共查询到20条相似文献,搜索用时 593 毫秒
1.
The formation of nitrite reductase and cytochrome c in Micrococcus denitrificans was repressed by O 2. The purified nitrite reductase utilized reduced forms of cytochrome c, phenazine methosulphate, benzyl viologen and methyl viologen, respectively, as electron donors. The enzyme was inhibited by KCN, NaN 3 and NH 2OH each at 1 mM, whereas CO and bathocuproin, diethyl dithiocarbamate, o-phenanthroline and ,'-dipyridyl at 1 mM concentrations were relatively ineffective. The purified enzyme contains cytochromes, probably of the c and a2 types, in one complex. A Km of 46 μM for NO 2− and a pH optimum of 6.7 were recorded for the enzyme. The molecular weight of the enzyme was estimated to be around 130000, and its anodic mobility was 6.8·10 −6 cm 2·sec −1·V −1 at pH 4.55. The most highly purified nitrite reductase still exhibited cytochrome c oxidase activity with a Km of 27 μM for O2. This activity was also inhibited by KCN, NaN3 and NH2OH and by NO2−. A constitutive cytochrome oxidase associated with membranes was also isolated from cells grown anaerobically with NO2−. It was inhibited by smaller amounts of KCN, NaN3 and NH2OH than the cytochrome oxidase activity of the nitrite reductase enzyme and also differed in having a pH optimum of about 8 and a Km for O2 of less than 0.1 μM. Spectroscopically, cytochromes b and c were found to be associated with the constitutive oxidase in the particulate preparation. Its activity was also inhibited by NO2−. The physiological role of the cytochrome oxidase activity associated with the purified nitrite reductase is likely to be of secondary importance for the following reasons: (a) it accounts for less than 10% of total cytochrome c oxidase activity of cell extracts; (b) the constitutive cytochrome c oxidase has a smaller Km for O2 and would therefore be expected to function more efficiently especially at low concentrations of O2. 相似文献
2.
A novel nutrient removal/waste heat utilization process was simulated using semicontinuous cultures of the thermophilic cyanobacterium Fischerella. Dissolved inorganic carbon (DIC)-enriched cultures, maintained with 10 mg l −1 daily productivity, diurnally varying temperature (from 55°C to 26–28°C), a 12:12 light cycle (200 μE sec −1 m −2) and 50% biomass recycling into heated effluent at the beginning of each light period, removed > 95% of NO 3− + NO 2−−N, 71% of NH 3-N, 82% of PO 43− −P, and 70% of total P from effluent water samples containing approximately 400 μg l −1 combined N and 60 μg l −1 P. Nutrient removal was not severely impaired by an altered temperature gradient, doubled light intensity, or DIC limitation. Recycling 75% of the biomass at the end of each light period resulted in unimpaired NO 3− + NO 2− removal, 38–45% P removal and no net NH 3 removal. Diurnally varying P removal, averaging 50–60%, and nearly constant > 80% N removal, are therefore projected for a full-scale process with continuous biomass recycling. 相似文献
3.
A field study measured the rate of soil mineral N supply and its effects on plant biomass and N accumulation in a 13-year-old, naturally regenerating, calcareous grassland. Gross rates of N mineralisation (2 μg g −1 day −1, i.e. 0.69 kg ha −1 day −1), assessed using 15N pool dilution, were at the lower end of the range previously reported for grasslands. Weekly additions of liquid N fertiliser ([NH 4] 2SO 4, NH 4NO 3 or KNO 3) and, to a lesser extent the addition of water, increased plant growth substantially, demonstrating that the primary constraint to plant growth was low N availability. In plants that had received NO 3−, the activity of the inducible enzyme nitrate reductase in shoots initially increased in proportion to the amount of NO 3− supplied. However, as above-ground herbage accumulated, nitrate reductase activity declined to similar low levels in all treatments, despite the continuance of the constant NO 3− additions. The decline in NR specific activity reflected declining tissue NO 3− concentrations, although total plant NRA may have remained constant during the period of study. The study has shown that plant growth is limited by low N mineralisation rates and indeed the soil is a sink for much added N. Low water availability provides an additional constraint on N mineralisation in this calcareous grassland soil. Any disturbances in the N cycle which increase the availability of mineral N will result in a substantial increase in plant growth within this ecosystem. 相似文献
4.
