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1.
Because glyoxylate inhibits CO 2 fixation by intact chloroplasts and purified ribulose bisphosphate carboxylase/oxygenase, glyoxylate might be expected to exert some regulatory effect on photosynthesis. However, ribulose bisphosphate carboxylase activity and activation in intact chloroplasts from Spinacia oleracea L. leaves were not substantially inhibited by 10 millimolar glyoxylate. In the light, the ribulose bisphosphate pool decreased to half when 10 millimolar glyoxylate was present, whereas this pool doubled in the control. When 10 millimolar glyoxylate or formate was present during photosynthesis, the fructose bisphosphate pool in the chloroplasts doubled. Thus, glyoxylate appeared to inhibit the regeneration of ribulose bisphosphate, but not its utilization. The fixation of CO2 by intact chloroplasts was inhibited by salts of several weak acids, and the inhibition was more severe at pH 6.0 than at pH 8.0. At pH 6.0, glyoxylate inhibited CO2 fixation by 50% at 50 micromolar, and glycolate caused 50% inhibition at 150 micromolar. This inhibition of CO2 fixation seems to be a general effect of salts of weak acids. Radioactive glyoxylate was reduced to glycolate by chloroplasts more rapidly in the light than in the dark. Glyoxylate reductase (NADP+) from intact chloroplast preparations had an apparent Km (glyoxylate) of 140 micromolar and a Vmax of 3 micromoles per minute per milligram chlorophyll. 相似文献
2.
Magnesium was most inhibitory to photosynthetic reactions by intact chloroplasts when the magnesium was added in the dark before illumination. Two millimolar MgCl 2, added in the dark, inhibited CO 2-dependent O 2 evolution by Hordeum vulgare L. and Spinacia oleracea L. (C 3 plants) chloroplasts 70 to 100% and inhibited (pyruvate + oxaloacetate)-dependent O 2 evolution by Digitaria sanguinalis L. (C 4 plant) mesophyll chloroplasts from 80 to 100%. When Mg 2+ was added in the light, O 2 evolution was reduced only slightly. O 2 evolution in the presence of phosphoglycerate was less sensitive to Mg 2+ inhibition than was CO 2-dependent O 2 evolution. Magnesium prevented the light activation of several photosynthetic enzymes. Two millimolar Mg2+ blocked the light activation of NADP-malate dehydrogenase in D. sanguinalis mesophyll chloroplasts, and the light activation of phosphoribulokinase, NADP-linked glyceraldehyde-3-phosphate dehydrogenase, and fructose 1,6-diphosphatase in barley chloroplasts. The results suggest that Mg2+ inhibits chloroplast photosynthesis by preventing the light activation of certain enzymes. 相似文献
3.
The incorporation of 14CO 2 into glycolate by intact spinach leaf ( Spinacia oleracea L. var. Kyoho) chloroplasts exposed to 14CO 2 (NaH 14CO 3, 1 millimolar) in the light was determined as a function of O 2 concentrations in the reaction media. A hyperbolic saturation curve was obtained, apparent Km (O 2) of 0.28 millimolar, indicating that glycolate is produced predominantly by ribulose-1,5-bisphosphate carboxylase/oxygenase. A concentration gradient of glycolate was invariably observed between chloroplast stroma and the outside media surrounding chloroplasts during photosynthetic 14CO 2 fixation under an O 2 atmosphere. 相似文献
4.
CO 2 fixation during photosynthesis is regulated by the activity of ribulose bisphosphate carboxylase (Rubisco). This conclusion
became more apparent to me after CO 2-fixation experiments using isolated spinach chloroplasts and protoplasts, purified Rubisco enzyme, and intact leaves. Ribulose
bisphosphate (RuBP) pools and activation of Rubisco were measured and compared to 14CO 2 fixation in light. The rates of 14CO 2 assimilation best followed the changes in Rubisco activation under moderate to high light intensities. RuBP pool sizes regulated
14
2 assimilation only in very high CO 2 levels, low light and in darkness. Activation of Rubisco involves two separate processes: carbamylation of the protein and
removal of inhibitors blocking carbamylation or blocking RuBP binding to carbamylated sites before reaction with CO 2 or O 2.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
5.
