2) synthase, which is coupled more efficiently to cyclooxygenase 1.

Regulation in metabolic systems under homeostatic flux control

The general properties of metabolic systems under homeostatic flux control are analyzed. It is shown that the main characteristic point for an enzyme in such a system is a sharp transition from limitation outside the system to limitation by some enzyme inside the system. A method for the quantitative treatment of the experimental dependence of metabolic flux on enzyme content is presented. The conception of "nonlimiting," "near-limiting," and "limiting" enzymes is developed for these systems. It is pointed out that reactions close to a thermodynamic equilibrium under normal conditions can considerably limit the homeostatic fluxes. The rules for regulation of fluxes in such systems are illustrated by the data obtained for transgenic plants with reduced activities of some Calvin-cycle enzymes and further examples. A comparison is made between the developed quantitative description of metabolic fluxes under homeostatic flux control and the methods of metabolic control analysis.

Electron acceptors in isolated intact spinach chloroplasts act hierarchically to prevent over-reduction and competition for electrons

Electron fluxes in isolated intact spinach chloroplasts were analyzed under saturating light and under optimal CO(2) and P(i) supply. When CO(2) assimilation was the only ATP- and NADPH-consuming reaction, the DeltapH decreased and the chloroplasts showed clear evidence of over-reduction. This suggested that additional electron flow is required in order to maintain the DeltapH and the stromal NADPH/ATP ratio. The additional electron flow may be cyclic electron transport around Photosystem I and linear electron transport towards either oxaloacetate or O(2). The contributions of, and the interrelationships between, these three electron transfer pathways were analyzed by following the reactions of chloroplasts in their presence or absence, and by monitoring to what extent they were able to compensate for each other. Inhibition of cyclic electron flow by antimycin A caused strong over-reduction and decreased the DeltapH. Only oxaloacetate, but not O(2), was able to restore photosynthesis. In the presence of H(2)O(2), there was a rapid build-up of a high DeltapH, and the reduction of any other electron acceptor was prevented. It is concluded that the different electron acceptors in the stroma are organized in a hierarchical manner; this allows electron flux towards CO(2) and nitrite reduction to proceed without any competition for electrons, and any excess electrons to be taken by these additional non-assimilatory pathways. Hence, the DeltapH is maintained at the required level and over-reduction of the electron transport chain and the stromal redox components is avoided.

Regulation of the Calvin cycle for CO2 fixation as an example for general control mechanisms in metabolic cycles

The theory of a metabolic cycle with the main portion of its intermediates remaining inside the cycle during one turnover has been developed. On this basis, the regulation of the Calvin cycle is analyzed. It is demonstrated that not only the reactions of non-equilibrium enzymes, as the carboxylation of ribulose 1,5-bisphosphate, but reactions that operate close to a thermodynamic equilibrium, especially the reduction of 3-phosphoglycerate and the transketolase reaction can significantly influence the total turnover period in the Calvin cycle. The role of compensating mechanisms in the maintenance of the photosynthesis rate upon changes of environmental conditions and of enzyme contents is analyzed for the Calvin cycle. It is shown that the change of the total quantity of the metabolites is one of the main self-regulated mechanisms in the Calvin cycle. A change of the ATP/ADP ratio can be used by the cell to maintain the CO2 assimilation rate, when the total quantity of the metabolites is changed. The developed analysis permits to explain some experimental data obtained with transgenic plants with restricted efflux of carbon from the chloroplasts.

A novel, non-redox-regulated NAD-dependent malate dehydrogenase from chloroplasts of Arabidopsis thaliana L

We report a novel plastidic NAD-dependent malate dehydrogenase (EC 1. 1.1.37), which is not redox-regulated in contrast to its NADP-specific counterpart (EC 1.1.1.82). Analysis of isoenzyme patterns revealed a single NAD-MDH associated with highly purified chloroplasts isolated from Arabidopsis and spinach. A cDNA clone encoding the novel enzyme was found in the Arabidopsis EST data base by sorting all putative clones for NAD-dependent malate dehydrogenase. A derived amino acid sequence is very similar to mitochondrial and peroxisomal NAD-MDHs within the region coding for the mature protein but possesses a 80-amino acid long N-terminal domain with typical characteristics of a chloroplast transit peptide. In vitro synthesized labeled precursor protein was imported into the stroma of spinach chloroplasts and processed to a mature enzyme subunit of 34 kDa. Expressed in Escherichia coli, the recombinant enzyme exhibited the same distinctive isoelectric point of 5.35 as the original enzyme from Arabidopsis chloroplasts. Northern analysis revealed that the protein is expressed in both autotrophic and heterotrophic tissues. The findings reported here indicate that the "malate valve" operates not only in the illuminated chloroplasts but also in dark chloroplasts and in heterotrophic plastids and is therefore a general mechanism to maintain the optimal ratio between ATP and reducing equivalents in plastids.

