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1.
Abstract— The hypothesis that the ATPase and phosphatidyhnositol (PI) kinase activities of chromaffin vesicle membranes are catalysed by same enzyme was investigated. The two activities exhibited entirely different responses to variations in Mg2+ or Mn2+ concentrations. In the presence of 1 mM ATP, maximal ATPase activity occurred with 1 mM Mg2+ while maximal PI kinase activity required 100 mM Mg2+ Similar differences were observed with Mn2+ with the exception that maximal ATPase activity occurred with 0.5 mM Mn2+ and maximal PI kinase activity occurred with 5 mM Mn2+ Mn2+ was more effective than Mg2+ in stimulating PI kinase activity at low concentrations, but at optimal concentrations of each, the maximal activity obtained with Mg2+ was 5-fold greater than the maximal activity obtained with Mn2+ The heat stabilities of the two enzymes are vastly different. At 50°C the ATPase activity of the intact membranes was stable for up to 20 min while the t l/2 of PI kinase was less than 2 min. After solubilization in Lubrol PX or at higher temperatures both enzymes were less heat stable, but PI kinase was still inactivated at a much greater rate than the ATPase. The evidence suggests that the ATPase and the PI kinase are different proteins.
The major phosphorylated product was diphosphatidylinositol and once formed, it was stable. Phosphorylation of membrane protein accounted for less than 10% of the total32 P-incorporated into chromaffin vesicles. SDS gel electrophoresis of the solubilized membranes showed the presence of at least 2 major phosphorylated high molecular weight components. 相似文献
The major phosphorylated product was diphosphatidylinositol and once formed, it was stable. Phosphorylation of membrane protein accounted for less than 10% of the total
2.
C. Marco D. Gonzalez-Pacanowska A. Linares E. Garcia-Peregrin 《Neurochemical research》1983,8(6):711-721
The patterns of cholesterol content in chick brain and liver were studied during embryonic development and compared with the variations in the specific activities of mevalonate-activating enzymes during the same period. Total cholesterol content in both embryonic chick brain and liver increased during incubation. The relative percentage of free cholesterol was always maintained over 85% in brain, while in liver this percentage decreased to less than 10% during the later days of incubation. A straight parallelism was observed between free cholesterol and pyrophosphomevalonate decarboxylase activity in the embryonic brain. On the other hand, the hepatic decarboxylase exhibited a lower specific activity than in brain and did not show significant variations throughout the same period of incubation. Changes in brain pyrophosphomevalonate decarboxylase activity were more pronounced than those observed in both mevalonate kinase and phosphomevalonate kinase activities, in spite of that the specific activity of decarboxylase was the lowest of the three mevalonate-activating enzymes, suggesting that this reaction is one rate-limiting step for cholesterogenesis during myelination. 相似文献
3.
Abstract The multi-ionic equilibria between enzymes, substrates and monovalent and divalent cations are related in such a way that a change in concentration of one element modifies the repartition of all the concentrations of the other elements, leading to a mutual depletion system. The pyruvate kinase reaction is a good application of the mutual depletion model: this cytoplasmic enzyme utilizes magnesium (Mg) and potassium (K) as cofactors and reacts with free phosphoenolpyruvate and MgADP, substrates involved in the binding of protons, K+ and Mg2+ . Pyruvate kinase from Vibrio costicola , a moderately halophilic eubacterium, obeys the mutual depletion system and is competitively inhibited by physiological concentrations of potassium ions. This effect is relieved by manganese which forms more stable complexes than magnesium. Pyruvate kinase from Halobacterium cutirubrum cannot be described unambiguously by the mutual depletion model.
