共查询到20条相似文献,搜索用时 31 毫秒
1.
H. A. Blair 《The Journal of general physiology》1932,16(2):177-189
Equations are derived for the capacity-voltage relations for stimulation of tissue by condenser discharges, using the hypothesis that the local excitatory process p grows under the influence of an applied potential V according to the equation, See PDF for Equation where K and k are constants. It is further assumed that the local excitatory process becomes adequate when it attains a value h ± α V where h and α are constants and V is the applied potential at the particular instant that the adequate value is attained. The equations so obtained are applied to the data of several authors on several types of tissue and the agreements obtained are sufficiently good. It is shown in one case that the direct current equation and the condenser discharge equation each derived from the above bases are consistent when applied to data from the same preparation. 相似文献
2.
A study of lipid bilayer membrane stability using precise measurements of specific capacitance 总被引:1,自引:0,他引:1
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S H White 《Biophysical journal》1970,10(12):1127-1148
A method is described for measuring the specific capacitance (Cm) of lipid bilayer membranes with an estimated experimental error of only 1%. The gross capacitance was measured with an AC Wheatstone bridge and a photographic technique was used to determine the area of thin membrane. The results of measurements on oxidized cholesterol-decane membranes formed in 1 × 10-2 M KCl show that Cm depends upon temperature, voltage, time, and the age of the bulk membrane solutions. For a freshly thinned membrane (from 5 week old solution), Cm increases exponentially from an initial value of 0.432 ±0.021 (SD) μF/cm2 with a time constant of ~15 min. A 100 mv potential applied across the membrane for 10-20 min prior to making measurements eliminated this time dependence and produced final-state membranes. Cm of final-state membranes depends upon applied voltage (Va) and obeys the equation Cm = C0 + βVa2 where Va VDC + VrmsAC. C0 and β depend upon temperature; C0 decreases linearly with temperature while β increases linearly. At 20°C, C0 = 0.559 ±0.01 (SD) μF/cm2 and β = 0.0123 ±0.0036 (SD) (μF/cm2)/(mv2) and at 34°C, C0 = 0.472 ±0.01 and β = 0.0382 ±0.0039. These variations in Cm are interpreted as resulting from thickness changes. The possibility that they result from diffuse layer and/or membrane dielectric phenomena is discussed and found to be unlikely. The results are discussed in terms of membrane stability by constructing hypothetical potential energy vs. thickness curves. 相似文献
3.
Kinetics and mechanism of catalysis by proteolytic enzymes. A comparison of the kinetics of hydrolysis of synthetic substrates by bovine α- and β-trypsin
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Several esters of the α-N-toluene-p-sulphonyl and α-N-benzoyl derivatives of S-(3-aminopropyl)-l-cysteine and the methyl ester of S-(4-aminobutyl)-N-toluene-p-sulphonyl-l-cysteine were synthesized. The kinetics of hydrolysis of these and esters of the α-N-toluene-p-sulphonyl and α-N-benzoyl derivatives of l-arginine, l-lysine, S-(2-aminoethyl)-l-cysteine and esters of γ-guanidino-l-α-toluene-p-sulphonamidobutyric acid and α-N-toluene-p-sulphonyl-l-homoarginine by α- and β-trypsin were compared. On the basis of values of the specificity constants (kcat./Km), the two enzymes display similar catalytic efficiency towards some substrates. In other cases α-trypsin is less efficient than β-trypsin. It is possible that α-trypsin possesses greater molecular flexibility than β-trypsin. 相似文献
4.
