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1.
The Na+/l-glutamate (l-aspartate) cotransport system present at the level of rat intestinal brush-border membrane vesicles is specifically activated by the ions K+ and Cl?. The presence of 100 mM K+ inside the vesicles drastically enhances the uptake rate and the transient intravesicular accumulation (overshoot) of the two acidic amino acids. It has been demonstrated that the activation of the transport system depended only in the intravesicular K+ concentration and that in the absence of any sodium gradient, an outward K+ gradient was unable to influence the Na+/acidic amino acid transport system. It was also found that Cl? could specifically activate the Na+-dependent l-glutamate (l-aspartate) uptake either in the presence or in the absence of K+. Also the effect of Cl? was observed only in the presence of an inward Na+ gradient and it was noted to be higher when chloride ion was present on both sides of the membrane vesicles. No influence (activation or accumulation) was observed in the absence of the Na+ gradient and in the presence of chloride gradient. l-Glutamate uptake measured in the presence of an imposed diffusion potential and in the presence of K+ or Cl? did not show any translocation of net charge.  相似文献   

2.
The uptake of l-phenylalanine into brush border microvilli vesicles and basolateral plasma membrane vesicles isolated from rat kidney cortex by differential centrifugation and free flow electrophoresis was investigated using filtration techniques.Brush border microvilli but not basolateral plasma membrane vesicles take up l-phenylalanine by an Na+-dependent, saturable transport system. The apparent affinity of the transport system for l-phenylalanine is 6.1 mM at 100 mM Na+ and for Na+ 13 mM at 1 mM l-phenylalanine. Reduction of the Na+ concentration reduces the apparent affinity of the transport system for l-phenylalanine but does not alter the maximum velocity.In the presence of an electrochemical potential difference for Na+ across the membrane (ηNa0 >ηNa1) the brush border microvilli accumulate transiently l-phenylalanine over the concentration in the incubation medium (overshoot phenomenon). This overshoot and the initial rate of uptake are markedly increased when the intravesicular space is rendered electrically more negative by membrane diffusion potentials induced by the use of highly permeant anions, of valinomycin in the presence of an outwardly directed K+ gradient and of carbonyl cyanide p-trifluoromethoxyphenylhydrazone in the presence of an outward-directed proton gradient.These results indicate that the entry of l-phenylalanine across the brush border membrane into the proximal tubular epithelial cells involves cotransport with Na+ and is dependent on the concentration difference of the amino acid, on the concentration difference of Na+ and on the electrical potential difference. The exit of l-phenylalanine across the basolateral plasma membranes is Na+-independent and probably involves facilitated diffusion.  相似文献   

3.
In the presence of a Na+-gradient (out > in), l-glutamic acid and l-and d-aspartic acids were equally well concentrated inside the vesicles, while no transport above simple diffusion levels was seen by replacement of Na+ by K+. Equilibrium uptake values were found inversely proportional to the medium osmolarity, thus demonstrating uptake into an osmotically sensitive intravesicular space. The extrapolation of these lines to infinite medium osmolarity (zero space) showed only a small binding component in acidic amino-acid transport. When the same experiment was performed at saturating substrate concentrations, linear relationships extrapolating through the origin but showing smaller slope values were recorded, thus indicating that the binding component could be more important than suspected above. However, binding to the membrane was neglected in our studies as it was absent from initial rate measurements. Na+-dependent uphill transport of l-glutamic acid was stimulated by K+ present on the intravesicular side only but maximal stimulation was recorded under conditions of an outward K+-gradient (in > out). Quantitative and qualitative differences in the K+ effect were noted between pH 6.0 and 8.0. Initial uptake rates showed pH dependency in Na+-(out > in) + K+-(in > out) gradient conditions only with a physiological pH optimum between 7.0 and 7.5. It was also found that a pH-gradient (acidic outside) could stimulate both the Na+-gradient and the Na+ + K+-gradient-dependent transport of l-glutamic acid. However, pH- or K+-gradient alone were ineffective in stimulating uptake above simple diffusion level. Finally, it was found that increased rates of efflux were always observed with an acidic pH outside, whatever the conditions inside the vesicles. From these results, we propose a channel-type mechanism of l-glutamic acid transport in which Na+ and K+ effects are modulated by the surrounding pH. The model proposes a carrier with high or low affinity for Na+ in the protonated or unprotonated forms, respectively. We also propose that K+ binding occurs only to the unprotonated carrier and allows its fast recycling as compared to the free form of the carrier. Such a model would be maximally active and effective in the intestine in the in vivo physiological situations.  相似文献   