Cytoplasmic concentrations of potassium ions are elevated in halophilic bacteria and may thus inhibit the formation of the divalent cation complexes necessary in the enzymatic machinery of halophilic bacteria. Accordingly, the contents of the most abundant divalent cation, Mg2+ , and of the trace element manganese, Mn2+ , are higher in the halophilic bacteria, V. costicola, Halobacterium volcanii , and H. cutirubrum , and their increase is proportional to the ionic strength of the extracellular media. The Mn2+ increase is more marked than the Mg2+ increase, although the Mn2+ content is about two orders of magnitude lower than the Mg2+ content. 相似文献
Cytoplasmic concentrations of potassium ions are elevated in halophilic bacteria and may thus inhibit the formation of the divalent cation complexes necessary in the enzymatic machinery of halophilic bacteria. Accordingly, the contents of the most abundant divalent cation, Mg
4.
Barbara Tomaszewska Ryszard W. Schramm Anna Kokot Hanna Mazurowa 《Physiologia plantarum》1983,59(4):641-646
NADP+ -dependent malic enzyme (L-malate : NADP+ oxidoreductase, decarboxylating, EC 1.1.1.40) was extracted from the leaves of yellow lupine. The purification procedure included fractionation with (NH4 )2 SO4 and Sephadex G-25 chromatography, followed by purification on DEAE-cellulose and Sephadex G-200 columns. The enzyme was purified 122-fold. The enzyme affinity towards L-malate was found to be significantly higher with Mn2+ than with Mg2+ . The Hill coefficient for Mg2+ depended on concentration and was 1.6 for the lower and 3.9 for the higher concentrations. The dependence of the enzyme activity on NADP+ followed a hyperbolic curve. Km values and Hill coefficients for NADP+ were similar with both Mn2+ and Mg2+ . The enzyme activity was strictly dependent on divalent cations and followed a sigmoidal curve at least for Mg2+ . The enzyme had 4-fold higher affinity towards Mn2+ than towards Mg2+ , the Km values being 0.3 and 1.15 m M respectively. Of several tested organic acids, oxalate was the most effective inhibitor followed by oxaloacetate while succinate was the strongest activator. 相似文献
5.
W. F. LOOMIS C. KLEIN PH. BRACKET 《Differentiation; research in biological diversity》1979,12(1-3):83-89
Following the initiation of development, amoebae of Dictyostelium discoideum aggregate chemotactically toward cyclic AMP (cAMP). Adenyl cyclase, cAMP phosphodiesterase, and cAMP binding sites all increase 20–40 fold during the first few hours of development. It has been shown that addition of 1 mM EDTA and 5 mM MgCl2 accelerates the aggregation process. Likewise, the calcium ionophore, A23187, leads to precocious aggregation while 4 × 10−5 M progesterone considerably delays it These treatments have now been shown to result in increased accumulation of adenyl cyclase in the case of EDTA and Mg2+ or the ionophore and greatly decreased accumulation in the case of the steroid.
Treatment with EDTA and Mg2+ or the ionophore has been shown not only to accelerate aggregation in wild-type amoebae but to overcome complete blocks to aggregation in certain mutant strains. We have found that addition of Mn2+ will also permit aggregation of mutant cells otherwise unable to aggregate. This divalent ion, unlike EDTA and Mg2+ or the ionophore, was shown to directly stimulate adenyl cyclase. Calcium ions were also found to affect the enzyme such that at Ca2+ concentrations found within the cells the great majority of the activity is inhibited. Manganese ions can overcome the inhibition by Ca2+ .
These findings show that conditions which stimulate aggregation result in increased activity of adenyl cyclase either by increased accumulation of the enzyme or by increased activity of the available enzyme, and support the proposed central role of adenyl cyclase in aggregation. 相似文献
Treatment with EDTA and Mg
These findings show that conditions which stimulate aggregation result in increased activity of adenyl cyclase either by increased accumulation of the enzyme or by increased activity of the available enzyme, and support the proposed central role of adenyl cyclase in aggregation. 相似文献
6.
