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1.
Young leaves of salt-depleted Aeluropus litoralis Parl. plants show CO 2 fixation by the C 3-carbon fixation pathway. No detectable activity of phosphoenol pyruvate (PEP) carboxylase was found. When A. litoralis plants were exposed to a NaCl solution, the leaves showed a high activity of PEP carboxylase as well as a significant CO 2 fixation by the C 4-pathway. — Also in Zea mays L. and Chloris gayana Kunth., the presence of NaCl in the medium influences the balance between phosphoenol pyruvate carboxylase and ribulose-1,5-diphosphate carboxylase. 相似文献
2.
Suaeda monoica Frossk. ex J. F. Gmel is a C 4 plant with three different photosynthesizing cell layers. The outer chlorenchymatous layer shows a high activity of phosphoenolpyruvate (PEP) carboxylase but none of ribulose bisphosphate (RuBP) carboxylase. The electrophoretic protein band of RuBP carboxylase was missing in this layer. The second chlorenchymatous cells layer shows a very high activity of RuBP carboxylase and NAD malic enzyme and only traces of activity of PEP carboxylase. The third photosynthesizing cell type is comprised of the water tissue. It has moderate activities of RuBP carboxylase and PEP carboxylase. A model for carbon flow in Suaeda monoica leaves is proposed. 相似文献
3.
The activity of phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31
[EC]
)increased with leaf age in Kalanchoe daigremontiana. Polyacrylamidegel electrophoresis showed 3 bands of PEP carboxylase activity,one predominant in young leaves, another predominant in olderleaves. PEP carboxylase activities in desalted extract differedaccording to pH, at acidic pH the enzyme activity of young leavesdecreased drastically, whereas that of older leaves remainednearly constant. (Received August 2, 1982; Accepted September 27, 1982) 相似文献
4.
In vivo CO 2 fixation and in vitro phosphoenolpyruvate (PEP) carboxylase levels have been measured in lupin ( Lupinus angustifolius L.) root nodules of various ages. Both activities were greater in nodule tissue than in either primary or secondary root tissue, and increased about 3-fold with the onset of N 2 fixation. PEP carboxylase activity was predominantly located in the bacteroid-containing zone of mature nodules, but purified bacteroids contained no activity. Partially purified PEP carboxylases from nodules, roots, and leaves were identical in a number of kinetic parameters. Both in vivo CO 2 fixation activity and in vitro PEP carboxylase activity were significantly correlated with nodule acetylene reduction activity during nodule development. The maximum rate of in vivo CO 2 fixation in mature nodules was 7.9 nmol hour −1 mg fresh weight −1, similar to rates of N 2 fixation and reported values for amino acid translocation. 相似文献
5.
Summary Ribulose diphosphate (RuDP) and (PEP) phosphoenolpyruvate carboxylase enzyme activities were studied in young, mature, and senescent Portulaca oleracea leaves. While the absolute amount of both the C 3 (RuDP) and C 4 (PEP) carboxylase is less in senescent leaves than in mature leaves, RuDP carboxylase activity is reduced to a lesser degree. In senescent leaves, PEP carboxylase activity equals 10% of that in mature tissue, but RuDP carboxylase is 27% of that in mature leaves. The same ontogenetic series was also used to determine photorespiration rates and responses to several gas treatments. Young and mature leaves were unaffected by changes in the light regime or oxygen concentrations, and exhibited typical C 4-plant light/dark 14CO 2 evolution ratios. Senescent leaves, on the other hand, have photorespiration ratios similar to C 3-plants. In addition, senescent leaves were affected by minus CO 2, 100% O 2 and N 2 in a manner expected of C 3-plants, but not C 4-plants. These results are discussed in terms of a relative increase in activity of the C 3 cycle in later developmental stages in this plant.Abbreviation RuDP
ribulose diphosphate
- PEP
phosphoenolpyruvate
- PGA
phosphoglyceric acid 相似文献
6.
The activities of the carboxylating enzymes ribulose-1,5-biphosphate (RuBP) carboxylase and phosphoenolpyruvate (PEP) carboxylase in leaves of three-week old Zea mays plants grown under phytotron conditions were found to vary according to leaf position. In the lower leaves the activity of PEP carboxylase was lower than that of RuBP carboxylase, while the upper leaves exhibited high levels of PEP carboxylase. Carbon dioxide compensation points and net photosynthetic rates also differed in the lower and upper leaves. Differences in the fine structure of the lowermost and uppermost leaves are shown. The existence of both the C 3 and C 4 photosynthetic pathways in the same plant, in this and other species, is discussed.Abbreviations PEP
phosphoenolpyruvate
- RuBP
ribulose-1,5-biphosphate 相似文献
7.
