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1.
2.
The kinetics of the chlorophyll fluorescence rise induced by adding 20 mM MgCl2 to a suspension of isolated pea chloroplasts treated with 3-(3,4-dichlorophenyl)-1, 1-dimethylurea (DCMU) have been examined experimentally and theoretically as a function of temperature. The application of similarity arguments and particle aggregation theory to the experimental results suggests that at the first approximation, the salt-induced time-dependent fluorescence changes may be described by the diffusion-controlled lateral movement of Photosystem II pigment-protein complexes. From an analysis of the temperature dependence of the fluorescence changes, estimates obtained for the lateral diffusion coefficients were 1.85 · 10?12–3.08 · 10?11 cm2/s over the temperature range 10°C ? T?30°C. 相似文献
3.
Semin BK Davletshina LN Aleksandrov AY Lanchinskaya VY Novakova AA Ivanov II 《Biochemistry. Biokhimii?a》2004,69(3):331-339
Light-induced interaction of Fe(II) cations with the donor side of Mn-depleted photosystem II (PS II(–Mn)) results in the binding of iron cations and blocking of the high-affinity (HAZ) Mn-binding site. The pH dependence of the blocking was measured using the diphenylcarbazide/2,6-dichlorophenolindophenol test. The curve of the pH dependence is bell-shaped with pK 1 = 5.8 and pK 2 = 8.0. The pH dependence of the O2-evolution mediated by PS II membranes is also bellshaped (pK 2 = 7.6). The pH dependence of the process of electron donation from exogenous donors in PS II(–Mn) was studied to determine the location of the alkaline pH sensitive site of the electron transport chain. The data of the study showed that the decrease in the iron cation binding efficiency at pH > 7.0 during blocking was determined by the donor side of the PS II(–Mn). Mössbauer spectroscopy revealed that incubation of PS II(–Mn) membranes in a buffer solution containing 57Fe(II) + 57Fe(III) was accompanied by binding only Fe(III) cations. The pH dependence of the nonspecific Fe(III) cation binding is also described by the same bell-shaped curve with pK 2 = 8.1. The treatment of the PS II(–Mn) membranes with the histidine modifier diethylpyrocarbonate resulted in an increase in the iron binding strength at alkaline pH. It is suggested that blocking efficiency at alkaline pH is determined by competition between OH– and histidine ligand for Fe(III). Because the high-affinity Mn-binding site contains no histidine residue, this fact can be regarded as evidence that histidine is located at another (other than high-affinity) Fe(III) binding site. In other words, this means that the blockage of the high-affinity Mn-binding site is determined by at least two iron cations. We assume that inactivation of oxygen-evolving complex and inhibition of photoactivation in the alkaline pH region are also determined by competition between OH– and a histidine residue involved in coordination of manganese cation outside the high-affinity site. 相似文献
4.
Protection of Pyruvate,Pi Dikinase from Maize against Cold Lability by Compatible Solutes 总被引:6,自引:1,他引:5
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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.
Engelbert Weis 《Planta》1982,154(1):41-47
The heat-sensitivity of photosynthetic oxygen evolution of thylakoids isolated from spinach increases by increasing the pH above neutral value. The temperature for inactivation (transition temperature) is lowered from about 45° C (pH 6.0–7.4) to 33°C (pH 8.5). Similar results are obtained with intact chloroplasts. At pH 7.0 the transition temperature of washed thylakoids decreases by lowering the salt concentration below 20 mM with monovalent cations (Li+, Na+, K+) and below 3–4 mM with divalent cations (Mg2+, Ca2+, Sr2+). Illumination decreases the heat-sensitivity of oxygen evolution in intact chloroplasts, but even increases the heat-sensitivity in uncoupled chloroplasts. In intact chloroplasts the transition temperature of the heat-induced rise in chlorophyll fluorescence yield (Fo; see Schreiber and Armond 1978) decreases from 44° C to 38° C when the pH of the suspending medium is increased from 6.5 to 8.5. At 20° C, Fo is almost insensitive to pH (6.0–8.5). At 40° C, however, Fo is constant between 6.0 and 7.0, but strongly increases by increasing the pH above neutral value. The results are discussed in terms of a close relation between electrostatic forces at the thylakoid membrane and thermal sensitivity of photosynthetic apparatus. It is suggested that the heat-sensitivity of the photosystem II complex partially depends on the ionization state of fixed groups having alkaline pK. The packed volume of thylakoids suspended in a low salt medium increases when the temperature is increased above 30° C (pH 7.0) and above 20° C (pH 8.0), respectively. This result suggests a heat-induced increase in surface charge density of the thylakoid membrane.Abbreviations HEPES
N-2-hydroxyethylpiperazine-N-2-ethane sulfonic acid
- MES
morpholinoethane sulfonic acid
- MOPS
2-N-morpholinopropane sulfonic acid
- TRICIN
N-[tris(hydroxymethyl)-methyl] glycine 相似文献
6.