The Mg2+ -dependent activity of the tonoplast pyrophosphatase (PPase) was investigated by measuring proton transport and by using the acridine orange technique on intact vacuoles of the aquatic liverwort Riccia fluitans L. In solutions with both Mg2+ and pyrophosphate present, a number of complexes are formed, which could all influence the enzymatic and hence the transport activity of the PPase. Therefore, the individual concentrations of these complexes were calculated and their contributions to proton transport across the tonoplast were tested. From these experiments we conclude that Mg2+ has three different roles: (i) Mg2+ stimulates transport activity of the PPase. (ii) Mg2 PPi inhibits PPase-mediated H+ transport, (iii) MgPPi * (= MgPPi 2- + MgHPPi - ) is the substrate with an apparent K1/2 = 5–10 μM, with no discrimination between MgPPi 2- and MgHPPi - . 相似文献
7.
Abstract: Nations were found to inhibit the uptake of L-tryptophan into synaptosomes with a shallow dose-response curve. Almost maximal inhibition was obtained with 10 mM-Na+ . The divalent cations Ca2+ and Mg2+ were shown to be responsible for the increased uptake of L-tryptophan in the absence of Na+ ions. Other divalent cations also promoted tryptophan uptake under this condition (Ca2+ < Mg2+ < Mn2+ < Fe2+ < Zn2+ < Cu2+ ). It was concluded that monovalent chelate complexes were responsible for this enhancing effect. The measured L-tryptophan uptake was the net product of membrane bound and unbound tryptophan. Both bound and unbound tryptophan were increased in the presence of divalent cations. If no divalent cations were added to the incubation medium, Na+ ions decreased the unbound tryptophan but were without effect on bound tryptophan. Under these circumstances D-tryptophan had no effect on binding of the L-isomer and affected the transport of 1.-tryptophan only at very high does (100 x conc. L-tryptophan). These results suggest that I -tryptophan binds to a stereospecific transport carrier located in the synaptosomal membrane and that Na+ ions prevent the translocation of this carrier amino acid complex from the outer to the inner site of the neuronal membrane. 相似文献
8.
Protein Synthesis in a Cell-free System from Rat Brain Sensitive to ACTH-like Peptides 总被引:2,自引:2,他引:0
Abstract: Protein synthesis was measured using a cell-free system obtained from subcortical rat brain tissue. The concentrations of Mg2+ and K+ and the amount of tissue, during both the preparation and the final assay, were critical to the incorporation of amino acids as expressed per milligram protein. Even under optimal conditions mainly elongation of growing peptide chains was measured. Behaviorally active fragments of ACTH modulated the activity of the system in a biphasic manner; i.e., at a low concentration (10−8 M) of ACTH a stimulation of between 10 and 70% was found; a high concentration (10−4 M) was inhibitory (50 to 70%). Structure-activity studies revealed that the stimulatory effect was confined to the N-terminus of the peptide (1–24), whereas the C-terminal sequence was responsible for the inhibition. The stimulation by ACTH1–24 was dependent on Ca2+ and Mg2+ . Cyclic AMP (10−5 M) stimulated the amino acid incorporation too. When a similar cell-free extract was prepared from brain tissue of hypophysectomized rats, the lower in vivo protein synthesis in these animals was preserved in the present cell-free system. The data are discussed in terms of a possible direct intracellular effect of ACTH on brain protein synthesis. 相似文献
9.
Abstract— The effects of two sulfhydryl reagents, PCMBS ( p -chloromercuribenzene sulfonic acid) and NEM ( N -ethylmaleimide) on microtubule-associated Mg2+ -and Ca2+ -ATPase activity were studied in a MTP (microtubule proteins) preparation and in a MAP (microtubule-associated proteins) fraction. In the MTP preparation at pH 6.8, PCMBS stimulated the Mg2+ -ATPase activity at low concentrations and inhibited at higher, whereas the Ca2+ -ATPdse activity was only inhibited. NEM affected the activity in a similar way. At pH 8.0 PCMBS was only inhibitory. NEM showed stimulatory effects over a broader concentration range.
