A cytosolic ADP-glucose pyrophosphorylase is a feature of graminaceous endosperms, but not of other starch-storing organs

The occurrence of an extra-plastidial isoform of ADP-glucose (Glc) pyrophosphorylase (AGPase) among starch-storing organs was investigated in two ways. First, the possibility that an extra-plastidial isoform arose during the domestication of cereals was studied by comparing the intracellular distribution of enzyme activity and protein in developing endosperm of noncultivated Hordeum species with that previously reported for cultivated barley (Hordeum vulgare). As in cultivated barley, the AGPase of H. vulgare subsp. spontaneum and Hordeum murinum endosperm is accounted for by a major extra-plastidial and a minor plastidial isoform. Second, the ratio of ADP-Glc to UDP-Glc was used as an indication of the intracellular location of the AGPase activity in a wide range of starch-synthesizing organs. The ratio is expected to be high in organs in which UDP-Glc and ADP-Glc are synthesized primarily in the cytosol, because the reactions catalyzed by AGPase and UDP-Glc pyrophosphorylase will be coupled and close to equilibrium. This study revealed that ADP-Glc contents and the ratio of ADP-Glc to UDP-Glc were higher in developing graminaceous endosperms than in any other starch-storing organs. Taken as a whole the results indicate that an extra-plastidial AGPase is important in ADP-Glc synthesis in graminaceous endosperms, but not in other starch-storing organs.

Meeting the photosynthetic demand for inorganic carbon in an alga-invertebrate association: preferential use of CO2 by symbionts in the giant clam Tridacna gigas

Unlike most marine invertebrates which excrete respiratory CO2, giant clams (Tridacna gigas) must acquire inorganic carbon (Ci) in order to support their symbiotic population of photosynthetic dinoflagellates. Their capacity to meet this demand will be reflected in the Ci concentration of their haemolymph during periods of high photosynthesis. The Ci concentration in haemolymph was found to be inversely proportional to irradiance with a minimum Ci concentration of 0.75 mM at peak light levels increasing to 1.2 mM in the dark. The photosynthetic rate of isolated zooxanthellae under conditions that prevail in the haemolymph at peak light levels was significantly less than the potential Pmax (maximum photosynthetic rate) indicating that zooxanthellae are carbon limited in hospite. This is consistent with previous studies on the hermatypic coral symbiosis. The Pmax was not affected by pH but there was a dramatic increase in the half-saturation constant for Ci (K0.5 (Ci)) with increasing pH (6.5-9.0) and only a small decrease in K0.5 (CO2) over the same range. These results indicate that zooxanthellae in giant clams use CO2 as the primary source of their Ci in contrast to symbionts in corals, which use bicarbonate. The physiological implications are discussed and comparison is made with the coral symbiosis.

Characterization of transgenic potato (Solanum tuberosum) tubers with increased ADPglucose pyrophosphorylase

The aim of the work described in this paper was to characterize the tubers of potato (Solanum tuberosum var. Prairie) plants that had been transformed with the Escherichia coli ADPglucose pyrophosphorylase (EC 2.7.7.27) gene, glgC-16, under the control of a patatin promoter. Over 30 lines of transformed plants with increased ADPglucose pyrophosphorylase activity were obtained. The tubers of six of these lines were compared with those of control plants expressing the gene for beta-glucuronidase. The average increase in pyrophosphorylase activity was 200%, and the highest was 400%. Western immunoblotting of tuber extracts showed that the amounts of glgC-16 protein were linearly related to the extractable activity of the ADPglucose pyrophosphorylase. Cell fractionation studies showed that the increased activity of the pyrophosphorylase in the glgC-16 tubers had a similar intracellular location, the amyloplast fraction, to that found in the control tubers. No pleiotropic changes in the maximum catalytic activities of the following enzymes could be detected in the glgC-16 tubers: sucrose synthase, fructokinase, UDPglucose pyrophosphorylase, phosphofructokinase, soluble starch synthase, starch branching enzyme, phosphoglucomutase and alkaline inorganic pyrophosphatase. The glgC-16 tubers are held to be suitable for the study of the role of ADPglucose pyrophosphorylase in the control of starch synthesis.

