共查询到20条相似文献,搜索用时 15 毫秒
1.
1. Purified rabbit-muscle and -liver glucose phosphate isomerase, free of contaminating enzyme activities that could interfere with the assay procedures, were tested for inhibition by fructose, fructose 1-phosphate and fructose 1,6-diphosphate. 2. Fructose 1-phosphate and fructose 1,6-diphosphate are both competitive with fructose 6-phosphate in the enzymic reaction, the apparent Ki values being 1·37×10−3−1·67×10−3m for fructose 1-phosphate and 7·2×10−3−7·9×10−3m for fructose 1,6-diphosphate; fructose and inorganic phosphate were without effect. 3. The apparent Km values for both liver and muscle enzymes at pH7·4 and 30° were 1·11×10−4−1·29×10−4m for fructose 6-phosphate, determined under the conditions in this paper. 4. In the reverse reaction, fructose, fructose 1-phosphate and fructose 1,6-diphosphate did not significantly inhibit the conversion of glucose 6-phosphate into fructose 6-phosphate. 5. The apparent Km values for glucose 6-phosphate were in the range 5·6×10−4−8·5×10−4m. 6. The competitive inhibition of hepatic glucose phosphate isomerase by fructose 1-phosphate is discussed in relation to the mechanism of fructose-induced hypoglycaemia in hereditary fructose intolerance. 相似文献
2.
Form of inorganic carbon involved as a product and as an inhibitor of c(4) Acid decarboxylases operating in c(4) photosynthesis 总被引:1,自引:1,他引:0
下载免费PDF全文
![点击此处可从《Plant physiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
These studies demonstrated that CO2 rather than HCO3− is the inorganic carbon metabolite produced by the C4 acid decarboxylases involved in C4 photosynthesis (chloroplast located NADP malic enzyme, mitochondrial NAD malic enzyme, and cytosolic phosphoenolpyruvate [PEP] carboxykinase). The effect of varying CO2 or HCO3− as a substrate for the carboxylation reaction catalyzed by these enzymes or as inhibitors of the decarboxylation reaction was also determined. The KmCO2 was 1.1 millimolar for NADP malic enzyme and 2.5 millimolar for PEP carboxykinase. For these two enzymes the velocity in the carboxylating direction was substantially less than for the decarboxylating direction even with CO2 concentrations at the upper end of the range of expected cellular levels. Activity of NAD malic enzyme in the carboxylating direction was undetectable. The decarboxylation reaction of all three enzymes was inhibited by added HCO3−. For NADP malic enzyme CO2 was shown to be the inhibitory species but PEP carboxykinase and NAD malic enzyme were apparently inhibited about equally by CO2 and HCO3−. 相似文献
3.
Sulfur assimilation in c(4) plants: intercellular compartmentation of adenosine 5'-triphosphate sulfurylase in crabgrass leaves
下载免费PDF全文
![点击此处可从《Plant physiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The activity of adenosine 5′ triphosphate sulfurylase was determined in crabgrass mesophyll cells, bundle sheath strands, and whole leaf extracts. The enzyme was assayed by following molybdate-dependent pyrophosphate release from ATP, 35SO42− incorporation into adenosine 5′ phosphosulfate, and ATP synthesis dependent upon adenosine 5′ phosphosulfate and inorganic pyrophosphate. With all assays, greater than 90% of the activity was found in extracts from bundle sheath strands. The activities in whole leaf extracts were consistently intermediate between the activities of mesophyll and bundle sheath extracts and extract-mixing experiments gave no indication of enzyme activation or inhibition in vitro. Whole leaf activities were several hundred-fold less than concurrent measurements of ribulose 1,5-bisphosphate and phosphoenolpyruvate carboxylase activities, which is interpreted as being consistent with the relative amounts of elemental carbon and sulfur found in higher plants. A hypothesis is presented for the intercellular compartmentation of sulfur assimilation in relationship to NO3− and CO2 assimilation in leaves of C4 plants. 相似文献
4.