Preincubation in the presence of ATP counteracted the stimulatory effects of both PCMBS and NEM on Mg2+ -ATPase at pH 6.8.
In the MAP fraction at pH 6.8 PCMBS and NEM caused similar but less pronounced effects on the Mg2+ -and Ca2+ -ATPase.
The results show that brain microtubule-associated ATPase activity is similar to dynein and myosin ATPases with respect to biphasic alteration by sulfhydryl reagents. 相似文献
Preincubation in the presence of ATP counteracted the stimulatory effects of both PCMBS and NEM on Mg
In the MAP fraction at pH 6.8 PCMBS and NEM caused similar but less pronounced effects on the Mg
The results show that brain microtubule-associated ATPase activity is similar to dynein and myosin ATPases with respect to biphasic alteration by sulfhydryl reagents. 相似文献
10.
Abstract: The terminal phosphate group of ATP was transferred to ADP by an enzyme present in the soluble core proteins of adrenal medulla catecholamine storage vesicles. It was purified 10–30-fold by DEAE Sephadex chromatography (Fraction I). The enzyme required divalent metal ions for activation; Mn2+ was almost as effective as Mg2+ , but Ca2+ was only a weak activator. Activation by Mg2+ took place over a very narrow concentration range (0.5–3 m m ). The specificity of the enzyme activity to nucleoside triphosphates was broad, to the nucleoside diphosphates narrow, favouring adenosine diphosphate. In dependence on the pH the activity increased from pH 4 to pH 7 and remained constantly high between pH 7 and 9. The Arrhenius plot was linear between 5 and 70°C, with an activation energy of 11.1 kcal/mol. The phosphoryl group transfer reaction depended on the function of thiol groups; p -hydroxymercuribenzoate inhibited 50% of the enzyme activity; dithioerythritol reactivated it completely. Gel electrophoresis revealed that in Fraction I, a protein of molecular weight about 45,000, was enriched compared with the total soluble proteins. The enzyme-enriched Fraction I differed significantly in its relative amino acid composition from that of the total soluble proteins; in general, the acidic amino acids were reduced and the more basic acids enhanced. 相似文献
11.
Bert P. Stok Otto Gy. Tànczos Monika Kähr C. E. E. Stuiver ers Kylin Pieter J. C. Kuiper 《Physiologia plantarum》1981,52(1):115-123
Mg2+ - and Ca2+ -uptake was measured in dark-grown oat seedlings ( Avena sativa L. cv. Brighton) cultivated at two levels of mineral nutrition. In addition the stimulation of the ATPase activity of the microsomal fraction of the roots by Mg2+ was measured. Ca2+ -uptake by the roots was mainly passive. Mg2+ -uptake mainly active; the passive component of Mg2+ -uptake was accompanied by Ca2+ -efflux up to 60% of the Ca2+ present in the roots.
In general Mg2+ -uptake of oat roots was biphasic. The affinity of the second phase correspond well with that of the Mg2+ -stimulation of the ATPase activity, in low-salt roots as well as in high-salt roots and in roots of plants switched to the other nutritional condition. Linear relationships were observed when [phase 2] Mg2+ -uptake was plotted against Mg2+ -stimulation of the ATPase activity of the microsomal fraction of the roots. In 5 days old high-salt plants 1 ATP (hydrolysed in the presence of Mg2+ J corresponded with active uptake of a single Mg2+ ion, but in older high-salt roots and in low-salt roots more ATP was hydrolysed per net uptake of a Mg2+ ion. The results are discussed against the background of regulation of the Mg2+ -level of the cytoplasm of root cells by transport of Mg2+ by a Mg2+ -ATPase to the vacuole, to the xylem vessels, and possibly outwards. 相似文献
In general Mg
12.