Starch metabolism in tubers of transgenic potato (Solanum tuberosum) with increased ADPglucose pyrophosphorylase

The aim of this work was to use tubers from transgenic lines of potato (Solanum tuberosum) containing increased amounts of ADPglucose pyrophosphorylase to study the role of this enzyme in the control of starch synthesis. A 4-5-fold increase in activity of the enzyme, achieved by transformation with the Escherichia coli ADPglucose pyrophosphorylase gene glgC-16, had no detectable effect on the starch content of developing or mature tubers. No significant effects were found on the contents of ADPglucose, UDPglucose, glucose 1-phosphate, glucose 6-phosphate, PP1, ATP and ADP. Flux from [U-14C]sucrose, supplied to tubers still attached to the plant, to starch increased roughly in proportion to the increase in ADPglucose pyrophosphorylase activity. These measurements of flux gave a response coefficient close to 1 for the activity of the pyrophosphorylase in respect of starch synthesis. Pulse-chase experiments with [U-14C]sucrose showed that the increased flux into starch in the transformed tubers was accompanied by an increased rate of starch turnover. Further experiments suggested that the increased turnover was associated with an increase in the capacity of the tubers to degrade starch.

A mutant of Arabidopsis thaliana lacking the ability to transport glucose across the chloroplast envelope

At the end of a 12-h day leaves of the mutant of Arabidopsis thaliana L., TC265, contained 4-5 times more starch than those of the wild type. During a subsequent 12-h night the decline in the starch content of the leaves of the mutant was at least 50% of that of the wild-type leaves. Starch labelled in the light in a 30-min pulse in 14CO2 was rapidly broken down in a subsequent 12-h chase in the dark in air in the leaves of both mutant and wild type. Chloroplasts from leaves of the wild type took up [32P]Pi and [U-14C]glucose at 12 and 1.6 mumol/h per mg of chlorophyll respectively; chloroplasts from the mutant showed a similar rate for [32P]Pi but no uptake of [U-14C]glucose. The glucose content of freshly isolated chloroplasts from the mutant was twice that of chloroplasts from the wild type; this difference was accentuated when the isolated chloroplasts were incubated in the dark. SDS/PAGE of preparations of chloroplast envelopes showed that those from the mutant were deficient in a protein band of approximate molecular mass 40 kDa. It is suggested that in mutant TC265 the primary lesion is in a hexose transporter in the chloroplast envelope, and that this transporter moves the products of starch breakdown that are destined for sucrose synthesis from the chloroplast to the cytosol.

Plant physiology. Virtue on both sides

Experiments with transgenic plants are beginning to unravel how the products of photosynthesis get into the phloem from transport throughout the living tissues of the plant.

Pyrophosphate: fructose-6-phosphate 1-phosphotransferase and fructose 2,6-bisphosphate in the bundle sheath of maize leaves

The aim of this work was to discover whether the cells of the bundle sheath of the leaves of maize (Zea mays) contained pyrophosphate:fructose-6-phosphate 1-phosphotransferase (PFP) and fructose 2,6-bisphosphate (Fru-2,6-P2). Physiologically active preparations of bundle sheath cells from leaves of 4- to 6-week-old plants showed activities of PFP, 6-phosphofructo-2-kinase (6-PF-2-K), and fructose-2,6-bisphosphatase (Fru-2,6-Pase) of 38, 1.8, and 15 nmol min-1 mg-1 chlorophyll, respectively, and contained 75 pmol mg-1 chlorophyll Fru-2,6-P2. For the above enzymes, and marker enzymes for the bundle sheath and for mesophyll cells, the ratios of the activities in leaf extracts to those in bundle sheath extracts were determined. The ratios for PFP, 6-PF-2-K, Fru-2,6-Pase, and Fru-2,6-P2 were intermediate between those found for the mesophyll markers and bundle sheath markers. The distribution of PFP activity after nonaqueous fractionation of leaves differed from that of the bundle sheath and mesophyll marker enzymes. It is argued that maize bundle sheaths can contain significant activity of PFP and amounts of Fru-2,6-P2.