Rates of photosynthetic O 2 evolution, for measuring K 0.5(CO 2 + HCO 3−) at pH 7, upon addition of 50 micromolar HCO 3− to air-adapted Chlamydomonas, Dunaliella, or Scenedesmus cells, were inhibited up to 90% by the addition of 1.5 to 4.0 millimolar salicylhydroxamic acid (SHAM) to the aqueous medium. The apparent K1(SHAM) for Chlamydomonas cells was about 2.5 millimolar, but due to low solubility in water effective concentrations would be lower. Salicylhydroxamic acid did not inhibit oxygen evolution or accumulation of bicarbonate by Scenedesmus cells between pH 8 to 11 or by isolated intact chloroplasts from Dunaliella. Thus, salicylhydroxamic acid appears to inhibit CO 2 uptake, whereas previous results indicate that vanadate inhibits bicarbonate uptake. These conclusions were confirmed by three test procedures with three air-adapted algae at pH 7. Salicylhydroxamic acid inhibited the cellular accumulation of dissolved inorganic carbon, the rate of photosynthetic O 2 evolution dependent on low levels of dissolved inorganic carbon (50 micromolar Na-HCO 3), and the rate of 14CO 2 fixation with 100 micromolar [ 14C] HCO 3−. Salicylhydroxamic acid inhibition of O 2 evolution and 14CO 2-fixation was reversed by higher levels of NaHCO 3. Thus, salicylhydroxamic acid inhibition was apparently not affecting steps of photosynthesis other than CO 2 accumulation. Although salicylhydroxamic acid is an inhibitor of alternative respiration in algae, it is not known whether the two processes are related. 相似文献
6.
Addition of millimolar sodium glyoxylate to spinach ( Spinacia oleracea) chloroplasts was inhibitory to photosynthetic incorporation of 14CO 2 under conditions of both low (0.2 millimolar or air levels) and high (9 millimolar) CO 2 concentrations. Incorporation of 14C into most metabolites decreased. Labeling of 6-P-gluconate and fructose-1,6-bis-P increased. This suggested that glyoxylate inhibited photosynthetic carbon metabolism indirectly by decreasing the reducing potential of chloroplasts through reduction of glyoxylate to glycolate. This hypothesis was supported by measuring the reduction of [ 14C]glyoxylate by chloroplasts. Incubation of isolated mesophyll cells with glyoxylate had no effect on net photosynthetic CO 2 uptake, but increased labeling was observed in 6-P-gluconate, a key indicator of decreased reducing potential. The possibility that glyoxylate was affecting photosynthetic metabolism by decreasing chloroplast pH cannot be excluded. Increased 14C-labeling of ribulose-1,5-bis-P and decreased 3-P-glyceric acid and glycolate labeling upon addition of glyoxylate to chloroplasts suggested that ribulose-bis-P carboxylase and oxygenase might be inhibited either indirectly or directly by glyoxylate. Glyoxylate addition decreased 14CO 2 labeling into glycolate and glycine by isolated mesophyll cells but had no effect on net 14CO 2 fixation. Glutamate had little effect on net photosynthetic metabolism in chloroplast preparations but did increase 14CO 2 incorporation by 15% in isolated mesophyll cells under air levels of CO 2. 相似文献
7.
The exchange properties of the activator CO 2 of spinach ribulose-1,5-bisphosphate carboxylase/oxygenase were characterized both in vitro with the purified enzyme, and in situ within isolated chloroplasts. Carboxyarabinitol-1,5-bisphosphate, a proposed reaction intermediate analog for the carboxylase activity of the enzyme, was used to trap the activator CO 2 on the enzyme both in vitro and in situ. Modulation of ribulose-1,5-bisphosphate carboxylase/oxygenase activity in intact chloroplasts during a light/dark cycle was associated with a similar modulation in carboxyarabinitol-1,5-bisphosphate-trapped CO 2. The exchange kinetics of the activator CO 2 were monitored by activation of the enzyme to steady state in the presence of 12CO 2, followed by addition of 14CO 2 and determination of the amount of labeled CO 2 trapped on the enzyme by carboxyarabinitol-1,5-bisphosphate. Rate constants ( Kobs) for exchange with both the purified enzyme (0.45 min −1) and in illuminated chloroplasts (0.18 min −1) were comparable to the observed rate constants for enzyme activation under the two conditions. A similar exchange of the activator CO 2 was not observed in chloroplasts in the dark. Kinetic analysis of the exchange properties of the purified enzyme were consistent with an equilibrium between active and inactive forms of the enzyme during steady state activation. 相似文献
8.
