Regulation of phosphoenolpyruvate carboxylase of Zea mays by metabolites

Phosphoenolpyruvate carboxylase from pennywort (Umbilicus rupestris). Changes in properties after exposure to water stress

Umbilicus rupestris (pennywort) switches from C3 photosynthesis to an incomplete form of crassulacean acid metabolism (referred to as 'CAM-idling') when exposed to water stress (drought). This switch is accompanied by an increase in the activity of phosphoenolpyruvate carboxylase. This enzyme also shows several changes in properties, including a marked decrease in sensitivity to acid pH, a lower Km for phosphoenolpyruvate, very much decreased sensitivity to the allosteric inhibitor malate, and increased responsiveness to the allosteric effector glucose 6-phosphate. The Mr of the enzyme remains unchanged, at approx. 185 000. These changes in properties of phosphoenolpyruvate carboxylase are discussed in relation to the roles of the enzyme in C3 and in CAM plants.

Role and location of NAD malic enzyme in thermogenic tissues of Araceae

This work was done to discover how those nonphotosynthetic tissues of the Araceae that become thermogenic release, as CO2, carbon recently fixed by phosphoenolpyruvate carboxylase. Extracts of clubs of the spadix of Arum maculatum showed no activity for phosphoenolpyruvate carboxykinase and low activities of NADP malic enzyme. NAD malic enzyme activity in the above extracts and in those of thermogenic tissues of other Araceae was appreciable. Analysis of homogenates of clubs of Typhonium giraldii by differential centrifugation and sucrose gradients showed that NAD malic enzyme was confined to mitochondria. Centrifugation of mitochondria after freezing and thawing left all the NAD malic enzyme in the supernatant. NAD malic enzyme in isolated, intact mitochondria was completely latent, and was completely protected from exogenous trypsin. The responses of this latency and protection to different concentrations of Triton X-100 suggested that none of the NAD malic enzyme was accessible from either the outside or the intermembrane space of the mitochondria. Treatment of excised clubs of A. maculatum with 2-N-butylmalonate largely prevented the development of the rapid respiration responsible for thermogenesis, and severely inhibited dark fixation of 14CO2. The conclusion is that in mature clubs of the Araceae phosphoenolpyruvate is converted to malate in the cytosol by phosphoenolpyruvate carboxylase and NAD malate dehydrogenase, and that this malate then enters the mitochondrial matrix where it is converted to pyruvate by NAD malic enzyme.

Light-dependent reduction of dehydroascorbate and uptake of exogenous ascorbate by spinach chloroplasts

A reconstituted spinach chloroplast system containing thylakoids, stroma and 0.1 mM NADPH supported O2 evolution in the presence of oxidised glutathione (GSSG). The properties of the reaction were consistent with light-coupled GSSG-reductase activity involving H2O as eventual electron donor. The reconstituted system also supported dehydroascorbate-dependent O2 evolution in the presence of 0.6 mM reduced glutathione (GSH) and 0.1 mM NADPH with the concomitant production of ascorbate. The GSSG could replace GSH in which case the production of GSH preceded the accumulation of ascorbate. The data are consistent with the light-dependent reduction of dehydroascorbate using H2O as eventual electron donor via the sequence H2O→NADP→GSSG→dehydroascorbate. Approximately 30% of the GSH-dehydrogenase activity of spinach leaf protoplasts is localised in chloroplasts: this could not be attributed to contamination of chloroplasts by activity from the extrachloroplast compartment. Washed intact chloroplasts supported the uptake of ascorbate but the uptake mechanism had a very low affinity for ascorbate (Km approximately 20 mM). The rate of uptake of ascorbate was less than the rate of light-dependent reduction of dehydroascorbate and too slow to account for the rate of H2O2 reduction by washed intact chloroplasts.