Activation of Respiration to Support Dark NO(3) and NH(4) Assimilation in the Green Alga Selenastrum minutum
下载免费PDF全文
![点击此处可从《Plant physiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Short-term changes in pyridine nucleotides and other key metabolites were measured during the onset of NO3− or NH4+ assimilation in the dark by the N-limited green alga Selenastrum minutum. When NH4+ was added to N-limited cells, the NADH/NAD ratio rose immediately and the NADPH/NADP ratio followed more slowly. An immediate decrease in glutamate and 2-oxoglutarate indicates an increased flux through the glutamine synthase/glutamate oxoglutarate aminotransferase. Pyruvate kinase and phosphoenolpyruvate carboxylase are rapidly activated to supply carbon skeletons to the tricarboxylic acid cycle for amino acid synthesis. In contrast, NO3− addition caused an immediate decrease in the NADPH/NADP ratio that was accompanied by an increase in 6-phosphogluconate and decrease in the glucose-6-phosphate/6-phosphogluconate ratio. These changes show increased glucose-6-phosphate dehydrogenase activity, indicating that the oxidative pentose phosphate pathway supplies some reductant for NO3− assimilation in the dark. A lag of 30 to 60 seconds in the increase of the NADH/NAD ratio during NO3− assimilation correlates with a slow activation of pyruvate kinase and phosphoenolpyruvate carboxylase. Together, these results indicate that during NH4+ assimilation, the demand for ATP and carbon skeletons to synthesize amino acid signals activation of respiratory carbon flow. In contrast, during NO3− assimilation, the initial demand on carbon respiration is for reductant and there is a lag before tricarboxylic acid cycle carbon flow is activated in response to the carbon demands of amino acid synthesis. 相似文献
5.
1. An F−-insensitive 3′-nucleotidase was purified from spinach leaf tissue; the enzyme hydrolysed 3′-AMP, 3′-CMP and adenosine 3′-phosphate 5′-sulphatophosphate but not adenosine 5′-nucleotides nor PPi. The pH optimum of the enzyme was 7.5; Km (3′-AMP) was approx. 0.8mm and Km (3′-CMP) was approx. 3.3mm. 3′-Nucleotidase activity was not associated with chloroplasts. Purified Mg2+-dependent pyrophosphatase, free from F−-insensitive 3′-nucleotidase, catalysed some hydrolysis of 3′-AMP; this activity was F−-sensitive. 2. Adenosine 5′-sulphatophosphate kinase activity was demonstrated in crude spinach extracts supplied with 3′-AMP by the synthesis of the sulphate ester of 2-naphthol in the presence of purified phenol sulphotransferase; purified ATP sulphurylase and pyrophosphatase were also added to synthesize adenosine 5′-sulphatophosphate. Adenosine 5′-sulphatophosphate kinase activity was associated with chloroplasts and was released by sonication. 3. Isolated chloroplasts synthesized adenosine 3′-phosphate 5′-sulphatophosphate from sulphate and ATP in the presence of a 3′-nucleotide; the formation of adenosine 5′-sulphatophosphate was negligible. In the absence of a 3′-nucleotide the synthesis of adenosine 3′-phosphate 5′-sulphatophosphate was negligible, but the formation of adenosine 5′-sulphatophosphate was readily detected. Some properties of the synthesis of adenosine 3′-phosphate 5′-sulphatophosphate by isolated chloroplasts are described. 4. Adenosine 3′-phosphate 5′-sulphatophosphate, synthesized by isolated chloroplasts, was characterized by specific enzyme methods, electrophoresis and i.r. spectrophotometry. 5. Isolated chloroplasts catalysed the incorporation of sulphur from sulphate into cystine/cysteine; the incorporation was enhanced by 3′-AMP and l-serine. It was concluded that adenosine 3′-phosphate 5′-sulphatophosphate is an intermediate in the incorporation of sulphur from sulphate into cystine/cysteine. 相似文献
6.
Plants under Climatic Stress: VI. Chilling and Light Effects on Photosynthetic Enzymes of Sorghum and Maize
下载免费PDF全文
![点击此处可从《Plant physiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The activity of several photosynthetic enzymes was unaltered by exposure of sorghum or maize to low temperatures (10 C) and light (170 w m−2). Two light-activated C4-pathway enzymes, NADP-malate dehydrogenase and pyruvate Pi dikinase, were reduced in activity, and this was largely attributable to a loss of enzyme rather than to incomplete enzyme activation. Loss of NADP-malate dehydrogenase was more marked in sorghum than in maize, and in both species no loss occurred at 10 C when light levels were reduced from 170 to 50 w m−2. A light-dependent, low temperature-induced loss of catalase activity was also observed in maize leaves. 相似文献
7.