The light-dependent accumulation of radioactively labeled inorganic carbon in isolated spinach ( Spinacia oleracea L.) chloroplasts was determined by silicone oil filtering centrifugation. Intact chloroplasts, dark-incubated 60 seconds at pH 7.6 and 23°C with 0.5 millimolar sodium bicarbonate, contained 0.5 to 1.0 millimolar internal inorganic carbon. The stromal pool of inorganic carbon increased 5- to 7-fold after 2 to 3 minutes of light. The saturated internal bicarbonate concentration of illuminated spinach chloroplasts was 10- to 20-fold greater than that of the external medium. This ratio decreased at lower temperatures and with increasing external bicarbonate. Over one-half the inorganic carbon found in intact spinach chloroplasts after 2 minutes of light was retained during a subsequent 3-minute dark incubation at 5°C. Calculations of light-induced stromal alkalization based on the uptake of radioactively labeled bicarbonate were 0.4 to 0.5 pH units less than measurements performed with [ 14C]dimethyloxazolidine-dione. About one-third of the binding sites on the enzyme ribulose 1,5-bisphosphate carboxylase were radiolabeled when the enzyme was activated in situ and 14CO 2 bound to the activator site was trapped in the presence of carboxypentitol bisphosphates. Deleting orthophosphate from the incubation medium eliminated inorganic carbon accumulation in the stroma. Thus, bicarbonate ion distribution across the chloroplast envelope was not strictly pH dependent as predicted by the Henderson-Hasselbach formula. This finding is potentially explained by the presence of bound CO 2 in the chloroplast. 相似文献
9.
Ribulose-1,5-bisphosphate (RuBP) carboxylase in lysed spinach ( Spinacia oleracea L. cv virtuosa) chloroplasts that had been partly inactivated at low CO 2 and Mg 2+ by incubating in darkness with 4 millimolar partially purified RuBP was reactivated by light. If purified RuBP was used to inhibit dark activation of the enzyme, reactivation by light was not observed unless fructose-1,6-bisphosphate, ATP, or ADP plus inorganic phosphate were also added. Presumably, ADP plus inorganic phosphate acted as an ATP-generating system with a requirement for the generation of ΔpH across the thylakoid membrane. When the RuBP obtained from Sigma Chemical Co. was used, light did not reactivate the enzyme. There was no direct correlation between ΔpH and activation. Therefore, thylakoids are required in the ribulose-1,5-bisphosphate carboxylase activase system largely to synthesize ATP. Inactivation of RuBP carboxylase in isolated chloroplasts or in the lysed chloroplast system was not promoted simply by a transition from light to dark conditions but was caused by low CO 2 and Mg 2+. 相似文献
10.
Millimolar concentrations of Mg 2+ inhibited CO 2-dependent O 2 evolution by barley ( Hordeum vulgare L.) chloroplasts and also prevented the activation of NADP-glyceraldehyde-3-phosphate dehydrogenase, ribulose-5-phosphate kinase, and fructose-1,6-diphosphatase by light in intact chloroplasts. When added in the dark, 3-phosphoglycerate prevented the inhibition of O 2 evolution by Mg 2+ and reduced the Mg 2+ inhibition of enzyme activation by light. Fructose 1,6-diphosphate and ribulose 5-phosphate also prevented the inhibition of O 2 evolution by Mg 2+ whereas glucose 1-phosphate, glucose 6-phosphate, ribulose 1,5-diphosphate, and citrate had no effect. Phosphoenolpyruvate gave an intermediate response. Metabolites that prevented the Mg 2+ inhibition of O 2 evolution shortened the lag phase of CO 2-dependent O 2 evolution in the absence of M 2+. Loading chloroplasts in the dark with 3-phosphoglycerate reduced both the lag phase of O 2 evolution and the inhibition of O 2 evolution by Mg 2+. The results suggested that Mg 2+ inhibition was lessened either by external metabolites that compete with inorganic phosphate for transport into the chloroplast or by a high concentration of internal metabolites. 相似文献
11.
