Search

The Malpighian (renal) tubule is capable of transporting fluid at remarkable rates. This review will focus on recent insights into the mechanisms by which these high rates are achieved and controlled, with particular reference to the tubules of Drosophila melanogaster, in which the combination of physiology and genetics has led to particularly rapid progress. Like many vertebrate epithelia, the Drosophila tubule has specialized cell types, with active cation transport confined to a large, metabolically active principal cell; whereas the smaller intercalated stellate cell controls chloride and water shunts to achieve net fluid secretion. Recently, the genes underlying many of these processes have been identified, functionally validated and localized within the tubule. The imminent arrival of new types of post-genomic data (notably single cell sequencing) will herald an exciting era of new discovery.

Maintenance of the appropriate intracellular concentrations of monovalent cations, mostly potassium and sodium, is a requirement for living cells, and this is particularly relevant in cells that, like unicellular fungi, are directly exposed to a changing environment. These organisms are endowed with a variety of cation influx and efflux systems. Import of potassium, which is accumulated intracellularly, is achieved by means of at least three different kind of transporters, whereas removal of the surplus of sodium (a relatively toxic cation) is accomplished by a combination of Na + -ATPases and antiporters. In addition, toxic cations can be confined in intracellular organelles, such as the vacuole, with the aid of intracellular transporters. Fine regulation of these elements allows a relatively constant ionic intracellular composition in spite of sudden changes in the external medium.

Cation transporters/channels are key players in a wide range of physiological functions in plants, including cell signaling, osmoregulation, plant nutrition and metal tolerance. The recent identification of genes encoding some of these transport systems has allowed new studies toward further understanding of their integrated roles in plant. This review summarizes recent discoveries regarding the function and regulation of the multiple systems involved in cation transport in plant cells. The role of membrane transport in the uptake, distribution and accumulation of cations in plant tissues, cell types and subcellular compartments is described. We also discuss how the knowledge of inter- and intra-species variation in cation uptake, transport and accumulation as well as the molecular mechanisms responsible for these processes can be used to increase nutrient phytoavailability and nutrients accumulation in the edible tissues of plants. The main trends for future research in the field of biofortification are proposed.

Maintenance of monovalent cation homeostasis (mainly K+ and Na+) is vital for cell survival, and cation toxicity is at the basis of a myriad of relevant phenomena, such as salt stress in crops and diverse human diseases. Full understanding of the importance of monovalent cations in the biology of the cell can only be achieved from a systemic perspective. Translucent is a multinational project developed within the context of the SysMO (System Biology of Microorganisms) initiative and focussed in the study of cation homeostasis using the well-known yeast Saccharomyces cerevisiae as a model. The present review summarize how the combination of biochemical, genetic, genomic and computational approaches has boosted our knowledge in this field, providing the basis for a more comprehensive and coherent vision of the role of monovalent cations in the biology of the cell., Work supported by grants GEN2006-27748-C2-1 and EUI2009-04147 (Ministerio de Ciencia e Innovación, Spain, and FEDER) to J. A.; grants 826.06.004 and 826.09.006 (Netherlands Organization for Scientific Research (NWO)—Earth and Life Sciences (ALW)) to G. P.H. v. H.; grants GA CR P503/10/0307, GA AS CR IAA500110801 and MSMT COSTLD13037 to H. S. and BBSRC grants to C. P.

Maintenance of monovalent cation homeostasis (mainly K(+) and Na(+)) is vital for cell survival, and cation toxicity is at the basis of a myriad of relevant phenomena, such as salt stress in crops and diverse human diseases. Full understanding of the importance of monovalent cations in the biology of the cell can only be achieved from a systemic perspective. Translucent is a multinational project developed within the context of the SysMO (System Biology of Microorganisms) initiative and focussed in the study of cation homeostasis using the well-known yeast Saccharomyces cerevisiae as a model. The present review summarize how the combination of biochemical, genetic, genomic and computational approaches has boosted our knowledge in this field, providing the basis for a more comprehensive and coherent vision of the role of monovalent cations in the biology of the cell.

Planar lipid bilayers made from synthetic lipids and incorporating ionophores such as valinomycin or gramicidin, provide a useful tool for the study of membrane transport. Fusion of vesicles from native cell membranes into planar lipid bilayers provides information concerning ion channels that complements what is available from patch-clamp studies. Selective ion channels in planar lipid bilayers display high transport rates and low temperature coefficients, characteristics that distinguish ion channels from other modes of mediated transport. Finally, the features of cation transport by gramicidin in planar lipid bilayers are summarized. Detailed kinetic studies have shown that transport of cations and water through gramicidin occurs by means of single-file diffusion in which the ions and the waters cannot pass each other in the narrow part of the pore. The cation binds at a site near either end of the pore and then is driven over an energy barrier in the pore by the electrochemical gradient. The rate-limiting step, however, is the dissociation of the ion from the channel.

