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1.
The (K+,Mg2+)-ATPase was partially purified from a plasma membrane fraction from corn roots (WF9 × Mol7) and stored in liquid N2 without loss of activity. Specific activity was increased 4-fold over that of the plasma membrane fraction. ATPase activity resembled that of the plasma membrane fraction with certain alterations in cation sensitivity. The enzyme required a divalent cation for activity (Co2+ > Mg2+ > Mn2+ > Zn2+ > Ca2+) when assayed at 3 millimolar ATP and 3 millimolar divalent cation at pH 6.3. When assayed in the presence of 3 millimolar Mg2+, the enzyme was further activated by monovalent cations (K+, NH4+, Rb+ Na+, Cs+, Li+). The pH optima were 6.5 and 6.3 in the absence and presence of 50 millimolar KCl, respectively. The enzyme showed simple Michaelis-Menten kinetics for the substrate ATP-Mg, with a Km of 1.3 millimolar in the absence and 0.7 millimolar in the presence of 50 millimolar KCl. Stimulation by K+ approached simple Michaelis-Menten kinetics, with a Km of approximately 4 millimolar KCl. ATPase activity was inhibited by sodium orthovanadate. Half-maximal inhibition was at 150 and 35 micromolar in the absence and presence of 50 millimolar KCl. The enzyme required the substrate ATP. The rate of hydrolysis of other substrates, except UDP, IDP, and GDP, was less than 20% of ATP hydrolysis. Nucleoside diphosphatase activity was less than 30% of ATPase activity, was not inhibited by vanadate, was not stimulated by K+, and preferred Mn2+ to Mg2+. The results demonstrate that the (K+,Mg2+)-ATPase can be clearly distinguished from nonspecific phosphohydrolase and nucleoside diphosphatase activities of plasma membrane fractions prepared from corn roots.  相似文献   

2.
Regulation of ADP-Glucose Pyrophosphorylase from Chlorella vulgaris   总被引:1,自引:1,他引:0  
ADP-glucose pyrophosphorylase was partially purified from Chlorella vulgaris 11h. 3-Phosphoglycerate activated the enzyme by lowering the Michaelis constant for glucose-1-phosphate (from 0.97 to 0.36 millimolar in the presence of 2 millimolar phosphoglycerate) and ATP (from 0.23 to 0.10 millimolar), as well as increasing the Vmax. Saturation curves for 3-phosphoglycerate were hyperbolic and the activator concentration at half Vmax value for 3-phosphoglycerate was 0.41 millimolar either in the presence or absence of phosphate. Phosphate inhibited the enzyme in a competitive manner with respect to glucose-1-phosphate, but did not affect the Michaelis constant value for ATP. 3-Phosphoglycerate changed neither the inhibitor concentration at half Vmax value of 1.0 millimolar for phosphate nor the hyperbolic inhibition kinetics for phosphate. The enzyme required divalent cations for its activity. The activation curves for Mn2+ and Mg2+ were highly sigmoidal. The activator concentration at half Vmax values for Mn2+ and Mg2+ were 2.8 and 3.7 millimolar, respectively. With optimal cations, the Michaelis constant values for ATP-Mn and ATP-Mg were 0.1 and 0.4 millimolar, respectively.  相似文献   