Flux control of the malate valve in leaf cells

The coupled processes of the chloroplast trans-envelope transport of malate and oxaloacetate and their interconversion as catalyzed by the stromal NADP-linked malate dehydrogenase are quantitatively analyzed by means of a steady-state model. The equation for the NADP-malate dehydrogenase reaction is developed. The empirical dependence of enzyme activity on NADPH and NADP+ is used to determine its actual activity. The trans-envelope counter exchange of malate and oxaloacetate is described by a kinetic model of the translocator. Kinetic parameters are derived from known data, except for the Km value and the maximum rate for oxaloacetate transport, which are estimated from oxaloacetate-dependent malate formation in isolated intact chloroplasts. Using the kinetic properties of the system and the known metabolite concentrations, the model demonstrates that photosynthetically generated NADPH can be exported efficiently from the chloroplasts to the cytosol by the malate-valve system. The transfer capacity of the malate valve is estimated not to exceed 20 mumol (mg Chl)-1 h-1 (or 5% of the electron transport) under normal physiological conditions. The possible role of the malate valve in leaf cells under normal conditions and during stress is discussed.

Identification of the cysteine residues involved in redox modification of plant plastidic glucose-6-phosphate dehydrogenase

The cDNA sequences encoding cytosolic and light-modulated plastidic glucose-6-phosphate dehydrogenase (G6PDH) from potato were modified by polymerase chain reaction and subsequently overexpressed in Escherichia coli. Characterization of the recombinant enzymes showed that they closely resembled their native counterparts. Treatment with reduced dithiothreitol or glutathione led to inactivation of plastidic G6PDH, whereas the activity of the cytosolic isoenzyme was not influenced by reduction. As for the native enzyme, inactivation of recombinant plastidic G6PDH was accelerated by thioredoxin m and could be fully reversed by subsequent addition of oxidant. To identify the residues which are involved in redox regulation of plastidic G6PDH, each of the six cysteines in the mature protein sequence was exchanged separately for serine by site-directed mutagenesis. Two mutant proteins exhibited characteristics of the reduced wild-type enzyme. Exchange of either Cys149 or Cys157 to serine abolished the regulatory properties, suggesting that these cysteine residues are the sites responsible for redox-mediated inactivation of plastidic G6PDH.

Cloning and sequence analysis of cDNAs encoding plant cytosolic malate dehydrogenase

Here we report the first complete sequence of plant cytosolic malate dehydrogenase (EC 1.1.1.37). The phylogenetic relationships between malate dehydrogenases from different cell compartments are discussed. The constructed phylogenetic tree shows that cytosolic NAD-MDH and chloroplast NADP-MDH have evolved through gene duplication of the pre-existing nuclear gene.

Transgenic Tobacco Plants Expressing Pea Chloroplast Nmdh cDNA in Sense and Antisense Orientation (Effects on NADP-Malate Dehydrogenase Level, Stability of Transformants, and Plant Growth)

A full-length cDNA encoding light-activated chloroplast NADP-malate dehydrogenase (NADP-MDH) (EC 1.1.1.82) from pea (Pisum sativum L.) was introduced in the sense and antisense orientation into tobacco (Nicotiana tabacum L.). Transgenic plants with decreased or increased expression levels were obtained. Because of substantial age-dependent differences in individual leaves of a single plant, standardization of NADP-MDH levels was required first. Then, extent and stability of over- or under-expression of Nmdh, the gene encoding NADP-MDH, was characterized in the various transformants. Frequently, cosuppression effects were observed, indicating sufficient homology between the endogenous tobacco and the heterologous pea gene. Analysis of the T1 and T2 progeny of a series of independent transgenic lines revealed that NADP-MDH capacity ranged between 10% and [greater than or equal to]10-fold compared with the wild type. Under ambient conditions whole-plant development, growth period, and fertility were unaffected by NADP-MDH reduction to 20% of the wild-type level; below this threshold plant growth was retarded. A positive growth effect was registered in young plants with stably enhanced NADP-MDH levels within a defined developmental window.

Transforming growth factor-beta1 induces activation of Ras, Raf-1, MEK and MAPK in rat hepatic stellate cells

The transdifferentiation of hepatic stellate cells into myofibroblast-like cells and the proliferation of the transdifferentiated cells are controlled by TGF-beta1. Little is known about the intracellular signal transducers of TGF-beta1. In this paper we show that in cultured hepatic stellate cells TGF-beta1 induces activation of Ras, Raf-1, MEK and MAPK p42 and p44. The activation of MAPK depends on the activation of MEK. Our data exclude that the observed effects are mediated by a bFGF or PDGF autocrine loop.

Functional studies of chloroplast glyceraldehyde-3-phosphate dehydrogenase subunits A and B expressed in Escherichia coli: formation of highly active A4 and B4 homotetramers and evidence that aggregation of the B4 complex is mediated by the B subunit carboxy terminus