Isolated tomato fruit cuticles were subjected to low dose (80 Gy) γ-irradiation, as a potential methodology to prevent harvested fruit and vegetables spoilage. Both irradiated and non-irradiated samples have been morphologically and chemically characterized by scanning electron (SEM), atomic force (AFM), attenuated total reflectance Fourier transform infrared (ATR-FTIR) and X-ray photoelectron (XPS) spectroscopies. Additionally, electrochemical measurements comprising membrane potential and diffusive permeability were carried out to detect modifications in transport properties of the cuticle as the fruit primary protective membrane. It has been found that low dose γ-irradiation causes some textural changes on the surface but no significant chemical modification. Texture modification is found to be due to a partial removal of outermost (epicuticular) waxes which is accompanied by mild changes of electrochemical parameters such as the membrane fixed charge, cation transport number and salt permeability. The modification of such parameters indicates a slight reduction of the barrier properties of the cuticle upon low dose γ-irradiation. © 2012 Elsevier Ireland Ltd.

The growth of barrier-type anodic films at high efficiency on a range of sputtering-deposited Al–Hf alloys, containing from 1 to 95 at.% Hf, has been investigated in ammonium pentaborate electrolyte. The alloys encompassed nanocrystalline and amorphous structures, the latter being produced for alloys containing from 26 to 61 at.% Hf. Except at the highest hafnium content, the films were amorphous and contained units of HfO 2 and Al 2 O 3 distributed relatively uniformly through the film thickness. Boron species were confined to outer regions of the films. The boron distributions suggest that the cation transport number decreases progressively with increasing hafnium concentration in the films, from ∼0.4 in anodic alumina to ∼0.2 for a film on an Al–61 at.% Hf alloy. The distributions of Al 3+ and Hf 4+ ions in the films indicate their similar migration rates, which correlates with the similarity of the energies of Al 3+ –O 2− and Hf 4+ –O 2− bonds. For an alloy containing ∼95 at.% Hf, the film was largely nanocrystalline, with a thin layer of amorphous oxide, of non-uniform thickness, at the film surface. The formation ratios for the films on the alloys changed approximately in proportion to the hafnium content of the films between the values for anodic alumina and anodic hafnia, ∼1.2 and 1.8 nm V −1 respectively.

Publisher Summary This chapter discusses the hormonal modulation of autoimmune diseases, glucocorticoids (GCs). GCs are the most effective and most used immune modulators in systemic auto-immune diseases. GCs mediate fascinating anti-inflammatory and immunomodulatory effects when used therapeutically. There are many specific effects of the commonly used GC drugs, which include prednisone, prednisolone, methylprednisolone, or dexamethasone. GCs at high concentrations are able to intercalate into cellular membranes, such as plasma and mitochondrial membrane, and change their properties. This is the basis for non-specific non-genomic effects, possibly mediated by changes in the cation transport through the plasma membrane and in the proton leak of the mitochondria. These physicochemical interactions with biological membranes are very likely to be the key to the very rapid immunosuppressive and anti-inflammatory effects of high-dose GCs. These high GC concentrations are achieved by intra-articular GC injections or intravenous GC pulse therapy. Several risk factors are involved in the pathogenesis of chronic inflammatory diseases, including genetics, chronic infections, sex hormones, and stress.

The sarco-endoplasmic reticulum Ca 2+ ATPase (SERCA) utilizes ATP for active transport of cytosolic Ca 2+ into membrane-bound intracellular compartments. The catalytic and transport cycle of SERCA includes formation of a phosphorylated enzyme intermediate and protein conformational changes, leading to vectorial translocation of Ca 2+ across the membrane. Intracellular storing of Ca 2+ by SERCA is an integral part of signaling mechanisms for a large number of cellular functions. Ca 2+ signaling and excitation-contraction coupling in cardiac muscle offer a prominent example of SERCA involvement in physiology and pathology.

