共查询到20条相似文献,搜索用时 31 毫秒
1.
C.R. Bridges A.C. Taylor R.J.A. Atkinson 《Journal of experimental marine biology and ecology》1982,59(1):51-60
The respiratory properties of the whole blood of the burrowing red band fish Cepola rubescens L. were investigated. Oxygen dissociation curves constructed at 15°C were found to be close to hyperbolic in shape with a mean value for the cooperativity coefficient at half-saturation (n50) of 1.56. Half-saturation oxygen tension (P50) for pH = 7.56 (mean in vivo pH of venous blood) was 27 Torr. The blood showed a marked Bohr effect () and also a Root effect which at the in vivo pH reduced oxygen carrying capacity by 20%. The PvCO2 was 3.2 Torr and the buffering power of the blood was low, the buffer value of true plasma averaging 5.43 mmol · 1?1 · pH?1. It is suggested that the large Bohr effect coupled with the low buffer value confers on the haemoglobin a flexibility, in terms of oxygen affinity, to withstand changes which occur in environmental oxygen tensions. 相似文献
2.
Oxygen inhibits CO2 fixation by mesophyll protoplast extracts of the C3 plants, and , but stimulates the pyruvate induced CO2 fixation by mesophyll protoplast extracts of the C4 plants and . The former is reversed by increased levels of bicarbonate, whereas the latter effect is independent of bicarbonate concentration. The results are consistent with the proposal that oxygen inhibits C3 photosynthesis by competing with CO2 in the RuDP carboxylase/oxygenase system. The oxygen enhancement of C4 mesophyll photosynthesis is proposed to be due to pseudocyclic electron flow supplying additional ATP for the CO2 fixation process. 相似文献
3.
《BBA》1986,849(3):355-365
Rapid CO2 gas exchange by Helianthus leaves was analysed kinetically using a computer model which distinguished different components of the gas exchange by different time constants. A rapid phase of CO2 uptake was ascribed to the solubilization of CO2 in all leaf compartments and to the conversion of the dissolved CO2 to HCO3− in the chloroplast stroma which contains carbonic anhydrase. From stromal ratios the stroma pH of darkened leaves was estimated to be close to 7.5. Occasionally, values as high as 8 or as low as 7 were also obtained. If fast HCO3− formation also occurs in the cytosol, pH values may be lower by about 0.3 pH units than those calculated under the assumption that carbonic anhydrase is localized in chloroplasts only. Illumination with a light intensity close to saturation of photosynthesis caused an increase in CO2 solubilization which indicated the alkalization of the chloroplast stroma by about 0.6 pH units. This is an underestimation, if the pH of cytosol decreases in the light liberating CO2 by the action of carbon anhydrase. An alkalization of the stroma by 0.6 pH units indicates the export of about 450 nmol H+/mg chlorophyll from the stroma. This forms the basis of a large transthylakoid pH gradient which drives light-dependent ATP synthesis. A pH gradient between stroma and cytosol is capable of supporting secondary gradients between these compartments in the light, such as a gradient in the ratio. On darkening, the stroma alkalization was reversed. The rate of stroma acidification was much higher in the presence of CO2 than in its absence. 相似文献
4.
The respiratory requirements of Plasmodium falciparum were studied in vitro in continuous cultivation. The cultures were held in petri dishes containing the parasites incubated in different gas mixtures for periods of 72 to 144 hr with daily media changes. Atmospheres were combinations of 0.5 to 21% O2 mixed with 1 to 5% CO2 diluted with N2. Gas concentrations and the pH of media were measured with an analyzer. Best growth was realized in all cases at 3% O2 and 1 to 2% CO2. The culture appeared to be selfperpetuating in O2 concentrations as low as 0.5% providing the CO2 was not over 2%. Oxygen concentrations of 21% proved deleterious to growth. The parasite however, failed to grow in the highly reducing atmosphere of anaerobic “Brewer Jars,” suggesting that P. falciparum is an obligate microaerophile. 相似文献
5.
The dd-carboxypeptidase-endopeptidase of low penicillin sensitivity that is excreted by Streptomyces albus G has been crystallized from a polyethylene glycol (Mr 6000 to 7500) solution at pH 8.0. X-ray examination of the prismatic crystals shows that the space group is P21 with unit cell dimensions and one molecule in the asymmetric unit. A crystal suspension made in 50 mm-Tris · HCl buffer (pH 8.0) supplemented with 5 mm-MgCl2 and 16% () polyethylene glycol exhibits enzyme activity on the substrate Ac2-l-Lys-d-Ala-d-Ala. 相似文献
6.
