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1.
Kinetic properties of soybean net photosynthetic CO 2 fixation and of the carboxylase and oxygenase activities of purified soybean ( Glycine max [L.] Merr.) ribulose 1, 5-diphosphate carboxylase (EC 4.1.1.39) were examined as functions of temperature, CO 2 concentration, and O 2 concentration. With leaves, O 2 inhibition of net photosynthetic CO 2 fixation increased when the ambient leaf temperature was increased. The increased inhibition of CO 2 fixation at higher temperatures was caused by a reduced affinity of the leaf for CO 2 and an increased affinity of the leaf for O 2. With purified ribulose 1,5-diphosphate carboxylase, O 2 inhibition of CO 2 incorporation and the ratio of oxygenase activity to carboxylase activity increased with increased temperature. The increased O 2 sensitivity of the enzyme at higher temperature was caused by a reduced affinity of the enzyme for CO 2 and a slightly increased affinity of the enzyme for O 2. The similarity of the effect of temperature on the affinity of intact leaves and of ribulose 1,5-diphosphate carboxylase for CO 2 and O 2 provides further evidence that the carboxylase regulates the O 2 response of photosynthetic CO 2 fixation in soybean leaves. Based on results reported here and in the literature, a scheme outlining the stoichiometry between CO 2 and O 2 fixation in vivo is proposed. 相似文献
2.
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
3.
Summary Sugar cane chloroplasts isolated in simple media possessed little photochemical activity, but showed rapid O 2 uptake, independent of light. A similar rapid consumption of O 2 was observed with brei prepared from cane leaves. This was not observed in brei of spinach leaves. Authentic polyphenols and cane leaf extracts stimulated the consumption of O 2 by cane preparations and inhibited photosynthesis in chloroplasts isolated from spinach. Chlorogenic acid and caffeic acid were the major o-diphenols in extracts of cane leaves. These compounds inhibited reactions associated with CO 2 fixation by the photosynthetic carbon reduction cycle. Assimilation of CO 2 due to phosphoenol pyruvate carboxylase activity was less sensitive to inhibition by o-diphenols. Mechanisms are discussed whereby o-diphenols may inhibit cane chloroplasts during their isolation. 相似文献
4.
The nature of the inducible formation of enzymes engaged in the photosynthetic CO 2 fixation was examined in Chromatium vinosum during its autotropic development. Although the activity of RuBP carboxylase was the lowest among several enzyme activities examined, it was enhanced 2.5 times during a 5-hr incubation, while other enzyme activities were little altered. The enhancement of the RuBP carboxylase activity was dependent on the presence of reduced sulfur compounds in the incubation medium and illumination (>100 lx). The increase in enzyme activity, however, was repressed by CO 2 or pyruvate. Furthermore, O 2 markedly reduced the enzyme activity. In order to prove whether or not the enhancement of RuBP carboxylase activity was attributable to the biosynthesis of the enzyme, the incorporation of [ 35S]methionine into RuBP carboxylase was followed by immunoprecipitation analysis. The incorporation was dependent on the reduced sulfur compounds, and was repressed by elevating the CO 2 level. 相似文献
5.