Perchlorate (ClO 4–) is a major ground water pollutant of public health concern. ClO 4– reductase is the key enzyme in the pathway of ClO 4– breakdown. ClO 4– reductase from cell-free extracts of the ClO 4–-respiring bacterium perc1ace was purified 10-fold by ion-exchange and molecular exclusion fast protein liquid chromatography (FPLC). The ClO 4– reductase catalyzed the reduction of ClO 4– at a Vmax and Km of 4.8 U mg protein –1 and 34.5 M, respectively. ClO 4– reduction was achieved in the temperature range of 20 to 40C and with optimum activity at 25C to 30C and pH 7.5 to 8.0. Molecular masses of two subunits of ClO 4– reductase were determined by SDS-PAGE to be 35 kDa and 75 kDa. MALDI-TOF/MS analysis of a trypsin digest of the 35 kDa subunit, revealed several tryptic peptides. Amino acid sequences of 22 tryptic peptides of the 35 kDa ClO 4– reductase subunit were obtained by electrospray mass spectrometry. GenBank protein Blast analysis of the amino acid sequences revealed relevant similarity to reductases, dehydrogenases and heme proteins. Data obtained are useful towards the identification of the overall genetic determinants of ClO 4– reduction and specific in situ detection of ClO 4– as well as NO 3-reducing bacteria in ground water. 相似文献
5.
Nitrogen dioxide (NO 2•) is a key biological oxidant. It can be derived from peroxynitrite via the interaction of nitric oxide with superoxide, from nitrite with peroxidases, or from autoxidation of nitric oxide. In this study, submicromolar concentrations of NO 2• were generated in < 1 μs using pulse radiolysis, and the kinetics of scavenging NO 2• by glutathione, cysteine, or uric acid were monitored by spectrophotometry. The formation of the urate radical was observed directly, while the production of the oxidizing radical obtained on reaction of NO 2• with the thiols (the thiyl radical) was monitored via oxidation of 2,2′-azino-bis-(3-ethylthiazoline-6-sulfonic acid). At pH 7.4, rate constants for reaction of NO 2• with glutathione, cysteine, and urate were estimated as 2 × 10 7, 5 × 10 7, and 2 × 10 7 M −1 s −1, respectively. The variation of these rate constants with pH indicated that thiolate reacted much faster than undissociated thiol. The dissociation of urate also accelerated reaction with NO 2• at pH > 8. The thiyl radical from GSH reacted with urate with a rate constant of 3 × 10 7 M −1 s −1. The implications of these values are: (i) the lifetime of NO 2• in cytosol is < 10 μs; (ii) thiols are the dominant ‘sink’ for NO 2• in cells/tissue, whereas urate is also a major scavenger in plasma; (iii) the diffusion distance of NO 2• is 0.2 μm in the cytoplasm and < 0.8 μm in plasma; (iv) urate protects GSH against depletion on oxidative challenge from NO 2•; and (v) reactions between NO 2• and thiols/urate severely limit the likelihood of reaction of NO 2• with NO• to form N 2O 3 in the cytoplasm. 相似文献
6.
