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1.
The oxyhydrogen reaction in the presence and absence of CO 2 was studied in H 2-adapted Scenedesmus obliquus by monitoring the initial rates of H 2, O 2, and 14CO 2 uptake and the effect of inhibitors on these rates with gas-sensing electrodes and isotopic techniques. In the presence of 0.02 atmosphere O 2, the pH 2 was varied from 0 to 1 atmosphere. Whereas the rate of O 2 uptake increased by only 30%, the rate of H 2 uptake increased severalfold over the range of pH 2 values. At 0.1 atmosphere H 2 and 0.02 atmosphere O 2, rates for H 2 and O 2 uptake were between 15 and 25 micromoles per milligram chlorophyll per hour. As the pH 2 was changed from 0 to 1 atmosphere, the quotient H 2:O 2 changed from 0 to roughly 2. This change may reflect the competition between H 2 and the endogenous respiratory electron donors. Respiration in the presence of glucose and acetate was also competitive with H 2 uptake. KCN inhibited equally respiration (O 2 uptake in the absence of H 2) and the oxyhydrogen reaction in the presence and absence of CO 2. The uncoupler carbonyl cyanide p-trifluoromethoxyphenylhydrazone accelerated the rate of respiration and the oxyhydrogen reaction to a similar extent. It was concluded that the oxyhydrogen reaction both in the presence and absence of CO 2 has properties in common with components of respiration and photosynthesis. Participation of these two processes in the oxyhydrogen reaction would require a closely linked shuttle between mitochondrion and chloroplast. 相似文献
2.
The hydrogenase-dependent processes, photoreduction and the dark oxyhydrogen reaction, both of which can support CO 2 assimilation, were compared with aerobic photosynthesis and respiration for their sensitivity to electron transport inhibitors in cells and intact chloroplasts of Chlamydomonas reinhardii 11-32/6. Photoreduction but not photosynthesis was inhibited in chloroplasts and the oxyhydrogen reaction detected only in cells was inhibited up to 75 and 90%, respectively, by 150 micromolar rotenone, indicating the involvement of a NAD(P)H-plastoquinone oxidoreductase in the hydrogen utilizing pathways. The oxyhydrogen reaction coupled to CO 2 fixation was inhibited more than 95% by 10 micromolar 2,5 - dibromo - 3 - methyl - 6 - isopropyl - p - benzoquinone (DBMIB), a concentration which did not affect respiratory activity. In cells, both photoreduction and the oxyhydrogen reaction exhibited a similar sensitivity to salicylhydroxamic acid (SHAM) showing approximately 90% inhibition by 7 millimolar concentration. Photosynthesis was inhibited only 30% by the same concentration of SHAM. Antimycin A (18 micromolar, 10 micrograms per milliliter) inhibited both photoreduction (80%) and the oxyhydrogen reaction (92%) in cells with the oxyhydrogen reaction being approximately 10 times more sensitive to lower concentrations of the inhibitor. Antimycin A at 18 micromolar concentration did not inhibit photosynthetic CO 2 fixation unless the cells were adapted to an atmosphere of N 2 and the reaction conducted anaerobically. Photosynthesis, photoreduction, and the oxyhydrogen reaction coupled to CO 2 fixation were all inhibited greater than 90% by 10 micromolar carbonylcyanide- p-trifluoromethoxyphenylhydrazone. ATP added to chloroplasts adapted to an atmosphere of H 2 could support CO 2 uptake in the dark. These results are interpreted as evidence that photoreduction and the oxyhydrogen reaction involve some common components of thylakoidal electron transport pathways in Chlamydomonas including NAD(P)H-plastoquinone oxidoreductase and the plastoquinone pool. An O 2-consuming thylakoidal or mitochondrial reaction is an additional component of the oxyhydrogen reaction. 相似文献
3.