Glycolysis at the climacteric of bananas

This work was carried out to investigate the relative roles of phosphofructokinase and pyrophosphate-fructose-6-phosphate 1-phosphotransferase during the increased glycolysis at the climacteric in ripening bananas (Musa cavendishii Lamb ex Paxton). Fruit were ripened in the dark in a continuous stream of air in the absence of ethylene. CO2 production, the contents of glucose 6-phosphate, fructose 6-phosphate, fructose 1,6-bisphosphate, phosphoenolpyruvate and PPi; and the maximum catalytic activities of pyrophosphate-fructose-6-phosphate 1-phosphotransferase, 6-phosphofructokinase, pyruvate kinase and phosphoenolpyruvate carboxylase were measured over a 12-day period that included the climacteric. Cytosolic fructose-1,6- bisphosphatase could not be detected in extracts of climacteric fruit. The peak of CO2 production was preceded by a threefold rise in phosphofructokinase, and accompanied by falls in fructose 6-phosphate and glucose 6-phosphate, and a rise in fructose 1,6-bisphosphate. No change in pyrophosphate-fructose-6-phosphate 1-phosphotransferase or pyrophosphate was found. It is argued that phosphofructokinase is primarily responsible for the increased entry of fructose 6-phosphate into glycolysis at the climacteric.

Radiorespirometry evidence for the discrimination between 13C-enriched glucose and unlabelled glucose molecules by Paracoccus denitrificans

Paracoccus denitrificans was grown on either unlabelled glucose, [1-13C]glucose or [6-13C]glucose as the sole carbon source for growth. The cells were then incubated with a range of 14C-glucose substrates to compare the 14CO2-evolution rates between cells grown on the glucose and the 13C-labelled glucose. Cells grown on 13C-glucose had significantly faster rates of 14CO2-evolution than those grown on unlabelled glucose. The % yields of 14CO2, per [1-14C]-, [6-14C]- and [U-14C]glucose supplied were also substantially greater than those measured for cells grown on unlabelled glucose. The data indicated that growth of Paracoccus on 13C-enriched glucose substrates resulted in cells with notably different 14C-glucose oxidation metabolism compared to that observed in cells grown on unlabelled glucose.

Apparent equilibrium constant and mass-action ratio for sucrose-phosphate synthase in seeds of Pisum sativum

The aim of this work was to use preparations from germinating seeds of Pisum sativum to determine the apparent equilibrium constant of the reaction catalysed by sucrose-phosphate synthase (EC 2.4.1.14) and to compare this with the mass-action ratio of the reaction in the seeds. The apparent equilibrium constant ranged from 5.3 at 0.25 mM-MgCl2, pH 7.0, to 62 at 10 mM-MgCl2, pH 7.5. The sucrose phosphate content of the seeds, 23 nmol/g fresh wt., was determined by separating sucrose phosphate from sucrose by ion-exchange chromatography and then measuring the sucrose released by alkaline phosphatase. Comparison of equilibrium constants and mass-action ratios in the cotyledons of 38 h-germinated seeds showed that the reactions catalysed by glucose-6-phosphate isomerase, phosphoglucomutase and UDP-glucose pyrophosphorylase are close to equilibrium, and those catalysed by sucrose-phosphate synthase and sucrose phosphatase are considerably displaced from equilibrium in vivo.