The adsorption behavior of Cs+ and Ba2+ cations on magnesite has been studied as a function of time, cation concentration and temperature, utilizing both the radiotracer method and X-ray photoelectron spectroscopy (XPS). Saturation was approached in about I day for both cations. The sorption data were found to follow Freundlich type isotherms. Sorption of both Cs+ and Ba2+ cations were found to be exothermic in nature with ΔH0 (kJ/mol) of -37, -13 and ΔS0 (kJ/mol·K) of -0.09, -0.009, respectively. Negative ΔG0 values were obtained for both cations, indicating the spontaneity of their sorption on magnesite. The magnitude of ΔG0 suggest that ion exchange is the dominating sorption mechanism. The adsorption behavior of Cs+ and Ba2+ cations on magnesite has been studied as a function of time, cation concentration and temperature, utilizing both the radiotracer method and X-ray photoelectron spectroscopy (XPS). Saturation was approached in about 1 day for both cations. The sorption data were found to follow Freundlich type isotherms. Sorption of both Cs+ and Ba2+ cations were found to be exothermic in nature with ΔH0 (kJ/mol) of -37, -13 and ΔS0 (kJ/mol·K) of -0.09, -0.009, respectively. Negative ΔG0 values were obtained for both cations, indicating the spontaneity of their sorption on magnesite. The magnitude of ΔG0 suggest that ion exchange is the dominating sorption mechanism.

The effects of olive oil (OO)- and high-oleic sunflower oil (HOSO)-enriched diets on erythrocyte membrane cholesterol distribution (by means of cholesterol oxidation after continuous cholesterol oxidase treatment) and Na+Li+ countertransport activity in control subjects and patients with untreated essential hypertension (with or without concomitant hypercholesterolemia) have been examined. The participants were 12 normotensive and sixteen hypertensive women who consumed in randomized order the two monosaturated fatty acid (MUFA) diets over 4-week periods separated by a 4-week washout period. The half-times for cholesterol oxidation were significantly higher in hypertensive women, ranging from an increase of 38 to 57% in the normo-(20.6 ± 2.8 min; P < 0.001) and hyper-(23.4 ± 4.2 min; P < 0.001) cholesterolemic groups. There was a general decrease by 75% in the half-time for cholesterol oxidation after HOSO diet. Interestingly, the oxidation rates were almost normalized after OO diet. The activity of Na+Li+ countertransport was found to be significantly higher in hypertensive women, ranging the increase from 22 to 57% in the normo-(0.314 ± 0.043 mmol × [h × liter cell]−1; P < 0.01) and hyper-(0.405 ± 0.086 mmol × [h × liter cell]−1; P < 0.01) cholesterolemic groups. This transport system was further activated after HOSO diet, while almost restored after OO diet. These findings suggest that dietary OO, but not HOSO, is helpful for normalizing the impaired distribution of membrane cholesterol and reducing elevated activity of Na+Li+ countertransport in erythrocyte of hypertensive patients. This action of OO also indicates that alterations in membrane cholesterol distribution may be relevants for the pathogenesis of hypertension. The effects, however, cannot be exclusively attributed to the content of MUFAs (mainly oleic acid) in the diet, as HOSO was unable to induce favorable changes.

Hypertension is associated with some abnormalities in cell membrane structure, including an impaired distribution of cholesterol into the monolayers of erythrocyte membrane. Transbilayer movement of membrane cholesterol modulates the formation of these structural cholesterol domains. We tested whether the rate of cholesterol movement may influence on the erythrocyte Na+-Li+ countertransport, that is a marker of human essential hypertension. In single regression analysis, the half-time for the decrease in specific radioactivity of cholestenone (inverse of membrane cholesterol transbilayer movement) was negatively related to the erythrocyte cation flux mediated by Na+-Li+ countertransport (r = -0.8983, P, Supported by a grant (AL1950073) from Comision Interministerial de Ciencia y Tecnologia (CICYT), and the Andalusian Heart Service.

Publisher Summary Each of the cation transport cycles in mitochondria is driven by the vectorial proton-pumping redox enzymes. Therefore, each cycle is a futile cycle that diverts energy away from adenosine triphosphate (ATP) synthesis. However, these energy expenditures are not wasted. Each of these cycles serves a vital function in cell regulation and homeostasis. This chapter describes protocols currently in use in the laboratory for the study of mitochondrial cation porters. The methods entail partial purification, reconstitution into liposomes, and assay of ion flux using fluorescent probes. They yield partially purified, reconstitutively active proteins that are free of the complexities of the intact organelle and are highly useful for the study of the mechanisms of transport and regulation. The chapter discusses the mitochondrial proton cycle, mitochondrial potassium cycle, and mitochondrial calcium cycle and the various methods and material used for obtaining the mitochondrial porters. It concludes with defining various methods for the reconstitution of cation porters into liposomes.