Cyclic nucleotide phosphodiesterase (3',5'-cyclic nucleotide nucleotidohydrolase, EC 3.1.4.17) activity isolated from Phaseolus vulgaris L. cv. Limberg seedlings was partially purified and characterized by fractional (NH4 )2 SO4 precipitation, DEAE-cellulose chromatography, chromatography on 3',5'-cAMP-agarose, gel permeation chromatography and chromatofocusing. A crude enzyme preparation, a 30–65% (NH4 )2 SO4 pellet, showed an acidic pH optimum. The enzyme activity was stimulated by imidazole and divalent cations such as Ca2+ , Mg2+ and Mn2+ , whereas NaF, PPi and Fe3+ were inhibitory. Isobutylmethylxanthine had no significant effect on the plant enzyme. An MI of 42 000 was estimated by gel permeation high performance liquid chromatography. By chromatography on 3',5'-cAMP-agarose a phosphodiesterase was resolved that produced 5'-AMP as sole reaction product. 相似文献
13.
Purification and characterization of adenine phosphoribosyltransferase from Arabidopsis thaliana 总被引:1,自引:1,他引:0
Adenine phosphoribosyltransferase (APRT; EC 2. 4,2. 7) from Arabidopsis thaliana was purified approximately 3800-fold, to apparent homogeneity. The purification procedure involved subjecting a leaf extract to heat denaturation, (NH4 )2 SO4 precipitation, Sephadex G-25 salt separation, ultracentrifugation and liquid chromatography on Diethylaminoethyl Sephacel, Phenyl Sepharose CL-4B, Blue Sepharose CL-6B and adenosine 5'-monophosphate-Agarose. The purified APRT was a homodimer of approximately 54 kDa and it had a specific activity of approximately 300 μmol (mg total protein)-1 min-1 . Under standard assay conditions, the temperature optimum for APRT activity was 65°C and the pH optimum was temperature dependent. High enzyme activity was dependent upon the presence of divalent cations (Mn2+ or Mg2+ ). In the presence of MnCl2+ other divalent cations (Mg2+ , Ca2+ , Ba2+ , Hg2+ and Cd2+ ) inhibited the APRT reaction. Kinetic studies indicated that 5-phosphoribose-1-pyrophosphate (PRPP) caused substrate inhibition whereas adenine did not. The Km for adenine was 4.5±1.5 μ M , the Km for PRPP was 0.29±0.06 m M and the Ki for PRPP was 1.96±0.45 m M . Assays using radiolabelled cytokinins showed that purified APRT can also catalyze the phosphoribosylation of isopentenyladenine and benzyladenine. The Km for benzyladenine was approximately 0.73±0.06 m M 相似文献
14.
Seedlings of Euphorbia lathyris L. were grown in the dark at 25°C. Levels of amino acids, sugars and soluble phosphate in the endosperm increased upon germination after 4 days of imbibition, while the amounts of mineral reserves Mg2+ and K+ started decreasing. Ca2+ was not translocated from the endosperm to the seedling. Maximum values for amino acids were found on day 7, and the highest amounts of sugars were present on day 10. The endosperm was completely depleted by day 12.
Before germination (days 1–3) a low level of sterol synthesis in the embryo was detected with labeled sucrose and serine and to a lesser extent with labeled pyruvate. The label of [2-14 C]-mevalonate proceeded exclusively to squalene. Laticifers started the synthesis of their triterpenes upon germination (day 4), using sucrose as a main substrate. A concurrent increase of sterol synthesis outside the laticifers was traced with labeled serine. Radioactive triterpenes, 4 α-methylsterols and sterols were detected in growing seedlings after [14 C]-mevalonate uptake, but most of its label accumulated in squalene. The use of labeled mevalonate in sterol synthesis in growing seedlings is discussed. 相似文献
Before germination (days 1–3) a low level of sterol synthesis in the embryo was detected with labeled sucrose and serine and to a lesser extent with labeled pyruvate. The label of [2-
15.