Phosphoenolpyruvate (PEP) carboxylase (EC 4.1.1.31) and ribulose-1,5-bisphospate (RuBP) carboxylase (EC 4.1.1.39) activities in leaves of different maize hybrids grown under field conditions (high light intensity) and in a growth chamber (low light intensity) were determined. Light intensity and leaf age affected PEP carboxylase activity, whereas RuBP carboxylase was affected by leaf age only at low light intensity. PEP carboxylase/RuBP carboxylase activity ratio decreased according to light intensity and leaf age. Results demonstrate that Zea mays grown under field conditions is a typical C 4 species in all leaves independently from their position on the stem, whereas it may be a C 3 plant when it is grown in a growth chamber at low light intensityAbbreviations PEP
phosphoenolpyruvate
- RuBP
ribulose-1,5-bisphosphate 相似文献
8.
The aim of this work was to discover the extent of interference by phospho enolpyruvate (PEP) phosphatase in spectrophotometric assays of PEP carboxylase (EC 4.1.1.31) in crude extracts of plant organs. The presence of PEP phosphatase and lactate dehydrogenase (EC 1.1.1.27) in extracts leads to PEP-dependent NADH oxidation that is independent of PEP carboxylase activity, and hence to overestimation of PEP carboxylase activity. In extracts of three organs of pea ( Pisum sativum L.: leaves, developing embryos, and Rhizobium nodules), two organs of wheat ( Triticum aestivum L.: developing grain and endosperm), and leaves of Moricandia arvensis (L.) D.C., lactate dehydrogenase activity was at most only 16% of that of PEP carboxylase at the pH optimum for PEP carboxylase activity. Endogenous PEP phosphatase and lactate dehydrogenase are thus unlikely to interfere seriously with the assay for PEP carboxylase at its optimum pH. Addition of lactate dehydrogenase to PEP carboxylase assays— a proposed means of correcting for nonenzymic decarboxylation of oxaloacetate to pyruvate—resulted in increases in PEP-dependent NADH oxidation from zero ( Rhizobium nodules) to 131% (wheat grains). There was no obvious relationship between the magnitude of this increase and conditions in the assay that might promote oxaloacetate decarboxylation. However, the magnitude of the increase was highly positively correlated with the activity of PEP phosphatase in the extract. Addition of lactate dehydrogenase to PEP carboxylase assays can thus result in very large overestimations of PEP carboxylase activity, and should only be used as a means of correction for oxaloacetate decarboxylation for extracts with negligible PEP phosphatase activity. 相似文献
9.
Activity of ribulose 1,5-bisphosphate (RuBP) carboxylase in leaf extracts of the constitutive Crassulacean acid metabolism (CAM) plant Kalanchoe pinnata (Lam.) Pers. decreased with increasing leaf age, whereas the activity of phosphoenolpyruvate (PEP) carboxylase increased. Changes in enzyme activities were associated with changes in the amount of enzyme proteins as determined by immunochemical analysis, sucrose density gradient centrifugation, and SDS gel electrophoresis of leaf extracts. Young developing leaves of plants which received high amounts of NO
3
-
during growth contained about 30% of the total soluble protein in the form of RuBP carboxylase; this value declined to about 17% in mature leaves. The level of PEP carboxylase in young leaves of plants at high NO
3
-
was an estimated 1% of the total soluble protein and increased to approximately 10% in mature leaves, which showed maximum capacity for dark CO 2 fixation. The growth of plants at low levels of NO
3
-
decreased the content of soluble protein per unit leaf area as well as the extractable activity and the percentage contribution of both RUBP carboxylase and PEP carboxylase to total soluble leaf protein. There was no definite change in the ratio of RuBP carboxylase to PEP carboxylase activity with a varying supply of NO
3
-
during growth. It has been suggested (e.g., Planta 144, 143–151, 1978) that a rhythmic pattern of synthesis and degradation of PEP carboxylase protein is involved in the regulation of -carboxylation during a day/night cycle in CAM. No such changes in the quantity of PEP carboxylase protein were observed in the leaves of Kalanchoe pinnata (Lam.) Pers. or in the leaves of the inducible CAM plant Mesembryanthemum crystallinum L.Abbreviations CAM
Crassulacean acid metabolism
- RuBP
ribulose 1,5-bisphosphate
- PEP
phosphoenolpyruvate
- G-6-P
glucose-6-phosphate 相似文献
10.