Laboratory-scale experiments were conducted to examine the N 2O emission during the denitrification process. For each of the 6 runs carried out, synthetic effluent was fed in a 10 l batch mixed liquor to investigate the effect of nitrite on N 2O emission and Helium was continuously bubbled through the reactor at constant rate (0.12 l/min) to favour N 2O transfer and detection. An increasing COD/NO 3−-N influent ratio from 3 to 7 was firstly applied (runs 1–3). Secondly, NO 2− pulse additions were performed during run 4 and 5 (10 and 20 mg N/l, respectively). Finally, the reactor was fed with influent containing both NO 2− and NO 3−. We showed that N 2O emission was detected shortly after NO 2− accumulation, few minutes after the substrate feeding. The highest emission occurred at the lower COD/NO 3−-N ratio (=3) and at the higher NO 2− addition (20 mg N/l). In addition, the higher nitrogen conversion to N 2O gas (14.4%) was obtained with an influent containing initially both NO 2− and NO 3−. Our results suggest a direct effect of the NO 2− concentration on the N 2O emission. We have also confirmed the inhibitory effect of NO 2− concentration on N 2O reduction. 相似文献
7.
Phaseolus vulgaris L. cv. Kinghorn Wax seedlings, supplied with nutrient solution containing either 0 or 5 mM nitrate as sole N source, were exposed to 0.25 μl/l NO 2 for 6 hr each day for 10 days at continuous photosynthetic photon flux (PPF) of 100, 300, 500 or 700 μmol m −2 sec −1. There was a significant interaction of PPF and nitrate. Shoot and root dry weights increased with increasing PPFs only when nitrate was supplied. The main effects of NO 2 on plant growth were significant; none of the interactions involving NO 2 were significant. Exposure to NO 2 decreased shoot and root dry weight in both the presence and absence of nutrient N and at all PPF levels. All interactions were significant for in vitro leaf nitrate reductase activity (NRA), which increased markedly at PPFs above 100 μmol m −2 sec −1 when nitrate was supplied. Treatment with NO 2 strongly inhibited enzyme activity in the presence of nitrate, particularly at the 300 μmol m −2 sec −1 PPF level. These experiments demonstrated that PPF level does not modify the effect of NO 2 on growth but does have a major effect on NRA and on NO 2 effects on NRA in the presence of nutrient nitrate. 相似文献
8.
1. Difference spectra, at room and liquid N 2 temperatures, of S 2O 42−-, and NO 2−-reduced intact cells and cell-free preparations of Nitrobacter agilis demonstrated the presence of cytochromes of the c- and a-types. Reduction of cytochromes by succinate, and to a limited extent, by NADPH also occurred, provided KCN (0.1 mM) was also present. 2. A particulate, heat-labile nitrite oxidase having an absolute requirement for O2 was prepared from N. agilis cells using sonic oscillation and differential centrifugation. The particles also possessed NADH oxidase, succinoxidase, formate oxidase and traces of NADPH oxidase activity. The stoichiometry of the nitrite oxidase reaction approached the theoretical value of 2 moles of NO2− consumed per mole of O2 consumed. The pH optimum of the nitrite oxidase system shifted to progressively more alkaline values as the NO2− concentration was increased, changing from a pH value of 6.8 at 0.6 mM KNO2 to pH 8.0 at 0.01 M KNO2 with apparent Km's of 0.2 and 1.2 mM NO2−, respectively. Computations of the HNO2 concentrations present under the above conditions showed an approx. 500-fold greater affinity for HNO2 which was independent of pH, suggesting the involvement of HNO2 as both a substrate and an inhibitor (at higher concentrations) of the nitrite oxidase system. The marked inhibition by NaN3, NaCN and Na2S, as well the light-reversible inhibition by CO, indicated the presence of cytochrome oxidase which was subsequently characterized. NO2− proved to be a competitive inhibitor of the nitrite oxidase system. 3. The particulate preparation also possessed a heat-labile nitrite-cytochrome c reductase activity which was energy independent and routinely measured under anaerobic conditions. As in the case of nitrite oxidase, the affinity of the enzyme for NO3− increased as the pH was lowered, but the pH optimum remained unaffected. In terms of calculated HNO2 concentration an approximately constant Km of about 0.2 μM was estimated at the several pH's examined. The inhibition by NO3− was shown to be competitive. The marked sensitivity of the reductase to several metal-binding agents implicated a metal component in the electron transport chain at the site prior to cytochrome c. 4. The membrane-like composition of the nitrite oxidase system is indicated. 相似文献
9.