4.
The active transport of neutral amino acids into Streptomyces hydrogenans is inhibited by external Na+. There is no indication that in these cells amino acid accumulation is driven by an inward gradient of Na+. The extent of transport inhibition by Na+ depends on the nature of the amino acid. It decreases with increasing chain length of the amino acid molecules i.e. with increasing non-polar properties of the side chain. Kinetic studies show that Na+ competes with the amino acid for a binding site at the amino acid carrier. There is a close relation between the Ki values for Na+ and the number of C atoms of the amino acids. Other cations also inhibit neutral amino acid uptake competitively; the effectiveness decreases in the order Li+ > Na+ > K+ > Rb+ > Cs+. Anions do not have a significant effect on the uptake of neutral amino acids. After prolonged incubation of the cells with 150 mM Na+, in addition to the competitive inhibition of transport Na+ induces an increase in membrane permeability for amino acids.  相似文献   

5.
Na+-independent l-arginine uptake was studied in rabbit renal brush border membrane vesicles. The finding that steady-state uptake of l-arginine decreased with increasing extravesicular osmolality and the demonstration of accelerative exchange diffusion after preincubation of vesicles with l-arginine, but not d-arginine, indicated that the uptake of l-arginine in brush border vesicles was reflective of carrier-mediated transport into an intravesicular space. Accelerative exchange diffusion of l-arginine was demonstrated in vesicles preincubated with l-lysine and l-ornithine, but not l-alanine or l-proline, suggesting the presence of a dibasic amino acid transporter in the renal brush border membrane. Partial saturation of initial rates of l-arginine transport was found with extravesicular [arginine] varied from 0.005 to 1.0 mM. l-Arginine uptake was inhibited by extravesicular dibasic amino acids unlike the Na+-independent uptake of l-alanine, l-glutamate, glycine or l-proline in the presence of extravesicular amino acids of similar structure. l-Arginine uptake was increased by the imposition of an H+ gradient (intravesicular pH<extravesicular pH) and H+ gradient stimulated uptake was further increased by FCCP. These findings demonstrate membrane-potential-sensitive, Na+-independent transport of l-arginine in brush border membrane vesicles which differs from Na+-independent uptake of neutral and acidic amino acids. Na+-independent dibasic amino acid transport in membrane vesicles is likely reflective of Na+-independent transport of dibasic amino acids across the renal brush border membrane.  相似文献   

6.
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 Km value for Ca2+ measured at pH 10 was about 7 μM, and the Km 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   

7.
The uptake of d-glucose, 2-aminoisobutyric acid and glycine was studied with intestinal brush border membrane vesicles of a marine herbivorous fish: Boops salpa. The uptake of these three substances is stimulated by an Na+ electrochemical gradient (CoutCin). For glucose, an increase of the electrical membrane potential generated by a concentration gradient of the liposoluble anion, SCN?, increases the Na+-dependent transport. This responsiveness to the membrane potential was confirmed by valinomycin. Differently from glucose, uptake of glycine and 2-aminoisobutyric acid requires, besides the Na+ gradient, the presence of Cl? on the external side of the vesicles. In the absence of Cl?, amino acid uptake is not stimulated by the Na+ gradient and is not influenced by an electrical membrane potential generated by SCN? gradient (Cout>Cin) or by a K+ diffusion potential (Cin>Cout). This Cl? requirement differs from the Na+ requirement, since a Cl? gradient (Cout>Cin) does not result in an accumulation of glycine or 2-aminoisobutyric acid similar to that produced by an Na+ gradient.  相似文献   