Effects of 2,4-dinitrophenol and anoxia on the inorganic-pyrophosphate content of the spadix of Arum maculatum and the root apices of Pisum sativum

This work was done to determine whether the inorganic-pyrophosphate (PPi) content of plant tissues changes when the rate of glycolysis is altered. Treatment of excised clubs of the spadix of Arum maculatum L. and root apices of Pisum sativum L. with 2,4-dinitrophenol increased the rates of respiration but had no detectable effects on PPi contents. When the two tissues were subjected to up to 60 min anoxia, no changes in PPi were detected. Anoxia was shown to lead to a fall in ATP and concomitant rises in ADP and AMP in pea roots. It is argued (i) that variation in the rate of glycolysis was not accompanied by detectable changes in PPi content, (ii) that this observation does not favour the view that pyrophosphate fructose 6-phosphate 1-phosphotransferase mediates appreciable entry into glycolysis, and (iii) that PPi content can be maintained when respiratory-chain phosphorylation is inhibited.

Fructose 2,6-bisphosphate and the climacteric in bananas

This work was done to test the view that there is a marked rise in the content of fructose 2,6-bisphosphate during the climacteric of the fruit of banana (Musa cavendishii Lamb ex. Paxton). Bananas were ripened in the dark in a continuous stream of air in the absence of exogenous ethylene. CO2 production and the contents of fructose 2,6-bisphosphate and sucrose were monitored over a 15-day period. A range of extraction procedures for fructose 2,6-bisphosphate were compared. Recovery of fructose 2,6-bisphosphate added to samples of unripe fruit varied from poor to unmeasurable. Recoveries from samples of ripe fruit were high. It is argued that this differential recovery of fructose 2,6-bisphosphate undermines claims that the amount of this compound increases at the climacteric. When recoveries are taken into account, our data suggest that there is no major change in fructose 2,6-bisphosphate content during the onset of the climacteric in bananas.

Enzymic capacities of amyloplasts from wheat (Triticum aestivum) endosperm

Lysates of protoplasts from the endosperm of developing grains of wheat (Triticum aestivum) were fractionated on density gradients of Nycodenz to give amyloplasts. Enzyme distribution on the gradients suggested that: (i) starch synthase and ADP-glucose pyrophosphorylase are confined to the amyloplasts; (ii) pyrophosphate: fructose-6-phosphate 1-phosphotransferase and UDP-glucose pyrophosphorylase are confined to the cytosol; (iii) a significant proportion (23-45%) of each glycolytic enzyme, from phosphoglucomutase to pyruvate kinase inclusive, is in the amyloplast. Starch synthase, ADP-glucose pyrophosphorylase and each of the glycolytic enzymes showed appreciable latency when assayed in unfractionated lysates of protoplasts. No activity of fructose-1,6-bisphosphatase was found in amyloplasts or in homogenates of endosperm. Antibody to plastidic fructose-1,6-bisphosphatase did not react positively, in an immunoblot analysis, with any protein in extracts of wheat endosperm. It is argued that wheat endosperm lacks significant plastidic fructose-1,6-bisphosphatase and that carbon for starch synthesis does not enter the amyloplast as a C-3 compound but probably as hexose phosphate.

Effects of low temperature on the respiratory metabolism of carbohydrates by plants