Chloroplast glyceraldehyde-3-phosphate dehydrogenase (phosphorylating, E.C. 1.2.1.13) (GAPDH) of higher plants exists as an A2B2 heterotetramer that catalyses the reductive step of the Calvin cycle. In dark chloroplasts the enzyme exhibits a molecular mass of 600 kDa, whereas in illuminated chloroplasts the molecular mass is altered in favor of the more active 150 kDa form. We have expressed in Escherichia coli proteins corresponding to the mature A and B subunits of spinach chloroplast GAPDH (GapA and GapB, respectively) in addition to a derivative of the B subunit lacking the GapB-specific C-terminal extension (CTE). One mg of each of the three proteins so expressed was purified to electrophoretic homogeneity with conventional methods. Spinach GapA purified from E. coli is shown to be a highly active homotetramer (50-70 U/mg) which does not associate under aggregating conditions in vitro to high-molecular-mass (HMM) forms of ca. 600 kDa. Since B4 forms of the enzyme have not been described from any source, we were surprised to find that spinach GapB purified from E. coli was active (15-35 U/mg). Spinach GapB lacking the CTE purified from E. coli is more highly active (130 U/mg) than GapB with the CTE. Under aggregating conditions, GapB lacking the CTE is a tetramer that does not associate to HMM forms whereas GapB with the CTE occurs exclusively as an aggregated HMM form. The data indicate that intertetramer association of chloroplast GAPDH in vitro occurs through GapB-mediated protein-protein interaction.

C-terminal truncation of spinach chloroplast NAD(P)-dependent glyceraldehyde-3-phosphate dehydrogenase prevents inactivation and reaggregation

Chloroplast NAD(P)-dependent glyceraldehyde-3-phosphate dehydrogenase (NAD(P)-GAPDH; EC 1.2.1.13) consists of two types of subunits: GapA and GapB, which are rather similar, except that GapB carries an unique C-terminal sequence extension. Here, we report evidence that this sequence extension might be responsible for aggregation and dark inactivation of the enzyme in vivo. Recently, it had been demonstrated that upon limited proteolysis of the purified 600 kDa enzyme, using the Staphylococcus aureus V8 endoproteinase (Zapponi et al. (1993) Biol. Chem. Hoppe-Seyler 374, 395-402), the C-terminus of GapB can be removed, giving rise to the 150 kDa form. Based on these findings, we analyzed the changed catalytic properties of the enzyme after proteolysis and its ability to reaggregate. The time-course of proteolysis is paralleled by a strong increase in enzyme activity and the appearance of the tetrameric enzyme form, the increase of apparent activity preceding disaggregation. The proteolyzed enzyme is characterized by its increased affinity towards the substrate 1,3-bisphosphoglycerate and thus resembles the fully activated intact enzyme. In contrast to the effector-mediated activation of the intact enzyme, both proteolytic activation and the resulting disaggregation of the high-molecular-weight form cannot be reversed, even by incubation with NAD.

Reductive modification and nonreductive activation of purified spinach chloroplast NADP-dependent glyceraldehyde-3-phosphate dehydrogenase

Spinach chloroplast NAD(P)-glyceraldehyde-3-phosphate dehydrogenase (NAD(P)-GAPDH; EC, 1.2.1.13) was purified as the 600-kDa oligomer of low specific activity. Incubation of the enzyme with either a reductant or a 1,3-bisphosphoglycerate (1,3bisPGA) generating system, but most effectively with both, resulted in an increase of the apparent NADPH-dependent activity. Only the 1,3bisPGA treatment caused dissociation and yielded the 150-kDa heterotetramer (A2B2). The higher activity of the tetramer is largely due to a decreased KM value for the substrate 1,3bisPGA. Reductive treatment alone does not dissociate the enzyme. Reduction was equally effective with glutathione as with dithiothreitol or with reduced thioredoxin f. The concentration of 1,3bisPGA required to obtain 50% activity (K alpha) was 19.5 +/- 4.1 microM for the untreated enzyme and 2.0 +/- 1.4 microM for the thiol-pretreated enzyme. Thus, in vitro 1,3bisPGA, alone or--at much lower concentrations--together with a reductant can activate (and dissociate) NAD(P)-GAPDH. The enzyme exhibits similar K alpha values in its reduced and its oxidized form for ATP (1-2 mM), NADP (50-200 microM), and NADPH (0.3-0.5 mM) as positive effectors, but these effectors do not lead to any activation when present together with 0.14 mM NAD. Only 1,3bisPGA retained its characteristic effect in the presence of NAD. The dissociated enzyme reaggregates upon removal of the positive effectors. From these results it is concluded (i) that the role of the reduction of the NAD(P)-GAPDH in vivo is to increase its sensitivity toward the activator 1,3bisPGA and (ii) that the actual activation (and aggregation) state of the enzyme in chloroplasts in the light is regulated by the concentration of 1,3bisPGA as activator in the stroma and its actual activity by the availability of 1,3bisPGA as substrate.