Illumination of previously darkened maize ( Zea mays L. cv Golden Cross Bantam T51) leaves had no effect on the concentration of phosphoenolpyruvate (PEP) carboxylase protein, but increased enzyme activity about 2-fold when assayed under suboptimal conditions (pH 7.0 and limiting PEP). In addition, sensitivity to effectors of PEP carboxylase activity was significantly altered; e.g. malate inhibition was reduced and glucose-6-phosphate activation was increased. Consequently, 10- to 20-fold differences in PEP carboxylase activity were observed during dark to light transitions when assayed in the presence of effectors. At pH 7.0 activity of purified PEP carboxylase was not proportional to enzyme concentrations. Below 0.7 microgram PEP carboxylase protein per milliliter, enzyme activity was disproportionately reduced. Including polyethylene glycol plus potassium chloride in the reaction mixture eliminated this discontinuity and substantially increased PEP carboxylase activity and reduced malate inhibition dramatically. Inclusion of polyethylene glycol in the assay mixture specifically increased the activity of PEP carboxylase extracted from dark leaves, and reduced malate inhibition of the enzyme from both light and dark leaves. Collectively, the results suggest that PEP carboxylase in maize leaves is subjected to some type of protein modification that affects both activity and effector sensitivity. We postulate that changes in quaternary structure (dissociation or altered subunit interactions) may be involved. 相似文献
11.
Incubation of the submersed aquatic macrophyte, Hydrilla verticillata Royle, for up to 4 weeks in growth chambers under winter-like or summer-like conditions produced high (130 to 150 μl CO 2/1) and low (6 to 8 μl CO 2/l) CO 2 compensation points (Γ), respectively. The activities of both ribulose bisphosphate (RuBP) and phosphoenolpyruvate (PEP) carboxylases increased upon incubation but the major increase was in the activity of PEP carboxylase under the summer-like conditions. This reduced the ratio of RuBP/PEP carboxylases from 2.6 in high Γ plants to 0.2 in low Γ plants. These ratios resemble the values in terrestrial C 3 and C 4 species, respectively. Kinetic measurements of the PEP carboxylase activity in high and low Γ plants indicated the Vmax was up to 3-fold greater in the low Γ plants. The Km (HCO 3 ?) values were 0.33 and 0.22 mM for the high and low Γ plants, respectively. The Km (PEP) values for the high and low Γ plants were 0.23 and 0.40 mM, respectively; and PEP exhibited cooperative effects. Estimated Km (Mg 2+) values were 0.10 and 0.22 mM for the high and low Γ plants, respectively. Malate inhibited both PEP carboxylase types similarly. The enzyme from low Γ plants was protected by malate from heat inactivation to a greater extent than the enzyme from high Γ plants. The results indicated that C 4 acid inhibition and protection were not reliable methods to distinguish C 3 and C 4 PEP carboxylases. The PEP carboxylase from low Γ plants was inhibited more by NaCl than that from hight Γ plants. These analyses indicated that Hydrilla PEP carboxylases had intermediate characteristics between those of terrestrial C 3 and C 4 species with the low Γ enzyme being different from the high Γ enzyme, and closer to a C 4 type. 相似文献
12.
In this report, the effects of light on the activity and allosteric properties of phospho enolpyruvate (PEP) carboxylase were examined in newly matured leaves of several C 3 and C 4 species. Illumination of previously darkened leaves increased the enzyme activity 1.1 to 1.3 fold in C 3 species and 1.4 to 2.3 fold in C 4 species, when assayed under suboptimal conditions (pH 7) without allosteric effectors. The sensitivities of PEP carboxylase to the allosteric effectors malate and glucose-6-phosphate were markedly different between C 3 and C 4 species. In the presence of 5 mM malate, the activity of the enzyme extracted from illuminated leaves was 3 to 10 fold higher than that from darkened leaves in C 4 species due to reduced malate inhibition of the enzyme from illuminated leaves, whereas it increased only slightly in C 3 species. The Ki(malate) for the enzyme increased about 3 fold by illumination in C 4 species, but increased only slightly in C 3 species. Also, the addition of the positive effector glucose-6-phosphate provided much greater protection against malate inhibition of the enzyme from C 4 species than C 3 species. Feeding nitrate to excised leaves of nitrogen deficient plants enhanced the degree of light activation of PEP carboxylase in the C 4 species maize, but had little or no effect in the C 3 species wheat. These results suggest that post-translational modification by light affects the activity and allosteric properties of PEP carboxylase to a much greater extend in C 4 than in C 3 species. 相似文献
13.