The role of the oxyhydrogen reaction in the nitrogen metabolism of Anabaena cylin-drica, particularly under conditions of dinitrogen starvation, was investigated. It was shown that although this reaction supports nitrogenase activity in the dark, when the cells are deprived of nitrogen the rate of hydrogen uptake is little changed. Measurements of ammonia excretion into the medium in the presence of methionine sulfoximine under such conditions indicated that hydrogen uptake supported the turnover of cell protein as an alternative source of nitrogen. In the absence of H 2 and O 2 in the dark, nitrogenase activity was negligible but protein turnover continued. In their presence nitrogenase activity was greatly stimulated; turnover was also stimulated but to a greater extent in the absence of nitrogenase substrates. The oxyhydrogen reaction also stimulated uptake of ammonium ions by intact filaments in argon in the dark. Only at very low hydrogen tensions can net hydrogen formation be obtained in argon/CO 2 in the light, casting considerable doubt on the suitability of hydrogenase-containing organisms for biophotolytic hydrogen formation. Addition of exogenous ammonia to the cultures incubated in argon resulted in a pronounced stimulation of H 2 uptake; nitrate and its derivatives had no such effect, nor did various amino acid derivatives of ammonia. 相似文献
4.
The hydrogenase system which catalyzes the oxyhydrogen reaction in soybean nodules produced by strains of is located in the bacteroids. The hydrogenase complex in intact bacteroids has an apparent K m for H 2 of 2.8 μM and an apparent K m for O 2 of 1.3 μM. The addition of hydrogen to bacteroids increases oxygen uptake but decreases respiratory CO 2 production, indicating a conservation of endogenous substrates. After correction for the effect of hydrogen on endogenous respiration a ratio of 1.9 ± 0.1 for H 2 to O 2 uptake was determined. Bacteroids from greenhouse or field-grown soybeans that evolved hydrogen showed no measurable oxyhydrogen reaction activity whereas consistent activity was demonstrated by bacteroids from soybean nodules that evolved little or no H 2. 相似文献
5.
Isolated intact chloroplasts of Chlamydomonas reinhardii were found to catalyze photoreduction of CO 2 in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea when adapted under an atmosphere of H 2 demonstrating the association of a hydrogenase and anaerobic adaptation system with these plastids. The specific activity of photoreduction was approximately one third that detected in cells and protoplasts. Photoreduction was found to have a lower osmoticum optimum relative to aerobically maintained chloroplasts (50 millimolar versus 120 millimolar mannitol). 3-Phosphoglycerate (3-PGA) stimulated photoreduction up to a peak at 0.25 millimolar beyond which inhibition was observed. In the absence of 3-PGA, inorganic phosphate had no effect on photoreduction but in the presence of 3-PGA, inorganic phosphate also stimulated the reaction. Carbonyl cyanide- p-trifluoromethoxyphenylhydrazone and 2,5-dibromo-3-methyl-6-isopropyl- p-benzoquinone inhibited photoreduction but inhibition by the former could be partially overcome by exogenously added ATP. The intact plastid can also catalyze photoevolution of H 2 while lysed chloroplast extracts catalyzed the reduction of methyl viologen by H 2. Both reactions occurred at rates approximately one-third of those found in cells. The oxyhydrogen reaction in the presence or absence of CO 2 was not detected. 相似文献
6.
Reactions of the uptake hydrogenase from Anabaena 7120 (A.T.C.C. 27893, Nostoc muscorum) were examined in whole filaments, isolated heterocysts, and membrane particles. Whole filaments or isolated heterocysts that contained nitrogenase consumed H 2 in the presence of C 2H 2 or N 2 in a light-dependent reaction. If nitrogenase was inactivated by O 2 shock, filaments catalyzed H 2 uptake to an unidentified endogenous acceptor in the light. Addition of NO 3− or NO 2− enhanced these rates. Isolated heterocysts consumed H 2 in the dark in the presence of electron acceptors with positive midpoint potentials, and these reactions were not enhanced by light. With acceptors of negative midpoint potential, significant light enhancement of H 2 uptake occurred. Maximum rates of light-dependent uptake were approximately 25% of the maximum dark rates observed. Membrane particles prepared from isolated heterocysts showed similar specificity for electron acceptors. These particles catalyzed a cyanide-sensitive oxyhydrogen reaction that was inactivated by O 2 at O 2 concentrations above 2%. Light-dependent H 2 uptake to low potential acceptors by these particles was inhibited by dibromothymoquinone but was insensitive to cyanide. In the presence of O 2, light-dependent H 2 uptake occurred simultaneously with the oxyhydrogen reaction. The pH optima for both types of H 2 uptake were near 7.0. These results further clarify the role of uptake hydrogenase in donating electrons to both the photosynthetic and respiratory electron transport chains of Anabaena. 相似文献
7.
