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 28,634 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The purple membrane of Halobacterium halobium acts as a light-driven proton pump, ejecting protons from the cell interior into the medium and generating electrochemical proton gradient across the cell membrane. However, the type response of cells to light as measured with a pH electrode in the medium consists of an initial net inflow of protons which subsides and is then replaced by a net outflow which exponentially approaches a new lower steady state pH level. When the light turned off a small transient acidification occurs before the pH returns to the original dark level. We present experiments suggesting that the initial inflow of protons is triggered by the beginning ejection of protons through the purple membrane and that the initial inflow rate is larger than the continuing light-driven outflow. When the initial inflow has decreased exponentially to a small value, the outflow dominates and causes the net acidification of the medium. The initial inflow is apparently driven by a pre-existing electrochemical gradient across the membrane, which the cells can maintain for extended times in the absence of light and oxygen. Treatments which collapse this gradient such as addition of small concentrations of uncouplers abolish the initial inflow. The triggered inflow occurs through the ATPase and is accompanied by ATP synthesis. Inhibitors of the ATPase such as N,N'-dicyclohexylcarbodiimide (DCCD) inhibit ATP synthesis and abolish the inflow. They also abolish the transient light-off acidification, which is apparently caused by a short burst of ATP hydrolysis before the enzyme is blocked by its endogenous inhibitor. Similar transient inflows and outflows of protons are also observed when anaerobic cells are exposed to short oxygen pulses.
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PMID:Light-driven proton translocations in Halobacterium halobium. 0 22

In a Na-rich bathing system, addition of amiloride to the mucosal fluid of turtle bladders produces decreased in the transepithelial potential difference (PD), short-circuiting current (I-sc), and conductance. Removal of amiloride results in complete reversal of these changes; and this reversibility is incomplete in amiloride-blocked bladders exposed to ouabain. In a Na-free bathing system, step increased in mucosal [Na] evoke rapid initial spikes in PD, Isc, and conductance, the magnitudes of which are independent of prior ouabain treatment. After these spikes, PD and Isc in the ouabain-treated bladder rapidly decay, while conductance remains unchanged and high. This unchanging conductance, plus the fact that ouabain inhibits half the microsomal (Na plus K). ATPase of this tissue within 1 min, suggests that ouabain inhibits Na pumping without changing tissue conductance. The delayed decrease in conductance (beginning 30 min after ouabain addition), a nonspecific and secondary effect of ouabain, is due to a concomitant collapse of the intercellular channels.
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PMID:Effects of ouabain and amiloride on Na pathways in turtle bladders. 12 25

A controlled exchange of calcium between the extracellular space (mM Ca2+) and the neuroplasm (microM Ca2+) is considered to be an essential prerequisite for almost every stage of neuronal activity. Our research interest is focused on those compounds, which due to their physico-chemical properties and localization within the synaptic membrane might fulfill the task as neuromodulators for functional synaptic proteins. Because of this specific binding properties towards calcium and their peculiar interactions with calcium in model systems gangliosides (amphiphilic sialic acid containing glycosphingolipids) are favorite candidates for a functional involvement in synaptic transmission of information. In this study we used monolayers to investigate the molecular packing and surface potential at the air/water interface, the interaction of gangliosides with the depsipeptide valinomycin (= monovalent ion carrier), and its influenceability by calcium. Furthermore we looked at calcium effects on the single channel conductance and mean channel life-time of the monovalent ion channel gramicidin A in mixed PC/ganglioside bilayers. In pure ganglioside monolayers the addition of 0.01 mM Ca2+ induces monolayer condensation, a rise in collapse pressure (= higher film stability), a shift of phase transition (= change of conformation), and a more negative head group potential (change of electric properties). In mixed ganglioside-valinomycin monolayers the addition of Ca2+ causes phase separation and/or aggregate formation between the ganglioside and the peptide. Single channel conductance fluctuations as well as mean channel life-time were analyzed for gramicidin A incorporated into binary mixed black lipid membranes of negatively charged gangliosides (GM1, GD1a, GT1b, GMix) and neutral lecithin (DOPC) in different molar ratios. At monovalent electrolyte concentrations up to < 250 mM CsCl the single channel conductance was significantly larger in the negatively charged mixed DOPC/ganglioside membranes than in the neutral DOPC membrane. Additionally, in the presence of gangliosides the mean channel life-time is increased. The addition of calcium (0.05 mM) induced a reduction of single channel conductance of gramicidin A in DOPC- and mixed DOPC/ganglioside membranes. These physico-chemical data in connection with new electromicroscopical evidences for a precise localization of calcium, a calcium pump (Ca(2+)-ATPase), a clustered arrangement of gangliosides in synaptic terminals, and biochemical results with regard to activatory nature of exogenous gangliosides for neuronal protein phosphorylation and ATPases, support the hypothesis of a modulatory function of gangliosides in synaptic transmission.
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PMID:Calcium-ganglioside interactions and synaptic plasticity: effect of calcium on specific ganglioside/peptide (valinomycin, gramicidin A)-complexes in mixed mono- and bilayers. 128 79