Incubation of the submersed aquatic macrophyte, Hydrilla vertieillata Royle, for up to 4 weeks in growth chambers under winter-like or summer-like conditions produced high (130 to 150 μl CO 2/l) and low (6 to 8 μl CO 2/l) CO 2 compensation points (Γ), respectively. The activities of both ribulose bisphosphate (RuBP) and phosphoenolpyruvate (PEP) carboxylases increased upon incubation but the major increase was in the activity of PEP carboxylase under the summer-like conditions. This reduced the ratio of RuBP/PEP carboxylases from 2.6 in high Γ plants to 0.2 in low Γ plants. These ratios resemble the values in terrestrial C 3 and C 4 species, respectively. Kinetic measurements of the PEP carboxylase activity in high and low Γ plants indicated the Vmax was up to 3-fold greater in the low Γ plants. The Km (HCO 3 -) values were 0.33 and 0.22 mM for the high and low Γ plants, respectively. The Km (PEP) values for the high and low Γ plants were 0.23 and 0.40 mM, respectively; and PEP exhibited cooperative effects. Estimated Km (Mg 2+) values were 0.10 and 0.22 mM for the high and low Γ plants, respectively. Malate inhibited both PEP carboxylase types similarly. The enzyme from low Γ plants was protected by malate from heat inactivation to a greater extent than the enzyme from high Γ plants. The results indicated that C 4 acid inhibition and protection were not reliable methods to distinguish C 3 and C 4 PEP carboxylases. The PEP carboxylase from low Γ plants was inhibited more by NaCl than that from high Γ plants. These analyses indicated that Hydrilla PEP carboxylases had intermediate characteristics between those of terrestrial C 3 and C 4 species with the low Γ enzyme being different from the high Γ enzyme, and closer to a C 4 type. 相似文献
14.
The leaf anatomy and certain photosynthetic properties of nitrate- and ammonia-grown plants of Moricandia arvensis (L.) DC., a species previously reported to be a C 3-C 4 intermediate, were investigated. Nitrate-grown plants had a high level of malate in the leaves while ammonia-grown plants had low levels of malate. In young leaves of nitrate-grown plants, there was a diurnal fluctuation of malate content, increasing during the day and decreasing during the night. Titratable acidity remained low in leaves of both nitrate- and ammonia-grown plants. In nitrate-grown plants, the activity of phosphoenolpyruvate (PEP) carboxylase was about 2-fold higher than in ammonia-grown plants, the latter having activity typical of C3 species. Also, in nitrate-grown plants, the ratio of activities of ribulose 1,5-bisphosphate (RuBP) carboxylase/PEP carboxylase was lower than in ammonia-grown plants. Nitrate reductase activities were higher in nitrate- than in ammonia-grown plants and the greatest activity was found in younger leaves. With nitrate-grown plants, during a pulse-chase experiment the label in malate, as a percentage of the total labeled products, increased from about 7% after a 10-second pulse with 14CO2 up to 17% during a 5-minute chase with 12CO2. The pattern of 14C labeling in various metabolites suggests the primary carboxylation is through RuBP carboxylase with a secondary carboxylation through PEP carboxylase. In similar experiments, with ammonia-grown plants, the percentage label in malate was only 0% to 4% with no increase in malate labeling during the chase period. The CO2 compensation point was lower in nitrate-grown than ammonia-grown plants. There was no evidence of Kranz-like anatomy in either the nitrate or ammonia-grown plants. Mitochondria of bundle-sheath cells were strikingly positioned along the inner tangential wall. This might allow the chloroplasts of these cells to fix the mitochondrial photorespired CO2 more effectively and contribute to the low CO2 compensation point in the species. Chloroplasts of bundle-sheath cells and contiguous mesophyll cells were similar in size and structure in plants grown on different media, although chloroplast thylakoids and stromata of the ammonia-grown plants stained more intensely than those of nitrate-grown plants. In addition, irregular clusters of phytoferritin particles occurred in the chloroplasts of the ammonia-grown plants. The results indicate that the substantial activity of PEP carboxylase, incorporation of CO2 into malate, the high malate content, and in part the relatively low CO2 compensation point in Moricandia arvensis may be accounted for by metabolism of nitrate rather than by a state of C3-C4 intermediacy. 相似文献
15.