Molecular characterization of the plastidic glucose-6-phosphate dehydrogenase from potato in comparison to its cytosolic counterpart

We report on the cloning of a plastidic glucose-6-phosphate dehydrogenase (EC 1.1.1.49) from higher plants. The complete sequence of the plastidic enzyme was obtained after rapid amplification of cDNA ends and comprises a putative plastidic transit peptide. Sequences amplified from leaf or root poly(A+) RNA are identical. In contrast to the cytosolic enzyme, the plastidic isoform is subject to redox modulation, i.e. thioredoxin-mediated inactivation by light. But when the plastidic enzyme is compared to a cyanobacterial homolog, none of the cysteine residues is conserved. The recombinant enzyme was used to raise antibodies in rabbits. Gene expression was studied in potato (Solanum tuberosum L.), at both the RNA and protein levels, revealing different patterns for the isoforms. The gene encoding the cytosolic enzyme was transcribed in all tissues tested, and the highest transcription was detected in tubers. In contrast, expression of the gene encoding the plastidic enzyme was confined to green tissues. Wounding of leaves resulted in a slight increase in the expression of the gene encoding the cytosolic isoform and a shutdown of the plastidic counterpart. Compared to the situation in soil, elevated transcription of the gene encoding the plastidic enzyme is found in roots of hydroponically grown potato plants, which is in agreement with the postulated role for this isoform in nitrite reduction.

Induction of Hexose-Phosphate Translocator Activity in Spinach Chloroplasts

Many environmental and experimental conditions lead to accumulation of carbohydrates in photosynthetic tissues. This situation is typically associated with major changes in the mRNA and protein complement of the cell, including metabolic repression of photosynthetic gene expression, which can be induced by feeding carbohydrates directly to leaves. In this study we examined the carbohydrate transport properties of chloroplasts isolated from spinach (Spinacia oleracea L.) leaves fed with glucose for several days. These chloroplasts contain large quantities of starch, can perform photosynthetic 3-phosphoglycerate reduction, and surprisingly also have the ability to perform starch synthesis from exogenous glucose-6-phosphate (Glc-6-P) both in the light and in darkness, similarly to heterotrophic plastids. Glucose-1-phosphate does not act as an exogenous precursor for starch synthesis. Light, ATP, and 3-phosphoglyceric acid stimulate Glc-6-P-dependent starch synthesis. Short-term uptake experiments indicate that a novel Glc-6-P-translocator capacity is present in the envelope membrane, exhibiting an apparent Km of 0.54 mM and a Vmax of 2.9 [mu]mol Glc-6-P mg-1 chlorophyll h-1. Similar results were obtained with chloroplasts isolated from glucose-fed potato leaves and from water-stressed spinach leaves. The generally held view that sugar phosphates transported by chloroplasts are confined to triose phosphates is not supported by these results. A physiological role for a Glc-6-P translocator in green plastids is presented with reference to the source/sink function of the leaf.

Development of A-type (axonless) horizontal cells in the rabbit retina

The development of A-type horizontal cells (HC) was studied in the rabbit retina between embryonic day (E)24 and adulthood [the day of birth was called postnatal day (P)1 and corresponds to E31-32]. The cells were visualized by several methods 1) by immunolabeling with antibodies to neurofilament 70,000 (NF-70kD), 2) by immunolabeling with antibodies to a calcium binding protein (CaBP-28kD), 3) by two different methods of silver impregnation, and 4) by histochemical demonstration of NADH-diaphorase activity. Most methods labeled A-type HC only in the dorsal retina; thus, our study is restricted to HC of this region. HC densities were determined at each developmental stage. The cells were drawn at scale, and size, quotient of symmetry, and topographical orientation of dendritic trees were studied by image analysis. The growth of HC dendritic fields was correlated with data on the postnatal local retinal expansion, which is known to be driven by the intraocular pressure (after cessation of retinal cell proliferation at P9). This expansion was evaluated in an earlier paper (Reichenbach et al. [1993] Vis. Neurosci. 10:479-498) by using local subpopulations of Müller cells as "markers" of distinct topographic regions of the retinae. After E24, when the final number of HC is established, we can discriminate three distinct developmental stages of A-type HC. During the first stage, between E24 and E27, the young cells are often vertically oriented and may extend their first short dendrites within (the primordia of) both plexiform layers. The irregular HC mosaic at E24 shows a significant difference to all other stages. The second stage begins after birth when the dendritic trees of the cells are already restricted to the outer plexiform layer. Between P3 and P9, their dendritic trees enlarge more than the surrounding retinal tissue expands, and the coverage factor almost doubles from 2.5 to 4.4. The third stage occurs after P9 when the growth rate of dendritic tree areas corresponds to that of the local retinal tissue expansion caused by "passive stretching" of the postmitotic tissue, and the coverage factor remains constant. This is compatible with the view that mature synaptic connections of A-type HC are mostly established after the first week of life and are then maintained.

Redox equilibria between the regulatory thiols of light/dark-modulated chloroplast enzymes and dithiothreitol: fine-tuning by metabolites