(1) In isolated chloroplasts (class B) electron flow is controlled mainly by the intrathylakoid pH (pHin). A decrease in pHin due to the light-driven injection of protons inside the thylakoid leads to the retardation of electron flow between two photosystems. This effect can be abolished by uncouplers or under photophosphorylation conditions (addition of Mg2+-ADP with Pi); Mg2+-ATP does not influence the steady-state rate of electron flow, (2) The steady-state pH difference, ΔpH, across the thylakoid membrane was estimated from quantitative analysis of the rate of P-700+ reduction. In chloroplasts, without adding Mg2+-ADP, ΔpH increases from 1.6 to 3.2 as the external pH rises from 6 to 9.5. Under the photophosphorylation conditions, ΔpH decreases showing a minimum at the external pH 7.5 (). (3) The value of photosynthetic control, K, measured as the ratio of the steady-state rates of P-700+ reduction in the presence of Mg2+-ADP (with Pi) and without adding Mg2+-ADP is dependent on external pH variations, showing a maximum value of at pHout 7.5. This pH dependence coincides with that of the ADP-stimulated ΔpH decrease. (4) Experiments with spin labels provide evidence that the light-induced changes in the thylakoid membrane are sensitive to the addition of uncouplers and are affected only slightly by the addition of Mg2+-ADP and Pi. 相似文献
7.
D N Silverman 《Archives of biochemistry and biophysics》1973,155(2):452-457
Oxygen-18 exchange techniques were applied to the dehydration of bicarbonate catalyzed by human carbonic anhydrase C. The rates of depletion of oxygen-18 from labeled bicarbonate were measured for both the catalyzed and uncatalyzed reactions at pH 9.4 and 25 °C. The equilibrium dissociation constant of the enzyme-substrate complex K is 0.321 ± 0.040 m and is (8.3 ± 1.9) × 105m?1 sec?1 under these conditions. On the basis of these results it is demonstrated that the oxygen-18 exchange technique is capable of measuring K and kenz for the carbonic anhydrase catalyzed dehydration of bicarbonate at a high pH range in which other kinetic techniques are not effective.It was also shown that the oxygen-18 exchange technique is an effective micromethod for the determination of carbonic anhydrase. Rates of isotopic depletion of labeled bicarbonate (in solutions of the enzyme) which fall outside the limits of error for the uncatalyzed rate of depletion demonstrate that this technique can detect concentrations of human carbonic anhydrase C as low as 5 × 10?11m. 相似文献
8.
Crystals of calotropin DI (Mr 23,400), have been prepared by microdialysis against 5% (w/v) polyethylene glycol 20,000 in water, pH 7.0. They have orthorhombic space group P212121 with cell parameters . Crystals of calotropin DII (Mr 24,000), prepared by the same technique against 5% (w/v) polyethylene glycol 20,000 in phosphate buffer of low ionic strength, pH 7.0, display monoclinic space group C2 with cell parameters . In both cases, there is only one molecule in the asymmetric unit. 相似文献
9.
Alfred Jokumsen Hans Jørgen Fyhn 《Journal of experimental marine biology and ecology》1982,61(2):189-203
pH and PO2, in Mytilus edulis L. and Modiolus modiolus L. decrease, while PCO2 and the concentrations of Ca2+ and Cl? increase during air exposure. The increase in the concentration of Ca2+ is 10 times greater than that of Cl?. There is a linear correlation between the concentration of Ca2+ and the haemolymph PCO2.The significance of shell valve movements and of the buffering by the CaCO3 shell in response to air exposure is discussed. 相似文献
10.
Leaves of the C3 plants Brassica oleracea L., Datura suaveolens Humb. & Bonpl. ex Willd., Helianthus annuus L. and Nicotiana tabacum L. with open stomata were exposed in a leaf chamber in the dark to CO2 concentrations varying from 1 to 20% in air. When they were transferred back into CO2-free air, CO2 was rapidly released. It originated from dissolved CO2 and from the bicarbonate in the chloroplast stroma, since vacuoles are acidic and chloroplasts contain carbonic anhydrase which rapidly liberates CO2 from bicarbonate. The data were fitted to a model which accounts for the CO2/bicarbonate equilibrium in buffers with different CO2 concentrations and initial pH values. From this, pH values and CO2-dependent pH changes in the chloroplast stroma were calculated. The full range of external CO2 concentration caused acidic shifts up to 1 pH unit. The best fits of the data points were obtained with stromal buffer concentrations ranging from 45 to 65 mM and stromal pH values at low CO2 between 7.5 and 7.9. Calculated buffer capacities ranged from 23 to 31 mM H+ per pH unit. The work shows that measurements of solubilized CO2 are useful to investigate proton buffering and pH regulation in the chloroplast stroma of intact leaves.Abbreviation Chl
chlorophyll
This work was supported by Sonderforschungsbereich 251 of the University of Würzburg and the Volkswagenstiftung grant I-67762. We are very grateful to Dr. V. Oja for helpful advice. 相似文献
11.