Photosynthetic carbon metabolism of the cool-temperate C4 grass Spartina anglica Hubb

The aim of this work was to describe the photosynthetic carbon metabolism of the cooltemperate C4 grass Spartina anglica. With the exception of pyruvate, phosphate dikinase and pyruvate kinase, the maximum catalytic activities in leaves of putative enzymes of the C4 cycle of a phosphoenolpyruvate-carboxykinase C4 plant were considerably in excess of the observed, steady-state rate of photosynthesis, and were comparable with the maximum catalytic activities of key enzymes of the reductive pentose-phosphate pathway. Radioactive carbon from (14)CO2 supplied to attached leaves during steady-state photosynthesis appeared first in malate and aspartate from which it moved to intermediates of the reductive pentose-phosphate pathway, and then to sucrose. These experiments show that photosynthetic carbon metabolism in this cool-temperate C4 plant is similar to that of C4 plants of hotter climates.

Changes in properties of phosphoenolpyruvate carboxylase from the CAM plant Sedum praealtum D.C. upon dark/light transition and their stabilization by glycerol

A prenounced decrease in phosphoenolpyruvate earboxylase (PEPC) activity is observed upon dark/light transition in Sedum praealtum D.C., only when glycerol is included in the extraction medium. If glycerol is omitted, the activity extracted in light is initially low, but soon reaches night levels. The stabilization of the light-induced form of the enzyme by glycerol, in crude or desalted extracts, made it possible to study its kinetic properties in comparison to those of the dark form. The behaviour towards substrate (PEP) changes from hyperbolic (dark) to sigmoid (light), S0.5 is increased and the enzymic activity becomes more sensitive to malate inhibition. Quite different activity/pH profiles are also obtained for the two forms of PEPC.It is inferred that the in vivo regulation of PEPC in CAM is effected by a concerted action of light, malate and pH shifting.

The intracellular distribution of adenylate kinase in the leaves of spinach, wheat and barley

Of the total adenylate-kinase activity in 10-d-old barley and wheat leaves, 40-50% is localised in the chloroplasts, while in mature spinach leaves 50-70% of the enzyme is chloroplastic. The extra-chloroplastic adenylate-kinase activity is associated with the mitochondria, very little, if any, is freely soluble in the cytoplasm. The adenylate pool of the cytoplasm could have access to adenylate-kinase activity in the intermitochondrial space because of the free permeation of adenylates across the outer mitochondrial membrane. Thus the adenylate pool of the cytoplasm could be subject to adenylate-kinase equilibrium. The mitochondrial adenylate kinase appeared to the localised exclusively in the intermembrane space.

Purification and regulatory properties of mung bean (vigna radiata L.) serine hydroxymethyltransferase

Serine hydroxymethyltransferase, the first enzyme in the pathway for interconversion of C(1) fragments, was purified to homogeneity for the first time from any plant source. The enzyme from 72-h mung bean (Vigna radiata L.) seedlings was isolated using Blue Sepharose CL-6B and folate-AH-Sepharose-4B affinity matrices and had the highest specific activity (1.33 micromoles of HCHO formed per minute per milligram protein) reported hitherto.The enzyme preparation was extremely stable in the presence of folate or l-serine. Pyridoxal 5'-phosphate, ethylenediaminetetraacetate and 2-mercaptoethanol prevented the inactivation of the enzyme during purification. The enzyme functioned optimally at pH 8.5 and had two temperature maxima at 35 and 55 degrees C. The K(m) values for serine were 1.25 and 68 millimolar, corresponding to V(max) values of 1.8 and 5.4 micromoles of HCHO formed per minute per milligram protein, respectively. The K(0.5) value for l-tetrahydrofolate (H(4)folate) was 0.98 millimolar. Glycine, the product of the reaction and d-cycloserine, a structural analog of d-alanine, were linear competitive inhibitors with respect to l-serine with K(i) values of 2.30 and 2.02 millimolar, respectively. Dichloromethotrexate, a substrate analog of H(4)folate was a competitive inhibitor when H(4)folate was the varied substrate. Results presented in this paper suggested that pyridoxal 5'-phosphate may not be essential for catalysis.The sigmoid saturation pattern of H(4)folate (n(H) = 2.0), one of the substrates, the abolition of sigmoidicity by NADH, an allosteric positive effector (n(H) = 1.0) and the increase in sigmoidicity by NAD(+) and adenine nucleotides, negative allosteric effectors (n(H) = 2.4) clearly established that this key enzyme in the folate metabolism was an allosteric protein. Further support for this conclusion were the observations that (a) serine saturation exhibited an intermediary plateau region; (b) partial inhibition by methotrexate, aminopterin, O-phosphoserine, dl-alpha-methylserine and dl-O-methylserine; (c) subunit nature of the enzyme; and (d) decrease in the n(H) value from 2.0 for H(4)folate to 1.5 in presence of l-serine.These results highlight the regulatory nature of mung bean serine hydroxymethyltransferase and its possible involvement in the modulation of the interconversion of folate coenzymes.