Masato Watanabe Toshifumi Taira Hiroshi Asai 《Biochemical and biophysical research communications》1980,95(3):1119-1125
Changes in the turbidity of suspensions of bovine rod outer segment fragments induced by rhodopsin bleaching were measured in the presence of various concentrations of divalent cations at acidic pH (4.7–5.4). Unlike the situation at neutral pH, the turbidity of the suspensions increased drastically by bleaching at acidic pH. It was found that the extent of turbidity change became maximum at a particular concentration of divalent cations (i.e., 5 mM CaCl2, 5 mM MgCl2, or 5 mM mixed divalent cations). However, the turbidity increment in the presence of 5 mM MgCl2 was greatly enhanced by the addition of a minute amount of CaCl2. These results evidently show that the membrane characteristic is abruptly changed by bleaching at acidic pH in particular. It is also suggested that there are two kinds of binding sites for Ca ions: one is a Ca2+ specific site, and the other is a nonspecific site to which Mg2+ can also bind. 相似文献
7.
Effects of the ionophore A23187 on isolated broken and intact chloroplasts in the pH range of 6.2 to 7.6 have been studied. In both types of chloroplasts, uncoupling of photosynthetic electron transport by A23187 (6–10 μm) was mediated either by Mg2+ or—in the absence of divalent cations (i.e., when EDTA was added to the medium)—by high concentrations of Na+, but not of K+ ions. At increased concentrations of the ionophore (above about 10 μm) and high pH (7.2 to 7.6), uncoupling in broken chloroplasts was also mediated by K+ ions. The inhibition of the energy-dependent slow decline of chlorophyll fluorescence in intact chloroplasts by the ionophore (which denotes uncoupling) is reversed by EDTA in the presence of K+, but not of Na+ ions. In 3-(3′,4′-dichlorophenyl)1,1-dimethylurea-poisoned intact chloroplasts, the yield of variable chlorophyll fluorescence is lowered by A23187 + EDTA and increased again by addition of NaCl or KCl. Chlorophyll fluorescence spectra at 77 °K of intact chloroplasts incubated with A23187 + EDTA indicated that the distribution of excitation energy had changed in favor of photosystem I, as expected from a depletion of Mg2+. This change was reversed by MgCl2+, KCl, or NaCl. From a comparison of low-temperature fluorescence spectra of broken and intact chloroplasts at different levels of Mg2+ in the medium, the concentration of free Mg2+ in the stroma of the intact chloroplasts at pH 7.6 in the dark was estimated at 1 to 4 mm. The results show that in chloroplasts the specificity of A23187 for divalent cations is limited. In the presence of EDTA, the ionophore mediates fast exchange across thylakoid membranes, whereas K+ is transferred much less efficiently. Both Na+ and K+ ions seem to be transported readily across the chloroplast envelope by the action of the ionophore, leading to an exchange of Mg2+ for monovalent cations at the thylakoid membrane surfaces in intact chloroplasts. 相似文献
8.