Jinn-Shiun Chen Yuan-Ming Hsu Chia-Chi Chen Li-Li Chen Chun-Chung Lee Tze-Sing Huang 《The Journal of biological chemistry》2010,285(33):25458-25466
HCT-8 colon cancer cells secreted heat shock protein 90α (HSP90α) and had increased invasiveness upon serum starvation. The concentrated conditioned medium of serum-starved HCT-8 cells was able to stimulate the migration and invasion of non-serum-starved cells, which could be prevented by treatment with an anti-HSP90α antibody. Recombinant HSP90α (rHSP90α) also enhanced HCT-8 cell migration and invasion, suggesting a stimulatory role of secreted HSP90α in cancer malignancy. HSP90α binding to CD91α and Neu was evidenced by the proximity ligation assay, and rHSP90α-induced HCT-8 cell invasion could be suppressed by the addition of anti-CD91α or anti-Neu antibodies. Via CD91α and Neu, rHSP90α selectively induced integrin αV expression, and knockdown of integrin αV efficiently blocked rHSP90α-induced HCT-8 cell invasion. rHSP90α induced the activities of ERK, PI3K/Akt, and NF-κB p65, but only NF-κB activation was involved in HSP90α-induced integrin αV expression. Additionally, we investigated the serum levels of HSP90α and the expression status of tumor integrin αV mRNA in colorectal cancer patients. Serum HSP90α levels of colorectal cancer patients were significantly higher than those of normal volunteers (p < 0.001). Patients with higher serum HSP90α levels significantly exhibited elevated levels of integrin αV mRNA in tumor tissues as compared with adjacent non-tumor tissues (p < 0.001). Furthermore, tumor integrin αV overexpression was significantly correlated with TNM (Tumor, Node, Metastasis) staging (p = 0.001). 相似文献
5.
H. A. Blair 《The Journal of general physiology》1935,18(5):755-766
Data on the electrical stimulation of sciatic-gastrocnemius preparations of the frog by both direct currents and condenser discharges at the same time are discussed in relation to the validity of the differential equation See PDF for Equation where p is the local excitatory process, V the stimulating current or voltage, and K and k are constants. It is concluded that the constant k is the same whether it is derived from the data of the one stimulus or the other when the same fibres are being stimulated. 相似文献
6.
Wei Yao Zhoufeng Shen Guanghong Ding 《International journal of biological sciences》2013,9(10):1050-1056
In this paper, we use Stokes, Brinkman and Darcy equations to approximate the porous continuum media of ligament tissues respectively, simulate the flow field with FLUENT software, and study the shear stress on the cell surface due to the interstitial fluid flow. Since the Brinkman equation approaches Stokes equation well in high hydraulic permeability (kp) condition (kp ≥1.0×10-8 m2 in our numerical simulation), and it is an approximation to Darcy model in low kp condition (kp ≤5.0×10-12 m2 in our numerical simulation), we used the Brinkman model to simulate the interstitial fluid flow in the ligament where kp is approximately 1.0×10-16 m2. It shows kp and anisotropic property have a little effect on the flow field, but have a great effect on the shear stress on the membrane of interstitial cells (τcell). There is a linear relationship between τcell and , when kp =1.0×10-16 m2 and the maximum τcell (τcell,max) is approximately 10 Pa. The anisotropic property will affect τcell''s distribution on the cell surface. When kx/ky>1, low τcell dominates the cell, while when kx/ky<1, high τcell dominants the cell. 相似文献
7.
The second-order rate constants for the microbial transformation of a series of phenols were correlated with the physicochemical properties of the phenols. The compounds studied were phenol, p-methylphenol, p-chlorophenol, p-bromophenol, p-cyanophenol, p-nitrophenol, p-acetylphenol, and p-methoxyphenol. Phenol-grown cells of Pseudomonas putida U transformed these compounds. Microbial transformation rate constants ranged from (1.5 ± 0.99) × 10−14 liter · organism−1 · h−1 for p-cyanophenol to (7.0 ± 1.3) × 10−12 liter · organism−1 · h−1 for phenol. Linear regression analyses of rate constants and electronic, steric, and hydrophobic parameters showed that van der Waal's radii gave the best coefficient of determination (r2 = 0.956). Products identified by thin-layer chromatography and liquid chromatography indicated that the phenols were microbially oxidized to the corresponding catechols. 相似文献
8.
