Characterization of dicarboxylate stimulation of ammonia, glutamine, and 2-oxoglutarate-dependent o(2) evolution in isolated pea chloroplasts

Three decades in transport business: studies of metabolite transport in chloroplasts - a personal perspective

This article gives an historical overview of our group's research on various metabolite translocators of chloroplasts, such as the translocators for phosphorylated intermediates of the Calvin-Benson cycle and of glycolysis, of ADP and ATP, of dicarboxylates, of pyruvate and of hexoses; how it began and where it led to. Wherever appropriate, references will be made to research in other laboratories.

A Metabolic Control Analysis of the Glutamine Synthetase/Glutamate Synthase Cycle in Isolated Barley (Hordeum vulgare L.) Chloroplasts

Ammonia assimilation in chloroplasts occurs via the glutamine synthetase/glutamate synthase (GS/GOGAT) cycle. To determine the extent to which these enzymes contribute to the control of ammonia assimilation, a metabolic control analysis was performed on isolated barley (Hordeum vulgare L.) leaf chloroplasts. Pathway flux was measured polarographically as ammonium-plus-2-oxoglutarate-plus-glutamine-dependent O2 evolution in illuminated chloroplasts. Enzyme activity was modulated by titration with specific, irreversible inhibitors of GS (phosphinothricin) and GOGAT (azaserine). Flux control coefficients (CJ0E0) were determined (a) by differentiation of best-fit hyperbolic curves of the data sets (flux versus enzyme activity), and (b) from estimates of the deviation indices (D/[prime]E0). Both analyses gave similar values for the coefficients. The control coefficient for GS was relatively high and the value did not change significantly with changes in 2-oxoglutarate concentration (C/0E0 = 0.58 at 5 mM 2-oxoglutarate and 0.40 at 20 mM 2-oxoglutarate). The control coefficient for GOGAT decreased with decreasing glutamine concentrations, from 0.76 at 20 mM glutamine to 0.19 at 10 mM glutamine. Thus, at high concentrations of glutamine, GOGAT exerts a major control over flux with a significant contribution also from GS. At lower concentrations of glutamine, however, GOGAT exerts far less control over pathway flux.

Chromatographic and immunological evidence that chloroplastic and cytosolic pea (Pisum sativum L.) NADP-isocitrate dehydrogenases are distinct isoenzymes

Two NADP-isocitrate dehydrogenase isoenzymes designated as NADP-IDH1 and NADP-IDH2 (EC 1.1.1.42) were identified in pea (Pisum sativum) leaf extracts by diethylaminoethylcellulose chromatography. The predominant form was found to be NADP-IDH1 while NADP-IDH2 represented only about 4% of the total leaf enzyme activity. These enzymes share few common epitopes as NADP-IDH2 was poorly recognized by the specific polyclonal antibodies raised against NADP-IDH1, and as a consequence NADP-IDH2 does not result from a post-translational modification of NADP-IDH1. Subcellular fractionation and isolation of chloroplasts through a Percoll gradient, followed by the identification of the associated enzymes, showed that NADP-IDH1 is restricted to the cytosol and NADP-IDH2 to the chloroplasts. Compared with the cytosolic isoenzyme, NADP-IDH2 was more thermolabile and exhibited a lower optimum pH. The data reported in this paper constitute the first report that the chloroplastic NADP-IDH and the cytosolic NADP-IDH are two distinct isoenzymes. The possible functions of the two isoenzymes are discussed.

Effect of sulphate on glutamate synthesis by intact spinach (Spinacia oleracea) chloroplasts

During the course of NH4+ (or NO2-)-plus-alpha-oxoglutarate-dependent O2 evolution in spinach (Spinacia oleracea) chloroplasts, glutamate was continuously excreted out of the chloroplasts. Under these conditions, for each molecule of NO2- or NH4+ which disappeared, one molecule of glutamate accumulated in the medium and the concentration of glutamate in the stroma space was maintained constant. SO4(2-) (or SO3(2-) behave as inhibitors of NH4+ incorporation into glutamate by intact chloroplasts. This considerable inhibition of glutamate synthesis by SO4(2-) was correlated with a rapid decline in the stromal Pi concentration. The reloading of stromal Pi with either external Pi or PPi4- relieved SO4(2-)-induced inhibition of glutamate synthesis by intact chloroplasts. It was concluded that SO4(2-) induced a rapid efflux of stromal Pi out of the chloroplast, leading to a limitation of ATP synthesis and therefore to an arrest of ATP-dependent glutamine synthetase functioning.