G E Edwards S P Robinson N J Tyler D A Walker 《Archives of biochemistry and biophysics》1978,190(2):421-433
In studying conditions for obtaining photosynthetically functional chloroplasts from mesophyll protoplasts of sunflower and wheat, a strong requirement for chelation was found. The concentration of chelator, either EDTA or pyrophosphate (PPi), required for maximum activation depended on the pH, the concentration of orthophosphate (Pi) in the assay, and the chelator used. Studies with EDTA indicate that including the chelator in the isolation, resuspension, and assay media, in the absence of divalent cations, was most effective. Increased concentration of EDTA from 1 to 10 mm broadened the pH response curve for photosynthesis, inasmuch as a higher concentration of chelator was required for activation of photosynthesis at lower pH.Either EDTA, PPi, or citrate could activate photosynthesis of sunflower chloroplasts isolated and assayed at pH 8.4. At pH 7.6, PPi and EDTA were equally effective at low Pi concentrations but PPi was particularly effective in shortening the induction period at high concentrations of Pi (2.5 mm) in the assay medium. Including 1 mm 3-phosphoglycerate in the assay medium with or without Pi could not replace the need for chelation. However, 3-phosphoglycerate + EDTA in the assay medium with 0.5 mm Pi, pH 7.6, gave a short induction period and rates of photosynthesis similar to those with 10 mm PPi. The results suggest that PPi can have a dual effect at the lower pH through chelation and inhibition of the phosphate transporter.Photosynthesis by sunflower chloroplasts isolated and assayed at pH 8.4 with 0.2 mm EDTA (+ 0.5 mm Pi in the assays) was severely inhibited by 2 mM CaCl2, MgCl2, or MnCl2. Wheat chloroplasts isolated and assayed at pH 8.4 without chelation, and assayed with 0.2 mm Pi, had low rates of photosynthesis (25 μmol O2 evolved mg?1 chlorophyll h?1) which were strongly inhibited by 2 to 4 mm MgCl2, MnCl2, or CaCl2. With inclusion of EDTA and Pi at optimum levels, isolated chloroplasts of sunflower and wheat have high rates of photosynthesis and PPi or divalent cations are not of benefit. 相似文献
9.
Shigeru Itoh Christine T. Yerkes Hiroyuki Koike Howard H. Robinson Antony R. Crofts 《BBA》1984,766(3):612-622
The role of Cl? in the electron transfer reactions of the oxidizing side of Photosystem II (PS II) has been studied by measuring the fluorescence yield changes corresponding to the reduction of P+-680, the PS II reaction center chlorophyll, by the secondary PS II donor, Z. In Cl?-depleted chloroplasts, a rapid rise in fluorescence yield was observed following the first and second flashes, but not during the third or subsequent flashes. These results indicate that there exists an additional endogenous electron donor beyond P-680 and Z in Cl?-depleted systems. In contrast, the terminal endogenous donor on the oxidizing side of PS II in Tris-washed preparations has previously been shown to be Z, the component giving rise to EPR signals IIf and IIvf. The rate of reduction of P+-680 in the Cl?-depleted chloroplasts was as rapid as that measured in uninhibited systems, within the time resolution of our instrument. Again, this is in contrast to Tris-washed preparations in which a dramatic decrease in the rate if this reaction has been previously reported. We have also carried out a preliminary study on the rate of rereduction of Z+ in the Cl?-depleted system. Under steady-state conditions, the reduction half-time of Z+ in uninhibited systems was about 450 μs, while in the Cl?-depleted chloroplasts, the reduction of Z+ was biphasic, one phase with a half-time of about 120 ms, and a slower phase with a half-time of several seconds. The appearance of the quenching state due to P+-680 observed following the third flash on excitation of Cl?-depleted chloroplasts was delayed by two flashed when low concentrations of NH2OH (20–50 μM) were included in the medium. Hydrazine at somewhat higher concentrations showed the same effect. This is taken to indicate that the reactions leading to PS II oxidation of NH2OH or NH2NH2 are uninhibited by Cl? depletion. Addition of NH2OH at low concentrations to Tris-washed chloroplasts did not alter the pattern of the fluorescence yield, indicating that the reactions leading to the NH2OH oxidation present in Cl?-depleted systems are absent following Tris inhibition. The results are discussed in terms of an inhibition by Cl? depletion of the reactions of the oxygen-evolving complex. It is suggested that no intermediary redox couple exists between the oxygen-evolving complex and Z, and that Z+ is reduced directly by Mn of the complex. In terms of the S-state model, Cl? depletion appears to inhibit the advancement of the mechanism beyond S2, but not to inhibit the transitions from S0 to S1, or from S1 to S2. 相似文献
10.