3.
Initial observations showed a 100% increase in H2-uptake (Hup) activity of Rhizobium leguminosarum strain 3855 in pea root nodules (Pisum sativum L. cv Alaska) on plants growing in a baked clay substrate relative to those growing in vermiculite, and an investigation of nutrient factors responsible for the phenomenon was initiated. Significantly greater Hup activity was first measured in the clay-grown plants 24 days after germination, and higher activity was maintained relative to the vermiculite treatment until experiments were terminated at day 32. The increase in Hup activity was associated with a decrease in H2 evolution for plants with comparable rates of acetylene reduction. Analyses of the clay showed that it contained more Na+ (29 versus 9 milligrams per kilogram) and less K+ (6 versus 74 milligrams per kilogram) than the vermiculite. Analyses of plants, however, showed a large increase in Na+ concentration of clay-grown plants with a much smaller reduction in K+ concentration. In tests with the same organisms in a hydroponic system with controlled pH, 40 millimolar NaCl increased Hup activity more than 100% over plants grown in solutions lacking NaCl. Plants with increased Hup activity, however, did not have greater net carbon or total nitrogen assimilation. KCl treatments from 5 to 80 millimolar produced slight increased in Hup activity at 10 millimolar KCl, and tests with other salts in the hydroponic system indicated that only Na+ strongly promoted Hup activity. Treating vermiculite with 50 millimolar NaCl increased Na+ concentration in pea plant tissue and greatly promoted Hup activity of root nodules in a manner analogous to the original observation with the clay rooting medium. A wider generality of the phenomenon was suggested by demonstrating that exogenous Na+ increased Hup activity of other R. leguminosarum strains and promoted Hup activity of R. meliloti strain B300 in alfalfa (Medicago sativa L.).  相似文献   

4.
Most C4 species are chilling sensitive and certain enzymes like pyruvate,Pi dikinase of the C4 pathway are also cold labile. The ability of cations and compatible solutes to protect maize (Zea mays) dikinase against cold lability was examined. The enzyme in desalted extracts at pH 8 from preilluminated leaves could be protected against cold lability (at 0°C) by the divalent cations Mn2+, Mg2+, and Ca2+. There was substantial protection by sulfate based salts but little protection by chloride based salts of potassium or ammonium (concentration 250 millimolar). The degree of protection against cold lability under limiting MgCl2 (5 millimolar) was pH sensitive (maximum protection at pH 8), but independent of ionic strength (up to 250 millimolar by addition of KCl). In catalysis Mg2+ is required and Mn2+ could not substitute as a cofactor. Several compatible solutes reduced or prevented the cold inactivation of dikinase (in desalted extracts and the partially purified enzyme), including glycerol, proline, glycinebetaine and trimethylamine-N-oxide (TMAO). TMAO and Mg2+ had an additive effect in protecting dikinase against cold inactivation. TMAO could largely substitute for the divalent cation and addition of TMAO during cold treatment prevented further inactivation. Cold inactivation was partially reversed by incubation at room temperature; with addition of TMAO reversal was complete. The temperature dependence of inactivation at pH 8 and 3 millimolar MgCl2 was evaluated by incubation at 2 to 17°C for 45 minutes, followed by assay at room temperature. At preincubation temperatures below 11°C there was a progressive inactivation which could be prevented by TMAO (450 millimolar). The results are discussed relative to possible effects of the solutes on the quaternary structure of this enzyme, which is known to dissociate at low temperatures.  相似文献   

5.
The enzyme, phenoloxidase, was isolated and partially purified as an inactive enzyme, a proenzyme, from plant cell cultures of Daucus carota, Nicotiana tabacum, and Haplopappus gracilis. The prophenoloxidase was found to be specifically activated by Ca2+ or Mn2+ ions in concentrations above 1 millimolar. Calmodulin was not involved in this activation. Concentrations of Ca2+ or Mn2+ below 1 millimolar could not induce activation of the prophenoloxidase, but if trypsin was added simultaneously with Ca2+ or Mn2+ at a concentration of 1 millimolar or below, the proenzyme was converted to its active form. The inactive form of phenoloxidase was found to be a soluble enzyme, whereas after activation the enzyme aggregated, and a significant amount of the enzyme activity could become pelleted.  相似文献   