8.
The effects of K+, Na+ and ATP on the gastric (H+ + K+)-ATPase were investigated at various pH. The enzyme was phosphorylated by ATP with a pseudo-first-order rate constant of 3650 min?1 at pH 7.4. This rate constant increased to a maximal value of about 7900 min?1 when pH was decreased to 6.0. Alkalinization decreased the rate constant. At pH 8.0 it was 1290 min?1. Additions of 5 mM K+ or Na+, did not change the rate constant at acidic pH, while at neutral or alkaline pH a decrease was observed. Dephosphorylation of phosphoenzyme in lyophilized vesicles was dependent on K+, but not on Na+. Alkaline pH increased the rate of dephosphorylation. K+ stimulated the ATPase and p-nitrophenylphosphatase activities. At high concentrations K+ was inhibitory. Below pH 7.0 Na+ had little or no effect on the ATPase and p-nitrophenylphosphatase, while at alkaline pH, Na+ inhibited both activities. The effect of extravesicular pH on transport of H+ was investigated. At pH 6.5 the apparent Km for ATP was 2.7 μM and increased little when K+ was added extravesicularly. At pH 7.5, millimolar concentrations of K+ increased the apparent Km for ATP. Extravesicular K+ and Na+ inhibited the transport of H+. The inhibition was strongest at alkaline pH and only slight at neutral or acidic pH, suggesting a competition between the alkali metal ions and hydrogen ions at a common binding site on the cytoplasmic side of the membrane. Two H+-producing reactions as possible candidates as physiological regulators of (H+ + K+)-ATPase were investigated. Firstly, the hydrolysis of ATP per se, and secondly, the hydration of CO2 and the subsequent formation of H+ and HCO3?. The amount of hydrogen ions formed in the ATPase reaction was highest at alkaline pH. The H+/ATP ratio was about 1 at pH 8.0. When CO2 was added to the reaction medium there was no change in the rate of hydrogen ion transport at pH 7.0, but at pH 8.0 the rate increased 4-times upon the addition of 0.4 mM CO2. The results indicate a possible co-operation in the production of acid between the H+ + K+-ATPase and a carbonic anhydrase associated with the vesicular membrane.  相似文献   

9.
Isolated muscle cells from adult rat heart have been used to study the relationship between myocardial glucose transport and the activity of the Na+/K+ pump. 86Rb+-uptake by cardiac cells was found to be linear up to 2 min with a steady-state reached by 40–60 min, and was used to monitor the activity of the Na+/K+ pump. Ouabain (10?3 mol/I) inhibited the steady-state uptake of 86Rb+ by more than 90%. Both, the ouabain-sensitive and ouabain-insensitive 86Rb+-uptake by cardiac cells were found to be unaffected by insulin treatment under conditions where a significant stimulation of 3-O-methylglucose transport occurred. 86Rb+-uptake was markedly reduced by the presence of calcium and/or magnesium, but remained unresponsive towards insulin treatment. Inhibition of the Na+/K+ pump activity by ouabain and a concomitant shift in the intracellular Na+:K+ ratio did not affect basal or insulin stimulated rates of 3-O-methylglucose transport in cardiac myocytes. The data argue against a functional relationship between the myocardial Na+/K+ pump and the glucose transport system.  相似文献   

10.
l-leucine uptake into full-grown oocytes of Xenopus laevis is a saturable process which is Na+ dependent and presumably coupled to Na+ gradient. Our results indicate that progesterone (10?6 M) blocks abruptly, around the germinal vesicle breakdown, the saturable transport of l-leucine. p-Chloromercuribenzoate (10?4 M) induces maturation and after a short lag of time strongly inhibits l-leucine uptake. Cycloheximide prevents progesterone-induced maturation and permeability changes.  相似文献   

11.
  • 1.1. Uptake of l-leucine, l-phenylalanine, l-proline and l-lysine into brush border membrane vesicles from rats fed either a medium-chain triglyceride (MCT) or a long-chain triglyceride (LCT) diet was studied under conditions of the presence or absence of a Na+ gradient.
  • 2.2. From the results of initial rate, Na+-dependent transport in LCT feeding were lower than in feeding MCT. The Na+-independent transport did not vary in either group except for l-lysine uptake.
  • 3.3. For l-leucine, l-phenylalanine and l-proline in Na+ dependence, kinetic analysis revealed 4–6-fold smaller Vmax values in LCT group than in MCT group. l-Lysine in Na+-independent transport was 10-fold lower in LCT group than in MCT group. The Km values were not affected by feeding the LCT or MCT diet.
  • 4.4. It is clear that amino acid transport is regulated by different types of dietary fat. We consider that the alteration of transport activity is attributable to the changes in number of membrane-bound transport carriers but not to their affinity.
  相似文献   

