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1.
The active transport and intracellular accumulation of HCO 3− by air-grown cells of the cyanobacterium Synechococcus UTEX 625 (PCC 6301) was strongly promoted by 25 millimolar Na +.Na +-dependent HCO 3− accumulation also resulted in a characteristic enhancement in the rate of photosynthetic O 2 evolution and CO 2 fixation. However, when Synechococcus was grown in standing culture, high rates of HCO 3− transport and photosynthesis were observed in the absence of added Na +. The internal HCO 3− pool reached levels up to 50 millimolar, and an accumulation ratio as high as 970 was observed. Sodium enhanced HCO 3− transport and accumulation in standing culture cells by about 25 to 30% compared with the five- to eightfold enhancement observed with air-grown cells. The ability of standing culture cells to utilize HCO 3− from the medium in the absence of Na + was lost within 16 hours after transfer to air-grown culture and was reacquired during subsequent growth in standing culture. Studies using a mass spectrometer indicated that standing culture cells were also capable of active CO 2 transport involving a high-affinity transport system which was reversibly inhibited by H 2S, as in the case for air-grown cells. The data are interpreted to indicate that Synechococcus possesses a constitutive CO 2 transport system, whereas Na +-dependent and Na +-independent HCO 3− transport are inducible, depending upon the conditions of growth. Intracellular accumulation of HCO 3− was always accompanied by a quenching of chlorophyll a fluorescence which was independent of CO 2 fixation. The extent of fluorescence quenching was highly dependent upon the size of the internal pool of HCO 3− + CO 2. The pattern of fluorescence quenching observed in response to added HCO 3− and Na + in air-grown and standing culture cells was highly characteristic for Na +-dependent and Na +-independent HCO 3− accumulation. It was concluded that measurements of fluorescence quenching provide an indirect means for following HCO 3− transport and the dynamics of intracellular HCO 3− accumulation and dissipation. 相似文献
2.
Carbon oxysulfide (COS) was reinvestigated as an inhibitor of active inorganic carbon transport in cells of Synechococcus PCC7942 adapted to growth at low inorganic carbon. COS inhibited both CO 2 and HCO 3− transport processes in a reversible (in the short term) and mixed competitive manner. The inhibition of COS was established using both silicone oil centrifugation experiments and O 2-evolution studies. The Ki for COS inhibition was 29 micromolar for CO 2 transport and 110 micromolar for HCO 3− transport. These results support a model of inorganic carbon transport with a central CO 2 pump and an inducible HCO 3− utilizing accessory protein which supplies CO 2 to the primary pump. 相似文献
3.
Mass-spectrometric disequilibrium analysis was applied to investigate CO 2 uptake and HCO 3− transport in cells and chloroplasts of the microalgae Dunaliella tertiolecta and Chlamydomonas reinhardtii, which were grown in air enriched with 5% (v/v) CO 2 (high-Ci cells) or in ambient air (low-Ci cells). High- and low-Ci cells of both species had the capacity to transport CO 2 and HCO 3−, with maximum rates being largely unaffected by the growth conditions. In high- and low-Ci cells of D. tertiolecta, HCO 3− was the dominant inorganic C species taken up, whereas HCO 3− and CO 2 were used at similar rates by C. reinhardtii. The apparent affinities of HCO 3− transport and CO 2 uptake increased 3- to 9-fold in both species upon acclimation to air. Photosynthetically active chloroplasts isolated from both species were able to transport CO 2 and HCO 3−. For chloroplasts from C. reinhardtii, the concentrations of HCO 3− and CO 2 required for half-maximal activity declined from 446 to 33 μm and 6.8 to 0.6 μm, respectively, after acclimation of the parent cells to air; the corresponding values for chloroplasts from D. tertiolecta decreased from 203 to 58 μm and 5.8 to 0.5 μm, respectively. These results indicate the presence of inducible high-affinity HCO 3− and CO 2 transporters at the chloroplast envelope membrane. 相似文献
4.