Three light/dark-modulated chloroplast enzymes, namely NADP-dependent malate dehydrogenase (EC 1.1.1.82), D-fructose 1,6-bisphosphatase (EC 3.1.3.11), and phosphoribulokinase (EC 2.7.1.19) were purified to apparent homogeneity from spinach leaves. Equilibrium constants for the covalent modification of the regulatory disulfide bonds of these enzymes in dithiothreitol (DTT)-redox buffer were determined according to a previously published method in the literature (Clancey and Gilbert (1987) J. Biol. Chem. 262, 13545-13549). The thiol/disulfide-redox potential (Kox) was defined as the ratio of reduced to oxidized dithiothreitol at which 50% of the maximal enzyme activity was observed after equilibrium had been established. All Kox values were very high, comparable to those of extracellular disulfide containing proteins: 0.23 +/- 0.02 for NADP-malate dehydrogenase, 0.59 +/- 0.17 for phosphoribulokinase, and 0.70 +/- 0.16 for D-fructose 1,6-bisphosphatase. The equilibrium constants for the reactions between these enzymes and the redox buffers were also determined in the presence of various concentrations of specific metabolites known to influence the rates of reduction and oxidation. Increasing concentrations of D-fructose 1,6-bisphosphate in the presence of Ca2+ shift the equilibrium constant between D-fructose 1,6-bisphosphatase and the DTT-redox buffer to much lower values. A decreasing NADPH/(NADP + NADPH) ratio increases the Kox of NADP-malate dehydrogenase in the redox buffer to very high values. For PRK, low concentrations of ATP result in a slight decrease of the Kox that is not further affected by higher ATP concentrations. The differences of the equilibrium constants of NADP-malate dehydrogenase and D-fructose 1,6-bisphosphatase as dependent upon the NADPH/(NADP + NADPH) ratio and the concentration of D-fructose 1,6-bisphosphate, respectively, reflect a mechanism of feed-back and feed-forward regulation by the product NADP and the substrate D-fructose 1,6-bisphosphate, respectively. Thus the actual activation state of these two key enzymes of chloroplast metabolism are determined in an independent manner. The relatively small effect of the ATP concentration upon the redox potential of phosphoribulokinase indicates that fine-regulation at this step might be achieved on another level (e.g., catalysis or aggregation state).

Cysteines of chloroplast NADP-malate dehydrogenase form mixed disulfides

Chloroplast NADP-malate dehydrogenase (NADP-MDH) from pea and from spinach was N-terminally truncated by limited proteolysis with Staphylococcus aureus protease V8. The resulting monomeric enzymes lacking, respectively, the 37 and 38 N-terminal amino acids were inactive. Reduction and addition of low concentrations of guanidine-HCl (50-100 mM) resulted in a highly active enzyme of 850 units per mg protein. Equilibration of the truncated enzyme with various glutathione (GSH) redox buffers and assaying its activity in the presence of guanidine-HCl was used to establish the existence of protein-GSH mixed disulfides. This finding was further confirmed using incorporation of radioactively labelled thiol. The possible function of such cysteine modifications under oxidative stress and their regeneration by the thioredoxin system in the light is discussed.

Competition between electron acceptors in photosynthesis: Regulation of the malate valve during CO2 fixation and nitrite reduction

For maximal rates of CO2 assimilation in isolated intact spinach chloroplasts the generation of the adequate NADPH/ATP ratio is achieved either by cyclic electron flow around photosystem I or by linear electron transport to oxaloacetate, nitrite or oxygen (Mehler-reaction). The interrelationships between these poising mechanisms turn out to be strictly hierarchical. In the presence of antimycin A, an inhibitor of ferredoxin-dependent cyclic electron transport, the reduction of both, oxaloacetate and nitrite, but not that of oxygen restores CO2 fixation. When oxaloacetate and nitrite are added at low concentrations simultaneously during steady-state CO2 fixation, the reduction of nitrite is clearly preferred over the reduction of oxaloacetate, but CO2 fixation is not influenced. Nitrite reduction is not decreased upon addition of oxaloacetate, but vice versa. This is due to the regulation of NADP-malate dehydrogenase activation by electron pressure via the ferredoxin/thioredoxin system on the one hand, and by the NADPH/(NADP+NADPH) ratio (anabolic reduction charge, ARC) on the other hand. Thus the closing of the 'malate valve' prevents drainage of reducing equivalents from the chloroplast (1) when a low ARC indicates a high demand for NADPH in the stroma and (2) when nitrite reduction reduces the electron pressure at ferredoxin. The 'malate valve' is opened when cyclic electron transport is inhibited by antimycin A. Under these conditions the rate of malate formation is higher than in the absence of the inhibitor even in the presence of oxaloacetate, thus indicating that the regulation of the 'malate valve' functions at various redox states of the acceptor side of Photosystem I.

[Light regulation of chloroplast enzymes]

The activity of various chloroplast enzymes is regulated by a covalent redox-modification cycle driven by photosynthetic electron flow and oxygen, and is fine-tuned by effectors specific for each target enzyme. The regulatory principle resembles the protein phosphorylation/dephosphorylation system. The primary structures of some redox-modulated enzymes reveal characteristic extra cysteine-carrying peptides in addition to very conserved sequences.