The effects of lowering the temperature from 25 degrees C to 2-8 degrees C on carbohydrate metabolism by plant cells are considered. Particular emphasis is placed on the mechanism of cold-induced sweetening in tubers of potato (Solanum tuberosum). Temperatures between 0 and 10 degrees C were shown to cause a marked reduction in the rate of respiration of a wide range of plant tissues. At these temperatures the ability of suspension cultures of soybean (Glycine max), and callus cultures and tubers of potato to metabolize [14C]glucose was appreciably diminished. The detailed distribution of 14C showed that lowering the temperature decreased the proportion of the metabolized [14C]glucose that entered the respiratory pathways and increased the proportion converted to sucrose. Pulse and chase experiments, in which [14C]glucose was supplied to potato tubers at 2 and 25 degrees C, showed that lowering the temperature led to accumulation of label in hexose 6-phosphates, which were subsequently converted to sucrose. The patterns of 14CO2 production from specifically labelled [14C]glucose supplied to soybean suspension cultures and disks of potato tuber suggested that lowering the temperature reduced the activity of glycolysis more than that of the oxidative pentose phosphate pathway. It is argued that the above experiments demonstrate that lowering the temperature not only reduces the rate of carbohydrate metabolism but also alters the relative activities of the different pathways involved. A disproportionate reduction in glycolysis at the lower temperatures is suggested. Mature tubers of many varieties of potato accumulate sucrose and hexose when stored between 2 and 10 degrees C. Starch is the source of carbon for this synthesis of sugar. We could not detect cytosolic fructose-1,6-bisphosphatase in potato tubers and suggest that carbon for sugar synthesis in the cold leaves the amyloplast, not as triose phosphate, but probably as a six-carbon compound. Evidence is presented that phosphofructokinase (EC 2.7.1.11) plays a major role in regulating the entry of hexose 6-phosphates into glycolysis in potato tubers. Phosphofructokinase was purified from potato tubers and shown to consist of four forms. Three of these forms were shown to have higher Q10 values over the range 2-6 degrees C than over the range 12-16 degrees C and are regarded as being cold-labile. No such cold-lability was detected for the key enzymes involved in sucrose synthesis and the oxidative pentose phosphate pathway.(ABSTRACT TRUNCATED AT 250 WORDS)

Measurement of the inorganic pyrophosphate in tissues of Pisum sativum L

Purified pyrophosphate: fructose 6-phosphate 1-phosphotransferase (EC 2.7.1.90) was used to measure the inorganic pyrophosphate in unfractionated extracts of tissues of Pisum sativum L. The fructose 1,6-bisphosphate produced by the above enzyme was measured by coupling to NADH oxidation via aldolase (EC 4.1.2.13), triosephosphate isomerase (EC 5.3.1.1) and glycerol-3-phosphate dehydrogenase (EC 1.1.1.8). Amounts of pyrophosphate as low as 1 nmol could be measured. The contents of pyrophosphate in the developing embryo of pea, and in the apical 2 cm of the roots, were appreciable; 9.4 and 8.9 nmol g(-1) fresh weight, respectively. The possibility that pyrophosphate acts in vivo as an energy source for pyrophosphate: fructose 6-phosphate 1-phosphotransferase and for UDPglucose pyrophosphorylase (EC 2.7.7.9) is considered.

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.

The role of nickel in urea assimilation by algae

Nickel is required for urease synthesis by Phaeodactylum tricornutum and Tetraselmis subcordiformis and for growth on urea by Phaeodactylum. There is no requirement for nickel for urea amidolyase synthesis by Chlorella fusca var. vacuolata. Neither copper nor palladium can substitute for nickel but cobalt partially restored urease activity in Phaeodactylum. The addition of nickel to nickel-deficient cultures of Phaeodactylum or Tetraselmis resulted in a rapid increase of urease activity to 7-30 times the normal level; this increase was not inhibited by cycloheximide. It is concluded that nickel-deficient cells over-produce a non-functional urease protein and that either nickel or the functional urease enzyme participates in the regulation of the production of urease protein.

Ammonium generation by nitrogen-starved cultures of Chlamydomonas reinhardii

Ammonium (NH 4 (+) ) assimilation by Chlamydomonas reinhardii was inhibited when cultures were incubated with methionine sulphoximine (MSO). Methionine sulphoximine inhibited glutamine synthetase acitvity in vitro in extracts from wild-type (2192) and mutant (CC419) cultures. Mutant cultures were insensitive to MSO inhibition in vivo. Nitrogen-starved, wild-type cultures excreted ammonium when they were incubated with MSO in light or in darkness. Ammonium generation was stimulated by glutamine, inhibited by CO2 and stoichiometrically related to loss of protein. Notrogen replete cultures treated with MSO excreted ammonium in light but little was excreted in darkness. Ammonium excretion in darkness, in the presence of MSO, was enhanced by either a period of nitrogen deprivation or by the addition of acetate. Nitrogen deprivation also diminished the lag before ammonium excretion commenced.

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.