Modeling Reduction of Uranium U(VI) under Variable Sulfate Concentrations by Sulfate-Reducing Bacteria 总被引:4,自引:2,他引:2
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The kinetics for the reduction of sulfate alone and for concurrent uranium [U(VI)] and sulfate reduction, by mixed and pure cultures of sulfate-reducing bacteria (SRB) at 21 ± 3°C were studied. The mixed culture contained the SRB Desulfovibrio vulgaris along with a Clostridium sp. determined via 16S ribosomal DNA analysis. The pure culture was Desulfovibrio desulfuricans (ATCC 7757). A zero-order model best fit the data for the reduction of sulfate from 0.1 to 10 mM. A lag time occurred below cell concentrations of 0.1 mg (dry weight) of cells/ml. For the mixed culture, average values for the maximum specific reaction rate, Vmax, ranged from 2.4 ± 0.2 μmol of sulfate/mg (dry weight) of SRB · h−1) at 0.25 mM sulfate to 5.0 ± 1.1 μmol of sulfate/mg (dry weight) of SRB · h−1 at 10 mM sulfate (average cell concentration, 0.52 mg [dry weight]/ml). For the pure culture, Vmax was 1.6 ± 0.2 μmol of sulfate/mg (dry weight) of SRB · h−1 at 1 mM sulfate (0.29 mg [dry weight] of cells/ml). When both electron acceptors were present, sulfate reduction remained zero order for both cultures, while uranium reduction was first order, with rate constants of 0.071 ± 0.003 mg (dry weight) of cells/ml · min−1 for the mixed culture and 0.137 ± 0.016 mg (dry weight) of cells/ml · min−1 (U0 = 1 mM) for the D. desulfuricans culture. Both cultures exhibited a faster rate of uranium reduction in the presence of sulfate and no lag time until the onset of U reduction in contrast to U alone. This kinetics information can be used to design an SRB-dominated biotreatment scheme for the removal of U(VI) from an aqueous source. 相似文献
9.
H. A. Blair 《The Journal of general physiology》1937,20(6):787-795
If it is assumed that the kinetics of the process of excitation in nerve is given by dp/dt = KI – kp, I being the actual exciting component of the current, p the state of excitation, and K and k constants, it is necessary to postulate that on application of a rectangular stimulus of voltage, V, the current, I, undergoes a transient exponential variation, usually a decrease, in order that the integral of the differential equation (above) may fit the strength-duration data in V and t. This hypothesis is substantiated by data by Sakamoto on single fibers of the sciatic nerve of the frog. The time constant of the postulated current transient is of the order of 10–4 sec. for single fibers and of the order of 10–5 sec. or less in the sciatic nerve trunk. The latter value is about the same as that found by Cole in the same tissue by purely physical measurements. Some criticisms by Rushton (1934) are discussed. 相似文献
10.
1. β-d-Galactopyranosyl pyridinium salts are well-behaved substrates for the β-galactosidase of Escherichia coli, catalysis occurring by the interaction of the salt itself with the normal active site of the protein. 2. logkcat. values for seven such salts show a linear relationship (correlation coefficient=−0.997) with the pKa of the parent pyridine. 3. The β-d-galactopyranosyl derivatives of pyridine and 4-bromoisoquinoline exhibit α-deuterium kinetic isotope effects of 1.136±0.040 and 1.187±0.046 on their enzymic hydrolysis, indicating formation of a galactopyranosyl cation in the rate-limiting step. 4. This behaviour of the pyridinium salts contrasts with the behaviour of aryl galactosides and this contrast can be accommodated by the β-galactosidase mechanism of Sinnott & Souchard (1973). 5. The α-deuterium kinetic isotope effect for the hydrolysis of β-d-galactopyranosyl azide is 1.098±0.033; comparison of the kcat. value of the azide with that of a pyridinium salt of the same aglycone pKa enables a maximum factor of 70 to be ascribed to the acceleration of the departure of azide by intracomplex general acid catalysis. 6. The possibility of the rate-limiting process in the glycosidase-catalysed hydrolysis of aryl glycosides being a conformation change is considered for a number of glycosidases where correlations of kcat. with aglycone acidity, reported in the literature, have been unsuccessful. 相似文献
11.