Evidence for Activation of the Oxidative Pentose Phosphate Pathway during Photosynthetic Assimilation of NO(3) but Not NH(4) by a Green Alga
下载免费PDF全文
![点击此处可从《Plant physiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Addition of NO3− to N-limited Selenastrum minutum during photosynthesis resulted in an immediate drop in the NADPH/NADP ratio and a slower increase of the NADH/NAD ratio. These changes were accompanied by a rapid decrease in glucose-6-phosphate and increase in 6-phosphogluconate, indicating activation of glucose-6-phosphate dehydrogenase and a role for the oxidation pentose phosphate pathway during photosynthetic NO3− assimilation. In contrast, the short-term changes in pyridine nucleotides and metabolites during photosynthetic assimilation of NH4+ were not consistent with a stimulation of the oxidative pentose phosphate pathway. 相似文献
8.
NO(3) Enhances the Kinase Activity for Phosphorylation of Phosphoenolpyruvate Carboxylase and Sucrose Phosphate Synthase Proteins in Wheat Leaves: Evidence from the Effects of Mannose and Okadaic Acid 总被引:1,自引:1,他引:0
下载免费PDF全文
![点击此处可从《Plant physiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The aim of this work was to determine which of the two reactions (i.e. phosphorylation or dephosphorylation) involved in the establishment of the phosphorylated status of the wheat leaf phosphoenolpyruvate carboxylase and sucrose phosphate synthase protein responds in vivo to NO3− uptake and assimilation. Detached mature leaves of wheat (Triticum aestivum L. cv Fidel) were fed with N-free (low-NO3− leaves) or 40 mm NO3− solution (high-NO3− leaves). The specific inhibition of the enzyme-protein kinase or phosphatase activities was obtained in vivo by addition of mannose or okadaic acid, respectively, in the uptake solution. Mannose at 50 mm, by blocking the kinase reaction, inhibited the processes of NO3−-dependent phosphoenolpyruvate carboxylase activation and sucrose phosphate synthase deactivation. Following the addition of mannose, the deactivation of phosphoenolpyruvate carboxylase and the activation of sucrose phosphate synthase, both due to the enzyme-protein dephosphorylation, were at the same rate in low-NO3− and high-NO3− leaves, indicating that NO3− had no effect per se on the enzyme-protein phosphatase activity. Upon treatment with okadaic acid, the higher increase of phosphoenolpyruvate carboxylase and decrease of sucrose phosphate synthase activities observed in high NO3− compared with low NO3− leaves showed evidence that NO3− enhanced the protein kinase activity. These results support the concept that NO3−, or a product of its metabolism, favors the activation of phosphoenolpyruvate carboxylase and deactivation of sucrose phosphate synthase in wheat leaves by promoting the light activation of the enzyme-protein kinase(s) without affecting the phosphatase(s). 相似文献
9.
In this study, dienelactone hydrolases (TfdEI and TfdEII) located on plasmid pJP4 of Cupriavidus necator JMP134 were cloned, purified, characterized and three dimensional structures were predicted. tfdEI and tfdEII genes were cloned into pET21b vector and expressed in E. coli BL21(DE3). The enzymes were purified by applying ultra-membrane filtration, anion-exchange QFF and gel-filtration columns. The enzyme activity was determined by using cis-dienelactone. The three-dimensional structure of enzymes was predicted using SWISS-MODEL workspace and the biophysical properties were determined on ExPASy server. Both TfdEI and TfdEII (Mr 25 kDa) exhibited optimum activity at 37°C and pH 7.0. The enzymes retained approximately 50% of their activity after 1 h of incubation at 50°C and showed high stability against denaturing agents. The TfdEI and TfdEII hydrolysed cis-dienelactone at a rate of 0.258 and 0.182 µMs−1, with a Km value of 87 µM and 305 µM, respectively. Also, TfdEI and TfdEII hydrolysed trans-dienelactone at a rate of 0.053 µMs−1 and 0.0766 µMs−1, with a Km value of 84 µM and 178 µM, respectively. The TfdEI and TfdEII kcat/Km ratios were 0.12 µM−1s−1and 0.13 µM−1s−1 and 0.216 µM−1s−1 and 0.094 µM−1s−1 for for cis- and trans-dienelactone, respectively. The kcat/Km ratios for cis-dienelactone show that both enzymes catalyse the reaction with same efficiency even though Km value differs significantly. This is the first report to characterize and compare reaction kinetics of purified TfdEI and TfdEII from Cupriavidus necator JMP134 and may be helpful for further exploration of their catalytic mechanisms. 相似文献
10.
