共查询到20条相似文献,搜索用时 31 毫秒
1.
Energy-dependent concentrative uptake of 14CH 3NH 3+ by cells of provides preliminary evidence for one or more transport systems for NH 4+ uptake. NH 4+, but not glutamic acid, inhibited the uptake of 14CH 3NH 3+. Varying the pH for the uptake assays exposed two apparent systems: one maximally functioning at pH 7 that was strongly inhibited by cyanide or by the uncoupler -chlorophenyl carbonylcyanide hydrazone and another maximally functioning at pH 9 and resistant to cyanide or -chlorophenyl carbonylcyanide hydrazone. Kinetic analysis showed considerable experimental variability from day to day. Often simple Michaelis-Menten kinetics were not followed, but NH 4+ was reproducibly a stronger inhibitor of uptake of 14CH 3NH 3+ than was nonradioactive CH 3NH 3+. 相似文献
2.
The experimental steady-state current-voltage relations for low concentrations of a neutral carrier and an ion may be fitted theoretically either by assuming a form for the potential dependence of the rate of transfer of complex across the membrane and adjusting the proposed nature of the association-dissociation reactions or by assuming equilibrium for the association and adjusting the potential dependence of the transfer process. Different dependences for the rate of transfer correspond, at least formally, to different shapes for the potential energy barrier which the complex must cross. By comparing measurements of the current-voltage relations for non-actin with Na +, K +, and NH 4+, it is possible to distinguish between the effeects of the various rates. For black lipid membranes made from glycerolmonooleate+ , the potential energy barrier is high with a narrow top, but the rate of association still becomes increasingly limiting for Na +, K + and NH 4+, in the order given. For bacterial phosphatidylethanolamine, with the barrier is much wider and no effect of the rate of association can be detected. 相似文献
3.
ADP is a competitive inhibitor with respect to ATP for pyruvate dehydrogenase kinase. Evidence is presented that K + or NH 4+ ions are required for inhibition of the kinase by ADP. K + at 30–90 mM and NH 4+ at 1–5 mM decrease markedly the apparent K i of bovine kidney pyruvate dehydrogenase kinase for ADP and also decrease, to a lesser extent, the apparent K m for ATP. Na + is less effective and, in addition, inhibits kinase activity. Since K + and NH 4+ are not required for kinase activity, their effect appears to be primarily of regulatory significance. K + and NH 4+ have little effect, if any, on pyruvate dehydrogenase phosphatase activity. When both the kinase and the phosphatase are present and functional, the near steady state activity of the pyruvate dehydrogenase complex is affected significantly by varying the concentration of K + or NH 4+ at a fixed ADP/ATP concentration ratio and by varying the ratio at a fixed concentration of monovalent cation. 相似文献
4.
50S ribosomal subunits show in the absence of 30S subunits and at low NH 4+ or K + high turnover activity in EF-G-dependent GTP hydrolysis which is inhibited by increasing concentrations of monovalent cations. At 80 mM NH 4+ or K + this activity is already 70–80% inhibited. This effect is reversed by 30S which are stimulatory with an optimum at about 80 mM for NH 4+ and 20–40 mM for K +. At low NH 4+ or K + (<5 mM) stimulation by 30S of maximal 50S activity depends on the [EF-G]/[50S]. Unlike EF-G, EF-T does not show any Phe-tRNA-dependent GTPase activity with 50S alone even at low concentrations of NH 4+ or K +. 相似文献
5.
Mutations which inactivate the NADP-glutamate dehydrogenase (anabolic GDHase) pleiotropically release the ammonia inhibition (NH 4+ effect) on a number of distinct catabolic activities. In addition to releasing inhibition on several permeability functions (1), these mutations suppress the NH 4+ effect on the synthesis of arginase, urea amidolyase and allantoinase. They do not affect the NH 4+ effect on the NAD-glutamate dehydrogenase.Two mechanisms of action of these mutations have to be considered, namely a modification of the process of (such as removal of inducer exclusion) and a suppression of . 相似文献
6.