12.
The uptake of l-glutamic acid into brush-border membrane vesicles isolated from rat renal proximal tubules is Na+-dependent. In contrast to Na+-dependent uptake of d-glucose, pre-equilibration of the vesicles with K+ stimulates l-glutamic acid uptake. Imposition of a K+ gradient ([Ki+] > [Ko+]) further enhances Na+-dependent l-glutamic acid uptake, but leaves K+-dependent glucose transport unchanged. If K+ is present only at the outside of the vesicles, transport is inhibited. Intravesicular Rb+ and, to a lesser extent, Cs+ can replace intravesicular K+ to stimulate l-glutamic acid uptake. Changes in membrane potential incurred by the imposition of an H+-diffusion potential or anion replacement markedly affect Na+-dependent glutamic acid uptake only in the presence of K+. Experiments with a potential-sensitive cyanine dye also indicate that, in the presence of intravesicular K+ a charge movement is involved in Na+-dependent transport of l-glutamic acid.The data indicate that Na+-dependent l-glutamic acid transport can be additionally energized by a K+ gradient. Furthermore, intravesicular K+ renders Na+-dependent l-glutamic acid transport sensitive to changes in the transmembrane electrical potential difference.  相似文献   

13.
The Cl? transport properties of the luminal border of bovine tracheal epithelium have been investigated using a highly purified preparation of apical plasma membrane vesicles. Transport of Cl? into an intravesicular space was demonstrated by (1) a linear inverse correlation between Cl? uptake and medium osmolarity and (2) complete release of accumulated Cl? by treatment with detergent. The rate of Cl? uptake was highly temperature-sensitive and was enhanced by exchange diffusion, providing evidence for a carrier-mediated transport mechanism. Transport of Cl? was not affected by the ‘loop’ diuretic bumetanide or by the stilbene-derivative anion-exchange inhibitors SITS (4-acetamido-4′-isothiocyanostilbene-2,2′-disulfonic acid) and DIDS (4,4′-diisothiocyanostilbene-2,2′-disulfonic acid). In the presence of the impermeant cation, tetramethylammonium (TMA+), uptake of Cl? was minimal; transport was stimulated equally by the substitution of either K+ or Na+ for TMA+. Valinomycin in the presence of K+ enhanced further Cl? uptake, while amiloride reduced Na+-stimulated Cl? uptake towards the minimal level observed with TMA+. These results suggest the following conclusions: (1) the tracheal vesicle membrane has a finite permeability to both Na+ and K+; (2) the membrane permeability to the medium counterion determines the rate of Cl? uptake; (3) Cl? transport is not specifically coupled with either Na+ or K+; and, finally (4) Cl? crosses the tracheal luminal membrane via an electrogenic transport mechanism.  相似文献   

14.
d-Glucose decreases phosphate reabsorption in rat proximal tubule. It is also postulated that some amino acids interact with phosphate reabsorption. To investigate the mechanism of these interactions, phosphate, d-glucose and l-alanine transport kinetics were measured in brush border membrane vesicles isolated from superficial rat kidney cortex by the calcium precipitation technique. At pH 7.4, Na+-dependent phosphate transport was inhibited in the presence of either d-glucose (39 mM) or l-alanine (2.4 mM). In this model, with d-glucose or with l-alanine the V value of the phosphate uptake was decreased, whereas the apparent Km for the phosphate uptake was not affected. However, some inhibition of phosphate transport was observed in the presence of l-glucose, d-alanine or d-glucose after phlorizin preincubation. A 30% Na+-dependent l-alanine (0.1 mM) transport inhibition was observed in the presence of 5 mM phosphate. d-Glucose (1 mM) was also inhibited by 20% when 5 mM phosphate was added to incubation medium. According to several authors, in our model, d-glucose decreased the l-alanine transport and vice versa. Moreover, when the membrane potential was abolished, a clear inhibition of d-glucose by l-alanine persisted. These multiple interactions could be explained by the accelerated dissipation of the Na+ gradient insofar as the rate of the Na+ uptake was increased with d-glucose, l-alanine or phosphate and since the absence of variations in membrane potential did not suppress these inhibitions.  相似文献   