The effect of 3,3-dichloro-2-(dihydroxyphosphinoylmethyl)-propenoate (DCDP), an analog of phosphoe nolpyruvate (PEP), on PEP carboxylase activity in crude leaf extracts and on photosynthesis of excised leaves was examined. DCDP is an effective inhibitor of PEP carboxylase from Zea mays or Panicum miliaceum; 50% inhibition was obtained at 70 or 350 micromolar, respectively, in the presence of 1 millimolar PEP and 1 millimolar HCO 3−. When fed to leaf sections via the transpiration stream, DCDP at 1 millimolar strongly inhibited photosynthesis in C 4 species (79-98% inhibition for a range of seven C 4 species), but only moderately in C 3 species (12-46% for four C 3 species), suggesting different mechanisms of inhibition for each photosynthetic type. The response of P. miliaceum (C 4) net photosynthesis to intercellular pCO 2 showed that carboxylation efficiency, as well as the CO 2 saturated rate, are lowered in the presence of DCDP and supported the view that carboxylation efficiency in C 4 species is directly related to PEP carboxylase activity. A fivefold increase in intercellular pCO 2 over that occurring in P. miliaceum under normal photosynthesis conditions only increased net photosynthesis rate in the presence of 1 millimolar DCDP from zero to about 5% of the maximal uninhibited rate. Therefore, it seems unlikely that direct fixation of atmospheric CO 2 by the bundle sheath cells makes any significant contribution to photosynthetic CO 2 assimilation in C 4 species. The results support the concept that C 4-selective herbicides may be developed based on inhibitors of C 4 pathway reactions. 相似文献
16.
Photosynthetic properties of photoautotrophic suspensions cultured in a minimal growth medium have been evaluated to determine whether changes have occurred in ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) activity, phosphoenol-pyruvate (PEP) carboxylase activity, chlorophyll content, or culture growth. Five photoautotrophic lines Amaranthus powellii, Datura innoxia, Glycine max, Gossypium hirsutum, and a Nicotiana tabacum-Nicotiana glutinosa fusion hybrid were grown in a medium without organic carbon other than phytohormones, and without vitamins. These photoautotrophic lines had total Rubisco activities ranging from 85 to 266 micromoles CO 2 fixed per milligram chlorophyll hour −1, with percent activation of Rubisco ranging from 16 to 53%. Inclusion of protease inhibitors in the homogenization buffer did not result in higher Rubisco activity. PEP carboxylase activity for cells cultured in minimal medium was found to range from 16 to 146 micromoles CO 2 per milligram chlorophyll hour −1, with no higher activity in the C 4Amaranthus cells compared with PEP carboxylase activity in the C 3 species assayed. Rubisco-to-PEP carboxylase ratios ranged from 2.2 to 1 up to 9.4 to 1. Chlorophyll contents increased in all but the Nicotiana cell line, and all of the photoautotrophic culture lines were capable of growth in vitamin-free medium with the exception of SB-P, which requires thiamine. 相似文献
17.
The intracellular localization of phosphoenolpyruvate (PEP) carboxylase in plants belonging to the C 4, Crassulacean acid metabolism (CAM) and C 3 types was invetigated using an immunocytochemical method with an immune serum raised against the sorghum leaf enzyme. The plants studied were sorghum, maize (C 4 type), kalanchoe (CAM type), french bean, and spinach (C 3 type). In the green leaves of C 4 plants, it was shown that the carboxylase was located in the mesophyll and stomatic cells, being largely cytosolic in the mesophyll cells. Similarly, in CAM plants, the enzyme was found mainly outside the chloroplasts. In contrast, in C 3 plants, the PEP carboxylase appeared to be distributed between the cytosol and the chloroplasts of foliar parenchyma. Examination of sections from etiolated leaves showed fluorescence emission from etioplasts and cytosol for the parenchyma of french bean as well as for the bundle sheath and mesophyll of sorghum leaves. This data indicated that during the greening process photoregulation and evolution of PEP carboxylase is dependent on the tissue and on the metabolic type of the plant considered.Abbreviations CAM
Crassulacean acid metabolism
- PEP
phosphoenolpyruvate 相似文献
18.