Glucose- and ADPGlc-dependent starch synthesis in isolated cauliflower-bud amyloplasts. Analysis of the interaction of various potential precursors

Recently, we have demonstrated that isolated cauliflower-bud amyloplasts incorporate glucose 6-phosphate at high rates into newly synthesized starch (Neuhaus et al. (1993) Plant Physiol. 101, 573-578). Here we have analyzed the incorporation of radioactively labeled glucose and ADPglucose into newly synthesized starch. It could be shown that glucose incorporation into starch exhibits a typical substrate saturation behaviour and is linear with time for at least 40 min. The incorporation of glucose is strongly dependent upon the intactness of the plastids and upon the presence of both, ATP and 3-phosphoglyceric acid. Using 4,4'-diisothiocyanostilbene-2,2'-disulfonate (DIDS) we showed that glucose is taken up into isolated cauliflower-bud amyloplasts as the free glucose molecule, rather than as glucose 6-phosphate. Glucose incorporation into newly synthesized starch is strongly inhibited in the presence of low concentrations of glucose 6-phosphate. The radioactively labeled glucose moiety of ADPglucose is also incorporated into starch. This incorporation can be saturated at increased concentrations of ADPglucose. ATP significantly inhibits the incorporation of the glucose moiety of ADPglucose into starch. This inhibition can be reinforced by the additional presence of glucose 6-phosphate. Glucose 6-phosphate-dependent starch synthesis is not strongly inhibited in the presence of glucose or ADPglucose indicating that glucose 6-phosphate is the precursor for starch synthesis in isolated cauliflower-bud amyloplasts.

Full-length cDNA sequences for both ferredoxin-thioredoxin reductase subunits from spinach (Spinacia oleracea L.)

Full-length cDNA clones for ferredoxin-thioredoxin reductase subunits A and B of Spinacia oleracea were obtained and their complete nucleotide sequences were determined. The results are compared with other known FTR sequences.

Purification, characterization, and cDNA sequence of glucose-6-phosphate dehydrogenase from potato (Solanum tuberosum L.)

Glucose-6-phosphate dehydrogenase (G6PDH, E.C. 1.1.1.49) has been purified from potato tuber at least 850-fold to apparent homogeneity as judged by SDS-PAGE. The enzyme was characterized by Km values of 260 microM for glucose-6-phosphate and 6 microM for NADP and a broad pH optimum between pH 7.5 and 9. NADPH, GTP, ATP, acetyl CoA and CoA inhibited G6PDH activity. Dithiothreitol (DTT) did not inactivate the enzyme. A highly specific antiserum was produced in a rabbit and used for immunodetection of G6PDH in Western blots. A cDNA library from potato leaves was screened with DNA probes produced by the polymerase chain reaction (PCR) in the presence of g6pdh-specific primers. A full-length cDNA clone was analyzed and the derived amino acid sequence compared with known G6PDH sequences from various sources. The homology of the plant sequence with G6PDH sequences from animals and yeast was found to be rather high (52%), whereas there was significantly lower homology with sequences of bacterial origin (37%). The lack of a plastidic signal sequence as well as the insensitivity of the recombinant enzyme towards reduced DTT, support the view that the cDNA sequence of a redox-independent cytosolic isoform was obtained.

Purification of highly intact plastids from various heterotrophic plant tissues: analysis of enzymic equipment and precursor dependency for starch biosynthesis

Starting with a protocol originally developed for the purification of intact plastids from cauliflower buds [Journet and Douce (1985) Plant Physiol. 79, 458-467] we have modified this method to obtain intact heterotrophic plastids from etiolated barley leaves (Hordeum vulgare) and pea (Pisum sativum) and maize (Zea mays) endosperm. Two subsequent centrifugation steps on Percoll gradients were performed, the first as an isopycnic, the second as zonal, centrifugation step in a swing-out rotor. Percoll density and centrifugation time were adjusted for the various tissues. The obtained plastid preparations are characterized by a low degree of contamination with other cellular components and an intactness of at least 90%. In isolated maize endosperm amyloplasts, starch synthesis is driven by exogenously applied hexose phosphates (glucose 6-phosphate and glucose 1-phosphate) rather than by dihydroxyacetone phosphate. The hexose-phosphate-dependent starch synthesis is strictly dependent upon the intactness of the plastids and is increased up to 9-fold when ATP and 3-phosphoglyceric acid are added to the incubation medium. The occurrence of fructose-1,6-bisphosphatase and malate dehydrogenases in some plastid types is discussed in relation to their possible role in starch synthesis.

Cloning, site-specific mutagenesis, expression and characterization of full-length chloroplast NADP-malate dehydrogenase from Pisum sativum

Chloroplast NADP-dependent malate dehydrogenase is regulated by a dithiol redox reaction. The assignment of the groups involved, requires the primary structure of the enzyme to be known. Using the polymerase chain reaction and the cDNA library of Pisum sativum, the sequence of the enzyme and its targeting signal was determined. The gene was cloned in Escherichia coli JM83 and expressed in E. coli JM83 and E. coli B at high yield. The determination of the physical properties of the gene product proves the recombinant protein to be indistinguishable from the enzyme purified from the plant. This holds true, in spite of the fact that the plant enzyme lacks 11 N-terminal residues. The lengths of the complete polypeptide chain of the recombinant enzyme and its transit peptide are 388 and 53 residues, respectively. The comparison of the sequences of the mature enzyme with those of known chloroplast NADP-MDH shows 83-95% identity, but with mitochondrial or bacterial MDH only approximately 20%. Reduction of the (inactive) oxidized enzyme with dithiothreitol allows mimicking of the in vivo activation. The reaction follows a consecutive second-order-kinetics mechanism. Guanidinium chloride (GdmCl) at concentrations below 0.4 M leads to a significant activation of the oxidized form of the enzyme. At [GdmCl] = 0.4-0.46 M, both oxidized and reduced NADP-MDH show highly cooperative changes in the hydrodynamic and spectral properties, indicating the synchronous breakdown of the quaternary, tertiary and secondary structures. Site-directed mutations C23A and C28A do not quench the regulatory properties of the enzyme; additional substitution of alanine for Cys206 and Cys376 renders the enzyme equally active in both the reduced and the oxidized state. Therefore, one can consider these residues, either alone or in combination with Cys23 and Cys28, as responsible for enzyme activation.