1. The oxyhydrogen reaction of Anacystis nidulans was studied manometrically and polarographically in whole cells and in cell-free preparations; the activity was found to be associated with the particulate fraction.2. Besides O 2, the isolated membranes reduced artificial electron acceptors of positive redox potential; the reactions were unaffected by O 2 levels <10–15%; aerobically the artificial acceptors were reduced simultaneously with O 2.3. H 2-supported O 2 uptake was inhibited by CO, KCN and 2- n-heptyl-8-hydroxyquinoline- N-oxide. Inhibition by CO was partly reversed by strong light. Uncouplers stimulated the oxyhydrogen reaction.4. The kinetic properties of O 2 uptake by isolated membranes were the same in presence of H 2 and of other respiratory substrates.5. Low rates of H 2 evolution by the membrane preparations were found in presence of dithionite; methyl viologen stimulated the reaction.6. The results indicate that under certain growth conditions Anacystis synthesizes a membrane-bound hydrogenase which appears to be involved in phosphorylative electron flow from H 2 to O 2 through the respiratory chain. 相似文献
8.
The establishment and maintenance of high rates of photosynthetic CO 2 incorporation in mesophyll cells of Papaver somniferum (opium poppy) depend on a regime of dark and light periods immediately following isolation, as well as carefully adjusted conditions of isolation. Analysis of the incorporation pattern of 14CO 2 by the isolated cells indicates an initial “stress-response” period of approximately 20 hours characterized by increased respiratory-type metabolism and diminished photosynthesis. Under the favorable regime, this period is followed by rapid recovery and the reinstatement of a metabolic state strikingly similar to that of intact leaves in which the initial rate of CO 2 incorporation is between 110 and 175 μmoles CO 2 fixed per mg chlorophyll per hour. The photosynthetic viability of these cells can be maintained for up to 80 hours. 相似文献
9.
Isolated heterocysts of Anabaena 7120 evolve H 2 in an ATP-dependent nitrogenase-catalyzed process that is inhibited by N 2 and C 2H 2. Heterocysts have an active uptake hydrogenase that only requires an electron acceptor of positive redox potential, e.g., methylene blue, dichlorophenolindophenol or potassium ferricyanide. O 2 supplied at low partial pressures is a very effective physiological oxidant for H 2 uptake. High concentrations of O 2 are inhibitory to H 2 uptake. The oxyhydrogen reaction in heterocysts appears to be mediated by a cytochrome-cytochrome oxidase system, and it supports ATP synthesis via oxidative phosphorylation. Attempts to demonstrate acetylene reduction in isolated heterocysts employing H 2 as an electron donor were unsuccessful. It is suggested that the uptake hydrogenase functions to conserve reductant that otherwise would be dissipated via nitrogenase-catalyzed H 2 evolution. 相似文献
10.