6.
To clarify the kinetic characteristics and ionic requirements of the tonoplast H+-translocating inorganic pyrophosphatase (H+-PPiase), PPi hydrolysis and PPi-dependent H+ transport were studied in tonoplast vesicles isolated from leaf mesophyll tissue of Kalanchoë daigremontiana Hamet et Perrier de la Bâthie. The tonoplast H+-PPiase showed an absolute requirement for a monovalent cation and exhibited hyperbolic kinetics with respect to cation concentration. H+-PPiase activity was maximal in the presence of K+ (K50 approximately 3 millimolar), with PPi-dependent H+ transport being more selective for K+ than PPi hydrolysis. When assayed in the presence of 50 millimolar KCl at fixed PPi concentrations, H+-PPiase activity showed sigmoidal kinetics with respect to total Mg concentration, reflecting a requirement for a Mg/PPi complex as substrate and free Mg2+ for activation. At saturating concentrations of free Mg2+, H+-PPiase activity exhibited Michaelis-Menten kinetics towards MgPPi2− but not Mg2PPi, demonstrating that MgPPi2− was the true substrate of the enzyme. The apparent Km (MgPPi2−) for PPi hydrolysis (17 micromolar) was significantly higher than that for PPi-dependent H+ transport (7 micromolar). Free Mg2+ was shown to be an allosteric activator of the H+-PPiase, with Hill coefficients of 2.5 for PPi hydrolysis and 2.7 for PPi-dependent H+ transport. Half-maximal H+-PPiase activity occurred at a free Mg2+ concentration of 1.1 millimolar, which lies within the range of accepted values for cytosolic Mg2+. In contrast, cytosolic concentrations of K+ and MgPPi2− appear to be saturating for H+-PPiase activity. We propose that one function of the H+-PPiase may be to act as an ancillary enzyme that maintains the proton-motive force across the vacuolar membrane when the activity of the tonoplast H+-ATPase is restricted by substrate availability. As ATP levels decline in the cytosol, free Mg2+ would be released from the MgATP2− complex, thereby activating the tonoplast H+-PPiase.  相似文献   

7.
Arora R  Palta JP 《Plant physiology》1986,82(2):625-629
Freezing injury, in onion bulb tissue, is known to cause enhanced K+ efflux accompanied by a small but significant loss of Ca2+ following incipient freezing injury and swelling of protoplasm during the postthaw secondary injury. The protoplasmic swelling of the cell is thought to be caused by the passive influx of extracellular K+ into the cell followed by water uptake. Using outer epidermal layer of unfrozen onion bulb scales (Allium cepa L. cv Big Red), we were able to stimulate the irreversible freezing injury symptoms, by bathing epidermal cells in 50 millimolar KCl. These symptoms were prevented by adding 20 millimolar CaCl2 to the extracellular KCl solution. Our results provide evidence that loss of cellular Ca2+ plays an important role in the initiation and the progression of freezing injury.  相似文献   

8.
Schwartz A 《Plant physiology》1985,79(4):1003-1005
Ca2+ (0.1-1.0 millimolar) accelerated dark-induced stomatal closure and reduced stomatal apertures in the light in epidermal peels of Commelina communis L. In contrast, ethyleneglycol-bis-(β-aminoethyl ether) N,N′tetraacetic acid (EGTA) (2 millimolar), a Ca2+ chelator, prevented closure in the dark and accelerated opening in the light. EGTA did not promote significant opening in the dark. It is therefore concluded that EGTA does not increase ion uptake into guard cells, but rather prevents ion efflux. Addition of EGTA to incubating solutions with 10 millimolar KCl resulted in steady state apertures of 15.6 micrometers, whereas in the absence of EGTA similar apertures required 55 millimolar KCl and 150 millimolar KCl was needed in the presence of 1 millimolar CaCl2. The results demonstrate the importance of Ca2+ in the regulation of stomatal closure and point to a role of Ca2+ in the regulation of K+ efflux from stomatal guard cells.  相似文献   