Purification and characterization of active-site components of the putative p-cresol methylhydroxylase membrane complex from Geobacter metallireducens
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Johannes J Bluschke A Jehmlich N von Bergen M Boll M 《Journal of bacteriology》2008,190(19):6493-6500
p-Cresol methylhydroxylases (PCMH) from aerobic and facultatively anaerobic bacteria are soluble, periplasmic flavocytochromes that catalyze the first step in biological p-cresol degradation, the hydroxylation of the substrate with water. Recent results suggested that p-cresol degradation in the strictly anaerobic Geobacter metallireducens involves a tightly membrane-bound PCMH complex. In this work, the soluble components of this complex were purified and characterized. The data obtained suggest a molecular mass of 124 ± 15 kDa and a unique αα′β2 subunit composition, with α and α′ representing isoforms of the flavin adenine dinucleotide (FAD)-containing subunit and β representing a c-type cytochrome. Fluorescence and mass spectrometric analysis suggested that one FAD was covalently linked to Tyr394 of the α subunit. In contrast, the α′ subunit did not contain any FAD cofactor and is therefore considered to be catalytically inactive. The UV/visible spectrum was typical for a flavocytochrome with two heme c cofactors and one FAD cofactor. p-Cresol reduced the FAD but only one of the two heme cofactors. PCMH catalyzed both the hydroxylation of p-cresol to p-hydroxybenzyl alcohol and the subsequent oxidation of the latter to p-hydroxybenzaldehyde in the presence of artificial electron acceptors. The very low Km values (1.7 and 2.7 μM, respectively) suggest that the in vivo function of PCMH is to oxidize both p-cresol and p-hydroxybenzyl alcohol. The latter was a mixed inhibitor of p-cresol oxidation, with inhibition constants of a Kic (competitive inhibition) value of 18 ± 9 μM and a Kiu (uncompetitive inhibition) value of 235 ± 20 μM. A putative functional model for an unusual PCMH enzyme is presented. 相似文献
12.
H. A. Blair 《The Journal of general physiology》1938,21(5):557-573
With a view to indicating that the α excitatory state in muscle is not of a special nature it is shown that the α strength-duration curves are of the same form as those determined for nerve and other tissues except that in about two-thirds of all cases the rheobase appears to be slightly too low. Also from experiments in latent addition it is found that the α excitatory state following an inadequate stimulus subsides exponentially at a rate which is related to the α excitability in the same way, approximately, as the subsidence rate in nerve is related to the nerve''s excitability. In both tissues the subsidence as measured directly is 2–3 times as fast as it appears to be from the strength-duration curve. The α refractory period is at least as short as that of nerve so the α chronaxie is unusually long compared to the refractory period. There is no reason at present, however, to consider this as having any bearing on the problem at issue. It is concluded therefore, that the α excitability differs from others in the rates of its reactions rather than in its fundamental nature and that any conclusions about excitability drawn from its study will probably be valid quite generally. 相似文献
13.
Arachidonic acid inhibition of L-type calcium (CaV1.3b) channels varies with accessory CaVβ subunits
Arachidonic acid (AA) inhibits the activity of several different voltage-gated Ca2+ channels by an unknown mechanism at an unknown site. The Ca2+ channel pore-forming subunit (CaVα1) is a candidate for the site of AA inhibition because T-type Ca2+ channels, which do not require accessory subunits for expression, are inhibited by AA. Here, we report the unanticipated role of accessory CaVβ subunits on the inhibition of CaV1.3b L-type (L-) current by AA. Whole cell Ba2+ currents were measured from recombinant channels expressed in human embryonic kidney 293 cells at a test potential of −10 mV from a holding potential of −90 mV. A one-minute exposure to 10 µM AA inhibited currents with β1b, β3, or β4 58, 51, or 44%, respectively, but with β2a only 31%. At a more depolarized holding potential of −60 mV, currents were inhibited to a lesser degree. These data are best explained by a simple model where AA stabilizes CaV1.3b in a deep closed-channel conformation, resulting in current inhibition. Consistent with this hypothesis, inhibition by AA occurred in the absence of test pulses, indicating that channels do not need to open to become inhibited. AA had no effect on the voltage dependence of holding potential–dependent inactivation or on recovery from inactivation regardless of CaVβ subunit. Unexpectedly, kinetic analysis revealed evidence for two populations of L-channels that exhibit willing and reluctant gating previously described for CaV2 channels. AA preferentially inhibited reluctant gating channels, revealing the accelerated kinetics of willing channels. Additionally, we discovered that the palmitoyl groups of β2a interfere with inhibition by AA. Our novel findings that the CaVβ subunit alters kinetic changes and magnitude of inhibition by AA suggest that CaVβ expression may regulate how AA modulates Ca2+-dependent processes that rely on L-channels, such as gene expression, enzyme activation, secretion, and membrane excitability. 相似文献
14.