Publisher Summary This chapter focuses on the cation transport probes that is it describes the various the amiloride series. Several recent studies have used radiolabeled amiloride analogs to identify and characterize binding sites of amiloride analogs in plasma membranes or microsomal membranes derived from cells known to have amilofide-sensitive transporters. Methods for assaying the binding of amiloride analogs to membranes and to detergent-solubllized proteins are outlined in this chapter. The use of amiloride and its analogs in the study of ion transport requires a knowledge of the pharmacology of inhibition of transport proteins, and of effects on enzymes, receptors, and other cellular processes, such as DNA, RNA, and protein synthesis, and cellular metabolism. This chapter reviews the pharmacology of inhibition of these processes by amiloride an its analogs, as well as the use of amiloride analogs as potential probes for the characterization of ion transport systems.

Publisher Summary This chapter presents experiments designed to establish the role of band 3 as the protein responsible for information transfer between the extracellular ouabain binding site of the Na, K-ATPase and the cytoplasmic glycolytic enzymes. The three components [Na, K-ATPase/band 3/G3PDH] are linked together so closely that conformational information can be transferred through this complex from the outside of the cell to the inside. In view of the presence of binding sites for G3PDH and aldolase on band 3, experiments are carried out to determine whether conformational information could be transferred between band 3 and the glycolytic enzymes, using the 31p resonance shift of 2,3-diphosphoglycerate (2,3-DPG) to report on the conformation of MPGM for which 2,3-DPG is an obligatory substrate. The inhibitor, 4, 4’-diisothiocyano-2, 2’-stilbenedisulfonic acid (DIDS) is a specific inhibitor of anion transport in band 3. Treatment of red cells with DIDS affects the resonance of 2, 3-DPG as does treatment with the cardiac glycoside cation transport inhibitor, ouabain, which is specific to the Na, K–ATPase. The DIDS experiments show that band 3 is coupled to the glycolytic enzyme system.

This chapter describes the O 2 -induced intramembrane H + movements in the mitochondrial membrane. Energy coupling to electron transport from substrate hydrogen, namely, NADH, succinate, and L-3-glycerolphosphate, to O 2 is mediated by the electron transport carriers and coupling enzymes located in the inner mitochondrial membrane. The transition of the membrane from the de-energized to the energized form is readily accomplished by addition of O 2 to an anaerobic suspension of membrane fragments called submitochondrial particles (SMPs). SMPs prepared from rabbit skeletal muscle mitochondria by high pressure disruption are open fragments of membrane that are infinitely leaky but retain the capacity for energy coupling to electron transport. They provide, therefore, a uniquely useful test system for studying movements of H + —postulated to be the currency of mitochondrial energy conservation—between membrane and medium, and within the membrane in the absence of net transmembrane cation transport. Energization of SMP by adding O 2 to the anerobic suspension containing either succinate or L-3-glycerolphosphate as substrate results in a biphasic efflux of H + into the medium; the more rapid initial phase requires 1–2 min and the slower phase occurs through the aerobic steady state. Uptake of H + on anerobiosis is also slow. The energy-linked H + efflux induced by O 2 is little affected by added valinomycin plus KCl or by added KSCN showing that net transmembrane H + or K + transport does not occur in these open-membrane fragments.

Publisher Summary This chapter focuses on the sodium-potassium-activated adenosinetriphosphatase. Most living animal cells are characterized by a high-intracellular K + concentration and a low Na + concentration, whereas the surrounding plasma and tissue fluid have a high Na + and a low K + concentration. These cation gradients, across the cell membrane, are gradually abolished upon cell death or upon inhibition of the cellular energy metabolism by cooling, withholding of glucose or application of suitable inhibitors. This suggests that the maintenance of the cation gradients requires metabolic energy. Radioisotope experiments have indicated that during steady-state conditions there is a passive influx of Na + ions and a passive efflux of K + ions. The chapter answers how this cation pump system is related to the cellular energy metabolism and what is the nature of the pump system. The findings suggest that the cation pump is not directly linked to some step in the glycolytic or oxidative phosphorylation chains, but rather that it is the adenosinetriphosphate (ATP) produced by these systems that provides the energy for cation transport. Direct evidence for this conclusion comes from experiments in which ATP is placed inside cells or cell membrane vesicles, which lack any other energy source.

Publisher Summary Monovalent-cation fluxes tightly coupled to the presence of chloride constitute a major portion of the ouabain-resistant (OR) “leak” cation transport in a variety of biological membranes. This chapter describes the properties and the occurrence of Cl-dependent cation transport in differentiated cells or cell lines and discusses their changes during cellular differentiation and development. One of the measures for changes throughout differentiation is the response of Cl-dependent cation transport during cell activation commissioned by a variety of stimuli, such as culture media, hormonal, and ionic effectors, which are used to gain insight into the role of other passive and active membrane transport systems, such as the Na–H exchanger or the Na–K pump in differentiation. Unlike the Na–H or K–H antiports, the K–Cl, Na–Cl, and Na–K–2Cl cotransporters do not play a primary role in the signal transduction for the cell to rise from the quiescent to the proliferating state and to grow and differentiate. Rather, the cotransport systems with their enormous mass transport capability serve to re-establish quickly the cellular volume perturbed by growth initiation events involving other transport systems.