9.
Chiang GG  Dilley RA 《Plant physiology》1989,90(4):1513-1523
Intact chloroplasts were compared to isolated thylakoids as to whether storage of the organelle in high KCl medium caused the energy coupling reactions to show a delocalized or a localized proton gradient energy coupling response. With isolated thylakoids, the occurrence of one or the other energy coupling mode can be reversibly controlled by the concentration of mono- and divalent cations used for the thylakoid storage media. Calcium was shown to be the key ion and previous evidence suggested a Ca2+-controlled gating of H+ fluxes in the thylakoid membrane system (G Chiang, RA Dilley [1987] Biochemistry 26: 4911-4916). Isolated, intact chloroplasts, which retained the outer envelope membranes during the 30 min or longer storage treatments in various concentrations of KCl and CaCl2 (with sorbitol to maintain iso-osmotic conditions), were osmotically burst in a reaction cuvette and within 3 minutes were assayed for either a localized or a delocalized proton gradient energy coupling (ATP formation) mode. The intact chloroplast system was analogous to isolated thylakoids, with regard to the effects of KCl and CaCl2 on the energy coupling mode. For example, adding 100 millimolar KCl to the intact organelle storage medium resulted in the subsequent ATP formation assay showing delocalized proton gradient coupling just as with isolated thylakoids. Adding 5 millimolar CaCl2 to the 100 millimolar KCl storage medium resulted in a localized proton gradient coupling mode. Suspending thylakoids in stromal material previously isolated from intact chloroplast preparations and testing the energy coupling response showed that the stromal milieu has enough Ca2+ to cause the localized coupling response even though there was about 80 millimolar K+ in the intact chloroplasts used in this study (determined by atomic absorption spectrophotometry). Extrapolating the intact chloroplast data to the whole leaf level, we suggest that proton gradient energy coupling is normally of the localized mode, but under certain conditions it could be either localized or delocalized, depending on factors that affect the putative Ca2+-regulated proton flux gating function.  相似文献   

10.
Two enzymes capable of hydrolyzing fructose-1,6-bisphosphate (FBP) have been isolated from the foliose lichen Peltigera rufescens (Weis) Mudd. These enzymes can be separated using Sephadex G-100 and DEAE Sephacel chromatography. One enzyme has a pH optimum of 6.5, and a substrate affinity of 228 micromolar FBP. This enzyme does not require MgCl2 for activity, and is inhibited by AMP. The second enzyme has a pH optimum of 9.0, with no activity below pH 7.5. This enzyme responds sigmoidally to Mg2+, with half-saturation concentration of 2.0 millimolar MgCl2, and demonstrates hyperbolic kinetics for FBP (Km = 39 micromolar). This enzyme is activated by 20 millimolar dithiothreitol, is inhibited by AMP, but is not affected by fructose-2-6-bisphosphate. It is hypothesized that the latter enzyme is involved in the photosynthetic process, while the former enzyme is a nonspecific acid phosphatase.  相似文献   

11.
The activity of cytidine 5′-diphosphate (CDP) choline: 1,2-diacylglycerol cholinephosphotransferase (EC 2.7.8.2) in developing soybean (Glycine max L. var Williams 82) seeds was 3 to 5 times higher in cotyledons grown at 20°C than in those grown at 35°C. Some characteristics of the enzyme from cotyledons cultured at 20 and 35°C were compared. In preparations from both growth temperatures, the enzyme showed a pH optimum of 7, Km of 7.0 micromolar for CDP-choline, and an optimum assay temperature of 45°C. Both enzyme preparations were stimulated by increasing concentrations of Mg2+ or Mn2+, up to 10 millimolar and 50 micromolar, respectively, though Mn2+ produced lower activities than Mg2+. Enzymes from both 20 and 35°C show the same specificity for exogenous diacylglycerol. No metabolic effectors were detected by addition of heat treated extracts to the assay mixture. The above findings suggest that the higher enzyme activity at 20°C can be attributed to a higher level of the enzyme rather than to the involvement of isozymes or metabolic effectors. Enzyme activity decreased rapidly during culture at 35°C, indicating a rapid turnover of the enzyme. The level of temperature modulation was found to be a function of seed developmental stage.  相似文献   