Chloroplastic respiration was monitored by measuring 14CO 2 from 14C glucose in the darkened Chlamydomonas reinhardtii F-60 chloroplast. The patterns of 14CO 2 evolution from labeled glucose in the absence and presence of the inhibitors iodoacetamide, glycolate-2-phosphate, and phospho enolpyruvate were those expected from the oxidative pentose phosphate cycle and glycolysis. The Km for glucose was 56 micromolar and for MgATP was 200 micromolar. Release of 14CO 2 was inhibited by phloretin and inorganic phosphate. Comparing the inhibition of CO 2 evolution generated by pH 7.5 with respect to pH 8.2 (optimum) in chloroplasts given C-1, C-2, and C-6 labeled glucose indicated that a suboptimum pH affects the recycling of the pentose phosphate intermediates to a greater extent than CO 2 evolution from C-1 of glucose. Respiratory inhibition by pH 7.5 in the darkened chloroplast was alleviated by NH 4Cl and KCl (stromal alkalating agents), iodoacetamide (an inhibitor of glyceraldehyde 3-phosphate dehydrogenase), or phospho enolpyruvate (an inhibitor of phosphofructokinase). It is concluded that the site which primarily mediates respiration in the darkened Chlamydomonas chloroplast is the fructose-1,6-bisphosphatase/phosphofructokinase junction. The respiratory pathways described here can account for the total oxidation of a hexose to CO 2 and for interactions between carbohydrate metabolism and the oxyhydrogen reaction in algal cells adapted to a hydrogen metabolism. 相似文献
11.
The unicellular cyanobacterium Chroococcidiopsis thermalis CALU 758 growing photoautotrophically synthesised a hydrogenase which catalysed an in vivo H 2 uptake in the oxyhydrogen reaction at a significant rate and showed only low level of in vitro MV-dependent H 2 evolution. The in vitro hydrogenase activity was not induced under microaerobic or nitrate-limiting conditions. Some correlation observed between the two activities indicated that the same enzyme may be involved in both H 2 uptake and H 2 evolution. Heterologous Southern hybridisations, using cyanobacterial hup and hox DNA fragments as probes, showed the presence of sequences similar to hox (encoding for a bidirectional hydrogenase) in C. thermalis CALU 758 with no indication for the presence of any sequences corresponding to an uptake hydrogenase. Further molecular experiments, using specific primers directed against different conserved regions of the large subunit ( hoxH) of the bidirectional hydrogenase confirmed the presence of corresponding sequences in C. thermalis CALU 758. Low-stringency Southern hybridisations detected only one copy of hoxH within the genome of C. thermalis CALU 758. 相似文献
12.
In nature, H 2- and CO 2-utilizing methanogenic archaea have to couple the processes of methanogenesis and autotrophic growth under highly variable conditions with respect to the supply and concentration of their energy source, hydrogen. To study the hydrogen-dependent coupling between methanogenesis and growth, Methanothermobacter thermautotrophicus was cultured in a fed-batch fermentor and in a chemostat under different 80% H 2-20% CO 2 gassing regimens while we continuously monitored the dissolved hydrogen partial pressures ( pH2). In the fed-batch system, in which the conditions continuously changed the uptake rates by the growing biomass, the organism displayed a complex and yet defined growth behavior, comprising the consecutive lag, exponential, and linear growth phases. It was found that the in situ hydrogen concentration affected the coupling between methanogenesis and growth in at least two respects. (i) The microorganism could adopt two distinct theoretical maximal growth yields ( YCH4 max), notably approximately 3 and 7 g (dry weight) of methane formed mol −1, for growth under low ( pH2 < 12 kPa)- and high-hydrogen conditions, respectively. The distinct values can be understood from a theoretical analysis of the process of methanogenesis presented in the supplemental material associated with this study. (ii) The in situ hydrogen concentration affected the “specific maintenance” requirements or, more likely, the degree of proton leakage and proton slippage processes. At low pH2 values, the “specific maintenance” diminished and the specific growth yields approached YCH4 max, indicating that growth and methanogenesis became fully coupled. 相似文献
13.