Publisher Summary This chapter discusses the monovalent cation transport and mechanisms of digitalis-induced inotropy. The studies presented in the chapter establish hypothesis that inhibition of the sodium pump is causally related to the positive inotropic effect of cardiac glycosides at subtoxic doses or concentrations in the intact animal. A hypothesis is that positive inotropic effects of digitalis glycosides are mediated by Na+–pump inhibition via increased [Na]i and consequently increased [Ca]i through the Na/Ca exchange carrier mechanism. For testing a hypothesis, a cardiac preparation is sought with properties, including (1) a substantial positive inotropic response to cardiac glycosides, and a means to monitor and record that response, (2) rapid onset of a stable inotropic response to glycoside exposure, with rapid offset following washout, (3) the means to assess, in as direct a way as possible, the effects of cardenolides on unidirectional and net fluxes of Na+, K+, and ca2+ with adequate temporal resolution under conditions identical to those under which inotropic state is determined, and (4) the means of resolving components of Ca fluxes in beating preparations under basal and glycoside-stimulated conditions.

Publisher Summary This chapter considers the systems responsible for cation transport in cyanobacteria, excluding proton and NH4 +3 transport. Cells use both primary and secondary transport systems for ion translocation across their membranes. Primary systems couple chemical energy directly to the performance of electrochemical work resulting in an ion gradient. In the reversed mode osmotic energy can be coupled to chemical bond formation. In bacteria as well as in eukaryotic cells, demonstration of the existence of a particular ion-transport system, and determination of its apparent kinetics and energy source, can be achieved in intact cells. Measurement of intracellular ion concentration, as dependent on extracellular concentration and on cellular metabolic state, is the routine experiment for identifying an active transport system in vivo. For calculation of intracellular ion concentration it is assumed that the intracellular ion is free in solution. The intracellular osmotic volume can be determined by any of the techniques already published and thoroughly discussed. An innovative approach for measuring intracellular volume has been recently developed in cyanobacteria. This method is based on spin-labeled, metabolically inert, small molecules that rapidly equilibrate across the cytoplasmic membrane.

It has been shown that: (a) the EPR-detectable distribution of a spin-labeled weak acid provides an accurate method to determine the intravesicular pH in H. halobium , whereas a spin-labeled amine is of more limited utility for such measurements because spectra obtained may need to be corrected by computer subtraction methods for amounts of the probe that are membrane bound; (b) precise quantitation of the pH gradient necessitates the simultaneous determination of the intravesicular volume, which is readily measured as the ferricyanide-inaccessible space using the spin probe Tempone; (c) spin probe movement rapidly monitors light-induced cation transport, provided this movement is linked to or causes proton movement. The quantitative data obtained with this technique reveal a maximum light-induced pH gradient of two units across H. halobium membranes at acidic bulk pH values.

Publisher Summary This chapter reviews that calcium pump of Sarcoplasmic Reticulum (SR) serves as a general model of cation transport across biological membranes, coupled to utilization of ATP. Coupling of ATPase transport activities occurs by long range interaction of phosphorylation (catalytic) and calcium binding sites within the ATPase protein. Kinetic resolution of calcium binding was obtained with the aid of radioactive tracer and a rapid filtration apparatus. A most interesting phenomenon is observed when SR vesicles preincubated with calcium are exposed to lanthanum. In this case, half of the bound calcium is displaced rather quickly, while the other half is displaced more slowly in several seconds. The idea that cations may be occluded within the ATPase protein during intermediate states of the catalytic and transport cycle was first suggested by studies on the Na + –K + ATPase and then on the Ca 2+ ATPase. Following formation of the phosphorylated enzyme intermediate, the two calcium ions bound lose their ability to be displaced by EGTA or lanthanum when added to the medium outside the SR vesicles.

It is shown that the electronic conductivity of a nonstoichiometric oxide that is an n-type semiconductor (i.e., a reduction semiconductor) can control the cation transport through the oxide, and hence, the rate of oxidation. This argument leads to a mathematical derivation of the logarithmic oxidation rate law. If the oxide is an oxidation semiconductor, it is p-type, and the hole conductivity will be sufficiently great that the rate-limiting property will be ion transport, and not electronic conduction. In this case, the Wagner ion-electron theory of the parabolic oxidation rate law will apply. The microscopic nature of the crystalline solids is discussed, including the gradients of composition and of conductivity across the oxide layer, and how these effects influence the oxidation rate.