A 42-kilodalton cytoplasmic membrane protein is synthesized when high CO 2-grown cells of Synechococcus PCC 7942 ( Anacystis nidulans R2) are exposed to low CO 2. The structural gene for this protein (cmpA) has been cloned and sequenced and shown to encode a 450 amino acid polypeptide with a molecular mass of 49 kilodalton. A deletion mutant lacking the 42-kilodalton protein was obtained by transformation of Synechococcus PCC 7942 following in vitro mutagenesis of the cloned gene. There were no significant differences between the mutant and wild-type cells in their growth rates under either low or high CO 2 conditions. The activity of inorganic carbon (C i) transport in the mutant was as high as that in the wild-type strain. In both types of cells, CO 2 was the main species of C i transported and the activities of CO 2 and HCO 3− transport increased when high CO 2-grown cells were exposed to low CO 2. We conclude that the 42-kilodalton protein is not directly involved in the C i-accumulating mechanism of Synechococcus PCC 7942. 相似文献
5.
The active transport of CO 2 in Synechococcus UTEX 625 was measured by mass spectrometry under conditions that preclude HCO 3− transport. The substrate concentration required to give one half the maximum rate for whole cell CO 2 transport was determined to be 0.4 ± 0.2 micromolar (mean ± standard deviation; n = 7) with a range between 0.2 and 0.66 micromolar. The maximum rates of CO 2 transport ranged between 400 and 735 micromoles per milligram of chlorophyll per hour with an average rate of 522 for seven experiments. This rate of transport was about three times greater than the dissolved inorganic carbon saturated rate of photosynthetic O 2 evolution observed under these conditions. The initial rate of chlorophyll a fluorescence quenching was highly correlated with the initial rate of CO 2 transport (correlation coefficient = 0.98) and could be used as an indirect method to detect CO 2 transport and calculate the substrate concentration required to give one half the maximum rate of transport. Little, if any, inhibition of CO 2 transport was caused by HCO 3− or by Na +-dependent HCO 3− transport. However, 12CO 2 readily interfered with 13CO 2 transport. CO 2 transport and Na +-dependent HCO 3− transport are separate, independent processes and the high affinity CO 2 transporter is not only responsible for the initial transport of CO 2 into the cell but also for scavenging any CO 2 that may leak from the cell during ongoing photosynthesis. 相似文献
6.
In high inorganic carbon grown (1% CO 2 [volume/volume]) cells of the cyanobacterium Synechococcus PCC7942, the carbonic anhydrase (CA) inhibitor, ethoxyzolamide (EZ), was found to inhibit the rate of CO 2 uptake and to reduce the final internal inorganic carbon (C i) pool size reached. The relationship between CO 2 fixation rate and internal C i concentration in high C i grown cells was little affected by EZ. This suggests that in intact cells internal CA activity was unaffected by EZ. High C i grown cells readily took up CO 2 but had little or no capacity for HCO 3− uptake. These cells appear to possess a CO 2 utilizing C i pump that has a CA-like function associated with the transport step such that HCO 3− is the species delivered to the cell interior. This CA-like step may be the site of inhibition by EZ. Low C i grown cells possess both CO 2 uptake and HCO 3− uptake activities and EZ inhibited both activities to a similar degree, suggesting that a common step in CO 2 and HCO 3− uptake (such as the C i pump) may have been affected. The inhibitor had no apparent effect on internal CO 2/HCO 3− equilibria (internal CA function) in low C i grown cells. 相似文献
7.