12.
Conditions for extraction and assay of ribulose-1,5-bisphophate carboxylase present in an in vivo active form (initial activity) and an inactive form able to be activated by Mg2+ and CO2 (total activity) were examined in leaves of soybean, Glycine max (L.) Merr. cv Will. Total activity was highest after extracts had preincubated in NaHCO3 (5 millimolar saturating) and Mg2+ (5 millimolar optimal) for 5 minutes at 25°C or 30 minutes at 0°C before assay. Initial activity was about 70% of total activity. Kact (Mg2+) and Kact (CO2) were approximately 0.3 millimolar and 36 micromolar, respectively. The carry-over of endogenous Mg2+ in the leaf extract was sufficient to support considerable catalytic activity. While Mg2+ was essential for both activation and catalysis, Mg2+ levels greater than 5 millimolar were increasingly inhibitory of catalysis. Similar inhibition by high Mg2+ was also observed in filtered, centrifuged, or desalted extracts and partially purified enzyme. Activities did not change upon storage of leaves for up to 4 hours in ice water or liquid nitrogen before homogenization, but were about 20% higher in the latter. Activities were also stable for up to 2 hours in leaf extracts stored at 0°C. Initial activity quickly deactivated at 25°C in the absence of high CO2. Total activity slowly declined irreversibly upon storage of leaf homogenate at 25°C.  相似文献   

13.
Rapid enrichment of CHAPS-solubilized UDP-glucose:(1,3)-β-glucan (callose) synthase from storage tissue of red beet (Beta vulgaris L.) is obtained when the preparation is incubated with an enzyme assay mixture, then centrifuged and the enzyme released from the callose pellet with a buffer containing EDTA and CHAPS (20-fold purification relative to microsomes). When centrifuged at high speed (80,000g), the enzyme can also be pelleted in the absence of substrate (UDP-Glc) or synthesis of callose, due to nonspecific aggregation of proteins caused by excess cations and insufficient detergent in the assay buffer. True time-dependent and substrate-dependent product-entrapment of callose synthase is obtained by low-speed centrifugation (7,000-11,000g) of enzyme incubated in reaction mixtures containing low levels of cations (0.5 millimolar Mg2+, 1 millimolar Ca2+) and sufficient detergent (0.02% digitonin, 0.12% CHAPS), together with cellobiose, buffer, and UDP-Glc. Entrapment conditions, therefore, are a compromise between preventing nonspecific precipitation of proteins and permitting sufficient enzyme activity for callose synthesis. Further enrichment of the enzyme released from the callose pellet was not obtained by rate-zonal glycerol gradient centrifugation, although its sedimentation rate was greatly enhanced by inclusion of divalent cations in the gradient. Preparations were markedly cleaner when product-entrapment was conducted on enzyme solubilized from plasma membranes isolated by aqueous two-phase partitioning rather than by gradient centrifugation. Product-entrapped preparations consistently contained polypeptides or groups of closely-migrating polypeptides at molecular masses of 92, 83, 70, 57, 43, 35, 31/29, and 27 kilodaltons. This polypeptide profile is in accordance with the findings of other callose synthase enrichment studies using a variety of tissue sources, and is consistent with the existence of a multi-subunit enzyme complex.  相似文献   

14.
Upon incubation of epidermal peels of Commelina communis in 1 millimolar KCl, a synergistic effect of light and low fusicoccin (FC) concentrations on stomatal opening is observed. In 1 millimolar KCl, stomata remain closed even in the light. However, addition of 0.1 micromolar FC results in opening up to 12 micrometers. The same FC concentration stimulates less than 5 micrometers of opening in darkness. The synergistic effect (a) decreases with increasing FC or KCl concentrations; (b) is dark-reversible; (c) like stomatal opening in high KCl concentrations (120 millimolar) is partially inhibited by the K+ channel blocker, tetraethyl-ammonium+ (20 millimolar). In whole-cell patch-clamp experiments with guard cell protoplasts of Vicia faba, FC (1 or 10 micromolar) stimulates an increase in outward current that is essentially voltage independent between - 100 and +60 millivolts, and occurs even when the membrane potential is held at a voltage (−60 millivolts) at which K+ channels are inactivated. These results are indicative of FC activation of a H+ pump. FC effects on the magnitude of inward and outward K+ currents are not observed. Epidermal peel and patch clamp data are both consistent with the hypothesis that the plasma membrane H+ ATPase of guard cells is a primary locus for the FC effect on stomatal apertures.  相似文献   