Identification of the putative hexose-phosphate translocator of amyloplasts from cauliflower buds

Starch synthesis in amyloplasts isolated from cauliflower buds is strongly inhibited by the addition of micromolar concentrations of 4,4'-di-isothiocyano-2,2'-stilbenedisulphonic acid (DIDS). Using [3H]DIDS it was possible to label specifically a 31.6 kDa membrane protein of the envelope fraction of isolated amyloplasts. The intensity of the radioactive label was decreased in the presence of glucose 6-phosphate or dihydroxyacetone phosphate, indicating that this protein might be the amyloplastic hexosephosphate translocator.

Effects of N-terminal truncations upon chloroplast NADP-malate dehydrogenases from pea and spinach

Using the purification procedure of Fickenscher and Scheibe (Biochim. Biophys. Acta 749 (1983), 249-254) and a modification of the method, we produced a series of NADP-MDH forms from spinach and pea-leaf extracts that were characterized by a stepwise shortening of the N-terminal sequences. Limited proteolysis of the enzymes resulted in the generation of even shorter forms. Immunoprecipitation of the NADP-MDH from crude extracts revealed that the sequences of the intact enzymes from pea, spinach and maize started at a position (Ser) identical with that established for the Sorghum enzyme (Crétin, C., et al. (1990) Eur. J. Biochem. 192, 299-303). Spinach NADP-MDH isolated by conventional methods was shown to represent the intact form. Thus, the kinetic, regulatory and structural properties of the various truncated forms could be compared with those of an intact form. Removal of 5 or 11 amino acids, as occurred during isolation of the pea NADP-MDH, was without any significant effect. The enzymes were all dimeric and still exhibited the characteristic redox-regulatory properties. However, removal of 31 and 37 amino acids using aminopeptidase K resulted in the formation of active monomers characterized by only slightly lowered affinities towards the substrates, a shift of their pH optimum from 8 to 7, the loss of oxaloacetate inhibition and an increased maximal velocity. Although these forms lacked most or all of the N-terminal extra-peptide, including the 2 cysteines involved in redox-modification, they were still sensitive to the redox-potential. However, the low concentration of thiol required for immediate and complete restoration of any lost activity (40 mM beta-mercaptoethanol) suggested that this reaction might not be relevant for redox-regulation in vivo.

Analysis of biophysical differences between oxidized and reduced chloroplast NADP-malate dehydrogenase

Various properties of purified chloroplast NADP-malate dehydrogenase were analyzed with respect to the redox state of the light/dark-modulated enzyme. The reduced enzyme is less resistant to heat, but the instability can be overcome by the addition of the coenzyme NADPH. Similarly, instability of the reduced NADP-MDH at high pH is alleviated by NADPH. The kinetics and protection characteristics of the alkylation of accessible thiols of NADP-MDH are used to describe the location of essential thiols relative to the active site, since again the coenzyme protects the active enzyme very effectively from inactivation by alkylation. The increased hydrophobicity of the reduced as opposed to the oxidized enzyme becomes apparent as the loss of activity from solutions due to adsorption to plastic surfaces. The kinetics and the solvent dependency of this process are analyzed and discussed, both from the practical (recovery of the purified enzyme) and the physiological point of view (in vivo protein/protein and protein/membrane interactions). The oxidized NADP-MDH has a lower tendency to bind to solid surfaces. NADP(H) efficiently prevents adsorption of the reduced form. Macromolecular solvents (polyethylene glycol), detergents (Triton X-100), or competing proteins also protect this otherwise very hydrophobic form from irreversible loss due to adsorption. Ribulosebisphosphate carboxylase/oxygenase and polyethylene glycol 10,000, however, used as competing substances only keep the oxidized, not the reduced, NADP-MDH in solution.

Characterization of Glucose-6-Phosphate Incorporation into Starch by Isolated Intact Cauliflower-Bud Plastids

Intact plastids from cauliflower (Brassica oleracea var Prince de Bretagne) buds were isolated according to the method described by Journet and Douce (E.P. Journet and R. Douce [1985] Plant Physiol 79: 458-467). Incubation of these plastids with various 14C-labeled compounds revealed that glucose-6-phosphate can act as a precursor for starch synthesis. However, significant rates (incorporation of 120 nmol glucose mg-1 protein h-1) could only be observed when both 3-phosphoglyceric acid and ATP were present as well. Starch synthesis in isolated plastids was strongly dependent upon the intactness of the organelle. The presence of a high-affinity ATP/ADP translocator with a Km for ATP of 12 [mu]M was demonstrated by uptake experiments with [14C]ATP. ADP inhibited both ATP uptake and effector-stimulated starch synthesis. Effector-stimulated glucose-6-phosphate-dependent starch synthesis was not significantly influenced by fructose-6-phosphate or 2-deoxyglucose-6-phosphate but was strongly inhibited by triose phosphate and inorganic phosphate. Starch synthesis was also inhibited by 4,4[prime]-diisothio-cyanostilbene-2,2[prime]-disulfonate, which is known to be a potent inhibitor of the chloroplast phosphate translocator. The data presented here support the view that starch biosynthesis in heterotrophic tissues is powered by increasing levels of cytosolic 3-phosphoglyceric acid and ATP when glucose-6-phosphate is available.

