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Liburdy 49 has detected an increase in cal-cium uptake into mitogen-stimulated rat thymocytes mature and human lymphocytes during exposure to 60 Hz ELF field or high-field NMR Nuclear Magnetic Resonance. As NMR fields contain a time-varying magnetic field, this result implies that the time-varying field of NMR is an operative com-ponent responsible for the effect on calcium transport.

Active transport - Wikipedia

The maximum flux density of the steady component of high-field MRI is about2T, similar to that produced by our electromagnet. Many investigators have also shown insignificant influences on various cellular functions of exposure to a similar magnitude static or DC magnetic field. However, MRI magnetic fields contain a non-homogeneous gradient component in addition to homogeneous static and time-varying components. Further investigation is needed on the effects on cellular functions of the gradient field that exerts Maxwell stress. We propose these sites based on our data and those of others and describe them in detail in the legend to Fig.

Liburdy et al. This is because electric field or eddy current induced by the magnetic field does not effectively penetrate the outer cell membrane. Since the cell membrane bilayer would act as an electric insulator. The ELF magnetic field will induce only a small eddy current inside the cells.

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Therefore, the eddy current induced in the medium could affect the surface electrical prop, Jpn J Physiol, 2. Biochim Biophys Acta, 3. Effects on fluorescence of a potential-sensitive cyanine dye.

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Biochim Biophys Acta, 4. J Biol Chem, Cell Struct Funct, 6. J Membrane Biol , 7. Biochim Biophys Acta , 8. Structure and kinetics.

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Academic Press, New York, , pp. J Cell Physiol , J Cell Physiol, Biomed Res, J Biol Chem , J Membr Biol , Biochim Biophys Acta , J Gen Physiol , Biochim Biophys Acta, Am J Physiol : CC, J Membrane Biol, J Mem-brane Biol, J Membrane Biol , Tokushima J exp Med , Am J Physiol CC, Jpn J Physiol SS, In : Ueno S, ed.

Although RND metal efflux systems seem to be primarily responsible for detoxification of periplasmic metals 57 , it has been suggested that the system would also transport cytosolic metal across the plasma membrane However, despite the large gradient, transport is quickly inhibited 20 s. Their importance is highlighted, for instance, by the embryonic lethal phenotype resulting from the Ctr1 gene knock-out in mice Ctr proteins are homotrimers.

However, this model still needs to be supported by strong experimental evidence and has to take into account the role of metal-accepting chaperones. The second site has not been identified, but likely candidates are the N-terminal Met-rich region or the HCH motif at the C terminus The functional roles of the N-terminal region, amino acids in the transmembrane region, and C-terminal HCH motifs have not been well defined.

However, mutation of transmembrane Met located in TM2 does not abolish metal flux, although it decreases the rate of transport In eukaryotes, they are localized in the plasma membrane and in organelles vacuole, endoplasmic reticulum, Golgi, etc.

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The functional forms of the transporters are homodimers. The topology of the subunit, six TMs and a cytoplasmic hydrophilic C-terminal domain, is well conserved among all family members Fig. A significant understanding of the mechanism of CDFs emerged from the biochemical characterization and structural studies of the E.

YiiP is a homodimer of two kDa subunits in 2-fold symmetry Fig. Site I binds the transported metal 15 , The site is composed of two Asp residues in TM2, one His residue, and one Asp residue in TM5, binding the metal with tetrahedral coordination. Mutation of these residues prevents metal transport 23 , 81 , This analysis also highlights the relatively low transport rate of metal transporters 2. Although it has been speculated that the conserved C-terminal domain site III might act as a metallochaperone 15 , there is no experimental evidence for this hypothesis.

Metal binding in this site contributes to the stabilization of the interaction between the C-terminal domains of each monomer. This appears to be a regulatory mechanism by which the functional transporter is assembled when excess substrate is present. In the last few years, the first high-resolution structures of representative members of some of the main metal transporter families have been obtained.

Further progress is expected in this direction with the structural characterization of other metal transporter families such as ZIP and Nramp and with further refinements of already determined structures in all their conformational stages. This will help establish the structural and functional determinants that lead to distinct metal transport mechanisms and transport specificity required by the cell to handle fundamental but highly toxic transition metal ions.

However, to validate the accuracy of novel models, similar advances in biochemical and biophysical studies will be required. Because of their importance in metalloprotein assembly and, consequently, in overall cell physiology, the determination of the precise interaction mechanism of metal transporters and metal-delivering and metal-accepting chaperones is one of the areas in which significant developments are likely. We thank Dr. Dempski and M. Emmert for critical reading of the manuscript and helpful discussions.

Kaplan, personal communication. Raimunda, T. Stemmler, and J. Valderrama, unpublished data. You'll be in good company. Journal of Lipid Research. Previous Section Next Section.