Extremely high activities of phosphoenolpyruvate carboxylase in thermogenic tissues of Araceae

Phosphoenolpyruvate carboxylase activity in extracts of a wide range of thermogenic tissues of the Araceae was shown to be in the range 10-100 μmol g(-1) fresh weight min(-1) (0.5-3.7 μmol mg(-1) protein min(-1)). Such high activities were not found in non-thermogenic tissues of the Araceae or in thermogenic tissues of Aristolochia brasiliensis Mart. and Zucc., Victoria amazonica Schomb. and Encephalartos barteri Carruth. During development and thermogenesis in the club of Arum maculatum L. the high activities of the carboxylase did not lead to any marked accumulation of citrate, isocitrate, 2-oxoglutarate, fumarate, malate and oxaloacetate. Clubs of Arum maculatum and of Arum italicum Miller readily fixed (14)CO2 in the dark, mostly into aspartate, malate, alanine and glutamate. Pulse and chase experiments showed that most of the fixed carbon was very rapidly metabolized to CO2. The detailed distribution suggest that this occurred largely by decarboxylation of C-4 acids. It is suggested that thermogenic tissues of the Araceae are characterized by very high activities of phosphoenolpyruvate carboxylase, and that in vivo this leads to synthesis of C-4 acids which are promptly decarboxylated.

Chlamydomonas reinhardtii cell preparation with altered permeability toward substrates of organellar reactions

Chlamydomonas reinhardtii cells disrupted under low pressure in a Yeda press yielded a preparation ("presate") with high permeability toward substrates for Class A chloroplasts and intact mitochondria. The stoichiometric rates of CO2 uptake and O2 photoevolution by the wild-type cell pressate were severely suppressed by 10 mM exogenous phosphate, and this suppression could be reversed by the addition of either 3-phosphoglycerate or dihydroxyacetone phosphate. A mutant, F60, which lacks phosphoribulokinase activity and hence CO2-dependent O2 photoevolution, was studied by using intact cells, pressate, and sonicated pressate. In the pressate, the rate of 3-phosphoglycerate-dependent O2 photoevolution was high, whereas that dependent on K3Fe(CN)6 was low; the opposite was true of the sonicated pressate. p-Benzoquinone supported high rates of O2 evolution in both the pressate and the sonicated pressate. The slow O2 uptake in the dark by the dark-adapted wild-type pressate could be increased by the addition of succinate and further stimulated by ADP. Addition of KCN resulted in rapid but only partial suppression of this activity. Dark O2 uptake by the unpressed preparation did not show similar responses. The procedure described here opens the possibility of in situ analysis of Class A chloroplasts from wild-type and mutant strains of C. reinhardtii.

Light induction of phosphoenolpyruvate carboxylase in etiolated maize leaf tissue

An antibody for phosphoenolpyruvate carboxylase was used to isolate and to quantitate the enzyme from greening maize (cv. KOU 6) leaves. The increase in enzyme activity during greening was due to de novo synthesis, which was paralleled by increases in enzyme protein and incorporation of leucine. The light-induced activity was due to one specific isoenzyme. The action spectrum for enzyme synthesis had red and blue peaks.

Uptake and subcellular compartmentation of gibberellin a(1) applied to leaves of barley and cowpea