Investment by bladderwort ( Utricularia foliosa) in carnivory, in terms of total C and N of bladders per leaf, was estimated in places with different nutrient concentrations from the Yahuarcaca Creek in the Colombian Amazon. The aims were to determine whether nutrient limiting conditions stimulate the investment in carnivory, and the relative balance between C and N invested in carnivory versus C and N obtained from prey. There were no significant differences either for phosphate (PO 43−) concentration or for ammonia (NH 4+) concentration among five sampling areas, along approximately 5 km long stretch of the creek, with a pooled mean ± S.D. of 0.19 ± 0.06 and 8.6 ± 3.0 μM, respectively. However, there were significant differences in the nitrate (NO 3−) concentration ranging from 0.6 to 2.5 μM. Total C and N of bladders per leaf increased with decreasing NO 3−. This corroborates the hypotheses that the carnivorous plant U. foliosa optimises its investment in carnivory according to nutrient availability in the water, and that N is a limiting factor that stimulates the investment in carnivory. The numbers of prey per bladder were also higher under NO 3− limitation, thus enhancing the input of nutrients toward the plant through the bladders. The ratio of total C of prey captured/total C invested in bladders was always lower than 1. However, the efficiency of N was higher since when NO 3− concentration was lower than 1 μM, the ratio of total N of prey captured/total N invested in bladders ranged between 0.97 and 1.67. 相似文献
10.
A procedure is described for preparing particles from cells of Micrococcus denitrificans which were broken osmotically after treatment with lysozyme. 1. 1. The preparations catalysed ATP synthesis coupled to O2 uptake or NO3− reduction. With NADH or succinate as the electron donors the P:O ratios were about 1.5 and 0.5, respectively; and the P:NO3− ratios were about 0.9 and 0.06, respectively. 2. 2. Addition of ADP or Pi to the reaction mixture increased the rates of NADH-dependent O2 uptake and NO3− reduction. Addition of 1 mM 2,4-dinitrophenol, which inhibited phosphorylation by 50–60%, increased the basal rates of electron transport. 3. 3. Evidence derived from spectrophotometry and from the differential inhibition by antimycin A of O2 and NO3− reduction leads to the conclusion that the nitrate reductase interacted with the respiratory chain in the region of the b-type cytochrome, and that the c-type cytochrome present was not involved in the reduction of NO3− to NO2−.
Abbreviations: TMPD; tetramethyl-p-phenylenediamine 相似文献
11.
Estimation of the ammonia production of the shrimp C. crangon in two littoral ecosystems (oligotrophic sand and eutrophic mud) was determined in winter and summer conditions from laboratory observations in experimental microcosms. The ammonia excretion rate of C. crangon was not influenced by either the sediment type or the ammonia concentration of the overlying water; on the other hand, the mean excretion rate and the response to initial handling stress increased markedly as shrimp were deprived of soft substratum. The daily ammonia production of C. crangon was 16 μmol NH3 · g −1 wet wt · day −1 in winter and 40 μmol in summer. A gross production of 12 μmol NH3 · m−2 · day −1 and 300–700 μmol μ m−2 · day−1, respectively, could be expected in the two ecosystems studied. This would account for 5% (winter) and 2–4% (summer) of the total NH+4 flux at the sediment-water interface. The contribution of the excretion of all macrofauna to the NH+4 flux from the sediment is discussed. 相似文献
12.