Transport Processes and Corresponding Changes in Metabolite Levels in Relation to Starch Synthesis in Barley (Hordeum vulgare L.) Etioplasts

Intact etioplasts with an intactness of 85% and with a cytosolic and a mitochondrial contamination of less than 10% were isolated from 8-d-old dark-grown barley (Hordeum vulgare) leaves. These plastids contained starch equivalent to 21.5 mumol of glucose per mg protein. From various likely precursors applied to isolated etioplasts, only dihydroxyacetone phosphate (DHAP) had significant effects on metabolite levels and on the internal ATP/ADP ratio. The concentration dependence of DHAP uptake exhibited saturation characteristics with half saturation at 0.36 mm DHAP and a maximal velocity of 6.6 mumol mg(-1) of protein h(-1). The transport was significantly inhibited by inorganic phosphate, pyridoxal-5'-phosphate, and 4,4'-diisothiocyano-2,2'-stilbenedisulfonate. The rate of glucose-6-phosphate uptake was much lower and not saturable up to a concentration of 10 mm. Exogenously applied [(14)C]DHAP was incorporated into starch at a rate of 0.14 mumol of DHAP mg(-1) of protein h(-1). Enzyme activities required to convert DHAP into starch were found to be present in etioplasts. Furthermore, enzymes generating ATP from DHAP for ADPglucose synthesis were also detected. Finally, a scheme is presented suggesting DHAP uptake to serve both as carbon skeleton and as energy source for starch synthesis, mediated by a translocator with properties similar to those of the triose phosphate translocator from chloroplasts.

Development of the rabbit retina. IV. Tissue tensility and elasticity in dependence on topographic specializations

A method is introduced for the quantification of specific compliance and the elasticity of small pieces of living retinal tissue. These pieces are fixed at their margins by means of tissue glue, and loaded with a small iron spherule the bending force of which can be gradually enhanced by the action of an electromagnet. Retinal bending caused by such calibrated forces is measured by a horizontal light microscope, and used for estimations of specific compliance and elasticity of the tissue. Three different particular regions of the rabbit retina--periphery, visual streak, and (prospective) medullary rays--were tested at several post-natal developmental stages. From very early stages on (day 2 p.p.) up to adulthood the peripheral retina was found to be significantly more tensile than the two other central regions. This can be shown to depend greatly on the thickness of the tissue which is lower in the retinal periphery. During early post-natal development, all retinal regions except the (prospective) medullary rays become thinner. The tensility of the tissue increases, with the exception of the medullary rays which reduce their compliance strongly. In the adult retina, however, the tensility of all retinal regions is reduced as compared with the neonatal tissue. This seems to be caused by a constant gradual increase of the elasticity of the retina during development which, in turn, may be caused by several developmental parameters, e.g. the formation of synapses, the outgrowth of glial side branches ensheathing neighbouring neuronal cells, or a reduction in extracellular clefts. It is proposed that these differences in tensility between different retinal regions, may be the cause for differential retinal expansion driven by the intraocular pressure. Thus, simple mechanical features of the tissue may contribute to the formation of important topographic specializations of the retina, e.g. the visual streak as the site of highest visual acuity.

Redox-modulation of chloroplast enzymes : a common principle for individual control

Assimilation of C, N, and S into organic compounds requires effective and flexible cooperation among the energy-converting, tightly coupled, thylakoid-bound processes and stromal metabolism. Fluctuations of light, temperature, and changing concentrations of the various reducible substrates pose unique regulatory problems to photoautotrophic plant cells. Covalent redox modification of enzyme proteins as mediated by the ferredoxin/thiore-doxin-system is suited to provide short-term adaptation of various enzymatic activities in the chloroplast. This mode of regulation is based on the continuous turnover of interconvertible enzyme forms, as in the systems driven by protein phosphorylation/dephosphorylation, but is particularly adapted to the unique conditions of a compartment performing oxygenic photosynthesis by depending on the simultaneous presence of reducing power and of oxygen. Individual fine control of each of the enzymes subjected to redox modification is achieved by specific metabolites acting as additional positive or negative effectors of the reductive (and/or oxidative) modification reaction. The biochemical prerequisite for such a control is the presence of regulatory (extra) sequences carrying cysteine residues which are subjected to reversible redox changes. Although no common amino acid sequence has yet been identified among the known regulatory peptides, in all cases the evolution of autotrophy should be related to the presence of extrasequences in otherwise very conserved enzyme molecules.