In vivo CO 2 fixation activity and in vitro phosphoenolpyruvate carboxylase activity were demonstrated in effective and ineffective nodules of alfalfa ( Medicago sativa L.) and in the nodules of four other legume species. Phosphoenolpyruvate carboxylase activity was greatly reduced in nodules from both host and bacterially conditioned ineffective alfalfa nodules as compared to effective alfalfa nodules. Forage harvest and nitrate application reduced both in vivo and in vitro CO2 fixation activity. By day 11, forage harvest resulted in a 42% decline in in vitro nodule phosphoenolpyruvate carboxylase activity while treatment with either 40 or 80 kilograms nitrogen per hectare reduced activity by 65%. In vitro specific activity of phosphoenolpyruvate carboxylase and glutamate synthase were positively correlated with each other and both were positively correlated with acetylene reduction activity. The distribution of radioactivity in the nodules of control plants (unharvested, 0 kilograms nitrogen per hectare) averaged 73% into the organic acid and 27% into the amino acid fraction. In nodules from harvested plants treated with nitrate, near equal distribution of radioactivity was observed in the organic acid (52%) and amino acid (48%) fractions by day 8. Recovery to control distribution occurred only in those nodules whose in vitro phosphoenolpyruvate carboxylase activity recovered. The results demonstrate that CO2 fixation is correlated with nitrogen fixation in alfalfa nodules. The maximum rate of CO2 fixation for attached and detached alfalfa nodules at low CO2 concentrations (0.13-0.38% CO2) were 18.3 and 4.9 nanomoles per hour per milligram dry weight, respectively. Nodule CO2 fixation was estimated to provide 25% of the carbon required for assimilation of symbiotically fixed nitrogen in alfalfa. 相似文献
6.
The nature and sequence of metabolic events during phase II (early morning) Crassulacean acid metabolism in Opuntia erinacea var columbiana (Griffiths) L. Benson were characterized. Gas exchange measurements under 2 and 21% O 2 revealed increased O 2 inhibition of CO 2 fixation with progression of phase II. Malate and titratable acidity patterns indicated continued synthesis of C 4 acids for at least 30 minutes into the light period. Potential activities of phosphoenolpyruvate carboxylase (PEPC) and NADP-malic enzyme exhibited little change during phase II, while light activation of NADP-malate dehydrogenase, pyruvate, orthophosphate dikinase, and ribulose-1,5-bisphosphate carboxylase was apparent. Short-term 14CO 2 fixation experiments showed that the per cent of 14C incorporated into C 4 acids decreased while incorporation into other metabolites increased with time. PEPC exhibited increased sensitivity to 2 millimolar malate, and the Ki(malate) for PEPC decreased markedly with time. Sensitivity of PEPC to malate inhibition was considerably greater at pH 7.5 than at 8.0. The results indicate that decarboxylation and synthesis of malate occur simultaneously during the early morning period, and that phase II acid metabolism is not limited by CO 2 diffusion through stomata. With progression of phase II, CO 2 fixation by PEPC decreases while fixation by ribulose-1,5-bisphosphate carboxylase increases. 相似文献
7.
A system has been developed for the study of photosynthetic CO 2 fixation by isolated spinach chloroplasts at air levels of CO 2. Rates of CO 2 fixation were typically 20 to 60 micromoles/milligrams chlorophyll per hour. The rate of fixation was linear for 10 minutes but then declined to less than 10% of the initial value by 40 minutes. Ribulose 1,5-bisphosphate (RuBP) levels remained unchanged during this period, indicating that they were not the cause for the decline. The initial activity of the RuBP carboxylase in the chloroplast was high for 8 to 10 minutes and then declined similar to the rate of CO 2 fixation, suggesting that the decline in CO 2 fixation may have been caused by deactivation of the enzyme. 相似文献
8.
Photosynthetic CO 2-fixation in isolated pea ( Pisum sativum L., cv Little Marvel) chloroplasts during induction is markedly inhibited by 0.4 millimolar sulfite. Sulfate at the same concentration has almost no effect. The 14CO 2-fixation pattern indicates that the primary effect of sulfite is inhibition of the reaction catalyzed by ribulose bisphosphate carboxylase and a stimulation of export of intermediates out of the chloroplasts. Inhibition of light modulation of stromal enzyme activity does not appear to account for the toxicity of SO 2 in this Pisum variety. Arsenite at 0.2 millimolar concentrations inhibits light activation and inhibits photosynthetic CO 2 fixation. The 14CO 2-fixation pattern indicates that the primary effect of arsenite is inhibition of light activation of reductive pentose phosphate pathway enzyme activity. 相似文献
9.