The rates of respiratory O 2 uptake have been studied in leaves, stems and whole shoots of several freshwater plants: 6 angiosperms, 2 bryophytes and one alga. For angiosperm leaves, rates varied widely with species (30–142 μmol O 2 (gDW) −1 h −1), were correlated with chlorophyll content and were higher than those of the stems (13–71 μmol O 2 (gDQ) −1 h −1). The rates for the shoots of bryophytes (53–66 μmol O 2 (gDW) −1 h −1) and for the alga Cladophora glomerata (L.) Kütz. (96 μmol O 2 (gDW) −1 h −1) were slightly higher than those of most angiosperm stems, but lower than those for most leaves. These plants had a significant cyanide-resistant respiration, suggesting the existence of an alternative pathway to the “classic” cytochrome system. This pathway was found to be active in all the species studied, as judged by responses to a specific inhibitor, SHAM (salicylhydroxamic acid). Measurement of electron-transport system (ETS) activity showed that there is a large electron-transport capacity which is not normally used by respiration in vivo. 相似文献
13.
Study on chlorate-resistants mutants of Escherichia coli K12. IV. Isolation, purification and study of nitrate-reductase restored in vitro by complementationBy mixing the cell-free extracts of the two mutants chl A− and chl B− of Escherichia coli K12, previously freed from particle membranes, we achieved restoration of nitrate reductase activity. The activity is restored first in a soluble form, then in a particulate form. This mechanism is called “complementation”. In the soluble state, the purified enzyme reduces NO3− and ClO3−, using reduced benzyl viologen or FMNH2 as electron donors. It is sensitive to KCN, NaN3, p-hydroxymercuribenzoate (1 mM) and N-ethylmaleimide (0.1 mM) The soluble form is sensitive neither to phospholipase C, nor to 2-n-heptyl-4-hydroxyquinoline-N-oxide; it associates with phospholipids and cytochrome b1 to form particles in which nitrate reductase activity is no longer sensitive to ethyl N-maleimide and p-hydroxymercuribenzoate, but, conversely, becomes sensitive to 2-n-heptyl-4-hydroxyquinoline-N-oxide. These results clearly demonstrate that it is possible to study the mechanism of integration of the enzyme leading to active membranes particles without any previous solubilisation of the original material. 相似文献
14.
Oxygenation of [Cu II(fla)(idpa)]ClO 4 (fla=flavonolate; IDPA=3,3′-iminobis( N, N-dimethylpropylamine)) in dimethylformamide gives [Cu II(idpa)( O-bs)]ClO 4 ( O-bs= O-benzoylsalicylate) and CO. The oxygenolysis of [Cu II(fla)(idpa)]ClO 4 in DMF was followed by electronic spectroscopy and the rate law −d[{Cu II(fla)(idpa)}ClO 4]/d t= kobs[{Cu II(fla)(idpa)}ClO 4][O 2] was obtained. The rate constant, activation enthalpy and entropy at 373 K are kobs=6.13±0.16×10 −3 M −1 s −1, Δ H‡=64±5 kJ mol −1, Δ S‡=−120±13 J mol −1 K −1, respectively. The reaction fits a Hammett linear free energy relationship and a higher electron density on copper gives faster oxygenation rates. The complex [Cu II(fla)(idpa)]ClO 4 has also been found to be a selective catalyst for the oxygenation of flavonol to the corresponding O-benzoylsalicylic acid and CO. The kinetics of the oxygenolysis in DMF was followed by electronic spectroscopy and the following rate law was obtained: −d[flaH]/d t= kobs[{Cu II(fla)(idpa)}ClO 4][O 2]. The rate constant, activation enthalpy and entropy at 403 K are kobs=4.22±0.15×10 −2 M −1 s −1, Δ H‡=71±6 kJ mol −1, Δ S‡=−97±15 J mol −1 K −1, respectively. 相似文献
15.