The outer hair cell is electromotile, its membrane motor identified as the protein SLC26a5 (prestin). An area motor model, based on two-state Boltzmann statistics, was developed about two decades ago and derives from the observation that outer hair cell surface area is voltage-dependent. Indeed, aside from the nonlinear capacitance imparted by the voltage sensor charge movement of prestin, linear capacitance (Clin) also displays voltage dependence as motors move between expanded and compact states. Naturally, motor surface area changes alter membrane capacitance. Unit linear motor capacitance fluctuation (δCsa) is on the order of 140 zeptofarads. A recent three-state model of prestin provides an alternative view, suggesting that voltage-dependent linear capacitance changes are not real but only apparent because the two component Boltzmann functions shift their midpoint voltages (Vh) in opposite directions during treatment with salicylate, a known competitor of required chloride binding. We show here using manipulations of nonlinear capacitance with both salicylate and chloride that an enhanced area motor model, including augmented δCsa by salicylate, can accurately account for our novel findings. We also show that although the three-state model implicitly avoids measuring voltage-dependent motor capacitance, it registers δCsa effects as a byproduct of its assessment of Clin, which increases during salicylate treatment as motors are locked in the expanded state. The area motor model, in contrast, captures the characteristics of the voltage dependence of δCsa, leading to a better understanding of prestin. 相似文献
15.
Regulation of vascular homeostasis depends upon collaboration between cells of the vessel wall and blood coagulation system. A direct interaction between integrin αVβ3 on endothelial cells and smooth muscle cells and prothrombin, the pivotal proenzyme of the blood coagulation system, is demonstrated and activation of the integrin is required for receptor engagement. Evidence that prothrombin is a ligand for αVβ3 on these cells include: (a) prothrombin binds to purified αVβ3 via a RGD recognition specificity; (b) prothrombin supports αVβ3-mediated adhesion of stimulated endothelial cells and smooth muscle cells; and (c) endothelial cells, either in suspension and in a monolayer, recognize soluble prothrombin via αVβ3. αVβ3-mediated cell adhesion to prothrombin, but not to fibrinogen, required activation of the receptor. Thus, the functionality of the αVβ3 receptor is ligand defined, and prothrombin and fibrinogen represent activation- dependent and activation-independent ligands. Activation of αVβ3 could be induced not only by model agonists, PMA and Mn2+, but also by a physiologically relevant agonist, ADP. Inhibition of protein kinase C and calpain prevented activation of αVβ3 on vascular cells, suggesting that these molecules are involved in the inside-out signaling events that activate the integrin. The capacity of αVβ3 to interact with prothrombin may play a significant role in the maintenance of hemostasis; and, at a general level, ligand selection by αVβ3 may be controlled by the activation state of this integrin. 相似文献
16.