The H 2-oxidizing complex in Rhizobium japonicum 122 DES bacteroids failed to catalyze, at a measurable rate, 2H 1H exchange from a mixture of 2H 2 and 1H 2 in presence of 2H 2O and 1H 2O, providing no evidence for reversibility of the hydrogenase reaction in vivo. In the H 2 oxidation reaction, there was no significant discrimination between 2H 2 and 1H 2, indicating that the initial H 2-activation step in the over-all H 2 oxidation reaction is not rate-limiting. By use of improved methods, an apparent Km for H 2 of 0.05 micromolar was determined. The H 2 oxidation reaction in bacteroids was strongly inhibited by cyanide (88% at 0.05 millimolar), theonyltrifluoroacetone, and other metal-complexing agents. Carbonyl cyanide m-chlorophenylhydrazone at 0.005 millimolar and 2,4-dinitrophenol at 0.5 millimolar inhibited H 2 oxidation and stimulated O 2 uptake. This and other evidence suggest the involvement of cytochromes and nonheme iron proteins in the pathway of electron transport from H 2 to O 2. Partial pressures of H 2 at 0.03 atmosphere and below had a pronounced inhibitory effect on endogenous respiration by bacteroid suspensions. The inhibition of CO 2 evolution by low partial pressures of H 2 suggests that H 2 utilization may result in conservation of oxidizable substrates and benefits the symbiosis under physiological conditions. Succinate, acetate, and formate at concentrations of 50 millimolar inhibited rates of H 2 uptake by 8, 29, and 25%, respectively. The inhibition by succinate was noncompetitive and that by acetate and formate was uncompetitive. A concentration of 11.6 millimolar CO 2 (initial concentration) in solution inhibited H 2 uptake by bacteroid suspensions by 18%. Further research is necessary to establish the significance of the inhibition of H 2 uptake by succinate, acetate, formate, and CO 2 in the metabolism of the H 2-uptake-positive strains of Rhizobium. 相似文献
14.
Bundle sheath strands capable of assimilating up to 68 μmoles CO 2 per mg chlorophyll per hr in the dark have been isolated from fully expanded leaves of Zea mays L. This dark CO 2-fixing system is dependent on exogenous ribose-5-phosphate, ADP or ATP, and Mg 2+ for maximum activity. The principal product of dark fixation in this system is 3-phosphoglycerate, indicating that the CO 2-fixing reaction is mediated by ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39). The rate of dark CO 2 uptake in the strands in the presence of saturating levels of ribose-5-phosphate plus ADP is inhibited by oxygen. The inhibitory effect of oxygen is rapidly and completely reversible, and is relieved by increased levels of CO 2. Glycolate is synthesized in this dark system in the presence of [U- 14C]ribose-5-phosphate, ADP, oxygen, and an inhibitor of glycolate oxidase (EC 1.1.3.1). Glycolate formation is completely abolished by heating the strands, and the rate of glycolate synthesis is markedly reduced by either lowering the oxygen tension or increasing the level of CO 2.These results, obtained with intact cells in the absence of light, indicate that the direct inhibitory effect of oxygen on photosynthesis is associated with photosynthetic carbon metabolism, probably at the level of ribulose-1,5-bisphosphate carboxylase, and not with photophosphorylation or photosynthetic electron transport. Furthermore, the findings indicate that the synthesis of glycolate from exogenous substrate can readily occur in the absence of photosynthetic electron transport, an observation consistent with the ribulose-1, 5-bisphosphate “oxygenase” scheme for glycolate formation during photosynthesis. 相似文献
15.
Oxyleghemoglobin was used to supply low concentrations of O 2 to H 2-oxidizing bacteroids from Rhizobium japonicum USDA 122 DES. The H 2 oxidation system of these bacteroids was capable of effectively utilizing O 2 at the low concentrations of O 2 expected to be found in soybean nodules. Apparent Km values of approximately 10 nanomolar O 2 have been calculated for the oxyhydrogen reaction. These values include the Km values for both H 2 oxidation and endogenous substrate oxidation. Even in the presence of oxyleghemoglobin, H 2 additions stimulated C 2H 2 reduction, reduced the rate of endogenous respiration and maintained the ATP contents of bacteroids. In our reconstituted oxyleghemoglobin and bacteriod system, we estimate that the H 2 oxidation system is capable of recycling all of the H 2 evolved during the N 2 fixation process. 相似文献
16.