The active transport of CO 2 in the cyanobacterium Synechococcus UTEX 625 was inhibited by H 2S. Treatment of the cells with up to 150 micromolar H 2S + HS − at pH 8.0 had little effect on Na +-dependent HCO 3− transport or photosynthetic O 2 evolution, but CO 2 transport was inhibited by more than 90%. CO 2 transport was restored when H 2S was removed by flushing with N 2. At constant total H 2S + HS − concentrations, inhibition of CO 2 transport increased as the ratio of H 2S to HS − increased, suggesting a direct role for H 2S in the inhibitory process. Hydrogen sulfide does not appear to serve as a substrate for transport. In the presence of H 2S and Na + -dependent HCO 3− transport, the extracellular CO 2 concentration rose considerably above its equilibrium level, but was maintained far below its equilibrium level in the absence of H 2S. The inhibition of CO 2 transport, therefore, revealed an ongoing leakage from the cells of CO 2 which was derived from the intracellular dehydration of HCO 3− which itself had been recently transported into the cells. Normally, leaked CO 2 is efficiently transported back into the cell by the CO 2 transport system, thus maintaining the extracellular CO 2 concentration near zero. It is suggested that CO 2 transport not only serves as a primary means of inorganic carbon acquisition for photosynthesis but also serves as a means of recovering CO 2 lost from the cell. A schematic model describing the relationship between the CO 2 and HCO 3− transport systems is presented. 相似文献
8.
The marine cyanobacterium, Synechococcus sp. Nägeli (strain RRIMP N1) changes its affinity for external inorganic carbon used in photosynthesis, depending on the concentration of CO 2 provided during growth. The high affinity for CO 2 + HCO 3− of air-grown cells (K ½ < 80 nanomoles [pH 8.2]) would seem to be the result of the presence of an inducible mechanism which concentrates inorganic carbon (and thus CO 2) within the cells. Silicone-oil centrifugation experiments indicate that the inorganic carbon concentration inside suitably induced cells may be in excess of 1,000-fold greater than that in the surrounding medium, and that this accumulation is dependent upon light energy. The quantum requirements for O 2 evolution appear to be some 2-fold greater for low CO 2-grown cells, compared with high CO 2-grown cells. This presumably is due to the diversion of greater amounts of light energy into inorganic carbon transport in these cells. A number of experimental approaches to the question of whether CO2 or HCO3− is primarily utilized by the inorganic carbon transport system in these cells show that in fact both species are capable of acting as substrate. CO2, however, is more readily taken up when provided at an equivalent concentration to HCO3−. This discovery suggests that the mechanistic basis for the inorganic carbon concentrating system may not be a simple HCO3− pump as has been suggested. It is clear, however, that during steady-state photosynthesis in seawater equilibrated with air, HCO3− uptake into the cell is the primary source of internal inorganic carbon. 相似文献
9.
A simple model based on HCO 3− transport has been developed to relate photosynthesis and inorganic carbon fluxes for the marine cyanobacterium, Synechococcus sp. Nägeli (strain RRIMP N1). Predicted relationships between inorganic carbon transport, CO 2 fixation, internal carbonic anhydrase activity, and leakage of CO 2 out of the cell, allow comparisons to be made with experimentally obtained data. Measurements of inorganic carbon fluxes and internal inorganic carbon pool sizes in these cells were made by monitoring time-courses of CO 2 changes (using a mass spectrometer) during light/dark transients. At just saturating CO 2 conditions, total inorganic carbon transport did not exceed net CO 2 fixation by more than 30%. This indicates CO 2 leakage similar to that estimated for C 4 plants. For this leakage rate, the model predicts the cell would need a conductance to CO2 of around 10−5 centimeters per second. This is similar to estimates made for the same cells using inorganic carbon pool sizes and CO2 efflux measurements. The model predicts that carbonic anhydrase is necessary internally to allow a sufficiently fast rate of CO2 production to prevent a large accumulation of HCO3−. Intact cells show light stimulated carbonic anhydrase activity when assayed using 18O-labeled CO2 techniques. This is also supported by low but detectable levels of carbonic anhydrase activity in cell extracts, sufficient to meet the requirements of the model. 相似文献
10.