Activities of key enzymes of Calvin cycle and C 4 metabolism, rate of 14CO 2 fixation in light and dark and the initial products of photosynthetic 14CO 2 fixation were determined in flag leaf and different ear parts of wheat viz. pericarp, awn and glumes. Compared to the activities of RuBP carboxylase and other Calvin cycle enzymes viz. NADP-glyceraldehyde-3-phosphate dehydrogenase, NAD-glyceraldehyde-3-phosphate dehydrogenase and ribulose-5-phosphate kinase, the levels of PEP carboxylase and other enzymes of C 4 metabolism viz. NADP-malate dehydrogenase, NAD-malate dehydrogenase, NADP-malic enzyme, NAD-malic enzyme, glutamate oxaloacetate transaminase genase, NADP-malic enzyme, NAD-malic enzyme, glutamate oxaloacetate transaminase and glutamate pyruvate transaminase, were generally greater in ear parts than in the flag leaf. In contrast to CO 2 fixation in light, the various ear parts incorporated CO 2 in darkness at much higher rates than flag leaf. In short term assimilation of 14CO 2 by illuminated ear parts, most of the 14C was in malate with less in 3-phosphoglyceric acid, whereas flag leaves incorporated most into 3-phosphoglyceric acid. It seems likely that ear parts have the capability of assimilating CO 2 by the C 4 pathway of photosynthesis and utilise PEP carboxylase for recapturing the respired CO 2. 相似文献
10.
Leaves of Peperomia camptotricha contain three distinct upper tissue layers and a one-cell thick lower epidermis. Light and dark CO 2 fixation rates and the activity of ribulose bisphosphate carboxylase/oxygenase and several C 4 enzymes were determined in the three distinct tissue layers. The majority of the C 4 enzyme activity and dark CO 2 fixation was associated with the spongy mesophyll, including the lower epidermis; and the least activity was found in the median palisade mesophyll. In contrast, the majority of the C 3 activity, that is ribulose bisphosphate carboxylase/oxygenase and light CO 2 fixation, was located in the palisade mesophyll. In addition, the diurnal flux in titratable acidity was greatest in the spongy mesophyll and lowest in the palisade mesophyll. The spatial separation of the C 3 and C 4 phases of carbon fixation in P. camptotricha suggests that this Crassulacean acid metabolism plant may have low photorespiratory rates when it exhibits daytime gas exchange (that is, when it is well watered). The results also indicate that this plant may be on an evolutionary path between a true Crassulacean acid metabolism plant and a true C 4 plant. 相似文献
11.
Mesophyll cells were isolated from sunflower leaves by an enzymic procedure. The cell suspensions possessed high photosynthesis rates. The products of cell photosynthesis were similar to the products of leaf disc photosynthesis. The relatively high radioactivity incorporated into malate after 14CO 2 feeding suggests that PEP carboxylase might participate in CO 2 fixation. Sunflower leaf extracts possessed a PEP carboxylase activity slightly higher than that of other C 3 species. Inhibition of PEP carboxylase by maleate decreased cell photosynthesis by only 15% and the first products of cell photosynthesis were phosphorylated compounds. It is concluded that the high photosynthesis rates displayed by sunflower are not due to a parallel C 4 pathway of photosynthesis but are rather dependent, at least in part, on the activity, or the amount, of RuBP carboxylase.Abbreviations PVP
polyvinylpyrrolidone
- PDS
potassium dextran sulfate
- DTT
dithiothreitol
- PEG
polyethyleneglycol
- RuBP
ribulose 1,5-bisphosphate
- PEP
phosphoenolpyruvate
- Mes
2-(N-morpholino) ethanesulfonic acid
- Hepes
N-2-hydroxyethylpiperazine-N-2-ethanesulfonic acid 相似文献
12.