A model of morphogenetic regulation in (1) is based on the assumption that NH 3 inhibits the synthesis and/or release of extracellular 3′,5′-cyclic AMP and that by topographical restriction of c-AMP production to specified zones within the cell aggregate, NH 3 is presumed to set up the conditions for apical dominance and directed morphogenetic movements. This study indicates that: exposure of preaggregative cells to exogenous NH 3 + NH 4+ inhibits the acquisition of c-AMP-induced properties associated with aggregation competence (accumulation of specific contact sites required to form EDTA resistant aggregates and the synthesis of extracellular and membrane-bound c-AMP phosphodiesterase); exposure of aggregation competent cells which are actively producing extracellular c-AMP to exogenous NH 3 + NH 4+ is followed by the immediate cessation of extracellular c-AMP release. The pH dependence of these effects suggests that the active species is NH 3. 相似文献
7.
When parsley [2Fe-2S] and 2[4Fe-4S] proteins in the normal oxidised state are reduced 1:1 with Cr(II) (15-aneN 4) (H 2O) 22+ the Cr(III) product remains attached to the protein and reduction is by an inner-sphere mechanism. With high potential [4Fe-4S] protein and rubredoxin the Cr(III) product is not attached to the protein and the mechanism is outer-sphere. Results are discussed in the context of protein crystallographic information. The Cr(III) product is not attached to the Fe 2S 2 core (extrusion experiments) or to the cysteinyl S-atoms (ESR). Negative patches close to the active site remain possible alternatives. 相似文献
8.
A new tetraamine was detected in the seed of sword bean and named canavalmine. The chemical structure was determined to be NH 2 (CH 2) 4NH(CH 2) 3NH(CH 2) 4NH 2 (1,13-diamino-5,9-diazatridecane) based on gas chromatography-mass spectrometry after derivatization of polyamines with pentafluoropropionic anhydride. The proof of identity was established by comparison of infrared and 1H-NMR spectra of the tetraamine isolated from sword bean with those of a synthetic compound. 相似文献
9.
NH 4
+ transport system of a psychrophilic marine bacterium Vibrio sp. strain ABE-1 ( Vibrio ABE-1) was examined by measuring the uptake of [ 14C]methylammonium ion ( 14CH 3NH 3
+) into the intact cells. 14CH 3NH 3
+ uptake was detected in cells grown in medium containing glutamate as the sole nitrogen source, but not in those grown in
medium containing NH 4Cl instead of glutamate. Vibrio ABE-1 did not utilize CH 3NH 3
+ as a carbon or nitrogen source. NH 4Cl and nonradiolabeled CH 3NH 3
+ completely inhibited 14CH 3NH 3
+ uptake. These results indicate that 14CH 3NH 3
+ uptake in this bacterium is mediated via an NH 4
+ transport system and not by a specific carrier for CH 3NH 3
+. The respiratory substrate succinate was required to drive 14CH 3NH 3
+ uptake and the uptake was completely inhibited by KCN, indicating that the uptake was energy dependent. The electrochemical
potentials of H + and/or Na + across membranes were suggested to be the driving forces for the transport system because the ionophores carbonylcyanide
m-chlorophenylhydrazone and monensin strongly inhibited uptake activities at pH 6.5 and 8.5, respectively. Furthermore, KCl
activated 14CH 3NH 3
+ uptake. The 14CH 3NH 3
+ uptake activity of Vibrio ABE-1 was markedly high at temperatures between 0° and 15°C, and the apparent K
m value for CH 3NH 3
+ of the uptake did not change significantly over the temperature range from 0° to 25°C. Thus, the NH 4
+ transport system of this bacterium was highly active at low temperatures.
Received: August 1, 1998 / Accepted: October 8, 1998 相似文献
10.