1. The effect of gossypol in the presence of K+ or Mg2+, or both, was studied on ATPase activity and respiration of rat liver mitochondria. 2. Respiration was uncoupled in the presence of gossypol, Mg2+, and K+, whereas in the presence of gossypol and Mg2+ a partial inhibition was observed. 3. Gossypol stimulated ATPase activity in the presence of K+ or Mg2+, but maximal activity was observed when both cations were in the incubation medium. 4. Stimulation of ATPase activity in the presence of Mg2+ was dose related. 5. EDTA reverted the stimulation produced by gossypol on ATPase activity. 6. Gossypol had no effect on the ATPase activity of submitochondrial particles, which suggests an indirect action of gossypol on the enzyme. 7. Mitochondrial membrane potential showed a higher collapse in the presence of gossypol and 1 mM MgCl2. 8. The observed effects of gossypol could be explained by the collapse of the mitochondrial membrane potential.
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PMID:The antifertility agent, gossypol, changes several mitochondrial functions in the presence of Mg2+. 136 Mar 78

The permeabilization of Trypanosoma brucei procyclic and bloodstream trypomastigotes with digitonin permitted the quantitative estimation of a mitochondrial membrane potential of the order of 130-140 mV, in both forms, using safranine O. Dependence on substrate oxidation and response of the procyclic mitochondrial membrane potential to phosphate, FCCP, valinomycin, and Ca2+ indicate that these mitochondria behave similarly to vertebrate mitochondria regarding the properties of their electrochemical proton gradient. In contrast, in bloodstream mitochondria, development of a membrane potential was independent of substrate oxidation and dependent on hydrolysis of ATP by the mitochondrial oligomycin-sensitive ATPase, as demonstrated by collapse of the membrane potential by oligomycin and its insensitivity to the respiratory chain-inhibitor antimycin A. Mitochondria of T. brucei bloodstream forms were also able to take up Ca2+ by an electrophoretic mechanism. This is the first report of the presence of a Ca2+ transport mechanism in an eukaryotic cell devoid of complete tricarboxylic acid cycle and respiratory chain, the activities of which are known to be regulated by changes in intramitochondrial calcium concentration in other cells.
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PMID:Energization-dependent Ca2+ accumulation in Trypanosoma brucei bloodstream and procyclic trypomastigotes mitochondria. 148 49

The centrifugal elongation of membranes to form extended tubular structures is a widespread form of intracellular organelle movement. Tubular lysosomes and the endoplasmic reticulum, for example, undergo such extension in association with microtubules, and this process has been mimicked in vitro by combining purified microtubules with isolated membranes and the mechanochemical ATPase kinesin. This, along with evidence that kinesin is associated with the endoplasmic reticulum, has led to the suggestion that kinesin provides the motive force for the formation and maintenance of elongated tubulovesicular structures in cells. We have addressed this hypothesis in murine macrophages, which have prominent tubular lysosomes whose form depends on the integrity of microtubules. Here we report that two antikinesin antibodies which disrupt in vitro motility will each cause centripetal collapse of the array of tubular lysosomes when scrape-loaded into macrophages. To our knowledge this provides the first in vivo evidence that kinesin is responsible for extension of tubulovesicular structures along microtubules.
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PMID:Radial extension of macrophage tubular lysosomes supported by kinesin. 169 3

The current study was done to test the hypotheses that parafollicular granules contain a vacuolar ATPase (V-ATPase) similar to that found in chromaffin granules, that the transport of H+ into granules mediated by this enzyme drives the granular uptake of 5-hydroxytryptamine (5-HT, serotonin), and that secretagogues stimulate both the acidification of parafollicular granules and their ability to take up 5-HT by opening an anion channel in the granular membrane. Our studies indicate that parafollicular granules contain a V-ATPase that is antigenically similar to that of the V-ATPase of adrenal chromaffin granules; however, the parafollicular granular membrane differs from that of chromaffin granules in permeability to Cl- and K+. The membranes of granules derived from resting parafollicular cells appear to be relatively impermeable to Cl- but permeable to K+. Parafollicular granules (and ghosts derived from them) manifest ATP-dependent transmembrane transport of 5-HT. This transport is more dependent on the pH difference (delta pH) than on the membrane potential component of the proton electrochemical gradient across the granular membrane. Transport of 5-HT is thus inhibited more by exposure of parafollicular granules to agents, such as nigericin, that collapse delta pH than by those, such as valinomycin, that decrease transmembrane difference in potential. ATP-dependent uptake of 5-HT by granules isolated from secretagogue-stimulated parafollicular cells is greater than that into granules isolated from unstimulated cells. Since secretagogues open a Cl- channel in parafollicular granule membranes, which enhances acidification of the granules, the facilitation of 5-HT uptake by secretagogues is probably due to an increase in delta pH.
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PMID:ATP-dependent uptake of 5-hydroxytryptamine by secretory granules isolated from thyroid parafollicular cells. 182 54