We have previously shown that crystals of calcium oxalate (COM) elicit a superoxide (O 2−) response from mitochondria. We have now investigated: (i) if other microparticles can elicit the same response, (ii) if processing of crystals is involved, and (iii) at what level of mitochondrial function oxalate acts. O 2− was measured in digitonin-permeabilized MDCK cells by lucigenin (10 μM) chemiluminescence. [ 14C]-COM dissociation was examined with or without EDTA and employing alternative chelators. Whereas mitochondrial O 2− in COM-treated cells was three- to fourfold enhanced compared to controls, other particulates (uric acid, zymosan, and latex beads) either did not increase O 2− or were much less effective (hydroxyapatite +50%, p < 0.01), with all at 28 μg/cm 2. Free oxalate (750 μM), at the level released from COM with EDTA (1 mM), increased O 2− (+50%, p < 0.01). Omitting EDTA abrogated this signal, which was restored completely by EGTA and partially by ascorbate, but not by desferrioxamine or citrate. Omission of phosphate abrogated O 2−, implicating phosphate-dependent mitochondrial dicarboxylate transport. COM caused a time-related increase in the mitochondrial membrane potential (Δψ m) measured using TMRM fluorescence and confocal microscopy. Application of COM to Fura 2-loaded cells induced rapid, large-amplitude cytosolic Ca 2+ transients, which were inhibited by thapsigargin, indicating that COM induces release of Ca 2+ from internal stores. Thus, COM-induced mitochondrial O 2− requires the release of free oxalate and contributes to a synergistic response. Intracellular dissociation of COM and the mitochondrial dicarboxylate transporter are important in O 2− production, which is probably regulated by Δψ m. 相似文献
16.
To determine whether centrally released vasopressin influences thirst, observations of osmotic thirst threshold, osmotic load excretion and postloading restitution of plasma osmolality were made in dogs in control experiments and during infusion of AVP antagonists into the third ventricle. Significant elevation of osmotic thirst threshold was elicited by infusion of d(CH 2) 5AVP at a rate of 0.2–2.0 μg·min −1 and of d(Et 2)AVP at a rate of 0.3 μg·min −1 (V 1 antagonists, weak V 2 agonists) as well as by administration of d(CH 2) 5[D-Ile 2,Abu 4]AVP at a rate of 0.4 μg·min −1 (potent V 2 antagonist, weak V 1 antagonist). Administration of d(CH 2) 5AVP at a rate of 2.0 μg·min −1 was associated with a significant suppression of the postloading water intake and osmotic load excretion and with a delay in restitution of plasma osmolality. These findings indicate that centrally released vasopressin may participate in the control of thirst. 相似文献
17.
The bioconversion of propionitrile to propionamide was catalysed by nitrile hydratase (NHase) using resting cells of Microbacterium imperiale CBS 498-74 (formerly, Brevibacterium imperiale). This microorganism, cultivated in a shake flask, at 28 °C, presented a specific NHase activity of 34.4 U mg DCW−1 (dry cell weight). The kinetic parameters, Km and Vmax, tested in 50 mM sodium phosphate buffer, pH 7.0, in the propionitrile bioconversion was evaluated in batch reactor at 10 °C and resulted 21.6 mM and 11.04 μmol min −1 mg DCW−1, respectively. The measured apparent activation energy, 25.54 kJ mol −1, indicated a partial control by mass transport, more likely through the cell wall. UF-membrane reactors were used for kinetic characterisation of the NHase catalysed reaction. The time dependence of enzyme deactivation on reaction temperature (from 5 to 25 °C), on substrate concentrations (from 100 to 800 mM), and on resting cell loading (from 1.5 to 200 μg
ml−1) indicated: lower diffusional control (Ea=37.73 kJ mol−1); and NHase irreversible damage caused by high substrate concentration. Finally, it is noteworthy that in an integral reactor continuously operating for 30 h, at 10 °C, 100% conversion of propionitrile (200 mM) was attained using 200 μg
ml−1 of resting cells, with a maximum volumetric productivity of 0.5 g l−1 h−1. 相似文献
18.
The growth of the freshwater microalga Scenedesmus obliquus was studied at 30°C in a mineral culture medium with phosphorus concentrations of between 0 and 372 μ
. The values for the specific growth rates, between
and
, fitted a semistructured substrate-limitation model with μ m1 = 0·0466 h −1, μ m2 = 0·0256 h −1 and
. The specific uptake rate of phosphorus reached a maximum value of qSm1 = 658·01 × 10 −4 μmol P mg −1 biomass h −1. 相似文献
19.