This study investigated the effect of cold plasma seed treatment on tomato bacterial wilt, caused by Ralstonia solanacearum (R. solanacearum), and the regulation of resistance mechanisms. The effect of cold plasma of 80W on seed germination, plant growth, nutrient uptake, disease severity, hydrogen peroxide (H2O2) concentration and activities of peroxidase (POD; EC 1.11.1.7), polyphenol oxidase (PPO; EC 1.10.3.2) and phenylalanine ammonia lyase (PAL; EC 4.3.1.5) were examined in tomato plants. Plasma treatment increased tomato resistance to R. solanacearum with an efficacy of 25.0%. Plasma treatment significantly increased both germination and plant growth in comparison with the control treatment, and plasma-treated plants absorbed more calcium and boron than the controls. In addition, H2O2 levels in treated plants rose faster and reached a higher peak, at 2.579 µM gFW−1, 140% greater than that of the control. Activities of POD (421.3 U gFW−1), PPO (508.8 U gFW−1) and PAL (707.3 U gFW−1) were also greater in the treated plants than in the controls (103.0 U gFW−1, 166.0 U gFW−1 and 309.4 U gFW−1, respectively). These results suggest that plasma treatment affects the regulation of plant growth, H2O2 concentration, and POD, PPO and PAL activity in tomato, resulting in an improved resistance to R. solanacearum. Consequently, cold plasma seed treatment has the potential to control tomato bacterial wilt caused by R. solanacearum. 相似文献
11.
Spinach Leaf Chloroplast CO(2) and NO(2) Photoassimilations Do Not Compete for Photogenerated Reductant: Manipulation of Reductant Levels by Quantum Flux Density Titrations 总被引:2,自引:2,他引:0
下载免费PDF全文
![点击此处可从《Plant physiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Robinson JM 《Plant physiology》1988,88(4):1373-1380
Potential competition between CO2 and NO2− photoassimilation for photogenerated reductant (e.g. reduced ferredoxin and NADPH) was examined employing isolates of mesophyll cells and intact chloroplasts derived from mature `source' spinach leaves. Variations in the magnitude of incident light energy were used to manipulate the supply of reductant in situ within chloroplasts. Leaf cell and plastid isolates were fed with saturating CO2 and/or NO2− to produce the highest demand for reductant by CO2 and/or NO2− assimilatory processes (enzymes). Even in the presence of CO2 fixation, NO2− reduction in intact leaf cell isolates as well as plastid isolates was maximal at light energies as low as 50 to 200 microeinsteins per second per square meter. Simultaneously, 500 to 800 microeinsteins per second per square meter were required to support maximal CO2 assimilation. Regardless of the magnitude of the incident light energy, CO2 assimilation did not repress NO2− reduction, nor were these two processes mutually repressed. These observations have been interpreted to mean that reduced ferredoxin levels in situ in the plastids of mature source leaf mesophyll cells were adequate to supply the concurrent maximal demands exerted by enzymes associated with CO2 as well as with inorganic nitrogen photoassimilation. 相似文献
12.
M. Oshiki S. Ishii K. Yoshida N. Fujii M. Ishiguro H. Satoh S. Okabe 《Applied and environmental microbiology》2013,79(13):4087-4093
We examined nitrate-dependent Fe2+ oxidation mediated by anaerobic ammonium oxidation (anammox) bacteria. Enrichment cultures of “Candidatus Brocadia sinica” anaerobically oxidized Fe2+ and reduced NO3− to nitrogen gas at rates of 3.7 ± 0.2 and 1.3 ± 0.1 (mean ± standard deviation [SD]) nmol mg protein−1 min−1, respectively (37°C and pH 7.3). This nitrate reduction rate is an order of magnitude lower than the anammox activity of “Ca. Brocadia sinica” (10 to 75 nmol NH4+ mg protein−1 min−1). A 15N tracer experiment demonstrated that coupling of nitrate-dependent Fe2+ oxidation and the anammox reaction was responsible for producing nitrogen gas from NO3− by “Ca. Brocadia sinica.” The activities of nitrate-dependent Fe2+ oxidation were dependent on temperature and pH, and the highest activities were seen at temperatures of 30 to 45°C and pHs ranging from 5.9 to 9.8. The mean half-saturation constant for NO3− ± SD of “Ca. Brocadia sinica” was determined to be 51 ± 21 μM. Nitrate-dependent Fe2+ oxidation was further demonstrated by another anammox bacterium, “Candidatus Scalindua sp.,” whose rates of Fe2+ oxidation and NO3− reduction were 4.7 ± 0.59 and 1.45 ± 0.05 nmol mg protein−1 min−1, respectively (20°C and pH 7.3). Co-occurrence of nitrate-dependent Fe2+ oxidation and the anammox reaction decreased the molar ratios of consumed NO2− to consumed NH4+ (ΔNO2−/ΔNH4+) and produced NO3− to consumed NH4+ (ΔNO3−/ΔNH4+). These reactions are preferable to the application of anammox processes for wastewater treatment. 相似文献
13.