Publisher Summary The involvement of the plasma membrane ATPase in the energization of solute transport in yeast is generally accepted. The way that ATPases transfer the energy from ATP to monovalent cation transport is still a matter of speculation. This chapter highlights the membrane potential of metabolizing cells of Saccharomyces cerevisiae, strain Delft 2, at varying medium pH values. The pmf was calculated from the pH difference between cells and medium and from the membrane potential. The cellular monovalent cation content can regulate the uptake of monovalent cations. During K+ absorption, the net uptake rate decreases gradually. This is caused by an increase in the K+ efflux. The uptake of K+ may be regulated by the cell pH. On increasing the K+ content of the cells, the cell pH increases, slowing down the influx of K+. There are no indications that the membrane potential is decreased much during K+ or Rb+ uptake. In plant roots, the Rb+ influx is decreased during K+ uptake.

Publisher Summary This chapter discusses concepts and techniques originating outside the bacterial world, and especially upon Peter Mitchell's chemiosmotie hypothesis. It aims to construct a framework to hang the present and future experimental data on. The chapter describes the balance of the charge account and with the physical state of the cytoplasmic ions. It discusses the use of ionophores. It also discusses the theoretical concepts, surveys methods for measuring membrane potentials and then turns to the emerging evidence for electrogenic proton transport as the metabolic basis of electrical potentials across bacterial membranes. The emerging evidence for electrogenic transport of protons by vectorial ATPase and a respiratory chain, with the generation of an electrical potential across the plasma membrane are discussed. The chapter discusses the transport of the alkali metal ions and the interrelationship of the cation fluxes. It also discusses the significance of the cations transport process.

Publisher Summary This chapter discusses the regulation of Na, K–ATPase by its biosynthesis and turnover. The regulation of alkali cation transport in a cell or tissue is in general brought about by the interaction of two processes: modulation of the number of copies of Na, K–ATPase and modulation of the activity of existing copies by appropriate ligands and substrates. Na, K–ATPase is a very dynamic protein. It is constantly being synthesized and turned over in many tissues and is susceptible to up and down regulation in number of copies in response to a variety of environmental stimuli. In the biosynthesis of this enzyme membrane and secretory proteins are generally believed to be synthesized in the rough endoplasmic reticulum (RER), being cotranslationally inserted partially (membrane proteins) or completely (secretory proteins) into the lumen of the RER. In the chapter, the small hydrophobic signal sequence is removed and common oligosaccharide precursors are transferred to the peptide backbone. RER-derived vesicles containing these proteins are then shuttled to the Golgi apparatus for further processing occurs. The proteins are then sorted via some unknown process and transferred to specific organelles, or in the case of secretory products, exported out of the cell by exocytosis. The translocation and covalent modification of the proteins accounts for a several-hour lag or “transit” time between translation and the arrival at their functional destination.

In this work, the effects of amphiphilic and other molecules with chain lengths from 2 to 8 C on K+ transport were studied; besides, several polar heads were tested. Guanidine being the most effective of these polar heads, the work was focused in the study of alkylguanidines, as well as some other guanidine derivatives. These molecules seem to be rather effective and specific inhibitors of K+ transport in yeast. KEYWORDS Potassium transport, yeast cation transport, monovalent cation transport, guanidines, alkylguanidines, transport in yeast.

Publisher Summary The characteristics of the Na,K-transport reaction lead one to assume that the translocation process involves motion of protein components in the pump. The α subunit with Mr 94–106,000 forms binding sites for nucleotides and cardiac glycosides, and is phosphorylated from ATP. The 3-subunit, a sialoglycoprotein with protein Mr 32-38,000 forms a part of the extracellular binding area for cardiac glycosides, but other catalytic functions have not been associated with this protein. Two principal conformations of the a-subunit, E1 and E2, are defined by two patterns of tryptic cleavage and by different intensities of fluorescence from tryptophan or extrinsic probes. This chapter discusses the conformational responses in preparations of soluble and fully active αβ-units in dodecyl-octa-ethylen-glycol-monoether (C12E8) to address whether the transitions between El and E2 forms consist of rearrangement within one α-subunit or if interaction between protomer αβ-units is required. The chapter discusses the use of selective proteolytic modification of the α-subunit to identify functional domains of the protein and to study the relationship between structural changes in the protein, saturation of high affinity ligand binding sites and flipping of cation sites between inside exposed and outside exposed states.