The uptake and subcellular accumulation of gibberellin A(1) (GA(1)) by leaves and protoplasts of barley (cv. Numar) and cowpea (cv. Blackeye pea No. 5) were investigated.Uptake of GA(1) by cowpea leaves is optimal at pH 5.8 and occurs by a saturable, probably carrier-mediated process having a half-maximal velocity at 10 to 20 micromolar. Uptake by both barley and cowpea leaves is inhibited by low temperature (+4 C) and the metabolic inhibitors 2,4-dinitrophenol and azide and is stimulated by ATP. Mesophyll protoplasts isolated from leaves fed radioactive GA(1) retain 20 to 80% of the radioactivity incorporated by excised leaves.The subcellular localization of the [(3)H]GA was determined by lysing protoplasts and separating subcellular organelles by density gradient centrifugation. Less than 5% of the incorporated [(3)H]GA was found associated with chloroplasts, mitochondria, nuclei, or other organelles or membranes with densities in sucrose gradients greater than 1.15 grams per cubic centimeter. Fifty to 100% of the [(3)H]GA was found in vacuoles. Isolated vacuoles were judged to be free of contamination by cytoplasm using phosphoenolpyruvate carboxylase as a marker enzyme. Osmotic breakage of vacuoles or protoplasts released > 95% of the [(3)H]GA, suggesting that GA is associated with the vacuolar sap rather than with the tonoplast membrane.

Kinetic analysis of effectors of phosphoenolpyruvate carboxylase from Bryophyllum fedtschenkoi

The activity of phosphoenolpyruvate carboxylase (orthophosphate:oxaloacetate carboxy-lyase (phosporylating) EC 4.1.1.31) purified from Bryophyllum fedtschenkoi has been measured in the presence of various concentrations of phosphoenolpyruvate, L-malate and glucose 6-phosphate. At high pH, the enzyme is competitively inhibited by L-malate and activated by glucose 6-phosphate. A reaction scheme describing the interaction of enzyme, substrate and effectors is proposed. Values for the appropriate equilibrium constants have been calculated for the enzyme acting at pH 7.8, which is one of its two pH optima. The kinetics are more complicated at low pH, partly because of non-linear reaction rates and partly because inhibition by L-malate is not competitive. Activation by glucose 6-phosphate is similar at high and low pH values. The behaviour of a wide range of other possible effectors is described briefly.

Phosphoenolpyruvate carboxylase from soybean nodule cytosol. Evidence for isoenzymes and kinetics of the most active component

Phosphoenolpyruvate carboxylase (orthophosphate:oxaloacetate carboxylase (phosphorylating), EC 4.1.1.31) from plant cells of soybean nodules was studied to assess its role in providing carbon skeletons for aspartate and asparagine synthesis. The enzyme was purified 119-fold by (NH4)2SO4 fractionation and DEAE-cellulose, BioGel A-1.5m, and hydroxyapatite chromatography. Five activity bands were resolved with discontinuous polyacrylamide gel electrophoresis. A small quantity of enzyme from the most active band was separated from the others by preparative electrophoresis. The apparent Michaelis constants of this enzyme for phosphoenolpyruvate and HCO3- were 9.4.10(-2) and 4.1.10(-1) mM, respectively. A series of metabolite tested at 1 mM had no significant effect on enzyme activity. These experiments indicate that the major factors directly controlling phosphoenolpyruvate carboxylase activity in vivo are phosphoenolpypyruvate and HCO3- concentrations.

Pathways of carbohydrate fermentation in the roots of marsh plants

We did this work to discover the pathways of carbohydrate fermentation in unaerated roots of three species of flood-tolerant plants, Ranunculus sceleratus, Glyceria maxima, and Senecio aquaticus. The experiments were done with the apical 1-2 cm of the roots and the results for the three species were similar. The maximum catalytic activities of alcohol dehydrogenase, lactate dehydrogenase, phosphoenolpyruvate carboxylase, NADP-dependent malic enzyme, and phosphofructokinase were appreciable and roughly comparable. Reduced aeration of the roots led to 1.5 to 5-fold increases in the maximum catalytic activities of alcohol dehydrogenase, small increases in those of lactate dehydrogenase in two species, and no increase in those of phosphoenolpyruvate carboxylase and phosphofructokinase. Phosphoenolpyruvate carboxykinase could not be detected. Metabolism of [U-(14)C]sucrose under anaerobic conditions by excised roots, grown without aeration, led to appreciable labelling of ethanol and alanine, slight but significant labelling of lactate, and minimal labelling of malate and related organic acids. Incubation of similar excised roots under anaerobic conditions for 4 h caused marked accumulation of ethanol, smaller accumulation of lactate, and no detectable accumulation of malate. We conclude that in all three species fermentation of carbohydrate results in the accumulation of predominant amounts of ethanol, smaller amounts of lactate, no significant quantities of malate, and probably appreciable amounts of alanine. Crawford's metabolic theory of flooding tolerance is held to be incompatible with these results.