The role of uptake hydrogenase in providing reducing power to nitrogenase was investigated in Rhizobium leguminosarum bacteroids from nodules of Pisum sativum L. (cv. Homesteader). H 2 increased the rate of C 2H 2 reduction in the absence of added substrates. Malate also increased nitrogenase (C 2H 2) activity while decreasing the effect of H 2. At exogenous malate concentrations above 0.05 mM no effect of H 2 was seen. Malate appeared to be more important as a source of reductant than of ATP. When iodoacetate was used to minimize the contribution of endogenous substrates to nitrogenase activity in an isolate in which H 2 uptake was not coupled to ATP formation, H 2 increased the rate of C 2H 2 reduction by 77%. In the presence of iodoacetate, an ATP-generating system did not enhance C 2H 2 reduction, but when H 2 was also included, the rate of C 2H 2 reduction was increased by 280% over that with the ATP-generating system alone. The data suggest that, under conditions of substrate starvation, the uptake hydrogenase in R. leguminosarum could provide reductant as well as ATP in an isolate in which the H 2 uptake is coupled to ATP formation, to the nitrogenase complex. 相似文献
17.
The effects of adenine nucleotides on phosphoenolypyruvate carboxylase were investigated using purified enzyme from the CAM plant, Crassula argentea. At 1 millimolar total concentration and with limiting phosphoenolpyruvate, AMP had a stimulatory effect, lowering the Km for phosphoenolpyruvate, ADP caused less stimulation, and ATP decreased the activity by increasing the Km for phosphoenolpyruvate. Activation by AMP was not additive to the stimulation by glucose 6-phosphate. Furthermore, AMP increased the Ka for glucose 6-phosphate. Inhibition by ATP was competitive with phosphoenolpyruvate. In support of the kinetic data, fluorescence binding studies indicated that ATP had a stronger effect than AMP on phosphoenolpyruvate binding, while AMP was more efficient in reducing glucose 6-phosphate binding. As free Mg 2+ was held constant and saturating, these effects cannot be ascribed to Mg 2+ chelation. Accordingly, the enzyme response to the adenylate energy charge was basically of the “R” type (involving enzymes of ATP regenerating sequences) according to D. E. Atkinson's (1968 Biochemistry 7: 4030-4034) concept of energy charge regulation. The effect of energy charge was abolished by 1 millimolar glucose 6-phosphate. Levels of glucose 6-phosphate and of other putative regulatory compounds of phosphoenolpyruvate carboxylase were determined in total leaf extracts during a day-night cycle. The level of glucose 6-phosphate rose at night and dropped sharply during the day. Such a decrease in glucose 6-phosphate concentration could permit an increased control of phosphoenolpyruvate carboxylase by energy charge during the day. 相似文献
18.