Light-dependent inorganic C (C i) transport and accumulation in air-grown cells of Synechococcus UTEX 625 were examined with a mass spectrometer in the presence of inhibitors or artificial electron acceptors of photosynthesis in an attempt to drive CO 2 or HCO 3− uptake separately by the cyclic or linear electron transport chains. In the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea, the cells were able to accumulate an intracellular C i pool of 20 mm, even though CO 2 fixation was completely inhibited, indicating that cyclic electron flow was involved in the C i-concentrating mechanism. When 200 μm N,N-dimethyl- p-nitrosoaniline was used to drain electrons from ferredoxin, a similar C i accumulation was observed, suggesting that linear electron flow could support the transport of C i. When carbonic anhydrase was not present, initial CO 2 uptake was greatly reduced and the extracellular [CO 2] eventually increased to a level higher than equilibrium, strongly suggesting that CO 2 transport was inhibited and that C i accumulation was the result of active HCO 3− transport. With 3-(3,4-dichlorophenyl)-1,1-dimethylurea-treated cells, C i transport and accumulation were inhibited by inhibitors of CO 2 transport, such as COS and Na 2S, whereas Li +, an HCO 3−-transport inhibitor, had little effect. In the presence of N,N-dimethyl- p-nitrosoaniline, C i transport and accumulation were not inhibited by COS and Na 2S but were inhibited by Li +. These results suggest that CO 2 transport is supported by cyclic electron transport and that HCO 3− transport is supported by linear electron transport. 相似文献
11.
The species of inorganic carbon (CO 2 or HCO 3−) taken up a source of substrate for photosynthetic fixation by isolated Asparagus sprengeri mesophyll cells is investigated. Discrimination between CO 2 or HCO 3− transport, during steady state photosynthesis, is achieved by monitoring the changes (by 14C fixation) which occur in the specific activity of the intracellular pool of inorganic carbon when the inorganic carbon present in the suspending medium is in a state of isotopic disequilibrium. Quantitative comparisons between theoretical (CO 2 or HCO 3− transport) and experimental time-courses of 14C incorporation, over the pH range of 5.2 to 7.5, indicate that the specific activity of extracellular CO 2, rather than HCO 3−, is the appropriate predictor of the intracellular specific activity. It is concluded, therefore, that CO 2 is the major source of exogenous inorganic carbon taken up by Asparagus cells. However, at high pH (8.5), a component of net DIC uptake may be attributable to HCO 3− transport, as the incorporation of 14C during isotopic disequilibrium exceeds the maximum possible incorporation predicted on the basis of CO 2 uptake alone. The contribution of HCO 3− to net inorganic carbon uptake (pH 8.5) is variable, ranging from 5 to 16%, but is independent of the extracellular HCO 3− concentration. The evidence for direct HCO 3− transport is subject to alternative explanations and must, therefore, be regarded as equivocal. Nonlinear regression analysis of the rate of 14C incorporation as a function of time indicates the presence of a small extracellular resistance to the diffusion of CO 2, which is partially alleviated by a high extracellular concentration of HCO 3−. 相似文献
12.
Mass spectrometry has been used to confirm the presence of an active transport system for CO 2 in Synechococcus UTEX 625. Cells were incubated at pH 8.0 in 100 micromolar KHCO 3 in the absence of Na + (to prevent HCO 3− transport). Upon illumination the cells rapidly removed almost all the free CO 2 from the medium. Addition of carbonic anhydrase revealed that the CO 2 depletion resulted from a selective uptake of CO 2, rather than a total uptake of all inorganic carbon species. CO 2 transport stopped rapidly (<3 seconds) when the light was turned off. Iodoacetamide (3.3 millimolar) completely inhibited CO 2 fixation but had little effect on CO 2 transport. In iodoacetamide poisoned cells, transport of CO 2 occurred against a concentration gradient of about 18,000 to 1. Transport of CO 2 was completely inhibited by 10 micromolar diethylstilbestrol, a membrane-bound ATPase inhibitor. Studies with DCMU and PSI light indicated that CO 2 transport was driven by ATP produced by cyclic or pseudocyclic photophosphorylation. Low concentrations of Na + (<100 microequivalents per liter), but not of K +, stimulated CO 2 transport as much as 2.4-fold. Unlike Na +-dependent HCO 3− transport, the transport of CO 2 was not inhibited by high concentrations (30 milliequivalents per liter) of Li +. During illumination, the CO 2 concentration in the medium remained far below its equilibrium value for periods up to 15 minutes. This could only happen if CO 2 transport was continuously occurring at a rapid rate, since the continuing dehydration of HCO 3− to CO 2 would rapidly raise the CO 2 concentration to its equilibrium value if transport ceased. Measurement of the rate of dissolved inorganic carbon accumulation under these conditions indicated that at least part of the continuing CO 2 transport was balanced by HCO 3− efflux. 相似文献
13.