Pathway of starch breakdown in photosynthetic tissues of Pisum sativum

1. The aim of this work was to discover the pathway of starch breakdown in the photosynthetic tissues of Pisum sativum. 2. Measurements of the starch in the leaves of plants grown in photoperiods of 12 or 18 h showed that starch, synthesized in the light, was rapidly metabolized in the dark at rates of 0.04--0.06 mumol glucose/min per g fresh weight. 3. The maximum catalytic activities of alpha-amylase, beta-amylase, hexokinase, alpha-glucan phosphorylase and phosphoglucomutase in extracts of leaves showed no diurnal variation in either photoperiod, and exceeded estimates of the rate of net starch breakdown in the dark. 4. Studies with intact chloroplasts, isolated from young shoots and from leaves, indicated that pea chloroplasts do not contain significant activities of alpha-amylase, beta-amylase and hexokinase, although some of the latter may be attached to the outside of the chloroplast envelope. These studies also showed that pea chloroplasts contained sufficient alpha-glucan phosphorylase and phosphoglucomutase to mediate the observed rates of starch breakdown. 5. It is proposed that starch breakdown in pea chloroplasts is phosphorolytic.

Identification of the regulatory steps in gluconeogenesis in cotyledons of Cucurbita pepo

1. The aim of this work was to discover the steps at which the conversion of oxaloacetate to glucose 6-phosphate during gluconeogenesis is regulated in the cotyledons of 5-day-old seedlings of Cucurbita pepo. 2. We estimated the maximum catalytic activities of all the enzymes in the above sequence and also the amounts of their substrates present in vivo. The results show that the reactions catalysed by fructose-1,6-bisphosphatase and phosphoenolpyruvate carboxykinase are the only ones in the sequence that are substantially displaced from equilibrium in vivo. 3. We also determined the effects of 3-mercaptopicolinic acid, an inhibitor of gluconeogenesis, on the amounts of the gluconeogenic intermediates present in vivo. The results show that the enzyme system, fructose-1,6-bisphosphatase: phosphofructokinase, and the system phosphoenolpyruvate carboxykinase: phosphoenolpyruvate carboxylase make major contributions to the regulation of gluconeogenesis in the cotyledons. 4. Possible mechanisms for the above regulation are discussed.

Dark fixation of CO2 during gluconeogenesis by the cotyledons of Cucurbita pepo L

We did this work to discover the pathway of CO2 fixation into sugars in the dark during gluconeogenesis by the cotyledons of 5-day-old seedlings of Cucurbita pepo L. We paid particular attention to the possibility of a contribution from ribulosebisphosphate carboxylase. The detailed distribution of (14)C after exposure of excised cotyledons to (14)CO2 in the dark was determined in a series of pulse and chase experiments. After 4s in (14)CO2, 89% of the (14)C fixed was in malate and aspartate. In longer exposures, and in chases in (12)CO2, label appeared in alanine, phosphoenolpyruvate, 3-phosphoglycerate and sugar phosphates, and accumulated in sugars. The transfer of label from C-4 acids to sugars was restricted by inhibition of phosphoenolpyruvate carboxykinase in vivo by 3-mercaptopicolinic acid. We conclude as follows. Initial fixation of CO2 in the dark is almost entirely into phosphoenolpyruvate, probably via phosphoenolpyruvate carboxylase (EC 4.1.1.31) which we showed to be present in appreciable amounts. Incorporation into sugars occurs chiefly, if not completely, as a result of randomization of the carboxyl groups of the C-4 acids and subsequent conversion of the oxaloacetate to sugars via the accepted sequence for gluconeogenesis. Ribulosebisphosphate carboxylase appears to make very little contribution to sugar synthesis from fat.