Salicylaldoxime (2 × 10 −3m and less) inhibits cyclic photophosphorylation in intact Chlorella cells severely whereas photosynthetic O 2-evolution and 14CO 2-fixation is hardly affected. Cyclic photophosphorylation in vivo was measured by following anaerobic light dependent glucose uptake. A similar difference in susceptibility has been observed with carbonylcyanide- p-trifluoromethoxyphenylhydrazone. Various controls exclude the possibility that the difference in inhibition was caused by differing experimental conditions or, in the case of glucose assimilation, by an inhibition of a reaction other than photophosphorylation. 相似文献
12.
The metabolism of 14C-labeled aspartic acid, diaminopimelic acid, malic acid and threonine by isolated pea ( Pisum sativum L.) chloroplasts was examined. Light enhanced the incorporation of [ 14C] aspartic acid into soluble homoserine, isoleucine, lysine, methionine and threonine and protein-bound aspartic acid plus asparagine, isoleucine, lysine, and threonine. Lysine (2 millimolar) inhibited its own formation as well as that of homoserine, isoleucine and threonine. Threonine (2 millimolar) inhibited its own synthesis and that of homoserine but had only a small effect on isoleucine and lysine formation. Lysine and threonine (2 millimolar each) in combination strongly inhibited their own synthesis as well as that of homoserine. Radioactive [1,7- 14C]diaminopimelic acid was readily converted into [ 14C]threonine in the light and its labeling was reduced by exogenous isoleucine (2 millimolar) or a combination of leucine and valine (2 millimolar each). The strong light stimulation of amino acid formation illustrates the point that photosynthetic energy is used in situ for amino acid and protein biosynthesis, not solely for CO 2 fixation. 相似文献
13.
Chloroplasts isolated from synchronous cultures of the unicellular green alga Chlamydomonas reinhardii, SAG 11-32/b (−), fix CO 2 at rates between 25 and 50 micromoles per milligram chlorophyll per hour. The upper value is approximately half of the rate of the intact cell. During storage in the dark on ice, the chloroplast preparation loses 30 to 50% of its CO2 fixing capability per hour. Under reducing conditions (+ 1 millimolar dithiothreitol), this loss of activity is about twice as fast. The same reducing conditions stimulate CO2 fixation in the light. High concentrations of inorganic phosphate (>2 millimolar) inhibit CO2 fixation. This inhibition is overcome by the addition of glycerate 3-phosphate. It is concluded that chloroplasts from C. reinhardii possess a higher plant type phosphate translocator. With respect to dependency upon light intensity, pH and Mg2+ concentration, the results were similar to that reported for chloroplasts from higher plants. However, in contrast to higher plant chloroplasts, maximum CO2 fixation is observed at the relatively low osmotic concentration of 0.12 molar mannitol in the reaction buffer. 相似文献
14.
Photosynthetic CO 2-fixation of mesophyll protoplasts of lambs lettuce [ Valerianella locusta (L.) Betcke] was inhibited by short time exposure to Cd +. Inhibition was due to uptake of the metal ion into the protoplasts and increased with increasing Cd 2+ concentrations and the time of preincubation. A 10 min pretreatment at 2 m M Cd 2+ reduced CO 2-fixation by 40–60%. Inhibition of photosynthesis was independent of the light intensity to which the protoplasts were exposed. Measurement of the lightinduced electrochromic pigment absorption change at 518nm and chlorophyll fluorescence studies revealed that primary photochemical reactions associated with the thylakoid membranes were not affected by the metal ion. Also, light activation of the ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39) was not inhibited by Cd 2+. Under rate-limiting CO 2 concentrations, inhibition of CO 2-fixation was smaller than at V max of CO 2 reduction indicating that the carboxylation reaction of the Calvin cycle is not susceptible to Cd 2+. Cd 2+ treatment of protoplasts significantly extended the lagphase of CO 2-supported O 2-evolution and partly inhibited light activation of the glyceraldehyde-3-phosphate dehydrogenase (EC 1.2.1.13) and the ribulose-5-phosphate kinase (EC 2.7.1.19). Measurement of relative concentrations of [ 14C]-labeled Calvin cycle intermediates showed that Cd 2+ caused a decrease in the 3-phosphoglycerate/triose phosphate ratio and an increase in the triose phosphate/ribulose-1,5-bisphosphate ratio. It is concluded that in protoplasts Cd 2+ affects photosynthesis mainly at the level of dark reactions and that the site of inhibition may be localized in the regenerative phase of the Calvin cycle. 相似文献
15.
The enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase displayed near-maximal activity in isolated, intact barley ( Hordeum vulgare L. cv. Pennrad) mesophyll protoplasts. The carboxylase deactivated 40 to 50% in situ when protoplasts were dark-incubated 20 minutes in air-equilibrated solutions. Enzyme activity was fully restored after 1 to 2 minutes of light. Addition of 5 millimolar NaHCO 3 to the incubation medium prevented dark-inactivation of the carboxylase. There was no permanent CO 2-dependent activation of the protoplast carboxylase either in light or dark. Activation of the carboxylase from ruptured protoplasts was not increased significantly by in vitro preincubation with CO 2 and Mg 2+. In contrast to the enzyme in protoplasts, the carboxylase in intact barley chloroplasts was not fully reactivated by light at atmospheric CO 2 levels. The lag phase in carbon assimilation was not lengthened by dark-adapting protoplasts to low CO 2 demonstrating that light-activation of the carboxylase was not involved in photosynthetic induction. Irradiance response curves for reactivation of the the carboxylase and for CO 2 fixation by isolated barley protoplasts were similar. The above results show that there was a fully reversible light-activation of the carboxylase in isolated barley protoplasts at physiologically significant CO 2 levels. 相似文献
16.
The activity of ribulose 1,5-bisphosphate (RuBP) car?ylase in intact spinach chloroplasts is shown to depend on light and CO 2. This activity was measured upon lysis of chloroplasts and assay of the initial activity using nonlimiting substrate concentrations. Incubation of chloroplasts at 25 °C in the absence of CO 2 results in a gradual inactivation of the RuBP car?ylase. In the presence of CO 2 the initial activity is preserved or increased. CO 2 is also able to reactivate the chloroplast car?ylase previously inactivated in the absence of CO 2. Upon illumination of the chloroplasts, additional activation was observed. This light activation results from an increased affinity for CO 2 of the chloroplast car?ylase. At pH 7.8, the enzyme in dark-adapted chloroplasts required 112 μ m CO 2 for half activation, while in the light it required 24 μ m CO 2. The light activation was inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea, carbonylcyanide 3-chlorophenylhydrazone, or dl-glyceraldehyde. Part of the light activation is most likely due to increased Mg 2+ in the stroma. dl-Glyceraldehyde inhibition also suggests that some intermediate of the photosynthetic carbon cycle is involved. These results suggest that photosynthetic CO 2 assimilation in the chloroplast depends upon the amount of activation of the RuBP car?ylase. This activation is regulated by CO 2 and light-induced changes in the chloroplast stroma such as pH, Mg 2+, and intermediates of the photosynthetic carbon cycle. 相似文献
17.
The concentration effect of aqueous CO 2 on the reaction velocity of spinach leaf ribulose-1,5-diphosphate carboxylase has been reevaluated. The homotropic effect of CO 2 in the enzyme reaction supports the previously reported allosteric nature of the enzyme in the CO 2-fixation process in chloroplasts. The concentration of CO 2 giving the half maximal reaction velocity, S 0.5, has been calculated to be 1.47 × 10 −5M. 相似文献
18.