1. Chloroplasts washed with Cl?-free, low-salt media (pH 8) containing EDTA, show virtually no DCMU-insensitive silicomolybdate reduction. The activity is readily restored when 10 mM Cl? is added to the reaction mixture. Very similar results were obtained with the other Photosystem II electron acceptor 2,5-dimethylquinone (with dibromothymoquinone), with the Photosystem I electron acceptor FMN, and also with ferricyanide which accepts electrons from both photosystems.2. Strong Cl?-dependence of Hill activity was observed invariably at all pH values tested (5.5–8.3) and in chloroplasts from three different plants: spinach, tobacco and corn (mesophyll).3. In the absence of added Cl? the functionally Cl?-depleted chloroplasts are able to oxidize, through Photosystem II, artificial reductants such as catechol, diphenylcarbazide, ascorbate and H2O2 at rates which are 4–12 times faster than the rate of the residual Hill reaction.4. The Cl?-concentration dependence of Hill activity with dimethylquinone as an electron acceptor is kinetically consistent with the typical enzyme activation mechanism: E(inactive) + Cl?ag E · Cl? (active), and the apparent activation constant (0.9 mM at pH 7.2) is unchanged by chloroplast fragmentation.5. The initial phase of the development of inhibition of water oxidation in Cl?-depleted chloroplasts during the dark incubation with NH2OH ( H2SO4) is 5 times slower when the incubation medium contains Cl? than when the medium contains NH2OH alone or NH2OH plus acetate ion. (Acetate is shown to be ineffective in stimulating O2 evolution.)6. We conclude that the Cl?-requiring step is one which is specifically associated with the water-splitting reaction, and suggests that Cl? probably acts as a cofactor (ligand) of the NH2OH-sensitive, Mn-containing O2-evolving enzyme. 相似文献
11.
A comparative study is made, at 15 °C, of flash-induced absorption changes around 820 nm (attributed to the primary donors of Photosystems I and II) and 705 nm (Photosystem I only), in normal chloroplasts and in chloroplasts where O2 evolution was inhibited by low pH or by Tris-treatment.At pH 7.5, with untreated chloroplasts, the absorption changes around 820 nm are shown to be due to P-700 alone. Any contribution of the primary donor of Photosystem II should be in times shorter than 60 μs.When chloroplasts are inhibited at the donor side of Photosystem II by low pH, an additional absorption change at 820 nm appears with an amplitude which, at pH 4.0, is slightly higher than the signal due to oxidized P-700. This additional signal is attributed to the primary donor of Photosystem II. It decays ( about 180 μs) mainly by back reaction with the primary acceptor and partly by reduction by another electron donor. Acid-washed chloroplasts resuspended at pH 7.5 still present the signal due to Photosystem II ( about 120 μs). This shows that the acid inhibition of the first secondary donor of Photosystem II is irreversible.In Tris-treated chloroplasts, absorption changes at 820 nm due to the primary donor of Photosystem II are also observed, but to a lesser extent and only after some charge accumulation at the donor side. They decay with a half-time of 120 μs. 相似文献
12.
The role of monovalent cations in the photosynthesis of isolated intact spinach chloroplasts was investigated. When intact chloroplasts were assayed in a medium containing only low concentrations of mono- and divalent cations (about 3 mval l-1), CO2-fixation was strongly inhibited although the intactness of chloroplasts remained unchanged. Addition of K+, Rb+, or Na+ (50–100 mM) fully restored photosynthesis. Both the degree of inhibition and restoration varied with the plant material and the storage time of the chloroplasts in low-salt medium. In most experiments the various monovalent cations showed a different effectiveness in restoring photosynthesis of low-salt chloroplasts (K+>Rb+>Na+). Of the divalent cations tested, Mg2+ also restored photosynthesis, but to a lesser extent than the monovalent cations.In contrast to CO2-fixation, reduction of 3-phosphoglycerate was not ihibited under low-salt conditions. In the dark, CO2-fixation of lysed chloroplasts supplied with ATP, NADPH, and 3-phosphoglycerate strictly required the presence of Mg2+ but was independent of monovalent cations. This finding excludes a direct inactivation of Calvin cycle enzymes as a possible basis for the inhibition of photosynthesis under low-salt conditions.Light-induced alkalization of the stroma and an increase in the concentration of freely exchangeable Mg2+ in the stroma, which can be observed in normal chloroplasts, did not occur under low-salt conditions but were strongly enhanced after addition of monovalent cations (50–100 mM) or Mg2+ (20–50 mM).The relevance of a light-triggered K+/H+ exchange at the chloroplast envelope is discussed with regard to the light-induced increase in the pH and the Mg2+ concentration in the stroma, which are thought to be obligatory for light activation of Calvincycle enzymes. 相似文献
13.
Thylakoids prepared from spinach (Spinacea oleracea L.) chloroplasts were exposed to osmotic stress in vitro in the presence or absence of different inorganic salts. By an hour after incubation in 1.0 M sorbitol and 10 mM (or more) MgCl2, the thylakoids lost approximately 80% of their photosystem (PS) II activity, but not PS I. The inhibition occurred only in presence of magnesium as indicated by the combinations of several cations/anions. The PS II activity was relatively insensitive to osmotic stress in the presence of diphenyl carbazide. We therefore conclude that under conditions of water stress in the presence of 10 mM or higher Mg2+, the oxygen evolving system in chloroplasts is rapidly inactivated.Abbreviations DCMU
3(3,4-dichlorophenyl)-1,1-dimethylurea
- DCPIP
2,6-dichlorophenol indophenol
- DPC
diphenyl carbazide
- MV
methyl viologen
- PS
photosystem
Part of this work was included in the thesis submitted by the first author of M.Phil.degree. 相似文献
14.