Heat inactivation of leaf phosphoenolpyruvate carboxylase: Protection by aspartate and malate in C4 plants

The activity of phosphoenolpyruvate (PEP) carboxylase EC 4.1.1.31 in leaf extracts of Eleusine indica L. Gaertn., a C4 plant, exhibited a temperature optimum of 35-37° C with a complete loss of activity at 50° C. However, the enzyme was protected effectively from heat inactivation up to 55° C by L-aspartate. Activation energies (Ea) for the enzyme in the presence of aspartate were 2.5 times lower than that of the control enzyme. Arrhenius plots of PEP carboxylase activity (±aspartate) showed a break in the slope around 17-20° C with a 3-fold increase in the Ea below the break. The discontinuity in the slopes was abolished by treating the enzyme extracts with Triton X-100, suggesting that PEP carboxylase in C4 plants is associated with lipid and may be a membrane bound enzyme. Depending upon the species, the major C4 acid formed during photosynthesis (malate or aspartate) was found to be more protective than the minor C4 acid against the heat inactivation of their PEP carboxylase. Oxaloacetate, the reaction product, was less effective compared to malate or aspartate. Several allosteric inhibitors of PEP carboxylase were found to be moderately to highly effective in protecting the C4 enzyme while its activators showed no significant effect. PEP carboxylase from C3 species was not protected from thermal inactivation by the C4 acids. The physiological significance of these results is discussed in relation to the high temperature tolerance of C4 plants.

Regulation of the Biosynthesis of Ent-Kaurene from Mevalonate in the Endosperm of Immature Marah macrocarpus Seeds by Adenylate Energy Charge

The rate of kaurene biosynthesis from mevalonate in a cell-free enzyme preparation from the endosperm of immature seeds of Marah macrocarpus is regulated by adenylate energy charge. The response curve is typical of a biosynthetic energy-utilizing sequence in which the rate of biosynthesis increases sharply as the energy charge is increased above 0.80. ADP proved to be an effective inhibitor of this process. AMP gave no inhibition at concentrations up to 2 mm and orthophosphate gave no inhibition up to 15 mm. Measurement of the pool sizes of intermediates in the sequence showed that the presence of ADP caused an increase in the levels of 5-phosphomevalonate and 5-pyrophosphomevalonate and a decrease in the levels of isopentenyl pyrophosphate and kaurene. These results indicate that pyrophosphomevalonate decarboxylase is the enzyme most subject to regulation by adenylate energy charge. The rate of conversion of isopentenyl pyrophosphate to kaurene and the rate of utilization of mevalonate by mevalonate kinase were not influenced by variations in the adenylate energy charge.

Ecophysiologic investigations in the family of the Mesembryanthemaceae : Occurrence of a CAM and Ion Content

Overnight accumulation of malate, citrate, and isocitrate in a large number of species of Mesembryanthemaceae grown under identical environmental conditions was studied. Of the 27 species investigated, 24 showed malate accumulation, which in 3 cases was accompanied by considerable overnight accumulation of citrate. In the leaves of the same plants, the Na⁺, K⁺, Cl^-, SO₄^2-, and PO₄^3- contents were determined. Although the plants were not exposed to substrates of high NaCl content, they exhibited extraordinarily high levels of Na⁺ and Cl^-. All plants accumulated, much more Na⁺ than K⁺. No readily discernible correlation between the amount of any particular ion and the extend of CAM was found. It is concluded that halophilism and CAM are widespread phenomena in the family of Mesembryanthemaceae that possess ecologic significance.

The regulation of potato phosphoenolpyruvate carboxylase in relation to a metabolic pH-stat

Phosphoenolpyruvate (PEP) carboxylase (E.C. 4.1.1.31.) was extracted from potato tubers (Solanum tuberosum L.) and investigated for regulatory response to metabolites. The enzyme was found to be activated by sugar phosphates and glycollate and non-competitively inhibited by succinate and fumarate. In both cases the effects were highly dependent on pH, being maximal between pH 7 and 7.6. Rapid extraction techniques demonstrated that the enzyme suffers a sharp decline in activity and sensitivity to metabolites during the first 2 h from extraction. The observed properties of PEP carboxylase were related to the possible role of the enzyme in a metabolic pH-stat.