Previous Section. Google Scholar. CrossRef Medline Google Scholar. Robinson N. Osman D. Outten C. Science , — Nies D. FEMS Microbiol. Blaustein M. Wandersman C. DiDonato R. Plant J. Noinaj N. Colangelo E. Plant Biol. Dumay Q. Nevo Y. Acta , — Medline Google Scholar. Gourdon P. Nature , 59 — Barry A. Raimunda D. Biometals 24 , — Aller S.

Liu J. Biochemistry 45 , — Biometals 20 , — Pearson R. CrossRef Google Scholar. Wei Y. Holm R. De Feo C. Considering the remarkable specificity of the transporters, it is not surprising that sometimes there are defects in transport systems. Nowadays, several different diseases known to be due to transport defects.

Cation Flux Across Biomembranes

In many of the cell membrane diseases, proteins do not transport materials properly. Some of the membrane transport disease are hereditary. An archetypical example of such transport diseases is Cystinuria , an inherited autosomal recessive disease that is characterized by abnormally high amino acid cystine concentration level in the urine, that may result in the formation of cystine stones in the kidneys.

Another example can be Cystic Fibrosis CF which is caused by a mutation in the cystic fibrosis transmembrane conductance regulator, CFTR, a protein that helps move salt and water across the membrane. It is a genetic disorder that affects mostly the lungs but also the pancreas, liver, kidneys, and intestine.

Long-term issues include breathing problems and coughing up mucus as a result of frequent lung infections. In a patient with CF, the cells do not secrete enough water; when it happens in the lungs, it causes the mucus to become extremely thick. It is also worth mentioning that most fatal toxins like Dendrotoxin black mamba snake of Africa and Batrachotoxin Colombian frog Phyllobates aurotaenia act directly on specific ion channels of the plasma membrane to disrupt the action potentials. To put it simply, this fatal toxin binds to anionic sites near the extracellular surface of the channel and physically blocks the path and ion conductance.

In a nutshell, batrachotoxin irreversibly binds to the sodium channels, enforcing them to remain open. The permeability of a membrane can be defined as the passive diffusion rate of permeated molecules across the biomembrane. It is unanimously accepted that permeability of any specific molecule depends mainly on charge number, polarity, size, and to some extent, to the molar mass of the molecule.

It should be noted though that both the nature of the bilayer and the prevalent environments can play a significant role too.

As mentioned before, because of the inevitable hydrophobic nature of the biomembranes, small uncharged molecules pass across the membrane more easily than charged, large ones [6]. With charged species e. Most cells are characterized by a membrane potential difference of mV V inside - V outside. Let us first consider an example of Cl - ion to clarify the issue.

So, there is a driving force of diffusion for Cl - to diffuse along the concentration gradient into the cell. Therefore, an equilibrium is achieved when influx and efflux of Cl - level each other. The membrane potential at which this equilibrium occurs is called equilibrium potential that can be calculated by Nernst equation [7]:. Note that this relation was obtained from ion transport equation for zero Gibbs free energy change i. By this definition, negative V DF means passive uptake and exit of cations and anions, respectively. In such conditions, passive protein channels or active transporters are required for the ion transfer.

Superscripts "aq" and "m" denote solute concentrations at bulk aqueous solutions and surfaces of the membrane, respectively. As it can be seen, the concentration gradient is considered to be from S 1 to S 2 , providing the chemical driving force of the transport. To mathematically describe the permeability, let us first introduce the useful concept of partition coefficient.

At thermodynamic equilibrium, the equality of the chemical potentials of solute j in two different intracellular and extracellular phases can be expressed as. Selectivity of Biomembranes When a membrane separates two aqueous compartments, some chemicals can move across the membrane while others cannot.

Based on the transport mechanism and permeability, solutes can be divided into three main groups as follows [2]: Small lipophilic lipid soluble molecules that transfer through the membrane by the sole diffusion. Molecules that cross the membrane with the aid of protein channels. Very large molecules that do not cross the membrane at all. Small Lipophilic Molecules Passive Diffusion Certain substances easily pass through the membrane by passive diffusion.

Polar and Charged Molecules Protein-Mediated Transfer Biological membranes are permeable not only to gases and small lipophilic molecules by passive diffusion processes , but also to many polar and charged molecules, including water, but through a different path. Large Molecules Membrane Barriers Very large molecules like proteins, polysaccharides or nucleic acids, do not diffuse across the cell membranes at all.

Passive and Active Transport Most biologically important solutes require protein carriers to cross cell membranes, by a process of either passive or active transport. Therefore, to summarize, transport of solutes across cell membranes by protein carriers can occur in one of two ways [2]: Downhill movement of solutes from regions of higher to lower concentration level, with the assistance of the protein carrier to pass through the membrane.

This process is called passive transport or facilitated diffusion, and does not require energy.