Net CO 2 dark fixation of Kalanchoë daigremontiana varies with night temperature. We found an optimum of fixation at about 15° C; with increasing night temperature fixation decreased. We studied the temperature dependence of the activity of phosphoenolpyruvate (PEP)-carboxylase, the key enzyme for CO 2 dark fixation. We varied the pH, the substrate concentration (PEP), and the L-malate and glucose-6-phosphate (G-6-P) concentration in the assay. Generally, lowering the pH and reducing the amount of substrate resulted in an increase in activation by G-6-P and in an increase in malate inhibition of the enzyme. Furthermore, malate inhibition and G-6-P activation increased with increasing temperature. Activity measurements between 10° C and 45°C at a given concentration of the effectors revealed that the temperature optimum and maximum activities at that optimum varied with the effector applied. Under the influence of 5 mol m -3 L-malate the temperature optimum and maximum activity dropped drastically, especially when the substrate level was low (at 0.5 mol m -3 PEP from 32° C to 20° C). G-6-P raised the temperature optimum and maximum activity when the substrate level was low. If both malate and G-6-P were present, intermediate values were measured. We suggest that changes in metabolite levels in K. daigremontiana leaves can alter the temperature features of PEP-carboxylase so that the observed in vivo CO 2 dark fixation can be explained on the basis of PEP-carboxylase activity.Abbreviations PEP-c
phosphoenolpyruvate carboxylase
- CAM
crassulacean acid metabolism
- PEP
phosphoenolpyruvate
- G-6-P
glucose-6-phosphate 相似文献
13.
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
14.
Chloroplasts isolated from pine needles were found to be inactive with respect to CO 2 fixation. Since it was suspected that pine needles may contain substances inhibitory to photosynthesis, studies were carried out using photosynthetically active isolated spinach chloroplasts and chloroplasts isolated from pine needles. When isolated pine chloroplasts were suspended in buffer and were added to isolated spinach chloroplasts they inhibited photosynthetic CO 2 fixation. When the pine chloroplasts were separated from the medium by centrifugation, the separated pine chloroplasts severely inhibited CO 2 fixation by isolated spinach chloroplasts, but the supernatant solution from the pine chloroplasts was not inhibitory. As little as 5% pine chloroplasts (based on chlorophyll content) produced 50% inhibition of CO 2 fixation by the spinach chloroplasts. Studies of fixation of 14C-labelled CO 2 by spinach chloroplasts were carried out in which after 5 min photosynthesis the pine chloroplasts were added. It was found that the subsequent inhibition of spinach CO 2 fixation was neither due to any effect on the rate of export of photosynthetic metabolites from the chloroplasts to the medium, nor to a direct effect on the RUBP carboxylase reaction. The principal effect was found to be an inhibition of the conversion of fructose-1,6-bisphosphate and sedoheptulose-1,7-bisphosphate to the respective monophosphates and inorganic phosphate. From this finding it was concluded that a principal effect of the inhibition by pine chloroplasts is probably an inhibition either directly or indirectly of the bisphosphatase enzymes in the spinach chloroplasts. Based on its distribution between organic and aqueous acidic or neutral solutions, the inhibitory factor of the pine chloroplasts must be lipophilic. Most of the factor could be transferred to an aqueous phase in a strongly alkaline solution. Following subsequent acidification of the aqueous phase the activity could be completely transferred back into the organic phase. This procedure allowed for separation of the inhibitory factor from most of the pigments and other lipophilic substances present in the pine chloroplasts and yielded a preparation which could be subsequently fractionated by thin layer chromatography. UV absorption was found in two fast moving spots and at the origin. The fastest running spot from the thin layer chromatography plate was found to be the one containing most of the inhibitory activity. 相似文献
15.