15.
Thomas F. Joos F. Alfred Anderer 《Biochimica et Biophysica Acta (BBA)/General Subjects》1979,584(1):104-115
Endogenous phosphorylation was studied with highly purified fractions of the plasma membrane and the endoplasmic reticulum of SV40-transformed mouse fibroblasts using [γ-32P]ATP and [γ-32P]GTP as precursors. With ATP maximum overall incorporation of 32P into both membrane fractions occured at pH 7.8 in the presence of 10 mM MgCl2 after incubation for 1 min. GTP could be utilized only by the plasma membrane fraction showing maximum incorporation of 32P at pH 7.8 and 10 mM MgCl2 after incubation for 3 min.The pattern of phosphoproteins of the plasma membrane is represented by more than 15 proteins whereas the endoplasmic reticulum essentially contained only one phosphorylated component of 35 000 molecular weight. The comparison of ATP- and GTP-specific phophorlation of the plasma membrane revealed GTP to be a less efficient precursor yielding a similar phosphoprotein pattern with one significant difference: the GTP-specific main component exhibited a molecular wieght of about 100 000 and the ATP-specific main component a molecular weight of 110 000.The relative distribution of individual phosphoproteins in the pattern of the plasma membrane was dependent on pH but not on MgCl2 concentration or time of incubation. Increasing concentrations of plasma membrane protein altered the patterns of phosphoproteins dramatically: At high protein concentrations the ATP-specific main component (Mr = 110 000) was no more phosphorylated whereas with GTP the main component Mr = 100 000 was essentially the sole phosphorylated protein. 相似文献
16.
Five active isoenzymes of mitochondrial malate dehydrogenase of 105,000, 91,000, 78,000, 65,000 and 39,000 daltons were observed when the enzyme was extracted from mycelia and centrifuged in a sucrose gradient with 5 mM tris-Cl at pH 9. Only one active species of 65,000 daltons was observed in either 100 mM tris-Cl, pH 9, or 5 or 100 mM sodium citrate at either pH 4 or 6. The addition of 10 mM of either MgCl2 or CaCl2 to the 5 mM tris-Cl pH 9 buffer reversed the aggregation and led to the occurrence of only the 65,000 daltons species. The addition of either 10, 50 or 100 mM KCl to the 5 mM tris-Cl pH 9 buffer yielded 4, 3 and 1 isoenzymes, respectively. The latter's molecular weight was 65,000. Thus, in alkaline solution, monovalent cations at 100 mM and divalent cations at 10 mM prevent the formation of multiple molecular weight species. 相似文献
17.
The presence of Ca2+ causes a twentyfold or greater increase in the rate of oxygen evolution by cell-free preparations of Phormidium luridum. The requirement for Ca2+ is specific; other divalent cations are much less effective or are inhibitory. The rate of the Hill reaction is maximal at 30 mM CaCl2 in both detergent-free and Brij 35 preparations. The 3-(3,4-dichlorophenyl)-1,1-dimethylurea-insensitive component of oxygen-evolving activity in each preparation also shows the requirement for added Ca2+. This indicates that Ca2+ is acting close to the oxygen-evolving reaction center of Photosystem II. Defatted bovine serum albumin increases the rate of oxygen evolution in the detergent-free preparation, but does not compete with Ca2+, discounting fatty acid mediation of the effects of Ca2+. Neither excess Hill acceptor nor uncouplers of photophosphorylation diminish the stimulatory effects of Ca2+. 相似文献
18.