When studying active CO 2 and HCO 3− transport by cyanobacteria, it is often useful to be able to inhibit concomitant CO 2 fixation. We have found that glycolaldehyde was an efficient inhibitor of photosynthetic CO 2 fixation in Synechococcus UTEX 625. Glycolaldehyde did not inhibit inorganic carbon accumulation due to either active CO 2 or HCO 3− transport. When glycolaldehyde (10 millimolar) was added to rapidly photosynthesizing cells, CO 2 fixation was stopped within 15 seconds. The quenching of chlorophyll a fluorescence remained high (≤ 82% control) when CO 2 fixation was completely blocked by glycolaldehyde. This quenching was relieved upon the addition of a glucose oxidase oxygentrap. This is consistent with our previous finding that q-quenching in the absence of CO 2 fixation was due to O 2 photoreduction. Photosynthetic CO 2 fixation was also inhibited by d,l,-glyceraldehyde but a sixfold higher concentration was required. Glycolaldehyde acted much more rapidly than iodoacetamide (15 seconds versus 300 seconds) and did not cause the onset of net O 2 evolution often observed with iodoacetamide. Glycolaldehyde will be a useful inhibitor when it is required to study CO 2 and HCO 3− transport without the complication of concomitant CO 2 fixation. 相似文献
14.
The Na + requirement for photosynthesis and its relationship to dissolved inorganic carbon (DIC) concentration and Li + concentration was examined in air-grown cells of the cyanobacterium Synechococcus leopoliensis UTEX 625 at pH 8. Analysis of the rate of photosynthesis (O 2 evolution) as a function of Na + concentration, at fixed DIC concentration, revealed two distinct regions to the response curve, for which half-saturation values for Na + ( K½[Na +]) were calculated. The value of both the low and the high K½(Na +) was dependent upon extracellular DIC concentration. The low K½(Na +) decreased from 1000 micromolar at 5 micromolar DIC to 200 micromolar at 140 micromolar DIC whereas over the same DIC concentration range the high K½(Na +) decreased from 10 millimolar to 1 millimolar. The most significant increases in photosynthesis occurred in the 1 to 20 millimolar range. A fraction of total photosynthesis, however, was independent of added Na + and this fraction increased with increased DIC concentration. A number of factors were identified as contributing to the complexity of interaction between Na + and DIC concentration in the photosynthesis of Synechococcus. First, as revealed by transport studies and mass spectrometry, both CO 2 and HCO 3− transport contributed to the intracellular supply of DIC and hence to photosynthesis. Second, both the CO 2 and HCO 3− transport systems required Na +, directly or indirectly, for full activity. However, micromolar levels of Na + were required for CO 2 transport while millimolar levels were required for HCO 3− transport. These levels corresponded to those found for the low and high K½(Na +) for photosynthesis. Third, the contribution of each transport system to intracellular DIC was dependent on extracellular DIC concentration, where the contribution from CO 2 transport increased with increased DIC concentration relative to HCO 3− transport. This change was reflected in a decrease in the Na + concentration required for maximum photosynthesis, in accord with the lower Na +-requirement for CO 2 transport. Lithium competitively inhibited Na +-stimulated photosynthesis by blocking the cells' ability to form an intracellular DIC pool through Na +-dependent HCO 3− transport. Lithium had little effect on CO 2 transport and only a small effect on the size of the pool it generated. Thus, CO 2 transport did not require a functional HCO 3− transport system for full activity. Based on these observations and the differential requirement for Na + in the CO 2 and HCO 3− transport system, it was proposed that CO 2 and HCO 3− were transported across the membrane by different transport systems. 相似文献
15.