Publisher Summary This chapter discusses the regulation of brain Na, K-ATPase by Norepinephrine and an endogenous inhibitor. One important question about Na, K-ATPase is whether there are specific mechanisms whereby populations of enzyme molecules having specialized physiologic functions are regulated. The possible presence of two enzyme forms raises the question of whether they are differentially regulated by physiologic enzyme effectors. A compound in brain extracts and plasma that reduces cardiac glycoside binding, cation transport, and enzyme activity is one candidate for such an effector, especially because the endogenous inhibitor appears to interact with the cardiac glycoside binding site and affinity for cardiac glycosides may distinguish the two enzyme forms. The experiments reported in the chapter examine: (1) the interactions of endogenous inhibitor and norepinephrine with high-affinity ouabain binding, (2) interactions between norepinephrine and endogenous inhibitor in vitro , and (3) the effects of persistent changes in exposure to norepinephrine in vivo on high-affinity ouabain binding. The material in brain extract reduced stimulation of enzyme activity by norepinephrine, decreasing maximal stimulation 3-fold and apparent affinity 10-fold.

Publisher Summary This chapter discusses the membrane transport events accompanying the cellular maturation of erythrocyte precursors into enucleate high potassium (HK) as well as low potassium (LK) red cells. This study is of interest because of its general aspect of the relationship between active and passive cation transport systems during cellular differentiation and maturation. The basic strategy of the study is to isolate cells by centrifugal elutriation and characterize by flow cytophotometry the reticulocyte population newly produced in response to massive bleeding. Then eluate is measured in the separated cells active K+ pump and passive K+ leak fluxes, the number of ouabain binding is also measured, this leads to pump sites per cell. In the course of the response of both HK and LK sheep to massive hemorrhage, the simultaneous appearance in the peripheral blood of two distinct reticulocyte populations was detected cyto­photometrically. As a result, a large macrocytic population (population M cells) and a small population with rather normocytic reticulocytes whose cell volume was indistinguishable from that of the older LK red cells.

Publisher Summary This chapter reviews that sensitive and accurate kinetic measurements of ionized metal ions are central for the understanding of ion transport in biological systems. Among the various cations interacting with biological materials, H + , Ca 2+ , and Mg 2+ have a key role in regulating enzyme activity and cellular functions. Hence, the study of H + , Ca 2+ , and Mg 2+ transport acquires particular significance for the understanding of (1) energy transduction and cellular events such as contraction, secretion, fusion, and excitation and (2) more complex events such as vision and hormone and neurotransmitter action. The chapter also discusses the methods for measuring H + , Ca 2+ , and Mg 2+ , which includes isotope distribution, atomic absorption spectroscopy, specific electrodes, photoluminescent, fluorescence, and absorbance indicators. All of these methods present advantages and disadvantages with respect to selectivity, specificity, time resolution, and sensitivity. It also reviews that metallochromic indicators are substances that undergo color changes when the concentration of free metal ion in the solution changes. The color difference between the free indicator and the indicator-metal ion complex is often very large and can be used for quantitative determination of the concentration of free metal ion in solution.

The bentonite-based engineered barrier or packing is a proposed component of several designs conceived to dispose of high-level nuclear waste in geologic repositories. Once radionuclides escape the waste package, they must first diffuse through the highly impermeable clay-rich barrier before they reach the host repository. To determine the effectiveness of the packing as a sorption barrier in the transient release period and as a mass-transfer barrier in the steady release period over the geologic time scales involved in nuclear waste disposal, a fundamental understanding of the diffusion of electrolytes in compacted clays is required. We present, and compare with laboratory data, a model quantifying the diffusion rates of cationic cesium and uncharged tritium in compacted montmorillonite clay. Neutral tritium characterizes the geometry (i.e., tortuosity) of the particulate gel. After accounting for cation exchange, we find that surface diffusion is the dominant mechanism of cation transport, with an approximate surface diffusion coefficient of 2 x 10 W cmS/s for cesium. This value increases slightly with increasing background ionic strength. The implications of this work for the packing as a migration barrier are twofold. During the transient release period, K/sub d/ values are of little importance in retarding ion migration. This ismore » because sorption also gives rise to a surface diffusion path, and it is surface diffusion which controls the diffusion rate of highly sorbing cations in compacted montmorillonite. During the steady release period, the presence of surface diffusion leads to a flux through the packing which is greatly enhanced. In either case, if surface diffusion is neglected, the appropriate diffusion coefficient of ions in compacted packing will be in considerable error relative to current design recommendations. 11 refs., 4 figs., 1 tab.« less