The present experiments were carried out to investigate the effect of increased fluxes of H + across the plasmalemma on glycolysis in heterotrophic cell suspension cultures of Chenopodium rubrum L. (1) Increased H + influx was produced by adding glucose, 6-deoxyglucose, 2-deoxyglucose, or sodium fluoride. The net influx decreased to zero after 3 min. This recovery was accompanied by an increase in the rate of O 2 uptake, but not of dark CO 2 fixation. When glucose or fluoride were added, the increase of O 2 uptake occurred without a decrease in the ATP/ADP ratio, and was large enough to provide the ATP that would be needed for compensatory H + extrusion via the plasmalemma H +-ATPase. When 2-deoxyglucose was added, the rise of respiration was restricted by sequestration of phosphate and depletion of phosphorylated metabolites, the ATP/ADP ratio declined, and a slow net H + influx started again after 4 min. (2) Alkalinisation of the medium to induce an H + efflux resulted in rapid activation of dark CO 2 fixation, but not of O 2-uptake. (3) A stimulation of respiration or dark CO 2 fixation was always accompanied by a decrease of phosphoenolpyruvate. This shows that the primary sites for regulation of glycolysis are pyruvate kinase and phosphoenolpyruvate carboxylase, respectively. (4) There was no consistent relation between glycolytic flux and triose-phosphates or hexose-phosphates. This shows that the reactions involved in carbohydrate mobilisation and the conversion of hexose-phosphates to triose-phosphates only have a secondary role in stimulation of glycolysis. (5) Phosphofructokinase will be stimulated as a consequence of the decrease in phosphoenolpyruvate. (6) The increase in glycolytic flux occurred independently of (in the case of 2-deoxyglucose and fluoride), or before (in the case of glucose), any increase of fructose-2,6-bisphosphate. When fructose-2,6-bisphosphate did increase (after supplying glucose), this was accompanied by an increase of triose-phosphate and fructose-1,6-bisphosphate, which otherwise remained very low. It is argued that fructose-2,6-bisphosphate increases as a consequence of the decrease of glycerate-3-phosphate, a known inhibitor of the synthesis of this regulator metabolite. However, activation of pyrophosphate fructose-6-phosphate phosphotransferase by fructose-2,6-bisphosphate does not play an obligatory role in the stimulation of glycolysis. 相似文献
19.
The anaerobic photodissimilation of acetate by Chlamydomonas reinhardii F-60 adapted to a hydrogen metabolism was studied utilizing manometric and isotopic techniques. The rate of photoanaerobic (N 2) acetate uptake was approximately 20 μmoles per milligram chlorophyll per hour or one-half that of the photoaerobic (air) rate. Under N 2, cells produced 1.7 moles H 2 and 0.8 mole CO 2 per mole of acetate consumed. Gas production and acetate uptake were inhibited by monofluoroacetic acid (MFA), 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea (DCMU) and by H 2. Acetate uptake was inhibited about 50% by 5% H 2 (95% N 2). H 2 in the presence of MFA or DCMU stimulated acetate uptake and the result was interpreted to indicate a transition from oxidative to reductive metabolism. Carbon-14 from both [1- 14C]- and [2- 14C]acetate was incorporated under N 2 or H 2 into CO 2, lipids, and carbohydrates. The methyl carbon of acetate accumulated principally (75-80%) in the lipid and carbohydrate fractions, whereas the carboxyl carbon contributed isotope primarily to CO 2 (56%) in N 2. The presence of H 2 caused a decrease in carbon lost from the cell as CO 2 and a greater proportion of the acetate was incorporated into lipid. The results support the occurrence of anaerobic and light-dependent citric acid and glyoxylate cycles which affect the conversion of acetate to CO 2 and H 2 prior to its conversion to cellular material. 相似文献
20.
The assay of the hydrogenase of glucose-grown cells of Chlorella pyrenoidosa, strain 7-11-05 by means of nitrite reduction with molecular hydrogen is described. The hydrogenase of Chlorella shows maximum activity immediately after equilibration in the hydrogen atmosphere. The hydrogenase mediated reduction of nitrite to ammonia requires the presence of CO 2. However, at pH 6.4. when the reaction proceeds optimally, there is apparently sufficient retention of metabolic CO 2 to support the reaction, which goes to completion, at near maximum rates. Reduction of nitrite in the hydrogenase system when CO2 is present results in the uptake of 3 moles of H2 per mole of nitrite and ammonia is the product. When CO2 is absent or limiting, ammonia is also formed from nitrite but with the uptake of less than the stoichiometric amount of H2. It is concluded that CO2 is essential for the uptake of H2, and that in the absence of CO2 internal hydrogen donors support nitrite reduction. The possibility that CO2 exerts a catalytic effect in all reductions mediated by hydrogenase in algae is considered, and a further hypothesis, that hydrogenase arises from that portion of the photosynthetic machinery which also shows a catalytic requirement for CO2, is proposed. 相似文献
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