The effects of guanosine 5'-triphosphate (GTP) and GTP-gamma-S, known activators of GTP binding proteins, on proton transport were investigated in endosome-enriched vesicles (endosomes). Endosomes were prepared from rabbit renal cortex following the intravenous injection of FITC-dextran. The rate of intravesicular acidification was determined by measuring changes in fluorescence of FITC-dextran. Both GTP and GTP-gamma-S stimulated significantly the initial rate of proton transport. In contrast, GDP-beta-S, which does not activate GTP binding proteins, inhibited proton transport. The rank order of stimulation was GTP-gamma-S greater than GTP greater than control greater than GDP-beta-S. GTP-gamma-S stimulation of proton transport was also observed under conditions in which chloride entry was eliminated, i.e., 0 mM external chloride concentration in the presence of potassium/valinomycin voltage clamping. GTP-gamma-S did not affect proton leak in endosomes as determined by collapse of H+ ATPase-generated pH gradients. ADP ribosylation by treatment of endosomal membranes with pertussis toxin revealed two substrates corresponding to the 39-41 kD region and comigrating with alpha i subunits. Pretreatment of the membranes with pertussis toxin had no effect on proton transport in the absence of GTP or GTP-gamma-S. However, pretreatment with pertussis toxin blocked the stimulation of proton transport by GTP. In contrast, as reported in other membranes by others previously, pertussis toxin did not prevent the stimulation of proton transport by GTP-gamma-S. These findings, taken together, indicate that GTP binding proteins are present in endosomal membranes derived from renal cortex and that activation of G protein by GTP and GTP-gamma-S stimulates proton transport in a rank order identical to that reported for other transport pathways modulated by Gi proteins. Therefore, these studies suggest that G proteins are capable of stimulating the vacuolar H ATPase of endosomes directly.
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PMID:A potential role for guanine nucleotide-binding protein in the regulation of endosomal proton transport. 185 Jul 57

In LLC-PK1 cells exposed to patulin (50 microM), lipid peroxidation, abrupt calcium influx, extensive blebbing, and total LDH release appeared to be serially connected events with each representing a step in the loss of structural integrity of the plasma membrane. The aforementioned patulin-induced events were prevented by concurrent incubation with butylated hydroxytoluene, deferoxamine, and cyclopiazonic acid, a fungal metabolite. Patulin also caused depletion of nonprotein sulfhydryls, increased 86Rb+ efflux, dome collapse, and eventually the loss of cell viability. These events were not prevented by antioxidants, results consistent with the hypothesis that they were also serially connected but occurring parallel to those previously mentioned. The earliest events observed in patulin-treated cells were the decrease in nonprotein sulfhydryls and increase in 86Rb+ efflux (5 min) which occurred before statistically significant alterations in protein-bound sulfhydryls. The increased potassium efflux (86Rb+ efflux) occurred via a pathway distinct from BaCl2, quinine, or tetraethylammonium sensitive potassium channels. This is the first published report of the antioxidant activity of indole tetramic acids (cyclopiazonic acid and cyclopiazonic acid imine). The protective effect of tetramic acids in LLC-PK1 cells was restricted to indole tetramic acids, and their prevention of lipid peroxidation did not involve iron chelation. The results of this study demonstrate that cyclopiazonic acid is a potent inhibitor of azide-insensitive, ATP-dependent, a23187-sensitive calcium uptake by the lysate of LLC-PK1 cells. This result is consistent with the hypothesis that the endoplasmic reticulum calcium transport ATPase is a sensitive target for cyclopiazonic acid in LLC-PK1 cells. These findings raise the interesting possibility that the antioxidant activity of indole tetramic acids may involve multiple novel mechanisms: surface charge alterations on the cytoplasmic surface of plasma membranes, alterations in calcium permeability in the plasma and endoplasmic reticulum membrane, and inhibition of the calcium-dependent ATPase of the endoplasmic reticulum.
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PMID:The mechanism of patulin's cytotoxicity and the antioxidant activity of indole tetramic acids. 203 42

(1) The affinity of the F0F1-ATPase from Paracoccus denitrificans for ATP during NADH-driven oxidative phosphorylation has been analyzed under different conditions by examining the type and extent of product inhibition. (2) A limited collapse of the protonmotive force (delta p) due to partial uncoupling does not increase the affinity for ATP at the active site(s) of the enzyme; instead, a partial noncompetitive inhibition becomes apparent, compatible with the binding of ATP to a noncatalytic site (or sites) with high affinity. (3) In contrast, partial inhibition of the electron-transport chain increases the extent of pure competitive product inhibition and, therefore, the affinity for ATP at the active site(s). (4) The results are interpreted as indicative of a modulation of the rate of ATP release from the active site(s) of the F0F1-ATPase which is controlled by the activity of the electron-transport chain and not by delta p.
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PMID:Kinetics of oxidative phosphorylation in Paracoccus denitrificans. 2. Evidence for a kinetic and thermodynamic modulation of F0F1-ATPase by the activity of the respiratory chain. 214 91


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