The mechanism of anion inhibition of the reaction of the pork heart extramitochondrial aspartate aminotransferase (EC 2.6.1.1) with erythro-β-hydroxy-l-aspartate was investigated. This reaction produces a mixture of complexes, one of which is characterized by an absorption maximum at 492 nm. Spectrophotometric analysis of equilibrium mixtures of aspartate aminotransferase and erythro-β-hydroxy-l-aspartate, in different buffers, indicated that acetate, chloride, and cacodylate were competitive inhibitors of hydroxyaspartate binding. Pyrophosphate, however, was not a competitive inhibitor. Between pH 4.5 and 9.0 the affinity of the enzyme for the monovalent anions decreased as the pH increased. The data indicated that the anion binding group had a pKa in the range from pH 6 to 7, depending upon the anion studied. From pH 4.5 to 9.0, the substrate dissociation constant and the distribution of enzyme-substrate complexes were both unaffected by pH. By stopped-flow spectrophotometry, an initial rapid relaxation () was associated with an increase in absorbance at 492 nm, and this rate depended upon both substrate and buffer concentrations. A slower relaxation () was associated with a decrease in the absorbance at 492 nm to approximately 70% of the value attained in the first rapid reaction. The rate of this slower reaction was largely independent of substrate and buffer concentrations. Kinetic analysis of the rates of the first relaxation in several different concentrations of Tris-acetate buffer of pH 8 showed that the rate of association decreased with increasing acetate concentration whereas the reaction rate for dissociation was unaffected. Thus, acetate appears to exert its inhibitory effect by preventing the formation of the enzyme-substrate complex rather than by displacing the substrate from the enzyme. 相似文献
12.
Acid dissociation constants (Ka), usually expressed as pKa (-logKa) can be considered as indices of acid-base equilibria in solution and their evaluation under the solution conditions that exist during the exposure of biological systems to low temperatures are as important as the measurement of pH per se. The assignment of pH1 standards to define pH1 scales in the binary mixed solvent, dimethyl sulfoxide-water (27), has provided the basis for measuring the pKa1 values of some biological buffers in mixtures of Me2SO and H2O which have particular relevance to studies which demonstrate the “pH-dependent” recovery of smooth muscle after low-temperature storage (9, 31). “Practical” ionization conslants in water (pK′a) and in 20% () and 30% () dimethylsulfoxide-water (pKa1) have been measured by potentiometric titration of a range of zwitterionic buffer compounds at 25, 0, ?5.5, and ?12 °C together with the respective buffer capacities and temperature coefficients. Measurements have been made with reference to the relevant standard states for each solvent system, thereby endowing the values with as much thermodynamic significance as possible. 相似文献
13.
Yoshio Yamamoto Kazuo Torii Nakamura Yoichi Iitaka Yukio Mitsui Kaoru Miyamoto Hisayuki Matsuo Kozo Narita Nobuo Yoshida 《Journal of molecular biology》1981,145(1):285-287
The crystals of ribonuclease St, the extracellular ribonuclease from Streptomyces erythreus, have been obtained from (NH4)2SO4 solution with acetate buffer (pH 4.1). The crystals belong to a monoclinic space group C2 with dimensions . There are two protein molecules per asymmetric unit. The crystals diffract beyond 2.0 Å resolution. 相似文献
14.
Ca2+ transport was studied in membrane vesicles of alkalophilic Bacillus. When Na+-loaded membrane vesicles were suspended in KHCO3/KOH buffer (pH 10) containing Ca2+, rapid uptake of Ca2+ was observed. The apparent value for Ca2+ measured at pH 10 was about 7 μM, and the value shifted to 24 μM when measured at pH 7.4. The efflux of Ca2+ was studied with Ca2+-loaded vesicles. Ca2+ was released when Ca2+-loaded vesicles were suspended in medium containing 0.4 M Na+.Ca2+ was also transported in membrane vesicles driven by an artificial pH gradient and by a membrane potential generated by K+-valinomycin in the presence of Na+.These results indicate the presence of Ca2+/Na+ and H+/Na+ antiporters in the alkalophilic Bacillus A-007. 相似文献
15.