[Changes in adenosine phosphates and energy charge in chloroplastic and nonchloroplastic compartments of wheat leaves (author's transl)]

Changes in the adenine nucleotides and energy charge (= (ATP)+1/2(ADP)/(AMP)+(ADP)+(ATP)) levels were studied in chloroplastic and non-chloroplastic compartments using non-aqueously isolated wheat leaves chloroplasts. The two adenine nucleotides pools (of chloroplasts and non-chloroplastic part of the cell), though distinct, are linked. This linkage substantiates an energy-rich bond exchange between the two compartments. When both photphosphorylation and oxidative phosphorylation occur simultaneously, energy charge takes high values, generally higher than 0.80. When neither oxidative phosphorylation nor photophosphorylation occur, energy charge is very low and takes values generally lower than 0.45. When one compartment alone produces approximately P, energy charge in the two compartments takes intermediate values which remain relatively high. Dark-light transition in nitrogen resulted in changes of the AMP, ADP and ATP levels which quickly reach a steady state. Chloroplast energy charge shifts rapidly from 0.45 to 0.75 in 10 s; after 1 min it reaches 0.86, a value that corresponds to a steady level. In the cytoplasm, energy charge changes from 0.44 to 0.71 in 1 min. Energy charge increase in the non-chloroplastic compartment substantiates an energy transfer from chloroplasts to the cytoplasm. On nitrogen-air transition in the dark, the cytoplasm energy charge reaches a steady level in 30 s. In chloroplasts, it also increases but slowly. There is indeed a transfer of energy from cytoplasm to chloroplasts. Darkening of the leaves in air causes a drastic and lasting drop of energy charge in the chloroplasts where it has a low value after 5 min in the dark. Then it increases again but slowly and is still lower than 0.70 after 10 min in the dark. Meanwhile, energy charge in cytoplasm keeps values higher than 0.75. Metabolic regulation by energy charge and control of adenine nucleotides level by adenylate kinase (EC 2.7.4.3) are discussed.

Chloroplast and Cytoplasmic Enzymes: VI. Pea Leaf 3-Phosphoglycerate Kinases

Pea (Pisum sativum) leaf chloroplastic and cytoplasmic 3-phosphoglycerate kinases (ATP: d-3-phosphoglycerate 1-phosphotransferase, EC 2.7.2.3) have similar Michaelis constants for ATP, 0.7 and 0.55 mm, for ADP, 0.18 and 0.22, and for 3-P-glycerate, 0.59 and 0.54 mm at low substrate concentrations, and 1.6 and 1.25 mm at high substrate concentrations. Both enzymes are inhibited by ADP and AMP in the ATP-utilizing direction and by ATP and AMP in the ATP-generating direction and are controlled by energy charge. Apparently, whether the cytoplasmic and chloroplastic kinases in the plant cell will participate in the reductive pentose phosphate cycle and gluconeogenesis or in glycolysis will be determined by the environment in the cell compartment and not by the differential properties of the enzymes themselves.

Development of C4 photosynthesis in sugar cane: Changes in properties of phosphoenolpyruvate carboxylase during greening

Phosphoenolpyruvate (PEP) carboxylase extracted from etiolated and greening sugar cane (a Saccharum hybrid) displayed different properties in terms of DEAE-cellulose elution profile and activation by glucose-6-phosphate. During the first 20 hours of greening, no increase in extractable PEP carboxylase activity was observed, but ionic and allosteric properties of the enzyme changed, becoming similar to those found for the enzyme from light-grown cane. Density labelling studies with deuterium oxide provided no evidence for either de novo synthesis or turnover of the enzyme during this period, or during the following 24 hours of greening. These results are discussed in relation to enzyme control mechanisms and the development of the C4 pathway in photosynthesis.

[The role of inorganic phosphate in the regulation of the phosphoenolpyruvate carboxylase of Mesembryanthemum crystallinum L]