Carbon isotope ratios demonstrate carbon flux from c(4) host to c(3) parasite 总被引:2,自引:0,他引:2
下载免费PDF全文
![点击此处可从《Plant physiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Carbon isotope ratios of mature leaves from the C3 angiosperm root hemiparasites Striga hermonthica (Del.) Benth (−26.7‰) and S. asiatica (L.) Kuntze (−25.6‰) were more negative than their C4 host, sorghum (Sorghum bicolor [L.] Moench cv CSH1), (−13.5‰). However, in young photosynthetically incompetent plants of S. hermonthica this difference was reduced to less than 1‰. Differences between the carbon isotope ratios of two C3-C3 associations, S. gesnerioides (Willd.) Vatke—Vigna unguiculata (L.) Walp. and Oryza sativa L.—Rhamphicarpa fistulosa (Hochst.) Benth differed by less than 1‰. Theoretical carbon isotope ratios for mature leaves of S. hermonthica and S. asiatica, calculated from foliar gas exchange measurements, were −31.8 and −32.0‰, respectively. This difference between the measured and theoretical δ13C-values of 5 to 6‰ suggests that even in mature, photosynthetically active plants, there is substantial input of carbon from the C4 host. We estimate this to be approximately 28% of the total carbon in S. hermonthica and 35% in S. asiatica. This level of carbon transfer contributes to the host's growth reductions observed in Striga-infected sorghum. 相似文献
14.
Short Term Studies of Nitrate Uptake into Barley Plants Using Ion-Specific Electrodes and ClO(3): II. Regulation of NO(3) Efflux by NH(4)
下载免费PDF全文
![点击此处可从《Plant physiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The influence of NH4+, in the external medium, on fluxes of NO3− and K+ were investigated using barley (Hordeum vulgare cv Betzes) plants. NH4+ was without effect on NO3− (36ClO3−) influx whereas inhibition of net uptake appeared to be a function of previous NO3− provision. Plants grown at 10 micromolar NO3− were sensitive to external NH4+ when uptake was measured in 100 micromolar NO3−. By contrast, NO3− uptake (from 100 micromolar NO3−) by plants previously grown at this concentration was not reduced by NH4+ treatment. Plants pretreated for 2 days with 5 millimolar NO3− showed net efflux of NO3− when roots were transferred to 100 micromolar NO3−. This efflux was stimulated in the presence of NH4+. NH4+ also stimulated NO3− efflux from plants pretreated with relatively low nitrate concentrations. It is proposed that short term effects on net uptake of NO3− occur via effects upon efflux. By contrast to the situation for NO3−, net K+ uptake and influx of 36Rb+-labeled K+ was inhibited by NH4+ regardless of the nutrient history of the plants. Inhibition of net K+ uptake reached its maximum value within 2 minutes of NH4+ addition. It is concluded that the latter ion exerts a direct effect upon K+ influx. 相似文献
15.