Characteristics of 2-oxoglutarate and glutamate transport in spinach chloroplasts : Studies with a double-silicone-layer centrifugation technique and in liposomes

The transport of glutamate, 2-oxoglutarate and malate in intact spinach chloroplasts was determined using a double-silicone-layer centrifugation technique in which the silicone layers stayed separated at the end of centrifugation. Glutamate was found to be transported via the dicarboxylate but not the 2-oxoglutarate translocator. Hence the kinetic parameters (i.e.K m,K i andV max) determined in glutamate-preloaded chloroplasts represent the kinetic constants of the dicarboxylate translocator. Measurements from malate- or succinate-preloaded chloroplasts represent the aggregate values of both the dicarboxylate and the 2-oxoglutarate translocators. Calculations showed that the 2-oxoglutarate and glutamate transport required to support the high fluxes of photorespiratory NH3 recycling could be achieved if the transport of these two dicarboxylates occurred on separate translocators. It is proposed that during photorespiration the transport of 2-oxoglutarate into and glutamate out of the chloroplast occurred via the 2-oxoglutarate and the dicarboxylate translocators, respectively. These transports are coupled to malate counter-exchange in a cascade-like manner resulting in a net 2-oxoglutarate/glutamate exchange with no net malate uptake.

The value of mutants unable to carry out photorespiration

Manipulation of the CO2 concentration of the atmosphere allows the selection of photorespiratory mutants from populations of seeds treated with powerful mutagens such as sodium azide. So far, barley lines deficient in activity of phosphoglycolate phosphatase, catalase, the glycine to serine conversion, glutamine synthetase, glutamate synthase, 2-oxoglutarate uptake and serine: glyoxylate aminotransferase have been isolated. In addition one line of pea lacking glutamate synthase activity and one barley line containing reduced levels of Rubisco are available. The characteristics of these mutations are described and compared with similar mutants isolated from populations of Arabidopsis. As yet, no mutant lacking glutamine synthetase activity has been isolated from Arabidopsis and possible reasons for this difference between barley and Arabidopsis are discussed. The value of these mutant plants in the elucidation of the mechanism of photorespiration and its relationships with CO2 fixation and amino acid metabolism are highlighted.

N-ammonia assimilation, 2-oxoglutarate transport, and glutamate export in spinach chloroplasts in the presence of dicarboxylates in the light

The direct incorporation of (15)NH(4)Cl into amino acids in illuminated spinach (Spinacia oleracea L.) chloroplasts in the presence of 2-oxoglutarate plus malate was determined. The amido-N of glutamine was the most highly labeled N-atom during (15)NH(4) assimilation in the presence of malate. In 4 minutes the (15)N-label of the amido-N of glutamine was 37% enriched. In contrast, values obtained for both the N-atom of glutamate and the amino-N of glutamine were only about 20% while that of the N-atom of aspartate was only 3%. The addition of malate during the assimilation of (15)NH(4)Cl and Na(15)NO(2) greatly increased the (15)N-label into glutamine but did not qualitatively change the order of the incorporation of (15)N-label into all the amino acids examined. This evidence indicates the direct involvement of the glutamine synthetase/glutamate synthase pathway for ammonia and nitrite assimilation in isolated chloroplasts. The addition of malate or succinate during ammonia assimilation also led to more than 3-fold increase in [(14)C]2-oxoglutarate transport into the chloroplast as well as an increase in the export of [(14)C]glutamate out of the chloroplast. Little [(14)C]glutamine was detected in the medium of the chloroplast preparations. The stimulation of (15)N-incorporation and [(14)C]glutamate export by malate could be directly attributed to the increase in 2-oxoglutarate transport activity (via the 2-oxoglutarate translocator) observed in the presence of exogenous malate.