15.
Sodium displaces Ca2+ from membranes (GR Cramer, A Läuchli, VS Polito Plant Physiol 1985 79: 207-211) and this can be related to the (Ca2+)/(Na+)2 activity ratio in the external solution (GR Cramer, A Läuchli 1986 J Exp Bot 37: 321-330). Supplemental Ca2+ is known to mitigate the adverse effects of salinity on plant growth. In this report we investigated the effects of NaCl (0-250 millimolar) and Ca2+ (0.4 and 10 millimolar) on the ion activities in solution and on root growth of cotton (Gossypium hirsutum L.). Ion activities were analyzed using the computer program, GEOCHEM. Most ion activities in a 0.1 modified Hoagland solution were significantly reduced by both NaCl and supplemental Ca2+. Ion-pair formation and precipitation were significant for some ions, especially phosphate. Root growth of 6-day-old seedlings was stimulated by low NaCl concentrations (25 millimolar). At higher NaCl concentrations, root growth was inhibited; the concentration at which this occurred depended on the Ca2+ concentration and the growth index used. Supplemental Ca2+ mitigated the inhibition of root growth caused by NaCl. There was a curvilinear relationship between root growth and the (Ca2+)/(Na+)2 ratio in the nutrient solution. The mechanisms by which Na+ and Ca2+ may affect root growth are discussed.  相似文献   

16.
Two soluble Ca2+-dependent protein kinases (enzymes I and II) have been extensively purified from silver beet leaf tissue by means of a protocol involving batch-wise elution from DEAE-cellulose, Ca2+-dependent binding to phenyl-Sepharose, gradient elution from DEAE-Sephacel, gel filtration and binding to Cibacron F3GA-Sepharose CL-6B. Protein kinases I and II are resolved on gradient elution from DEAE-Sephacel and are further distinguished by their different Km values for ATP and large differences in relative rates of phosphorylation of histone H1, casein and bovine serum albumin (the latter two proteins are relatively poor substrates for enzyme II but not enzyme I). Both enzymes have similar molecular weights as determined from gel filtration (56000 ± 2000 and 57000 ± 3000 for enzymes I and II, respectively). Both enzymes are absolutely dependent on free Ca2+ for activity with maximal histone H1 kinase activity being obtained at 0.5 μM free Ca2+. A millimolar concentration of Mg2+ is required in addition to a micromolar concentration Ca2+ for maximal activity. Both enzymes specifically phosphorylate serine residues of histone H1, are thiol activated and are inhibited by lanthanides and a range of calmodulin antagonists and inhibitors of protein kinase C.  相似文献   

17.
CDP-diglyceride:inositol transferase in endoplasmic reticulum fractions from castor bean (Ricinus communis) endosperm was partially characterized. The enzyme had a pH optimum of 8.5 and required Mn2+ for activity. Maximal activity was at 1.5 millimolar MnCl2. A Km of 0.30 mM was calculated for myo-inositol and 1.35 millimolar was estimated for CDP-dipalmitoylglyceride. Concentrations of CDP-dipalmitoylglyceride above 1.2 millimolar inhibited the enzyme. A deoxycholate concentration of 0.1% (w/v) stimulated the reaction slightly while Triton X-100 inhibited at all concentrations tested. Some incorporation of myo-inositol into phosphatidylinositol occurred in the absence of CDP-diglyceride.  相似文献   