Abstract— Intact neuroblastoma and glial cells in monolayer culture hydrolysed ATP added to their medium. Evidence is presented that ATP is cleaved outside of the permeability barrier of the plasma membrane and the product is liberated in the extracellular medium, i.e. the enzyme is an ecto-enzyme. Divalent cations such as Mg2+ , Ca2+ , Mn2+ and Co2+ activate the enzyme. In neuroblastoma cells, Ca2+ is the preferential cation for activation; Mg2+ in glial cells. Substrate specificity was very low when different nucleoside-5'-triphosphates were examined. Competition studies have revealed that all of the nucleoside triphosphates are hydrolysed by the same enzyme: divalent cation-activated ecto-nucleoside-5'-triphosphate phosphohydrolase.
Developmental pattern of the enzyme in several lines was established. The role of enzyme in the transport of divalent cations across the plasma membrane and/or in the physical properties of the membrane is suggested. 相似文献
Developmental pattern of the enzyme in several lines was established. The role of enzyme in the transport of divalent cations across the plasma membrane and/or in the physical properties of the membrane is suggested. 相似文献
16.
Effects of Microinjected Cations on the Early Event of Fertilization in the Medaka Egg 总被引:3,自引:3,他引:0
Effects of microinjected cations on the early events of fertilization were examined using eggs of Oryzias latipes . Microinjection of either Ca2+ , Ba2+ or Sr2+ into the thin cortical cytoplasm induced breakdown of cortical alveoli (vesicles) (CABD) under Ca-Mg-free conditions, but microinjection of Mg2+ , Mn2+ or Co2+ prevented CABD at the injected region when the eggs were inseminated in regular saline. Under Ca-Mg-free conditions, CABD could also be induced by microinjection of various solutions (NaCl, choline chloride, sucrose, pH buffer) without any divalent cations or ionophore A23187. Ca2+ microinjected into the cortical cytoplasm did not play a role in sperm penetration. Upon microinjection with either Ca2+ , Mg2+ or K+ , the resting membrane potential leakage was transiently observed. However, depolarization of the membrane followed by slow hyperpolarization was observed only upon microinjection of Ca2+ . From these experiments, it was inferred that microinjected divalent cations such as Ca2+ , Ba2+ or Sr2+ do not act directly upon the cortical alveolus membrane, but trigger the induction of CABD via depolarization of the membrne and increase in intracellular Ca2+ . 相似文献
17.
Relation of source and sink during grain filling period in wheat and some aspects of its regulation 总被引:1,自引:0,他引:1
H. Herzog 《Physiologia plantarum》1982,56(2):155-160
The effect of Mg2+ , Na+ , K+ , ouabain and pH on ATPase activity of purified membrane fractions enriched in plasmalemma fragments from Hordeum vulgare L. (glycophyte) and Halocnemum strobilaceum L. (halophyte) was studied. Membrane ATPases from both plants were synergistically activated by K+ and Na+ in the presence of Mg2+ . The maximum activity of the enzymes were observed at the ratio Na/K = 2–3. Ouabain (10-4 M) almost completely eliminated the (Na+ + K+ )-stimulated component of the ATPase activity. The Na, K, Mg-ATPase of Hordeum had a single pH optimum (pH 8), but that of the Halocnemum had two optima(pH 6 and 8). It appears that similar enzymes operate in the cells of both plants studied. The higher Na, K, Mg-ATPase activity of the halophyte compared to that of the glycophyte suggests the involvement of the enzyme in the extrusion of Na+ from the cytoplasm of cells of both plants. 相似文献
18.