A mass spectrometer was used to simultaneously follow the time course of photosynthetic O 2 evolution and CO 2 depletion of the medium by cells of the cyanobacterium Synechococcus leopoliensis UTEX 625. Analysis of the data indicated that both CO 2 and HCO 3− were simultaneously and continuously transported by the cells as a source of substrate for photosynthesis. Initiation of HCO 3− transport by Na + addition had no effect on ongoing CO 2 transport. This result is interpreted to indicate that the CO 2 and HCO 3− transport systems are separate and distinctly different transport systems. Measurement of CO 2-dependent photosynthesis indicated that CO 2 uptake involved active transport and that diffusion played only a minor role in CO 2 acquisition in cyanobacteria. 相似文献
16.
At low levels of dissolved inorganic carbon (DIC) and alkaline pH the rate of photosynthesis by air-grown cells of Synechococcus leopoliensis (UTEX 625) was enhanced 7- to 10-fold by 20 millimolar Na +. The rate of photosynthesis greatly exceeded the CO 2 supply rate and indicated that HCO 3− was taken up by a Na +-dependent mechanism. In contrast, photosynthesis by Synechococcus grown in standing culture proceeded rapidly in the absence of Na + and exceeded the CO 2 supply rate by 8 to 45 times. The apparent photosynthetic affinity ( K½) for DIC was high (6-40 micromolar) and was not markedly affected by Na + concentration, whereas with air-grown cells K½ (DIC) decreased by more than an order of magnitude in the presence of Na +. Lithium, which inhibited Na +-dependent HCO 3− uptake in air-grown cells, had little effect on Na +-independent HCO 3− uptake by standing culture cells. A component of total HCO 3− uptake in standing culture cells was also Na +-dependent with a K½ (Na +) of 4.8 millimolar and was inhibited by lithium. Analysis of 14C-fixation during isotopic disequilibrium indicated that standing culture cells also possessed a Na +-independent CO 2 transport system. The conversion from Na +-independent to Na +-dependent HCO 3− uptake was readily accomplished by transferring cells grown in standing to growth in cultures bubbled with air. These results demonstrated that the conditions experienced during growth influenced the mode by which Ssynechococcus acquired HCO 3− for subsequent photosynthetic fixation. 相似文献
17.
We report the changes in the concentrations and 18O contents of extracellular CO 2 and HCO 3− in suspensions of Synechococcus sp. (UTEX 2380) using membrane inlet mass spectrometry. This marine cyanobacterium is known to have an active uptake mechanism for inorganic carbon. Measuring 18O exchange between CO 2 and water, we have found the intracellular carbonic anhydrase activity to be equivalent to 20 times the uncatalyzed CO 2 hydration rate in different samples of cells that were grown on bubbled air (low-CO 2 conditions). This activity was only weakly inhibited by ethoxzolamide with an I 50 near 7 to 10 micromolar in lysed cell suspensions. We have shown that even with CO 2-starved cells there is considerable generation of CO 2 from intracellular stores, a factor that can cause errors in measurement of net CO 2 uptake unless accounted for. It was demonstrated that use of 13C-labeled inorganic carbon outside the cell can correct for such errors in mass spectrometric measurement. Oxygen-18 depletion experiments show that in the light, CO 2 readily passes across the cell membrane to the sites of intracellular carbonic anhydrase. Although HCO 3− was readily taken up by the cells, these experiments shown that there is no significant efflux of HCO 3− from Synechococcus. 相似文献
18.