The additional activation by monovalent cations of the ATPase (ATP phosphohydrolase, EC 3.6.1.3) in human erythrocyte membranes was studied.The Ca 2+-ATPase occurs in two different states. In the A-state the enzyme is virtually free of protein activator and the kinetics of Ca 2+ activation is characterized by low apparent Ca 2+ affinity and low maximum activity. In the B-state the enzyme is associated with activator and the kinetics is characterized by high Ca 2+ affinity and high maximum activity.At optimum concentrations of Ca 2+ the additional activation of the B-state by K +, NH 4+, Na + and Rb + exceeded the corresponding activations of the A-state, and half-maximum activations by K +, NH 4+, and Na + were achieved at lower concentrations in the B-state than in the A-state. Li + and Cs + activated the two states almost equally but maximum activation was obtained at lower cation concentrations in the B-state than in the A-state.The activation of the B-state by the various cations decreased in the order . The A-state was activated almost equally by K +, Na +, NH 4+, and Rb + and to a smaller extent by Li + and Cs +.At sub-optimum concentrations of Ca 2+ high concentrations of monovalent cations (100 mM) activated the Ca 2+-ATPase equally in the A-state and the B-state. In the absence of Ca 2+ the monovalent cations inhibited the Mg 2+-dependent ATPase in both types of membranes. This dependence on Ca 2+ indicates that the monovalent cations interact with the Ca 2+ sites in the B-state.The results suggest that K + or Na +, or both, contribute to the regulation of the Ca 2+ pump in erythrocytes. 相似文献
11.
A mutation, 19, which leads to inability to grow on glutamate as the sole nitrogen source but does not affect growth on glutamate as the sole source of carbon and nitrogen, is shown to result in increased repression of glutamate uptake by glucose. An allelic mutation, 102, results in insensitivity to glucose repression. Glutamate uptake is still sensitive to NH 4+ repression in the presence of glucose in these strains. Starvation for a carbon source leads to relief of NH 4+ repression. 相似文献
12.
Yeast phosphofructokinase (PFK) exists in two forms, an ATP-sensitive form, PFKs, and a desensitized form, PFKd(MgF +). PFKs exhibits sigmoidal kinetics with respect to Fru-6- P, whereby the S0.5, Fru-6-P is determined by [ATP]. This form of PFK is inhibited by ATP and citrate and allosterically activated by Fru-6- P and AMP. NH4+ activates PFKs and enhances its affinity for substrate Fru-6- P (1–3).PFKd(MgF +) in contrast is not inhibited by ATP and citrate, nor activated by Fru-6- P and AMP. Kinetics of the reaction with PFKd(MgF +) with respect to Fru6- P are hyperbolic, with of S0.5, Fm-6-P for PFKs. NH4+ strongly activates this form.In terms of the model of Monod et al. (4), PFKd(MgF +) corresponds to a fixed R-conformation, while PFKs is a limiting T-conformation. 相似文献
13.
Frozen aqueous suspensions of partially purified membrane-bound renal (Na + + K +)-ATPase have been irradiated at –135°C with high-energy electrons. (Na + + K +)-ATPase and K +-phosphatase activities are inactivated exponentially with apparent target sizes of , respectively. These values are significantly lower then found previously from irradiation of lyophilized membranes. After reconstitution of irradiated (Na + + K +)-ATPase into phospholipid vesicles the following transport functions have been measured and target sizes calculated from the exponential inactivation curves: ATP-dependent Na +?K + exchange, ; (ATP + P i)-activated Rb +?Rb + exchange, and ATP-independent Rb +?Rb + exchange, . The apparent size of the α-chain, judged by disappearance of Coomassie stain on SDS-gels, lies between 115 and 141 kDa. That for the β-glycoprotein, though clearly smaller, could not be estimated. We draw the following conclusions: (1) The simplest interpretation of the results is that the minimal functional unit for (Na + + K +)-ATPase is αβ. (2) The inactivation target size for (Na + + K +)-dependent ATP hydrolysis is the same as for ATP-dependent pumping of Na + and K +. (3) The target sizes, for K +-phosphatase (125 kDa) and ATP-independent Rb +?Rb + exchange (117 kDa) are indistinguishable from that of the α-chain itself, suggesting that cation binding sites and transport pathways, and the phosphate binding site are located exclusively on the α-chain. (4) ATP-dependent activities appear to depend on the integrity of an αβ complex. 相似文献
14.