Metabolic acidosis produces a phosphaturia which is independent of parathyroid hormone or dietary phosphorus intake. To study the underlying mechanism, inorganic phosphate (Pi) and glucose transport were studied in brush-border membrane vesicles prepared from the renal cortex of parathyroidectomized rats gavaged for three days with either 7.5 ml of 1.6% NaCl (control) or 1.5% NH4Cl (acidosis). At killing, blood pH and plasma bicarbonate were and , respectively, in control and () and () in acidotic rats. Serum Pi was similar in both groups, while 24 h urine Pi excretion was higher in the acidotic group (). Peak sodium-dependent uptake of Pi, measured after 1.5 min of incubation, was higher in controls than acidotic rats ( vs. protein, ), whereas peak glucose uptake at 1.5 min was not significantly different between the groups. Equilibrium values for Pi and glucose uptake were similar in the two groups. for Pi uptake in the control and acidotic animals were not different, 0.036 and 0.040 mM, respectively. By contrast, was higher in controls than in the acidotic group, 3.13 vs. 1.15 nmol/mg protein per 15 s. These results suggest that metabolic acidosis directly inhibits Pi uptake by the brush border of the proximal tubule by decreasing the availability of Pi carriers of the renal brush-border membrane. 相似文献
16.
Kidney slices either were exposed to the cryoprotectants for 1 hr at room temperature and subsequently washed and incubated in fresh KR buffer containing only the radioactive metabolic tracers, or were immediately incubated for 2 hr at 37 °C in KR buffer containing the cryoprotectant and the tracers. Exposure to glycerol by incubation of kidney slices in Krebs-Ringer bicarbonate buffer containing varying concentrations of glycerol from 0 to 70% () resulted in a pronounced inhibitory effect on the protein synthesizing activity, while thymidine incorporation into DNA and the α-aminoisobutyric acid uptake through the cell membranes were less affected. Exposure of the tissue to buffer containing dimethylsulfoxide (Me2SO) in concentrations of 10 to 20% () resulted in a stimulatory effect on metabolism. At higher concentrations, Me2SO was toxic resulting in damaging effects on protein and DNA synthesis as well as on membrane integrity. The stimulatory effects of exposure to low concentrations of Me2SO on protein and DNA synthesis in kidney slices were concluded to be the result of an increased transport of precursors through the cell membranes. 相似文献
17.
Toyonari Sugimoto Toshio Kokubo Yasuo Matsumura Jinsei Miyazaki Shigeo Tanimoto Masaya Okano 《Bioorganic chemistry》1981,10(1):104-113
The asymmetric reduction of alkyl aryl ketones with sodium borohydride (NaBH4) or sodium cyanoborohydride (NaBH3CN) in the presence of bovine serum albumin (BSA) was studied. The asymmetric induction in the product alcohols was largely dependent on the BSA concentration and the pH value of the buffer solution. The maximum optical yields obtained ranged over 14–78%, varying with the alkyl and aryl substituents, using ca. molar eq of BSA to the ketone and at pH > 9. With two exceptions, the predominant enantiomer had the R configuration in 13 ketones investigated. 相似文献
18.
The tetrasaccharide fraction obtained by gel chromatography after treatment of commercially available heparin with nitrous acid was reduced with NaB3H4 and then hydrolysed with 2m trifluoracetic acid at 70° for 3 days. By gel chromatography and electrophoresis, the 3H-labelled trisaccharide 1 bearing an unsubstituted 2-amino-2-deoxy-d-glucosyl group in the non-reducing position was obtained (18% from the 3H-labelled tetrasaccharide). By sequential, enzymic degradation, the structure was obtained for 1, which is a substrate for acetyl-CoA: 2-amino-2-deoxy-α-d-glucoside N-acetyltransferase, an enzyme that is deficient in the Sanfilippo C syndrome. In human-skin fibroblasts, the pH optimum of acetyl transfer onto 1 was between pH 5.5 and 7.0, and dependent on the buffer. An apparent Km for 1 of 0.14mM was found. 相似文献
19.
Beta-lytic protease from Myxobacter 495 was crystallized by dialysis with 1 m-sodium chloride and 0.1 m-sodium citrate (pH 5.95). The crystals were rhombic prisms with space group P212121 and unit cell parameters a = 54.1, b = 99.6 and . Considerations of the cell volume and molecular weight indicate two molecules of beta-lytic protease in the asymmetric unit. 相似文献
20.
Tonin is an enzyme isolated from the submaxillary glands of old rats. This enzyme is involved in the direct conversion of angiotensinogen or the tetradecapeptide renin substrate to the octapeptide angiotensin II, the agent implicated in hypertension. Large well-formed crystals of tonin were grown from 2.5 m-(NH4)2SO4, 0.01 m-phosphate buffer (pH 6.2) and 1% methylpentanediol. The crystals are trigonal, space group P3121 (P3221) with unit cell dimensions: . The value of corresponds to one molecule of molecular weight 32,000 per asymmetric unit. Definitive classification of tonin into one of the four proteolytic enzyme classes (carboxyl protease, serine protease, thiol protease or metalloprotease) has not been firmly established. 相似文献