Although many studies have focused on the effects of elevated atmospheric CO 2 on algal growth, few of them have demonstrated how CO 2 interacts with carbon absorption capacity to determine the algal competition at the population level. We conducted a pairwise competition experiment of Phormidium sp., Scenedesmus quadricauda, Chlorella vulgaris and Synedra ulna. The results showed that when the CO 2 concentration increased from 400 to 760 ppm, the competitiveness of S. quadricauda increased, the competitiveness of Phormidium sp. and C. vulgaris decreased, and the competitiveness of S. ulna was always the lowest. We constructed a model to explore whether interspecific differences in affinity and flux rate for CO 2 and HCO 3 − could explain changes in competitiveness between algae species along the gradient of atmospheric CO 2 concentration. Affinity and flux rates are the capture capacity and transport capacity of substrate respectively, and are inversely proportional to each other. The simulation results showed that, when the atmospheric CO 2 concentration was low, species with high affinity for both CO 2 and HCO 3 − (HCHH) had the highest competitiveness, followed by the species with high affinity for CO 2 and low affinity for HCO 3 − (HCLH), the species with low affinity for CO 2 and high affinity for HCO 3 − (LCHH) and the species with low affinity for both CO 2 and HCO 3 − (LCLH); when the CO 2 concentration was high, the species were ranked according to the competitive ability: LCHH > LCLH > HCHH > HCLH. Thus, low resource concentration is beneficial to the growth and reproduction of algae with high affinity. With the increase in atmospheric CO 2 concentration, the competitive advantage changed from HCHH species to LCHH species. These results indicate the important species types contributing to water bloom under the background of increasing global atmospheric CO 2, highlighting the importance of carbon absorption characteristics in understanding, predicting and regulating population dynamics and community composition of algae. 相似文献
19.
Studies on the nitrite uptake capability of a mutant of Synechococcus sp. strain PCC 7942 lacking the ATP-binding cassette-type nitrate-nitrite-bispecific transporter revealed the occurrence of a nitrite-specific active transport system with an apparent Km (NO 2−) of about 20 μM. Similar to the nitrate-nitrite-bispecific transporter, the nitrite-specific transporter was reversibly inhibited by ammonium in the medium. 相似文献
20.
Carbon oxysulfide (carbonyl sulfide, COS) is a close structural analog of CO 2. Although hydrolysis of COS (to CO 2 and H 2S) does occur at alkaline pH (>9), at pH 8.0 the rate of hydrolysis is slow enough to allow investigation of COS as a possible substrate and inhibitor of the active CO 2 transport system of Synechococcus UTEX 625. A light-dependent uptake of COS was observed that was inhibited by CO 2 and the ATPase inhibitor diethylstilbestrol. The COS taken up by the cells could not be recovered when the lights were turned off or when acid was added. It was concluded that most of the COS taken up was hydrolyzed by intracellular carbonic anhydrase. The production of H 2S was observed and COS removal from the medium was inhibited by ethoxyzolamide. Bovine erythrocyte carbonic anhydrase catalysed the stoichiometric hydrolysis of COS to H 2S. The active transport of CO 2 was inhibited by COS in an apparently competitive manner. When Na +-dependent HCO 3− transport was allowed in the presence of COS, the extracellular [CO 2] rose considerably above the equilibrium level. This CO 2 appearing in the medium was derived from the dehydration of transported HCO 3− and was leaked from the cells. In the presence of COS the return to the cells of this leaked CO 2 was inhibited. These results showed that the Na +-dependent HCO 3− transport was not inhibited by COS, whereas active CO 2 transport was inhibited. When COS was removed by gassing with N 2, a normal pattern of CO 2 uptake was observed. The silicone fluid centrifugation method showed that COS (100 micromolar) had little effect upon the initial rate of HCO 3− transport or CO 2 fixation. The steady state rate of CO 2 fixation was, however, inhibited about 50% in the presence of COS. This inhibition can be at least partially explained by the significant leakage of CO 2 from the cells that occurred when CO 2 uptake was inhibited by COS. Neither CS 2 nor N 2O acted like COS. It is concluded that COS is an effective and selective inhibitor of active CO 2 transport. 相似文献
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