Methods are described for purification of a vesicular membrane fraction of hog gastric mucosa using differential centrifugation, density gradient separation on zonal rotors and free-flow electrophoresis. As a result a fraction is obtained enriched 40-fold in terms of K +-ATPase and free of any other enzyme marker other than K +-activated phosphatase.the 5′-nucleotidase and basal Mg 2+-ATPase are clearly separated from the latter enzymes.Osmotic shock, Triton X-100 treatment or K + ionophores increased the K +-ATPase activity in isotonic conditions, but phosphatase is not affected by these treatments, nor is the ATPase activity in the presence of NH 4+. The results suggest that the electrophoretic fraction contains a major population of tight vesicles, whose permeability to K + is rate limiting for the ATPase activity but not for the phosphatase activity. It is concluded that K + site for the ATPase is internal whereas the K + site for the phosphatase is external, hence, the K + site must be mobile across the membrane. 相似文献
15.
Previous data in Egeria densa leaves demonstrated a strong inhibitory effect of Cs + on passive K + influx and on K +-induced, ATP-dependent electrogenic proton extrusion. In this paper we analyzed, using the same material, the effects of Cs + on ammonium (NH 4+) and methylammonium (CH 3NH 3+) transport in order to elucidate whether a common transport system for K + and NH 4+ could be demonstrated. The effects of Cs + on NH 4+- and CH 3NH 3+-induced titratable H + extrusion (–ΔH +) and on transmembrane electrical potential difference (E m) in E. densa leaves were analyzed in parallel. All experiments were run either in the absence or presence of fusicoccin, corresponding to low or high H +-ATPase activity and membrane hyperpolarization and leading, in this material, to respectively active or passive transport of K +. The results suggest the presence in E. densa leaves of two distinct pathways for NH 4+ uptake: one in common with NH 4+ and (with lower affinity) CH 3NH 3+, insensitive to Cs +, and a second system, operating at higher H +-ATPase activity and E m hyperpolarization, strongly inhibited by Cs + and impermeable to CH 3NH 3+. In agreement with this hypothesis, Xenopus laevis oocytes injected with the KAT1 RNA of Arabidopsis thaliana were permeable to K + and NH 4+, but not to CH 3NH 3+. 相似文献
16.
Hydrophobic anions of dipicrylamine and of sodium tetraphenylborate have been employed as probes of interfacial dipole potential variations in lipid bilayer membranes. Systematic variation of dipole potentials has been achieved by introduction of compounds incorporating N + and B ? charge centers. Distribution of hydrophilic and and hydrophobic groups relative to these charge centers has been shown to control the orientation in the membrane/solution interface of the electric dipole moment formed by these centers. Thus triphenyl-[4-trimethylphenylammonium] borate orients with the B ? center, surrounded by phenyl groups, embedded in the membrane, while the smaller methylated N + center is directed toward the aqueous phases. This orientation has been confirmed using dipicrylamine probe ions. Results obtained in this system have been interpreted quantitatively using a previously developed model incorporating discrete charge effects. A second class of compounds, borane (T nAB) complexes having the generic formula (C nH ) 3N +B ?H 3, have also been synthesized for this study, using even-carbon alkyls ranging from ethyl to decyl. Molecular orientation of the complex is with the N + center and its associated alkyl groups directed into the membranes, while the protonated B ? center is directed toward the aqueous phases, as confirmed by use of tetraphenylborate ions as probes. 相似文献
17.