The host-specific toxin produced by Helminthosporium maydis, race T, causes 50% inhibition of dark fixation of 14CO 2 by leaf discs of susceptible (Texas male sterile) corn when it is diluted to approximately 1/10,000 of the volume of the original fungus culture filtrate. Dilutions of 1/10 or less are required for equivalent inhibition of discs prepared from resistant (N) corn. Root growth and photosynthesis were considerably less sensitive (dilution values 1/3000 and 1/1200, respectively), as was leakage of 14C induced by toxin from preloaded discs. Based on literature values for dilutions causing ion leakage or inhibition of mitochondrial oxidation, toxin dilutions several orders of magnitude greater bring about inhibition of dark CO 2 fixation. Preincubation of discs in light increased sensitivity of dark fixation to toxin and an effect of light on symptom development was shown. Phosphoenolypruvate carboxylase activity in extracts of roots or leaves was not affected by toxin nor was the enzyme level altered in excised leaves treated with toxin. Inhibition of dark fixation of CO 2 provides a bioassaay for race T toxin which is both reliable and rapid. 相似文献
16.
Mesophyll cells and bundle sheath strands were isolated from Cyperus rotundus L. leaf sections infiltrated with a mixture of cellulase and pectinase followed by a gentle mortar and pestle grind. The leaf suspension was filtered through a filter assembly and mesophyll cells and bundle sheath strands were collected on 20-μm and 80-μm nylon nets, respectively. For the isolation of leaf epidermal strips longer leaf cross sections were incubated with the enzymes and gently ground as above. Loosely attached epidermal strips were peeled off with forceps. The upper epidermis, which lacks stomata, could be clearly distinguished from the lower epidermis which contains stomata. Microscopic evidence for identification and assessment of purity is provided for each isolated tissue.Enzymes related to the C 4-dicarboxylic acid cycle such as phosphoenolpyruvate carboxylase, malate dehydrogenase (NADP +), pyruvate, P i dikinase were found to be localized, ≥98%, in mesophyll cells. Enzymes related to operating the reductive pentose phosphate cycle such as RuDP carboxylase, phosphoribulose kinase, and malic enzyme are distributed, ≥99%, in bundle sheath strands. Other photosynthetic enzymes such as aspartate aminotransferase, pyrophosphatase, adenylate kinase, and glyceraldehyde 3- P dehydrogenase (NADP +) are quite active in both mesophyll and bundle sheath tissues.Enzymes involved in photorespiration such as RuDP oxygenase, catalase, glycolate oxidase, hydroxypyruvate reductase (NAD +), and phosphoglycolate phosphatase are preferentially localized, ≥84%, in bundle sheath strands.Nitrate and nitrite reductase can be found only in mesophyll cells, while glutamate dehydrogenase is present, ≥96%, in bundle sheath strands.Starch- and sucrose-synthesizing enzymes are about equally distributed between the mesophyll and bundle sheath tissues, except that the less active phosphorylase was found mainly in bundle sheath strands. Fructose-1,6-diP aldolase, which is a key enzyme in photosynthesis and glycolysis leading to sucrose and starch synthesis, is localized, ≥90%, in bundle sheath strands. The glycolytic enzymes, phosphoglyceromutase and enolase, have the highest activity in mesophyll cells, while the mitochondrial enzyme, cytochrome c oxidase, is more active in bundle sheath strands.The distribution of total nutsedge leaf chlorophyll, protein, and PEP carboxylase activity, using the resolved leaf components, is presented. 14CO 2 Fixation experiments with the intact nutsedge leaves and isolated mesophyll and bundle sheath tissues show that complete C 4 photosynthesis is compartmentalized into mesophyll CO 2 fixation via PEP carboxylase and bundle sheath CO 2 fixation via RuDP carboxylase. These results were used to support the proposed pathway of carbon assimilation in C 4-dicarboxylic acid photosynthesis and to discuss the individual metabolic characteristics of intact mesophyll cells, bundle sheath cells, and epidermal tissues. 相似文献
17.