V. A. Sopina 《Journal of Evolutionary Biochemistry and Physiology》2004,40(1):28-36
Tartrate-resistant acid phosphatase (TR-AcPh) from the ameba Amoeba proteus is represented by 3 bands (electromorphs) revealed after disk-electrophoresis in PAAG, using 2-naphthylphosphate as substrate. The presence of 50 mmol/l MgCl2 or CaCl2 in the incubation mixture increases activities of all electromorphs of TR-AcPh, while of ZnCl2, of two of them. The activity of the TR-AcPh electromorphs also rose after the 30-min incubation of the gels in MgCl2, CaCl2 or ZnCl2 (10 and 100 mM) before gel staining. However, 1 M ZnCl2, unlike 1 M CaCl2 or 1 M MgCl2, partly inactivated two out of three TR-AcPh electromorphs. The TR-AcPh electromorphs were inhibited by 1,10-phenanthroline (1,10-Ph), EDTA, and EGTA (all at a concentration of 5 mM) faster than by H2O2 (10 mM). The inactivation of the TR-AcPh electromorphs by the chelating agents did not depend (EGTA) or nearly did not depend (EDTA, 1,10-Ph) on their concentration (0.05, 0.5, and 5 mM). Out of 5 tested ions (Mg2+, Ca2+, Fe2+, Fe3+, and Zn2+), only Zn ions reactivated the TR-AcPh electromorphs inactivated by 1,10-Ph, EDTA or EGTA. The TR-AcPh electromorphs were reactivated worse after inactivation by EGTA than by EDTA or 1,10-Ph. It is suggested that the active site of TR-AcPh contains the zinc ion essential for catalytic activity of this enzyme, i.e., TR-AcPh of A. proteus is a metallophosphatase performing the phosphomonoesterase activity in acidic medium. 相似文献
19.
Bulychev A. A. Cherkashin A. A. Vredenberg W. J. Rubin A. B. Zykov V. S. Müller S. C. 《Russian Journal of Plant Physiology》2001,48(3):326-332
Continuous profiles of local pH near the cell surface of Chara corallinawere recorded during uniform longitudinal movement of an internodal cell relative to a stationary pH microelectrode. Under illumination, the pH profile consisted of alternating acid and alkaline bands with a pH difference of up to 3 pH units. After darkening, the bands disappeared and pH became uniformly distributed along the cell length. Chlorophyll fluorescence of chloroplasts was measured by microfluorometry at different locations within one cell, and significant differences were observed in close relation to light-dependent pH banding. The chlorophyll fluorescence yield was lower in zones of low external pH than in alkaline zones both under actinic and saturating light. The fluorescence parameters Fand F"
m
and the quantum yield of photosystem II (PSII) displayed variations along the cell length in accordance with pH changes in unstirred layers of the medium. The results show that PSII photochemical efficiency and the rate of noncyclic electron transport are higher in the chloroplasts of acid zones (zones of H+extrusion from the cell) than in alkaline zones. The dependence of photosynthetic electron transport on local pH near the cell surface may result from different contents of CO2in acid and alkaline regions. The acid zones are enriched with CO2that readily permeates through the membrane providing the substrate for the Calvin cycle. Conversely, a poorly permeating form, HCO–
3is predominant in alkaline zones, which may restrict the dark reactions and photosynthetic electron flow. 相似文献
20.
Estela Gottberg Laurent Diop Bernard Montreuil Tomás A. Reader 《Neurochemical research》1989,14(5):419-426
The effects of sodium, lithium, and magnesium on the in vitro binding properties of the D1 antagonist [3H]SCH23390 were examined with membrane preparations from rat neostriatum (CPU; caudate-putamen) and cerebral cortex (CTX). The saturation binding isotherms for both tissues performed in the presence of 120 mM of either Na+ or Li+ revealed an increase in the affinity, as compared to that observed when the incubation buffer was composed of Tris-Cl 50 mM with MgCl2 1 mM alone. For the CPU there were no changes in the maximum binding capacity (B
max) in the different buffers used. In the case of the CTX, there was a loss of [3H]SCH23390 binding sites when either Na+ or Li+ 120 mM were added to the incubations, suggesting a lack of selectivity of this ligand in the absence of group IA cations. The agonist state of the [3H]SCH23390 binding site was studied in competition experiments with dopamine. The highest agonist affinity was obtained in 50 mM Tris-Cl buffer with 1 mM MgCl2 while the addition of 120 mM of either Na+ or Li+ caused a 3- to 5-fold decrease in the potency of dopamine to compete with specific [3H]SCH23390 binding in both CPU and CTX. The presence of magnesium was essential for the competition experiments; i.e.: a concentration of 1 mM MgCl2 was optimum to obtain dopamine antagonism of ligand binding, while increasing Mg2+ to 2 or 5 mM did not appear to further improve the inhibitions. The results support both agonist and antagonist affinity shifts for the dopamine D1 receptor labeled with [3H]SCH23390. Receptor affinity studies should take into account that pharmacological specificity may vary with the incubation buffer utilized, especially when comparing binding data from different laboratories performed under varying ionic conditions. 相似文献