Publisher Summary This chapter discusses several topics concerning Na, K-ATPase. Na, K-ATPase is an entity that exchanges Na+ and K+ ions across the cell membrane against an electrochemical gradient, which is accompanied by ATP hydrolysis. This enzyme has many characteristics, some of which are specific for this enzyme. The enzyme binds ATP at a ratio of 1:1. The binding is not affected by the presence of Mg or Na, but K increases the dissociation constant and Na antagonizes the effect of K. The binding site is closely related to the active site according to Post, but there have been some experiments suggesting that ATP also acts aliosterically with Na on p-NPase activity. In the presence of a small amount of magnesium ion, or with NEM-treated enzyme, an ADP-ATP exchange react ion is observed and the reaction is accelerated by Na. The fact that cardiac glycosides specifically inhibit both cation active transport and Na, K-ATPase has been taken as strong evidence for the physiological entity of the enzyme in the active cation transport. The ratio between maximum binding of ouabain and the E-P in the presence of Na and Mg is 1:1.

Publisher Summary This chapter describes some recent developments in the field of alkali cation transport. It focuses on problems of sodium and potassium movements. Dean stated that the cell membrane is permeable for sodium as well as for potassium and that the leakage of sodium into the cells is compensated by a sodium pump located in the membrane. For those cell types in which the distribution of not only sodium but also potassium is far from equilibrium, the leak and pump concept possibly has to be extended to potassium, assuming the existence of a potassium pump. On the basis of various evidence, the conclusion was drawn that the leak and pump are spatially separated—leakage occurs by simple diffusion, possibly through aqueous channels, while the pump operates across a lipoid layer. The leak and pump system is utilized for specific cell functions, such as the excitation and recovery cycle in excitable tissues and the ion transports across epithelium cells in the frog skin and in the kidney.

Publisher Summary This chapter discusses electron transfer and energy conservation. Band theory applies well to metals and many covalent semiconductors and organic crystals. Proteins alone and groups of molecules of the chlorophyll or carotenoid type can act as metallic-type electron conductors. A plausible mechanism for electron transfer involves initial transfer of redox equivalents to or from the solvent or its ion-dissociation products, namely, OH − and H + . When no external energy is supplied, electron transfer goes downhill, but when it is applied, there is higher apparent activation energy than for the forward reaction. Porphyrins and chlorophylls can form complexes with acceptors in solution, and they are flat aromatic molecules peculiarly suitable to charge transfer interactions and reactions with other flat aromatic molecules. Weak acid anions that uncouple act in a highly specific way, suggesting that they act in the membrane and cannot act in the free solution. Cation transport does not require phosphorylation, and it is easy to set up transfer systems involving proton movements that could also transport cations. The structure of biological systems permits the nonequilibrium condition between different parts of space and this condition, which in turn permits energy transformations of a kind, is unknown in models

Publisher Summary This chapter discusses the influence of the plasma proteins on the transport of various cations in the blood and the intercellular space. Cations normally occur in the plasma in three different forms: (1) as free ions, (2) as complexes with substances of low molecular weight, and (3) as complexes with proteins. The free cations and the cations bound to substances of low molecular weight in the blood and in the extracellular compartment equilibrate rapidly. Both pass into the ultrafiltrate of the kidneys; however, they often differ in their capacity to penetrate cell membranes and in their effect on cellular function. The biological significance of the complex between calcium and specific plasma components is apparent from investigations of the kinetics of the reactions occurring in association with the coagulation of blood. The specific function of complexes between magnesium and specific plasma components is clear from examinations of the complement–properdin system. In these two cases, the calcium and magnesium are not biologically equivalent. This indicates more specific types of binding than for the bulk of the calcium and magnesium normally bound by the plasma proteins.

Publisher Summary Though other isomers occur naturally, the myo form of inositol (I) is most widely distributed in living tissues. It has been found in tapeworms, whales, land mammals, fruit flies, fish, spinach, bacteria, tobacco plants, yeasts, algae, and arid acorns. One important aspect of inositol function lies in the provision of these lipids that are almost as widely distributed as inositol itself. This chapter discusses the chemical structures, biosynthesis, and catabolism. The chapter focuses on the assessment of the possible relation between inositide metabolism and cation transport. It summarizes the present knowledge of the distribution of phosphoinositides among living organisms. A closer study of the relation between nervous activity arid the metabolism of the inositol lipids should be rewarding. In view of the widespread interest in mitochondrial function, particularly oxidative phosphorylation, there is no doubt that, studies of these lipids in mitochondria will be actively pursued.