In free-living Rhizobium japonicum cultures, the stimulatory effect of CO 2 on nitrogenase (acetylene reduction) activity was mediated through ribulose bisphosphate carboxylase activity. Two mutant strains (CJ5 and CJ6) of R. japonicum defective in CO 2 fixation were isolated by mitomycin C treatment. No ribulose bisphosphate carboxylase activity could be detected in strain CJ6, but a low level of enzyme activity was present in strain CJ5. Mutant strain CJ5 also exhibited pleiotropic effects on carbon metabolism. The mutant strains possessed reduced levels of hydrogen uptake, formate dehydrogenase, and phosphoribulokinase activities, which indicated a regulatory relationship between these enzymes. The CO 2-dependent stimulation of nitrogenase activity was not observed in the mutant strains. Both mutant strains nodulated soybean plants and fixed nitrogen at rates comparable to that of the wild-type strain. 相似文献
18.
The succulent leaf of the obligate Crassulacean acid metabolism plant Crassula falcata comprises two distinct types of parenchyma. The peripheral tissue is dark green, whereas the central tissue is relatively colorless. We have investigated whether the conventional interpretation of Crassulacean acid metabolism—simply, temporal separation of light and dark CO 2 fixation within individual cells—is sufficient. Ribulose-1,5-bisphosphate carboxylase/oxygenase (EC 4.1.1.39) and chlorophyll, indicating the photosynthetic-carbon-reduction pathway, were concentrated in peripheral tissue. Specific activities of P- enolpyruvate carboxylase (4.1.1.31) and of NAD +-malic enzyme (1.1.1.39), indicating capacity for dark CO 2 fixation and release, respectively, were high in both types of parenchyma. Measured directly as malic acid decline at the beginning of the photoperiod, CO 2 “storage” occurred in both tissues. These data indicate that there is a spatial component to Crassulacean acid metabolism in C. falcata. 相似文献
19.
In vivo CO 2 fixation and in vitro phosphoenolpyruvate (PEP) carboxylase levels have been measured in lupin ( Lupinus angustifolius L.) root nodules of various ages. Both activities were greater in nodule tissue than in either primary or secondary root tissue, and increased about 3-fold with the onset of N 2 fixation. PEP carboxylase activity was predominantly located in the bacteroid-containing zone of mature nodules, but purified bacteroids contained no activity. Partially purified PEP carboxylases from nodules, roots, and leaves were identical in a number of kinetic parameters. Both in vivo CO 2 fixation activity and in vitro PEP carboxylase activity were significantly correlated with nodule acetylene reduction activity during nodule development. The maximum rate of in vivo CO 2 fixation in mature nodules was 7.9 nmol hour −1 mg fresh weight −1, similar to rates of N 2 fixation and reported values for amino acid translocation. 相似文献
20.
Seven day old wheat and maize seedlings were exposed to 1300 or 2000 microeinsteins per square meter per second photosynthetically active radiation in CO 2-free air for 3 hours with either 1% O 2 in N 2 or N 2-only and then returned to normal air of 340 microliters per liter CO 2, 21% O 2 in N 2. Activity of the ribulose bisphosphate carboxylase and amount of the substrate, ribulose 1,5-bisphosphate, were measured during and following the CO 2-free treatments as was photosynthetic CO 2 fixation. Photoinhibition of photosynthesis was observed only with wheat seedlings following the N 2 only treatment. During the CO 2-free treatments, the levels of RuBP rose during all experiments except when wheat was photoinhibited. The activity of the ribulose bisphophate carboxylase, measured directly upon grinding the leaves, declined during the CO 2-free conditions. The carboxylase total activity increased in minutes in the leaf during and following the CO 2-free treatments. The specific activities of the wheat carboxylase went from 0.16 to 1.06 micromoles CO 2 fixed per milligram protein per minute while the maize carboxylase varied from 0.05 to 0.36 micromole CO 2 fixed per millogram protein per minute. This suggests that in these seedlings considerable inactive carboxylase must be stored in a form not activatable in extracts by CO 2 and Mg 2+. Possible mechanisms of regulation of photosynthesis by the ribulose bisphosphate carboxylase must consider not only the amount of active enzyme, but the amount of enzyme which the plant can make activatable upon demand. 相似文献
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