Sodium-dependent uptake of nitrate and urea by a marine diatom

Uptake of nitrate and urea by Phaeodactylum tricornutum is shown to be a sodium dependent process inhibited by lithium or potassium. The half-saturation constant for sodium (KNa) was 2.6 mM for nitrate uptake and 71 mM for urea uptake. It is suggested that sodium dependent uptake mechanisms may be characteristic of marine plants.

Carbohydrate breakdown by chloroplasts of Pisum sativum

1. The aims of this work were to discover the pathways of starch breakdown and carbohydrate metabolism in intact isolated chloroplasts from shoots of Pisum sativum. 2. 14C from starch, labelled by supplying [14C]glucose to chloroplasts, appeared, during starch breakdown, in CO2, maltose and the fraction of the acidic compounds that contained 3-phosphoglycerate and sugar phosphates. 3. When intact chloroplasts were incubated in the dark, 3-phosphoglycerate, triose phosphates and, to a lesser extent, hexose 6-phosphates accumulated in the medium at rates comparable to those of starch breakdown in leaves. This accumulation was dependent upon orthophosphate. 4. The patterns of 14CO2 production from specifically labelled [14C]glucose supplied to isolated chloroplasts were those expected of the oxidative pentose phosphate pathway with extensive recycling, and glycolysis. The respone of this pattern to lack of orthophosphate, addition of unlabelled intermediates, and 2-phosphoglycollate confirmed this view. 5. Starch breakdown in pea chloroplasts is held to be dominantly phosphorolytic with the products being metabolized via the oxidative pentose phosphate pathway and glycolysis to 3-phosphoglycerate, triose phosphates and CO2 that are exported to the cytoplasm.

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.

Phosphoenolpyruvate carboxykinase and gluconeogenesis in cotyledons of Cucurbita pepo

1. The aim of this work was to investigate the role of phosphoenolpyruvate carboxykinase (ATP:oxaloacetate carboxy-lyase (transphosphorylating) EC 4.1.1.49) in the conversion of fat to sugar by the cotyledons of seedlings of Cucurbita pepo. 2. The enzyme was partially purified from the cotyledons of 5-day-old seedlings. The Michaelis constants for oxaloacetate and ATP were 56 and 119 micron, respectively. The decarboxylation reaction was optimum at pH 7.4. A range of intermediary metabolites did not affect the activity of the enzyme, but 3-mercaptopicolinic acid at micron concentrations was an effective inhibitor. 3. Centrifugation of extracts of 5-day-old cotyledons sedimented appreciable proportions of the ribuloseibisphosphate carboxylase, isocitrate lyase and fumarate hydratase present but very little of the phosphoenolpyruvate carboxykinase. 4. Measurements of phosphoenolpyruvate carboxykinase of cotyledons during germination showed that the maximum catalytic activity exceeded, and changed coincidently with, the rate of gluconeogenesis. 5. 3-Mercaptopicolinic acid inhibited gluconeogenesis from [1-14C]- and [2-14C]acetate supplied to excised cotyledons. The detailed distribution of 14C indicated inhibition of the conversion of oxaloacetate to phosphoenolpyruvate. 6. It is concluded that in marrow cotyledons phosphoenolpyruvate carboxykinase is in the soluble phase of the cytoplasm and catalyses a component reaction of gluconeogenesis.

Antibiotic resistance in Pseudomonas aeruginosa from infections of the ear, nose and throat

1,000 strains of <i>Ps</i>. <i>aeruginosa</i> isolated from disease conditions of the ear, nose and throat, were examined for sensitivity to 9 antibiotics. Only 4.5 % of them were sensitive to all 9. The low incidence of strains resistant to polymixin B, colistin and carbenicillin makes them most suitable for the treatment of infections due to this organism, though the need for parenteral administration limits their systemic use to in-patients. A high level of cross-resistance exists between polymixin B and colistin, while there are various degrees of cross-resistance between the aminoglycoside antibiotics streptomycin, framycetin and neomycin. 63.9 % of strains resistant to oxytetracycline are cross-resistant to the chemically-unrelated chloramphenicol.