Inhibitors of glutamine synthetase cause derepression of nitrogenase biosynthesis in the presence of in the photosynthetic bacterium Rhodopseudomonas capsulata. A new derepressor of nitrogenase biosynthesis, β-N-oxalyl-L-α,β-diaminopropionic acid (ODAP), is here compared with the widely used L-methionine-DL-sulfoximine (MSX). With both compounds, a quantitative correlation has been observed between inhibition of glutamine synthetase and derepression of nitrogenase biosynthesis. We also find that both MSX and ODAP inhibit nitrogenase activity in vivo in R. capsulata. The latter effect seems to be indirect and related to the previously reported reversible inhibition of nitrogenase activity in vivo by . As a control it was observed that neither nor MSX nor ODAP inhibit nitrogenase activity in vivo in Clostridium pasteurianum. 相似文献
18.
Metabolite profiling is commonly performed by GC–MS of methoximated trimethylsilyl derivatives. The popularity of this technique owes much to the robust, library searchable spectra produced by electron ionization (EI). However, due to extensive fragmentation, EI spectra of trimethylsilyl derivatives are commonly dominated by trimethylsilyl fragments (e.g. m/z 73 and 147) and higher m/z fragment ions with structural information are at low abundance. Consequently different metabolites can have similar EI spectra, and this presents problems for identification of “unknowns” and the detection and deconvolution of overlapping peaks. The aim of this work is to explore use of positive chemical ionization (CI) as an adjunct to EI for GC–MS metabolite profiling. Two reagent gases differing in proton affinity (CH4 and NH3) were used to analyse 111 metabolite standards and extracts from plant samples. NH3-CI mass spectra were simple and generally dominated by [MH]+ and/or the adduct [M+NH4]+. For the 111 metabolite standards, m/z 73 and 147 were less than 3% of basepeak in NH3-CI and less than 30% of basepeak in CH4-CI. With CH4-CI, [MH]+ was generally present but at lower relative abundance than for NH3-CI. CH4-CI spectra were commonly dominated by losses of CH4 [M+1-16]+, 1–3 TMSOH [M+1-nx90]+, and combinations of CH4 and TMSOH losses [M+1-nx90-16]+. CH4-CI and NH3-CI mass spectra are presented for 111 common metabolites, and CI is used with real samples to help identify overlapping peaks and aid identification via determination of the pseudomolecular ion with NH3-CI and structural information with CH4-CI. 相似文献
19.
Metabolite profiling is commonly performed by GC–MS of methoximated trimethylsilyl derivatives. The popularity of this technique owes much to the robust, library searchable spectra produced by electron ionization (EI). However, due to extensive fragmentation, EI spectra of trimethylsilyl derivatives are commonly dominated by trimethylsilyl fragments (e.g. m/ z 73 and 147) and higher m/ z fragment ions with structural information are at low abundance. Consequently different metabolites can have similar EI spectra, and this presents problems for identification of “unknowns” and the detection and deconvolution of overlapping peaks. The aim of this work is to explore use of positive chemical ionization (CI) as an adjunct to EI for GC–MS metabolite profiling. Two reagent gases differing in proton affinity (CH 4 and NH 3) were used to analyse 111 metabolite standards and extracts from plant samples. NH 3-CI mass spectra were simple and generally dominated by [MH] + and/or the adduct [M+NH 4] +. For the 111 metabolite standards, m/ z 73 and 147 were less than 3% of basepeak in NH 3-CI and less than 30% of basepeak in CH 4-CI. With CH 4-CI, [MH] + was generally present but at lower relative abundance than for NH 3-CI. CH 4-CI spectra were commonly dominated by losses of CH 4 [M+1-16] +, 1–3 TMSOH [M+1- nx90] +, and combinations of CH 4 and TMSOH losses [M+1- nx90-16] +. CH 4-CI and NH 3-CI mass spectra are presented for 111 common metabolites, and CI is used with real samples to help identify overlapping peaks and aid identification via determination of the pseudomolecular ion with NH 3-CI and structural information with CH 4-CI. 相似文献
|