A Two-Translocator Model for the Transport of 2-Oxoglutarate and Glutamate in Chloroplasts during Ammonia Assimilation in the Light

This study examines the transport of 2-oxoglutarate (2-OG) and other dicarboxylates during ammonia assimilation in illuminated spinach chloroplasts. The transport of all dicarboxylates examined was strongly inhibited by NH(4)Cl preincubation in the light. Treatment with NH(4)Cl caused a rapid depletion of the endogenous glutamate pool and a corresponding increase in endogenous glutamine content. The inhibition of transport activity by NH(4)Cl was apparently linked to its metabolism in the light because inhibition of glutamine synthetase activity by the addition of l-methionine sulfoximine or carbonylcyanide-m-chlorophenylhydrazone abolished this affect. Measurements of endogenous metabolite pools showed that malate was most rapidly exchanged during the uptake of all exogenous dicarboxylates examined. Depending on the exogenous substrates used, the apparent half-times of efflux measured for endogenous malate, aspartate and glutamate were 10, 10 to 30, and 15 to 240 seconds, respectively. The transport of 2-OG was also inhibited by malate. But chloroplasts preincubated with malate in the presence or absence of NH(4)Cl were found to have high transport activity similar to untreated chloroplasts. A two-translocator model is proposed to explain the stimulation of 2-OG transport as well as the stimulation of (NH(3), 2-OG)-dependent O(2) evolution by malate (KC Woo, CB Osmond 1982 Plant Physiol 69: 591-596) in isolated chloroplasts. In this model the transport of 2-OG on the 2-OG translocator and glutamate on the dicarboxylate translocator is coupled to malate counter-exchange in a cascade-like manner. This results in a net 2-OG/glutamate exchange with no net malate transport. Thus, during NH(3) assimilation the transport of 2-OG into and the export of glutamate out of the chloroplast occurs via the 2-OG and the dicarboxylate translocators, respectively.

Characterization of a light-dependent glutamate synthase activity in Chlamydomonas reinhardtii

Photosynthetically active vesicles prepared from Chlamydomonas reinhardtii retained a light-dependent glutamate synthase activity which was highly specific for 2-oxoglutarate (Km=2.1 mM) and L-glutamine (Km=0.9 mM) as amido group acceptor and donor respectively. This activity was inhibited by azaserine, p-hydroxymercuribenzoate and 3-(p-chlorophenyl)-1,1-dimethyl urea.Light-dependent synthesis of glutamate was also obtained by coupling Chlamydomonas photosynthetic particles to purified ferredoxin-glutamate synthase, using ascorbate and 2,6-dichlorophenol-indophenol as electron donor. This system was also specific for 2-oxoglutarate (Km=1 mM) and L-glutamine (Km=0.8 mM) as substrates, and was stimulated by dithioerythritol. Azaserine and p-hydroxymercuribenzoate, but not 3-(p-chlorophenyl)-1,1-dimethyl urea, inhibited the reconstituted activity; high concentrations of 2-oxoglutarate were inhibitory.

Carbon and nitrogen metabolism in a barley (Hordeum vulgare L.) mutant with impaired chloroplast dicarboxylate transport

A mutant line, RPr79/2, of barley (Hordeum vulgare L. cv. Maris Mink) has been isolated that has an apparent defect in photorespiratory nitrogen metabolism. The metabolism of (14)C-labelled glutamine, glutamate and 2-oxoglutarate indicates that the mutant has a greatly reduced ability to synthesise glutamate, especially in air, although in-vitro enzyme analysis indicates the presence of wild-type activities of glutamine synthetase (EC 6.3.1.2) glutamate synthase (EC 1.4.7.1 and EC 1.4.1.14) and glutamate dehydrogenase (EC 1.4.1.2). Several characteristics of RPr79/2 are very similar to those described for glutamate-synthase-deficient barley and Arabidopsis thaliana mutants, including the pattern of labelling following fixation of (14)CO2, and the rapid rise in glutamine content and fall in glutamate in leaves on transfer to air. The CO2-fixation rate in RPr79/2 declines much more slowly on transfer from 1% O2 to air than do the rates in glutamate-synthase-deficient plants, and RPr79/2 plants do not die in air unless the temperature and irradiance are high. Analysis of (glutamine+NH3+2-oxoglutarate)-dependent O2 evolution by isolated chloroplasts shows that chloroplasts from RPr79/2 require a fivefold greater concentration of 2-oxoglutarate than does the wild-type for maximum activity. The levels of 2-oxoglutarate in illuminated leaves of RPr79/2 in air are sevenfold higher than in Maris Mink. It is suggested that RPr79/2 is defective in chloroplast dicarboxylate transport.