18.
A dihydroxyacetone phosphate (DHAP) reductase has been isolated in 50% yield from Dunaliella tertiolecta by rapid chromatography on diethylaminoethyl cellulose. The activity was located in the chloroplasts. The enzyme was cold labile, but if stored with 2 molar glycerol, most of the activity was restored at 30°C after 20 minutes. The spinach (Spinacia oleracea L.) reductase isoforms were not activated by heat treatment. Whereas the spinach chloroplast DHAP reductase isoform was stimulated by leaf thioredoxin, the enzyme from Dunaliella was stimulated by reduced Escherichia coli thioredoxin. The reductase from Dunaliella was insensitive to surfactants, whereas the higher plant reductases were completely inhibited by traces of detergents. The partially purified, cold-inactivated reductase from Dunaliella was reactivated and stimulated by 25 millimolar Mg2+ or by 250 millimolar salts, such as NaCl or KCl, which inhibited the spinach chloroplast enzyme. Phosphate at 3 to 10 millimolar severely inhibited the algal enzyme, whereas phosphate stimulated the isoform in spinach chloroplasts. Phosphate inhibition of the algal reductase was partially reversed by the addition of NaCl or MgCl2 and totally by both. In the presence of 10 millimolar phosphate, 25 millimolar MgCl2, and 100 millimolar NaCl, reduced thioredoxin causes a further twofold stimulation of the algal enzyme. The Dunaliella reductase utilized either NADH or NADPH with the same pH maximum at about 7.0. The apparent Km (NADH) was 74 micromolar and Km (NADPH) was 81 micromolar. Apparent Vmax was 1100 μmoles DHAP reduced per hour per milligram chlorophyll for NADH, but due to NADH inhibition highest measured values were 350 to 400. The DHAP reductase from spinach chloroplasts exhibited little activity with NADPH above pH 7.0. Thus, the spinach chloroplast enzyme appears to use NADH in vivo, whereas the chloroplast enzyme from Dunaliella or the cytosolic isozyme from spinach may utilize either nucleotide.  相似文献   

19.
The effect of K-deficiency on the putrescine biosynthetic enzyme, arginine decarboxylase (ADC), was investigated by growing oat (Avena sativa L. var Victory) plants on a low-K, but otherwise complete nutrient medium in washed quartz sand for up to 18 days. Enzyme activity rose as the concentration of KCl was dropped to 0.6 millimolar or below. However, growth was not inhibited significantly at 0.6 millimolar KCl. ADC activity increased in the whole shoot of K-deficient oats throughout the period of 6 to 18 days, but remained constant in normal plants. At 18 days, ADC activity in entire K-deficient shoots was 6 times greater than in normal shoots, while in the first (oldest) leaf, ADC specific activity increased to more than 30 times the specific activity in the first leaf of normal plants. This effect was due to a moderate rise in total ADC activity in the first leaf between 6 and 18 days, accompanied by a significant decline in protein content. Replacing K+ with Na+ or Li+ significantly inhibited the increase in ADC activity in K-deficient oats, while Rb+ depressed the specific activity to a level below that in normal plants. An alternative putrescine biosynthetic enzyme, ornithine decarboxylase, was also examined. The specific activity of a pelletable form of the enzyme was increased 2-fold in the shoots of K-deficient oats.  相似文献   

20.
Glycine decarboxylase has been successfully solubilized from pea (Pisum sativum) leaf mitochondria as an acetone powder. The enzyme was dependent on added dithiothreitol and pyridoxal phosphate for maximal activity. The enzyme preparation could catalyze the exchange of CO2 into the carboxyl carbon of glycine, the reverse of the glycine decarboxylase reaction by converting serine, NH4+, and CO2 into glycine, and 14CO2 release from [1-14C]glycine. The half-maximal concentrations for the glycine-bicarbonate exchange reaction were 1.7 millimolar glycine, 16 millimolar NaH14CO2, and 0.006 millimolar pyridoxal phosphate. The enzyme (glycine-bicarbonate exchange reaction) was active in the assay conditions for 1 hour and could be stored for over 1 month. The enzymic mechanism appeared similar to that reported for the enzyme from animals and bacteria but some quantitative differences were noted. These included the tenacity of binding to the mitochondrial membrane, the concentration of pyridoxal phosphate needed for maximum activity, the requirement for dithiothreitol for maximum activity, and the total amount of activity present. Now that this enzyme has been solubilized, a more detailed understanding of this important step in photorespiration should be possible.  相似文献   

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