15.
Right-side-out plasma membrane vesicles were isolated from wheat roots using an aqueous polymer two-phase system. The purity and orientation of the vesicles were confirmed by marker enzyme analysis. Membrane potential (Ψ)-dependent 22Na+ influx and sodium/proton (Na+/ H+) antiport-mediated efflux across the plasma membrane were studied using these vesicles. Membrane potentials were imposed on the vesicles using either K+ gradients in the presence of valinomycin or H+ gradients. The ΔΨ was quantified by the uptake of the lipophilic cation tetraphenylphosphonium. Uptake of Na+ into the vesicles was stimulated by a negative ΔΨ and had a Km for extrav-esicular Na+ of 34.8 ± 5.9 mol m3. The ΔΨ-dependent uptake of Na+ was similar in vesicles from roots of hexaploid (cv. Troy) and tetraploid (cv. Langdon) wheat differing in a K+/Na+ discrimination trait, and was also unaffected by growth in 50 mol m?3 NaCl. Inhibition of ΔΨ-dependent Na+ uptake by Ca2+ was greater in the hexaploid than in the tetraploid. Sodium/proton antiport was measured as Na+-dependent, amiloride-inhibited pH gradient formation in the vesicles. Acidification of the vesicle interior was measured by the uptake of 14C-methylamine. The Na+/H+ antiport had a Km, for intravesicular Na+ of between 13 and 19 mol m?3. In the hexaploid, Na+/H+ antiport activity was greater when roots were grown in the presence of 50 mol m?3NaCl, and was also greater than the activity in salt-grown tetraploid wheat roots. Antiport activity was not increased in a Langdon 4D chromosome substitution line which carries a trait for K+/Na+ discrimination. It is concluded that neither of the transport processes measured is responsible for the Na+/K+ discrimination trait located on the 4D chromosome of wheat.  相似文献   

16.
The presence of an Na+/Ca2+ exchange system in basolateral plasma membranes from rat small intestinal epithelium has been demonstrated by studying Na+ gradient-dependent Ca2+ uptake and the inhibition of ATP-dependent Ca2+ accumulation by Na+. The presence of 75 mM Na+ in the uptake solution reduces ATP-dependent Ca2+ transport by 45%, despite the fact that Na+ does not affect Ca2+-ATPase activity. Preincubation of the membrane vesicles with ouabain or monensin reduces the Na+ inhibition of ATP-dependent Ca2+ uptake to 20%, apparently by preventing accumulation of Na+ in the vesicles realized by the Na+-pump. It was concluded that high intravesicular Na+ competes with Ca2+ for intravesicular Ca2+ binding sites. In the presence of ouabain, the inhibition of ATP-dependent Ca2+ transport shows a sigmoidal dependence on the Na+ concentration, suggesting cooperative interaction between counter transport of at least two sodium ions for one calcium ion. The apparent affinity for Na+ is between 15 and 20 mM. Uptake of Ca2+ in the absence of ATP can be enhanced by an Na+ gradient (Na+ inside > Na+ outside). This Na+ gradient-dependent Ca2+ uptake is further stimulated by an inside positive membrane potential but abolished by monensin. The apparent affinity for Ca2+ of this system is below 1 μM. In contrast to the ATP-dependent Ca2+ transport, there is no significant difference in Na+ gradient-dependent Ca2+ uptake between basolateral vesicles from duodenum, midjejunum and terminal ileum. In duodenum the activity of ATP-driven Ca2+ uptake is 5-times greater than the Na+/Ca2+ exchange capacity but in the ileum both systems are of equal potency. Furthermore, the Na+/Ca2+ exchange mechanism is not subject to regulation by 1α,25-dihydroxy vitamin D-3, since repletion of vitamin D-deficient rats with this seco-steroid hormone does not influence the Na+/Ca2+ exchange system while it doubles the ATP-driven Ca2+ pump activity.  相似文献   

17.
Intestinal brush border vesicles of a Mediterranean sea fish (Dicentrarchus labrax) were prepared using the Ca2+-sedimentation method. The transport of glucose, glycine and 2-aminoisobutyric acid is energized by an Na+ gradient (out > in). In addition, amino acid uptake requires Cl? in the extravesicular medium (2-aminoisobutyric acid more than glycine). This Na+- and Cl?-dependent uptake is electrogenic, since it can be stimulated by negative charges inside the vesicles. The specific Cl? requirement of glycine and 2-aminoisobutyric acid transport is markedly influenced by pH, a change from 6.5 to 8.4 reducing the role played by Cl?. In the presence of Cl?, the Km of 2-aminoisobutyric acid uptake is reduced and its Vmax is enhanced. Cl? affects also a non-saturable Na+-dependent component of this amino acid uptake. Amino acid transport is also increased by intravesicular Cl? (2-aminoisobutyric acid less than glycine). This effect is more concerned with glucose uptake, which can be then multiplied by 2.3. A concentration gradient (in > out) as well as the presence of Na+ in the incubation medium seems to enter into this requirement. This intravesicular Cl? effect is not influenced by pH between 6.5 and 8.4.  相似文献   