Deoxyribonuclease activities in Myxococcus coralloides D 总被引:2,自引:1,他引:1
M. Magdalena Martínez-Cañamero J. Muñoz A.L. Extremera J.M. Arias 《Journal of applied microbiology》1991,71(2):170-175
Myxococcus coralloides D produced cell-bound deoxyribonucleases (DNases) during the exponential phase of growth in liquid medium. DNase activity was much higher than that detected in other myxobacterial strains and was fractionated into three different peaks by filtration through Sephadex G-200. The DNases were named G, M and P. The optimum temperatures were 37°C, 33°C and 25°C respectively, although high activities were recorded over the temperature range 20–45°C. The pH range of high activity was between 6·0 and 9·0, with an optimum for each DNase at 8·0. DNases M and P were strongly inhibited by low concentrations of NaCl, but activity of DNase G was less affected by NaCl. The three activities required divalent metal ions as cofactors (especially Mg2+ and Mn2+ ); however, other metal ions (Fe2+ , Ni2+ , Zn2+ ) were inhibitors. The molecular weights were estimated by gel filtration chromatography and SDS-PAGE as 44 kDa (DNase G), 49 kDa (DNase M) and 39 kDa (DNase P). 相似文献
19.
Kari R. Hoyt Sunita Rajdev Cheryl L. Fattman Ian J. Reynolds 《Journal of neurochemistry》1995,64(5):2049-2056
Abstract: We investigated the modulation of (±)-α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)-induced increases in intracellular free Ca2+ ([Ca2+ ]i ) and intracellular free Mg2+ ([Mg2+ ]i ) by cyclothiazide and GYKI 52466 using microspectrofluorimetry in single cultured rat brain neurons. AMPA-induced changes in [Ca2+ ]i were increased by 0.3–100 µ M cyclothiazide, with an EC50 value of 2.40 µ M and a maximum potentiation of 428% of control values. [Ca2+ ]i responses to glutamate in the presence of N -methyl- d -aspartate (NMDA) receptor antagonists were also potentiated by 10 µ M cyclothiazide. The response to NMDA was not affected, demonstrating specificity of cyclothiazide for non-NMDA receptors. Almost all neurons responded with an increase in [Ca2+ ]i to both kainate and AMPA in the absence of extracellular Na+ , and these Na+ -free responses were also potentiated by cyclothiazide. GYKI 52466 inhibited responses to AMPA with an IC50 value of 12.0 µ M . Ten micromolar cyclothiazide significantly decreased the potency of GYKI 52466. However, the magnitude of this decrease in potency was not consistent with a competitive interaction between the two ligands. Cyclothiazide also potentiated AMPA- and glutamate-induced increases in [Mg2+ ]i . These results are consistent with the ability of cyclothiazide to decrease desensitization of non-NMDA glutamate receptors and may provide the basis for the increase in non-NMDA receptor-mediated excitotoxicity produced by cyclothiazide. 相似文献
20.
Cathrine Lillo Lucy H. Smith Hugh G. Nimmo Malcolm B. Wilkins 《Physiologia plantarum》1996,98(1):140-146
Nitrate reductase (NR, EC 1.6.6.1) was tested in crude extracts of leaves from Bryophyllum fedtschenkoi plants growing under alternating light/darkness as well as in excised leaves kept in continuous light or darkness. In most extracts NR activity was inhibited 20–80% by 5 m M Mg2+ A light or darkness shift (30 min darkness) during the first part of the photoperiod gave an increase in the Mg2+ inhibition and a decrease in NR activity. Magnesium ion inhibition of NR also showed diurnal variations. Strongest inhibition was found in extracts made during the latter part of the photoperiod and start of the dark period. Pre-incubation of crude extracts with ATP increased Mg2+ inhibition, indicating that phosphorylation of NR is involved in regulation of NR in Crassulacean acid metabolism (CAM) plants. In continuous light an increase in Mg2+ inhibition occurred after 20 h and 40 h, indicating a rhythm in the phosphorylation of NR. A delay in the production of nitrite in the assay (hysteresis) was generally seen in extracts susceptible to Mg2+ inhibition. The rhythms related to NR activity showed the same period length (20 h) as the rhythm in CO2 exchange. However, in contrast to the rhythm in CO2 exchange, NR rhythms were strongly damped in continuous light. In constant darkness the rhythms were even more damped. The results show that post-translational modification of CAM NR is influenced by light/darkness and by an endogenous rhythm. 相似文献