The diverse function of human placental aromatase including estradiol 6-hydroxylase and cocaine N-demethylase activity are described, and the mechanism for the simultaneous metabolism of estradiol to 2-hydroxy- and 6-hydroxyestradiol at the same active site of aromatase is postulated. Comparison of aromatase activity is also made among the wild type and N-terminal sequence deleted forms of human aromatase which are recombinantly expressed in Escherichia coli. Aromatase cytochrome P450 was reconstituted and incubated with [6,7- 3H 2,4- 14C]estradiol, 7-ethoxycoumarin, and [N-methyl- 3H 3]cocaine. 6-Hydroxy[7- 3H,4- 14C]estradiol was isolated as the metabolite of estradiol and the 3H-water release method based on the 6- 3H label was established. The initial rate kinetics of the 6-hydroxylation gave K m of 4.3 μM, V max of 4.02 nmol min −1mg −1, and turnover rate of 0.27 min −1. Testosterone competed dose-dependently with the 6-hydroxylation and showed the K i of 0.15 μM, suggesting that they occupy the same binding site of aromatase. The deethylation of 7-ethoxycoumarin showed K m of 200 μM, V max of 12.5 nmol min −1mg −1 and turnover rate of 1.06 min −1. The N-demethylation of cocaine was analysed by the 3H-release method, giving K m of 670 μM, V max of 4.76 nmol min −1mg −1, and turnover rate of 0.49 min −1. All activity was dose-responsively suppressed by anti-aromatase P450 monoclonal antibody MAb3-2C2. The N-terminal 38 amino acid residue deleted form of aromatase P450 was expressed in particularly high yield giving a specific activity of 397 ± 83 pmol min −1mg −1 ( n = 12) of crude membrane-bound particulates with a turnover rate of 2.6 min −1. 相似文献
20.
High-pressure liquid-chromatography and microcalorimetry have been used to determine equilibrium constants and enthalpies of reaction for the disproportionation reaction of adenosine 5′-diphosphate (ADP) to adenosine 5′-triphosphate (ATP) andadenosine 5′-monophosphate (AMP). Adenylate kinase was used to catalyze this reaction. The measurements were carried out over the temperature range 286 to 311 K, at ionic strengths varying from 0.06 to 0.33 mol kg −1, over the pH range 6.04 to 8.87, and over the pMg range 2.22 to 7.16, where pMg = -log a(Mg 2+). The equilibrium model developed by Goldberg and Tewari (see the previous paper in this issue) was used for the analysis of the measurements. Thus, for the reference reaction: 2 ADp 3− (ao) AMp 2− (ao)+ ATp − (ao), K° = 0.225 ± 0.010, Δ G° = 3.70 +- 0.11 kJ mol −1, Δ H° = −1.5 ± 1. 5 kJ mol −1, °S ° = −17 ± 5 J mol −1 K−1, and ACP p°≈ = −46 J mo1l −1 K−1 at 298.15 K and 0.1 MPa. These results and the thermodynamic parameters for the auxiliary equilibria in solution have been used to model the thermodynamics of the disproportionation reaction over a wide range of temperature, pH, ionic strength, and magnesium ion morality. Under approximately physiological conditions (311.15 K, pH 6.94, [Mg 2+] = 1.35 × 10 −3 mol kg −1, and I = 0.23 mol kg −1) the apparent equilibrium constant ( KA′ = m(ΣAMP) m(ΣATP)/[ m(ΣADP)] 2) for the overall disproportionation reaction is equal to 0.93 ± 0.02. Thermodynamic data on the disproportionation reaction and literature values for this apparent equilibrium constant in human red blood cells are used to calculate a morality of 1.94 × 10 −4 mol kg −1 for free magnesium ion in human red blood cells. The results are also discussed in relation to thermochemical cycles and compared with data on the hydrolysis of the guanosine phosphates. 相似文献
|