18.
Analysis of the cation composition of growing Mycoplasma mycoides var. Capri indicates that these organisms have a high intracellular K+ concentration (Ki: 200–300 mM) which greatly exceeds that of the growth medium, and a low Na+ concentration (Nai+: 20 mM). Unlike Nai+, Ki+ varies with cell aging.The K+ transport properties studied in washed organisms resuspended in buffered saline solution show that cells maintain a steady and large K+ concentration gradient across their membrane at the expense of metabolic energy mainly derived from glycolysis. In starved cells, Ki+ decreases and is partially compensated by a gain in Na+. This substitution completely reverses when metabolic substrate is added (K+ reaccumulation process). Kinetic analysis of K+ movement in cells with steady K+ level shows that most of K+ influx is mediated by an autologous K+-K+ exchange mechanism. On the other hand, during K+ reaccumulation by K+-depleted cells, a different mechanism (a K+ uptake mechanism) with higher transport capacity and affinity drives the net K+ influx. Both mechanisms are energy-dependent.Ouabain and anoxia have no effect on K+ transport mechanisms; in contrast, both processes are completely blocked by dicyclohexylcarbodiimide, an inhibitor of the Mg2+-dependent ATPase activity.  相似文献   

19.
An axolemma-rich membrane vesicle fraction was prepared from the leg nerve of the lobster, Homerus americanus. In this preparation Ca2+ transport across the membrane was shown to require a Na+ gradient (Na+-Ca2+ exchange), and external K+ was found to facilitate this Na+-Ca2+ exchange activity. In addition, at high Ca2+ concentrations (20 mM) a Ca2+-Ca2+ exchange system was shown to operate, which is stimulated by Li+. The Na+-Ca2+ exchange system is capable of operating in the reverse direction, with Ca2+ uptake coupled with Na+ efflux. Such a vesicular preparation has the potential for providing useful experimental approaches to study the mechanism of this important Ca2+ extrusion system in the nervous system.  相似文献   

20.
Inhibition of the (Na+ + K+)-dependent ATPase by inorganic phosphate, Pi, was examined in terms of product inhibition of the various activities catalyzed by an enzyme preparation from rat brain, and considered in terms of the specific transport processes of the membrane Na+,K+-pump that these activities reflect. The K+-dependent phosphatase activity of the enzyme was most sensitive to Pi, and inhibition was competitive toward the substrate, nitrophenyl phosphate, as would be expected if Pi were released from the same enzyme form that bound substrate. However, this enzymatic activity does not seem to represent a transport process, and thus a cyclical discharge of K+ may not be involved. The Na+-dependent exchange activity was unaffected by Pi, in accord with the absence of Pi release in the reaction sequence. For the corresponding Na+/Na+ exchange function of the pump, which reportedly does not involve ATP hydrolysis either, prior release of Pi obviously cannot be required for Na+ discharge. With the Na+-dependent ATPase activity, measured using micromolar concentrations of ATP, Pi inhibited, but far less than with the phosphatase activity, and inhibition was not competitive toward ATP. Moreover, inhibition decreased as the Na+ concentration was raised from 10 to 100 mM. This elevated concentration of Na+ also led to substrate inhibition. For this ATPase activity, and the corresponding transport process, uncoupled Na+ efflux, the findings suggest that Na+ discharge follows Pi release, in contrast to Na+/Na+ exchange. The (Na+ + K+)-dependent ATPase activity, measured with millimolar concentrations of ATP and reflecting the coupled Na+,K+-transport function, was similarly sensitive to Pi, and again inhibition was not competitive toward ATP. However, in this case inhibition did not increase as the Na+ concentration was lowered. For this activity, and the associated transport process, the site of Na+ discharge in the overall reaction sequence remains unresolved.  相似文献   

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