Soybean plants (Glycine max [L.] Merr. cv Williams), which were symbiotic with Bradyrhizobium japonicum, and which grew well upon reduced nitrogen supplied solely through N2 fixation processes, often exhibited excess accumulation of starch and sucrose and diminished soluble protein in their source leaves. Nitrate and ammonia, when supplied to the nodulated roots of N2-fixing plants, mediated a reduction of foliar starch accumulation and a corresponding increase in soluble protein in the source leaves. This provided an opportunity to examine the potential metabolic adjustments by which NO3− and NH4+ (N) sufficiency or deficiency exerted an influence upon soybean leaf starch synthesis. When compared with soybean plants supplied with N, elevated starch accumulation was focused in leaf palisade parenchyma tissue of N2-fixing plants. Foliar activities of starch synthesis pathway enzymes including fructose-1,6-bisphosphate phosphatase, phosphohexoisomerase, phosphoglucomutase (PGM), as well as adenosine diphosphate glucose pyrophosphorylase (in some leaves) exhibited highest activities in leaf extracts of N2-fixing plants when expressed on a leaf protein basis. This was interpreted to mean that there was an adaptation of these enzyme activities in the leaves of N2-fixing plants, and this contributed to an increase in starch accumulation. Another major causal factor associated with increased starch accumulation was the elevation in foliar levels of fructose-6-phosphate, glucose-6-phosphate, and glucose-1-phosphate (G1P), which had risen to chloroplast concentrations considerably in excess of the Km values for their respective target enzymes associated with starch synthesis, e.g. elevated G1P with respect to adenosine diphosphate glucose pyrophosphorylase (ADPG-PPiase) binding sites. The cofactor glucose-1,6-bisphosphate (G1,6BP) was found to be obligate for maximal PGM activity in soybean leaf extracts of N2-fixing as well as N-supplemented plants, and G1,6BP levels in N2-fixing plant leaves was twice that of levels in N-supplied treatments. However the concentration of chloroplastic G1,6BP in illuminated leaves was computed to be saturating with respect to PGM in both N2-fixing and N-supplemented plants. This suggested that the higher level of this cofactor in N2-fixing plant leaves did not confer any higher PGM activation and was not a factor in higher starch synthesis rates. Relative to plants supplied with NO3− and NH4+, the source leaf glycerate-3-phosphate (3-PGA) and orthophosphate (Pi) concentrations in leaves of N2-fixing plants were two to four times higher. Although Pi is a physiological competitive inhibitor of leaf chloroplast ADPG-PPiase, and hence, starch synthesis, elevated chloroplast 3-PGA levels in N2-fixing plant leaves apparently prevented interference of Pi with ADPG-PPiase catalysis and starch synthesis. 相似文献
16.
Metabolic Activities in Extracts of Mesophyll and Bundle Sheath Cells of Panicum miliaceum (L.) in Relation to the C(4) Dicarboxylic Acid Pathway of Photosynthesis 总被引:7,自引:7,他引:0
下载免费PDF全文
![点击此处可从《Plant physiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The activities of certain enzymes related to the carbon assimilation pathway in whole leaves, mesophyll cell extracts, and bundle sheath extracts of the C4 plant Panicum miliaceum have been measured and compared on a chlorophyll basis. Enzymes of the C4 dicarboxylic acid pathway—phosphoenolpyruvate carboxylase and NADP-malic dehydrogenase—were localized in mesophyll cells. Carbonic anhydrase was also localized in mesophyll cell extracts. Ribose 5-phosphate isomerase, ribulose 5-phosphate kinase, and ribulose diphosphate carboxylase—enzymes of the reductive pentose phosphate pathway—were predominantly localized in bundle sheath extracts. High activities of aspartate and alanine transaminases and glyceraldehyde-3-P dehydrogenase were found about equally distributed between the photosynthetic cell types. P. miliaceum had low malic enzyme activity in both mesophyll and bundle sheath extracts. 相似文献
17.
Shireesha Sankella Abhimanyu Garg Jay D. Horton Anil K. Agarwal 《The Journal of biological chemistry》2014,289(8):4762-4777
In this study we examined the role of phosphatidic acid (PA) in hepatic glucose production (HGP) and development of hepatic insulin resistance in mice that lack 1-acylglycerol-3-phosphate O-acyltransferase 2 (AGPAT2). Liver lysophosphatidic acid and PA levels were increased ∼2- and ∼5-fold, respectively, in male Agpat2−/− mice compared with wild type mice. In the absence of AGPAT2, the liver can synthesize PAs by activating diacylglycerol kinase or phospholipase D, both of which were elevated in the livers of Agpat2−/− mice. We found that PAs C16:0/18:1 and C18:1/20:4 enhanced HGP in primary WT hepatocytes, an effect that was further enhanced in primary hepatocytes from Agpat2−/− mice. Lysophosphatidic acids C16:0 and C18:1 failed to increase HGP in primary hepatocytes. The activation of HGP was accompanied by an up-regulation of the key gluconeogenic enzymes glucose-6-phosphatase and phosphoenolpyruvate carboxykinase. This activation was suppressed by insulin in the WT primary hepatocytes but not in the Agpat2−/− primary hepatocytes. Thus, the lack of normal insulin signaling in Agpat2−/− livers allows unrestricted PA-induced gluconeogenesis significantly contributing to the development of hyperglycemia in these mice. 相似文献
18.