Dongil Keum Christina Baek Dong-Il Kim Hae-Jin Kweon Byung-Chang Suh 《The Journal of general physiology》2014,144(4):297-309
G protein–coupled receptors (GPCRs) signal through molecular messengers, such as Gβγ, Ca2+, and phosphatidylinositol 4,5-bisphosphate (PIP2), to modulate N-type voltage-gated Ca2+ (CaV2.2) channels, playing a crucial role in regulating synaptic transmission. However, the cellular pathways through which GqPCRs inhibit CaV2.2 channel current are not completely understood. Here, we report that the location of CaV β subunits is key to determining the voltage dependence of CaV2.2 channel modulation by GqPCRs. Application of the muscarinic agonist oxotremorine-M to tsA-201 cells expressing M1 receptors, together with CaV N-type α1B, α2δ1, and membrane-localized β2a subunits, shifted the current-voltage relationship for CaV2.2 activation 5 mV to the right and slowed current activation. Muscarinic suppression of CaV2.2 activity was relieved by strong depolarizing prepulses. Moreover, when the C terminus of β-adrenergic receptor kinase (which binds Gβγ) was coexpressed with N-type channels, inhibition of CaV2.2 current after M1 receptor activation was markedly reduced and delayed, whereas the delay between PIP2 hydrolysis and inhibition of CaV2.2 current was decreased. When the Gβγ-insensitive CaV2.2 α1C-1B chimera was expressed, voltage-dependent inhibition of calcium current was virtually abolished, suggesting that M1 receptors act through Gβγ to inhibit CaV2.2 channels bearing membrane-localized CaV β2a subunits. Expression of cytosolic β subunits such as β2b and β3, as well as the palmitoylation-negative mutant β2a(C3,4S), reduced the voltage dependence of M1 muscarinic inhibition of CaV2.2 channels, whereas it increased inhibition mediated by PIP2 depletion. Together, our results indicate that, with membrane-localized CaV β subunits, CaV2.2 channels are subject to Gβγ-mediated voltage-dependent inhibition, whereas cytosol-localized β subunits confer more effective PIP2-mediated voltage-independent regulation. Thus, the voltage dependence of GqPCR regulation of calcium channels can be determined by the location of isotype-specific CaV β subunits. 相似文献
17.
Yousra El Ghaleb Nadine J. Ortner Wilfried Posch Monica L. Fernndez-Quintero Wietske E. Tuinte Stefania Monteleone Henning J. Draheim Klaus R. Liedl Doris Wilflingseder Jrg Striessnig Petronel Tuluc Bernhard E. Flucher Marta Campiglio 《The Journal of general physiology》2022,154(9)
The skeletal muscle voltage-gated calcium channel (CaV1.1) primarily functions as a voltage sensor for excitation–contraction coupling. Conversely, its ion-conducting function is modulated by multiple mechanisms within the pore-forming α1S subunit and the auxiliary α2δ-1 and γ1 subunits. In particular, developmentally regulated alternative splicing of exon 29, which inserts 19 amino acids in the extracellular IVS3-S4 loop of CaV1.1a, greatly reduces the current density and shifts the voltage dependence of activation to positive potentials outside the physiological range. We generated new HEK293 cell lines stably expressing α2δ-1, β3, and STAC3. When the adult (CaV1.1a) and embryonic (CaV1.1e) splice variants were expressed in these cells, the difference in the voltage dependence of activation observed in muscle cells was reproduced, but not the reduced current density of CaV1.1a. Only when we further coexpressed the γ1 subunit was the current density of CaV1.1a, but not that of CaV1.1e, reduced by >50%. In addition, γ1 caused a shift of the voltage dependence of inactivation to negative voltages in both variants. Thus, the current-reducing effect of γ1, unlike its effect on inactivation, is specifically dependent on the inclusion of exon 29 in CaV1.1a. Molecular structure modeling revealed several direct ionic interactions between residues in the IVS3-S4 loop and the γ1 subunit. However, substitution of these residues by alanine, individually or in combination, did not abolish the γ1-dependent reduction of current density, suggesting that structural rearrangements in CaV1.1a induced by inclusion of exon 29 may allosterically empower the γ1 subunit to exert its inhibitory action on CaV1.1 calcium currents. 相似文献
18.