14CO 2 photoassimilation in the presence of MgATP, MgADP, and MgAMP was investigated using intact chloroplasts from Sedum praealtum, a Crassulacean acid metabolism plant, and two C 3 plants: spinach and peas. Inasmuch as free ATP, ADP, AMP, and uncomplexed Mg 2+ were present in the assays, their influence upon CO 2 assimilation was also examined. Free Mg 2+ was inhibitory with all chloroplasts, as were ADP and AMP in chloroplasts from Sedum and peas. With Sedum chloroplasts in the presence of ADP, the time course of assimilation was linear. However, with pea chloroplasts, ADP inhibition became progressively more severe, resulting in a curved time course. ATP stimulated assimilation only in pea chloroplasts. MgATP and MgADP stimulated assimilation in all chloroplasts. ADP inhibition of CO 2 assimilation was maximal at optimum orthophosphate concentrations in Sedum chloroplasts, while MgATP stimulation was maximal at optimum or below optimum concentrations of orthophosphate. MgATP stimulation in peas and Sedum and ADP inhibition in Sedum were not sensitive to the addition of glycerate 3-phosphate (PGA). PGA-supported O2 evolution by pea chloroplasts was not inhibited immediately by ADP; the rate of O2 evolution slowed as time passed, corresponding to the effect of ADP on CO2 assimilation, and indicating that glycerate 3-phosphate kinase was a site of inhibition. Likewise, upon the addition of AMP, inhibition of PGA-dependent O2 evolution became more severe with time. This did not mirror CO2 assimilation, which was inhibited immediately by AMP. In Sedum chloroplasts, PGA-dependent O2 evolution was not inhibited by ADP and AMP. In chloroplasts from peas and Sedum, the magnitude of MgADP and MgATP stimulation of PGA-dependent O2 evolution was not much larger than that given by ATP, and it was much smaller than MgATP stimulation of CO2 assimilation. Analysis of stromal metabolite levels by anion exchange chromatography indicated that ribulose 1,5-bisphosphate carboxylase was inhibited by ADP and stimulated by MgADP in Sedum chloroplasts. The appearance of label in the medium was measured when [U-14C] ADP-loaded Sedum chloroplasts were challenged with ATP, ADP, or AMP and their Mg2+ complexes. The rate of back exchange was stimulated by the presence of Mg2+. This suggests that ATP, ADP, and AMP penetrate the chloroplast slower than their Mg2+ complexes. A portion of the CO2 assimilation and O2 evolution data could be explained by differential penetration rates, and other proposals were made to explain the remainder of the observations. 相似文献
19.
Neither Dunaliella cells grown with 5% CO 2 nor their isolated chloroplasts had a CO 2 concentrating mechanism. These cells primarily utilized CO 2 from the medium because the K(0.5) (HCO 3−) increase from 57 micromolar at pH 7.0 to 1489 micromolar at pH 8.5, where as the K(0.5) CO 2 was about 12 micromolar over the pH range. After air adaptation for 24 hours in light, a CO 2 concentrating mechanism was present that decreased the K0.5 (CO 2) to about 0.5 micromolar and K0.5 (HCO 3−) to 11 micromolar at pH 8. These K0.5 values suggest that air-adapted cells preferentially concentrated CO 2 but could also use HCO 3− from the medium. Chloroplasts isolated from air-adapted cells had a K(0.5) for total inorganic carbon of less than 10 micromolar compared to 130 micromolar for chloroplasts from cells grown on high CO 2. Chloroplasts from air-adapted cells, but not CO 2-grown cells, concentrate inorganic carbon internally to 1 millimolar in 60 seconds from 240 micromolar in the medium. Maximum uptake rates occurred after preillumination of 45 seconds to 3 minutes. The CO 2 concentrating mechanism by chloroplasts from air-adapted cells was light dependent and inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) or flurocarbonyl-cyamidephenylhydrazone (FCCP). Phenazine-methosulfate at 10 micromolar to provide cyclic phosphorylation partially reversed the inhibition by DCMU but not by FCCP. One to 0.1 millimolar vanadate, an inhibitor of plasma membrane ATPase, inhibited inorganic carbon accumulation by isolated chloroplasts. Vanadate had no effect on CO 2 concentration by whole cells, as it did not readily cross the cell plasmalemma. Addition of external ATP to the isolated chloroplast only slightly stimulated inorganic carbon uptake and did not reverse vanadate inhibition by more than 25%. These results are consistent with a CO 2 concentrating mechanism in Dunaliella cells which consists in part of an inorganic carbon transporter at the chloroplast envelope that is energized by ATP from photosynthetic electron transport. 相似文献
20.
In isolated barley chloroplasts, the presence of 2 millimolar ZnSO 4 inhibits the electron transport activity of photosystem II, as measured by photoreduction of dichlorophenolindophenol, O 2 evolution, and chlorophyll a fluorescence. The inhibition of photosystem II activity can be restored by the addition of the electron donor hydroxylamine or diphenylcarbazide, but not by benzidine and MnCl 2. These observations suggest that Zn inhibits electron flow at the oxidizing side of photosystem II at a site prior to the electron donating site(s) of hydroxylamine and diphenylcarbazide. No inhibition of photosystem I-dependent electron transport by 3 millimolar ZnSO 4 is observed. However, with concentrations of ZnSO 4 above 5 millimolar, photosystem I activity is partially inactivated. Washing Zn 2+-treated chloroplasts partially restores the O 2-evolving activity. 相似文献
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