The activity of the phosphoenolpyruvate (PEP) carboxylase isolated from non-saline grown plants of the salt-tolerant plant Mesembryanthemum crystallinum is strongly inhibited by malate. This inhibition was found to depend on the pH (rising with increasing H(+)-concentrations) and on the concentration of malate used. The addition of inorganic phosphate (appropriate concentration 30 mM) to the in-vitro enzyme assay prior to malate addition results in a remarkable compensation of malate-caused inhibition of the enzyme activity. Again a dependency upon the pH can be observed. The ability of inorganic phosphate to restore malate-caused inhibition of the PEP-carboxylase increases with increasing pH.Another potent inhibitor of the PEP-carboxylase is NaCl, which shows a minimum inhibition at pH 7. At this pH a concentration of more than 60 mM NaCl is needed to reduce the activity of the enzyme below the control level with a 50% inhibition is reached at 150 mM. If the addition of NaCl is performed in the presence of 30 mM inorganic phosphate the inhibition is less pronounced. The enzyme now tolerates about 100 mM higher concentrations of NaCl without being inhibited.NaCl-treatment of Mesembryanthemum crystallinum plants results in an increase of inorganic phosphate in the cells with a concomitant establishment of a Crassulacean acid metabolism. The present results support evidence for a protective function of inorganic phosphate (compensation of NaCl-induced enzyme inhibition), possibly a commen reaction involved in the question of salt tolerance, and a more specific function (restriction of malatecaused inhibition of the PEP-carboxylase) providing the enzymatic background for the malate accumulation in Mesembryanthemum crystallinum.

Metabolic regulation in C4 photosynthesis: Phosphoenol pyruvate carboxylase and 3C intermediates of the photosynthetic carbon reduction cycle

The effects of 3C intermediates of the photosynthetic carbon reduction (PCR) cycle (glyceraldehyde 3 phosphate [G3P], dihydroxyacetone phosphate and 3 phosphoglyceric acid) on the activity of phosphoenol pyruvate (PEP) carboxylase from Pennisetum purpureum (a C4 plant) have been investigated. Addition of these compounds, at concentrations greater than about 3 mM, to standard assay mixtures (5 mM concentrations of PEP, HCO3 (-) and Mg(2+)) reduced the observed activity. The extent of inhibition was greater at lower (<5 mM) concentration of Mg(2+). G3P was the most potent inhibitor. During studies of saturation kinetics with PEP as variable substrate it was found that addition of these 3C compounds (2mM) reduced the sigmoid nature of Michaelis-Menten plots and reduced the value of n obtained from Hill plots from about 3 to near one. The results are disussed in terms of the possible regulation of C4 photosynthesis by intermediates of the PCR cycle which are easily leaked from chloroplasts.

Metabolic regulation in C4 photosynthesis: PEP-carboxylase and energy charge

The effects of ATP, ADP and AMP on the activity of PEP carboxylase from Pennisetum purpureum (a C4 plant) have been investigated. AMP caused slight changes in activity. Both ATP and ADP were inhibitory. The extent of inhibition was related to the concentration of Mg(2+). Kinetics of inhibition (with PEP as variable substrate) were investigated at limiting concentrations of Mg(2+) (2 mM); with excess Mg(2+) (5 mM); or with the concentration of Mg(2+) buffered at about 8 mM. At low Mg(2+) parabolic competitive-kinetics were observed. With higher Mg(2+) concentration linear competitive kinetics of inhibition were observed with a K i (slope) of 1.25 mM ATP. This was increased to 2.03 mM on addition of the allosteric activator glucose-6-P (5 mM). With Mg(2+) buffered ATP was an activator at low concentrations of PEP.Response to enegery charge was investigated using either calculated concentrations of all adenylates or mixtures of AMP and ATP pre-incubated with adenylate kinase. Inhibition at high charge could be reduced by increasing the concentration of Mg(2+) or by addition of the activator glucose-6-P. With enzyme-generated charge series and Mg(2+) buffered at 8 mM an increase in activity was observed at high charge values.These results are discussed in terms of a possible regulatory role of adenylates in the C4 dicarboxylic acid pathway of photosynthesis.

Regulation of 'malic' enzyme of Solanum tuberosum by metabolites

A purification of ;malic' enzyme from potato is described. The purified enzyme is specific for NADP and requires a bivalent cation for activity. At pH values below 7 the plot of rate versus malate concentration approximates to normal Michaelis-Menten kinetics. At pH values above 7 the plot of rate versus malate concentration is sigmoid. A number of dicarboxylic acids activate the enzyme and remove the sigmoidicity. The enzyme is inhibited by phosphate, triose phosphates and AMP. In general, effectors of the oxidative decarboxylation of malate behave in the same manner in the reductive carboxylation of pyruvate. The response of the enzyme to energy charge is reported and the physiological significance of the response to metabolites is discussed in relation to the proposed role of the enzyme in the control of pH.