Robert P. Bonin Agnieszka A. Zurek Jieying Yu Douglas A. Bayliss Beverley A. Orser 《PloS one》2013,8(3)
Changes in the expression of γ-aminobutyric acid type A (GABAA) receptors can either drive or mediate homeostatic alterations in neuronal excitability. A homeostatic relationship between α5 subunit-containing GABAA (α5GABAA) receptors that generate a tonic inhibitory conductance, and HCN channels that generate a hyperpolarization-activated cation current (Ih) was recently described for cortical neurons, where a reduction in Ih was accompanied by a reciprocal increase in the expression of α5GABAA receptors resulting in the preservation of dendritosomatic synaptic function. Here, we report that in mice that lack the α5 subunit gene (Gabra5−/−), cultured embryonic hippocampal pyramidal neurons and ex vivo CA1 hippocampal neurons unexpectedly exhibited a decrease in Ih current density (by 40% and 28%, respectively), compared with neurons from wild-type (WT) mice. The resting membrane potential and membrane hyperpolarization induced by blockade of Ih with ZD-7288 were similar in cultured WT and Gabra5−/− neurons. In contrast, membrane hyperpolarization measured after a train of action potentials was lower in Gabra5−/− neurons than in WT neurons. Also, membrane impedance measured in response to low frequency stimulation was greater in cultured Gabra5−/− neurons. Finally, the expression of HCN1 protein that generates Ih was reduced by 41% in the hippocampus of Gabra5−/− mice. These data indicate that loss of a tonic GABAergic inhibitory conductance was followed by a compensatory reduction in Ih. The results further suggest that the maintenance of resting membrane potential is preferentially maintained in mature and immature hippocampal neurons through the homeostatic co-regulation of structurally and biophysically distinct cation and anion channels. 相似文献
19.
Bert Billen Alexander Vassilevski Anton Nikolsky Sarah Debaveye Jan Tytgat Eugene Grishin 《The Journal of biological chemistry》2010,285(24):18545-18554
Spider venoms provide a highly valuable source of peptide toxins that act on a wide diversity of membrane-bound receptors and ion channels. In this work, we report isolation, biochemical analysis, and pharmacological characterization of a novel family of spider peptide toxins, designated β/δ-agatoxins. These toxins consist of 36–38 amino acid residues and originate from the venom of the agelenid funnel-web spider Agelena orientalis. The presented toxins show considerable amino acid sequence similarity to other known toxins such as μ-agatoxins, curtatoxins, and δ-palutoxins-IT from the related spiders Agelenopsis aperta, Hololena curta, and Paracoelotes luctuosus. β/δ-Agatoxins modulate the insect NaV channel (DmNaV1/tipE) in a unique manner, with both the activation and inactivation processes being affected. The voltage dependence of activation is shifted toward more hyperpolarized potentials (analogous to site 4 toxins) and a non-inactivating persistent Na+ current is induced (site 3-like action). Interestingly, both effects take place in a voltage-dependent manner, producing a bell-shaped curve between −80 and 0 mV, and they are absent in mammalian NaV channels. To the best of our knowledge, this is the first detailed report of peptide toxins with such a peculiar pharmacological behavior, clearly indicating that traditional classification of toxins according to their binding sites may not be as exclusive as previously assumed. 相似文献
20.
Isabella Tselnicker Vladimir A. Tsemakhovich Carmen W. Dessauer Nathan Dascal 《The Journal of biological chemistry》2010,285(27):20462-20471
Loss of neuronal protein stargazin (γ2) is associated with recurrent epileptic seizures and ataxia in mice. Initially, due to homology to the skeletal muscle calcium channel γ1 subunit, stargazin and other family members (γ3–8) were classified as γ subunits of neuronal voltage-gated calcium channels (such as CaV2.1-CaV2.3). Here, we report that stargazin interferes with G protein modulation of CaV2.2 (N-type) channels expressed in Xenopus oocytes. Stargazin counteracted the Gβγ-induced inhibition of CaV2.2 channel currents, caused either by coexpression of the Gβγ dimer or by activation of a G protein-coupled receptor. Expression of high doses of Gβγ overcame the effects of stargazin. High affinity Gβγ scavenger proteins m-cβARK and m-phosducin produced effects similar to stargazin. The effects of stargazin and m-cβARK were not additive, suggesting a common mechanism of action, and generally independent of the presence of the CaVβ3 subunit. However, in some cases, coexpression of CaVβ3 blunted the modulation by stargazin. Finally, the Gβγ-opposing action of stargazin was not unique to CaV2.2, as stargazin also inhibited the Gβγ-mediated activation of the G protein-activated K+ channel. Purified cytosolic C-terminal part of stargazin bound Gβγ in vitro. Our results suggest that the regulation by stargazin of biophysical properties of CaV2.2 are not exerted by direct modulation of the channel but via a Gβγ-dependent mechanism. 相似文献