Effect of Light and NO(3) on Wheat Leaf Phosphoenolpyruvate Carboxylase Activity: Evidence for Covalent Modulation of the C(3) Enzyme 总被引:3,自引:3,他引:0
下载免费PDF全文
![点击此处可从《Plant physiology》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Phosphoenolpyruvate carboxylase (PEPcase) activity was studied in excised leaves of wheat (Triticum aestivum L.) in the dark and in the light, in presence of either N-free (low-NO3− leaves) or 40 millimolar KNO3 (high-NO3− leaves) nutrient solutions. PEPcase activity increased to 2.7-fold higher than that measured in dark-adapted tissue (control) during the first 60 minutes and continued to increase more slowly to 3.8-fold that of the control. This level was reached after 200 minutes exposure of the leaves to light and high NO3−. In contrast, the lower rate of increase recorded for low-NO3− leaves ceased after 60 minutes of exposure to light at 2.3-fold the control level. The short-term NO3− effect increased linearly with the level of NO3− uptake. In immunoprecipitation experiments, the antibody concentration for PEPcase precipitation increased with the protein extracts from the different treatments in the order: control, illuminated low-NO3− leaves, illuminated high-NO3− leaves. This order also applied with regard to a decreasing sensitivity to malate and an increasing stimulation by okadaic acid (an inhibitor of P-protein phosphatases). Following these studies, 32P labeling experiments were carried out in vivo. These showed that the light-induced change in the properties of the PEPcase was due to an alteration in the phosphorylation state of the protein and that this effect was enhanced in high-NO3− conditions. Based on the responses of PEPcase and sucrose phosphate synthase in wheat leaves to light and NO3−, an interpretation of the role of NO3− as either an inhibitor of P-protein phosphatase(s) or activator of protein kinase(s) is inferred. In the presence of NO3−, the phosphorylation state of both PEPcase and sucrose phosphate synthase is increased. This causes activation of the former enzyme and inhibition of the latter. We suggest that NO3− modulates the relative protein kinase/protein phosphatase ratio to favor increased phosphorylation of both enzymes in order to redirect carbon flow away from sucrose synthesis and toward amino acid synthesis. 相似文献
19.
Oxygen-18 Exchange as a Measure of Accessibility of CO(2) and HCO(3) to Carbonic Anhydrase in Chlorella vulgaris (UTEX 263) 总被引:3,自引:3,他引:0
We have measured the exchange of 18O between CO2 and H2O in stirred suspensions of Chlorella vulgaris (UTEX 263) using a membrane inlet to a mass spectrometer. The depletion of 18O from CO2 in the fluid outside the cells provides a method to study CO2 and HCO3− kinetics in suspensions of algae that contain carbonic anhydrase since 18O loss to H2O is catalyzed inside the cells but not in the external fluid. Low-CO2 cells of Chlorella vulgaris (grown with air) were added to a solution containing 18O enriched CO2 and HCO3− with 2 to 15 millimolar total inorganic carbon. The observed depletion of 18O from CO2 was biphasic and the resulting 18C content of CO2 was much less than the 18O content of HCO3− in the external solution. Analysis of the slopes showed that the Fick's law rate constant for entry of HCO3− into the cell was experimentally indistinguishable from zero (bicarbonate impermeable) with an upper limit of 3 × 10−4 s−1 due to our experimental errors. The Fick's law rate constant for entry of CO2 to the sites of intracellular carbonic anhydrase was large, 0.013 per second, but not as great as calculated for no membrane barrier to CO2 flux (6 per second). The experimental value may be explained by a nonhomogeneous distribution of carbonic anhydrase in the cell (such as membrane-bound enzyme) or by a membrane barrier to CO2 entry into the cell or both. The CO2 hydration activity inside the cells was 160 times the uncatalyzed CO2 hydration rate. 相似文献
20.
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 μmol 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 μmol 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 [14C]DHAP was incorporated into starch at a rate of 0.14 μmol 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. 相似文献