Experiments were conducted with a tobacco ( Nicotiana tabacum) mutant with 40 to 50% greater catalase activity than wild type that is associated with a novel form of O 2-resistant photosynthesis. The apparent Km for H 2O 2 was the same in mutant and wild-type leaf extracts. Tobacco RNAs were hybridized with Nicotiana sylvestris catalase cDNA, and a threefold greater steady-state level of catalase mRNA was found in mutant leaves. Steady-state levels of ribulose-1,5-bisphosphate carboxylase small subunit mRNA were similar in mutant and wild type. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of partially purified catalase showed that the protein concentration in the band corresponding to catalase was higher in the mutant than in the wild type. Separation of leaf catalase proteins by isoelectric focusing revealed the presence of five major bands and one minor band of activity. The distribution of the catalase activity among these forms was similar in mutant and wild type, although the total activity was higher in the mutant in all five major bands. The results indicate that the enhanced catalase activity in mutant leaves is caused by an increase in synthesis of catalase protein and that this trait is mediated at the nucleic acid level. 相似文献
19.
Maize ( Zea mays L.) leaf phospho enopyruvate (PEP) carboxylase activity at subsaturating levels of PEP was increased by the inclusion of glycerol (20%, v/v) in the assay medium. The extent of activation was dependent on H + concentration, being more marked at pH 7 (with activities 100% higher than in aqueous medium) than at pH 8 (20% activation). The determination of the substrate concentration necessary to achieve half-maximal enzyme activity (S 0.5) (PEP) and maximal velocity ( V) between pH 6.9 and 8.2 showed a uniform decrease in S 0.5 in the presence of glycerol over the entire pH range tested, and only a slight decrease in V at pH values near 8. Including NaCl (100 millimolar) in the glycerol containing assay medium resulted in additional activation, mainly due to an increase in V over the entire range of pH. Glucose-6-phosphate (5 millimolar) activated both the native and the glycerol-treated enzyme almost to the same extent, at pH 7 and 1 millimolar PEP. Inhibition by 5 millimolar malate at pH 7 and subsaturating PEP was considerably lower in the presence of glycerol than in an aqueous medium (8% against 25%, respectively). Size-exclusion high performance liquid chromatography in aqueous buffer revealed the existence of an equilibrium between the tetrameric and dimeric enzyme forms, which is displaced to the tetramer as the pH was increased from 7 to 8. In the presence of glycerol, only the 400 kilodalton tetrameric form was observed at pH 7 or 8. However, dissociation into dimers by NaCl could not be prevented by the polyol. We conclude that the control of the aggregation state by the metabolic status of the cell could be one regulatory mechanism of PEP carboxylase. 相似文献
20.
Kinetic characteristics of phospho enolpyruvate carboxylase (PEPC) from the epiphytic C 3 or C 4: CAM intermediate plant, Peperomia camptotricha, were investigated. Few day versus night differences in Vmax, Km(PEP)), or malate inhibition were observed, even in extracts from water-stressed plants which characteristically perform CAM, regardless of efforts to stabilize day/night forms. The PEPC extracted from plants during the light period remained stable, without much of an increase or decrease in activity for at least 22 hours at 0 to 4°C. Extracts from mature, fully developed leaves had slightly greater PEPC activity than from very young, developing leaves. Generally, however, the kinetic properties of PEPC extracted from mature leaves of plants grown under short day (SD), long day (LD), or 1-week water-stress conditions, as well as from young, developing leaves, were similar. The PEPC inhibitor, l-malate, decreased the Vmax and increased the Km(PEP) for all treatments. Under specific conditions, malate did not inhibit PEPC rates in the dark extracts as much as the light. The PEPC activator, glucose-6-phosphate (G-6-P), lowered the Km(PEP) for all treatments. At saturating PEP concentrations, PEPC activity was independent of pH in the range of 7.5 to 9.0. At subsaturating PEP concentrations, the pH optimum was 7.8. The rates of PEPC activity were lower in the light period extracts than the dark, at pH 7.1, but day/night PEPC was equally active at pH 7.8. At pH 7.5 and a subsaturating PEP concentration, G-6-P significantly activated PEPC. At pH 8, however, only slight activation by G-6-P was observed. The lower pH of 7.5 combined with l-malate addition, greatly inhibited PEPC, particularly in extracts from young, developing leaves which were completely inhibited at an l-malate concentration of 1 millimolar. However, malate did not further inhibit PEPC activity in mature leaves when assayed at pH 7.1. The fairly constant day/night kinetic and regulatory properties of PEPC from P. camptotricha are unlike those of PEPC from CAM or C 4 species studied, and are consistent with the photosynthetic metabolism of this plant. 相似文献
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