Molecular Dynamics Study of Nanoribbon Formation by Encapsulating Cyclic Hydrocarbon Molecules inside Single-Walled Carbon Nanotube

Carbon nanotubes filled with organic molecules can serve as chemical nanoreactors. Recent experimental results show that, by introducing cyclic hydrocarbon molecules inside carbon nanotubes, they can be transformed into nanoribbons or inner tubes, depending on the experimental conditions. In this paper, we present our results obtained as a continuation of our previous molecular dynamics simulation work. In our previous work, the initial geometry consisted of independent carbon atoms. Now, as an initial condition, we have placed different molecules inside a carbon nanotube (18,0): C5H5 (fragment of ferrocene), C5, C5+H2; C6H6 (benzene), C6, C6+H2; C20H12 (perylene); and C24H12 (coronene). The simulations were performed using the REBO-II potential of the LAMMPS software package, supplemented with a Lennard-Jones potential between the nanotube wall atoms and the inner atoms. The simulation proved difficult due to the slow dynamics of the H abstraction. However, with a slight modification of the parameterization, it was possible to model the formation of carbon nanoribbons inside the carbon nanotube.

Formation of nanoribbons by carbon atoms confined in a single-walled carbon nanotube-A molecular dynamics study

Carbon nanotubes can serve as one-dimensional nanoreactors for the in-tube synthesis of various nanostructures. Experimental observations have shown that chains, inner tubes, or nanoribbons can grow by the thermal decomposition of organic/organometallic molecules encapsulated in carbon nanotubes. The result of the process depends on the temperature, the diameter of the nanotube, and the type and amount of material introduced inside the tube. Nanoribbons are particularly promising materials for nanoelectronics. Motivated by recent experimental results observing the formation of carbon nanoribbons inside carbon nanotubes, molecular dynamics calculations were performed with the open source LAMMPS code to investigate the reactions between carbon atoms confined within a single-walled carbon nanotube. Our results show that the interatomic potentials behave differently in quasi-one-dimensional simulations of nanotube-confined space than in three-dimensional simulations. In particular, the Tersoff potential performs better than the widely used Reactive Force Field potential in describing the formation of carbon nanoribbons inside nanotubes. We also found a temperature window where the nanoribbons were formed with the fewest defects, i.e., with the largest flatness and the most hexagons, which is in agreement with the experimental temperature range.

Melanin biopolymers from newly isolated Pseudomonas koreensis strain UIS 19 with potential for cosmetics application, and optimization on molasses waste medium

AIMS

Bacterial melanins are UV-absorber biopolymers with potential applications in cosmetics and pharmaceutical industries. However, the cost concern of these pigments remains a limiting factor for their commercial production. Hence, the present study was aimed to isolate a bacterium with high yield of melanin by optimization of an inexpensive waste sources.

METHODS AND RESULTS

Pseudomonas koreensis UIS 19 (accession number: MG548583), which displayed significant bioproduction of melanin along with high tyrosinase enzyme activity was isolated from soil and introduced as a melanin-producing bacterium. Scanning and transmission electron microscope studies revealed that melanin pigments accumulated inside as well as the extracellular space of the cells. Melanin was extracted from the isolated strain and its detection was investigated using NMR and HPLC analyses. Here, we showed that the DPPH radical scavenging activity of 20 mg ml^-1 melanin extracted from the isolated strain was >92·42% and its sun protection factor (SPF) value was 61·55. Melanin production by the UIS 19 in molasses medium showed sugar consumption (32 g l^-1 ) for biomass production (5·4 g dry wt) and melanin yield of 0·44 g dry wt g^-1 biomass from l-tyrosine. Some critical intermediated such tyramine, l-dopa, dopamine and dopaquinone related to the melanin Raper Mason pathway were detected by H-NMR. Furthermore, to achieve high bioproduction of melanin in inexpensive media include 5% molasses 5 Brix as an industrial waste, nutritional and environmental parameters were screened using response surface methodology by Box-Behnken design in which, maximum melanin yield of 5·5 g dry wt l^-1 was obtained.

CONCLUSIONS

The bioproduction of melanin as valuable compound from P. koreensis was performed using an optimized waste medium. The purified melanin showed high radical scavenging activity and high SPF value.

SIGNIFICANCE AND IMPACT OF THE STUDY

Pseudomonas koreensis UIS 19 is a promising candidate for industrial production of melanin as cosmetic skin-care product.

Foliar manganese spray induces the resistance of cucumber to Colletotrichum lagenarium

Micronutrients provide a potentially interesting alternative to fungicides for the protection of crops against fungal pathogens. Here we studied the effect of foliar-applied manganese (Mn) in the form of MnSO₄ on severity of anthracnose disease, caused by Colletotrichum lagenarium in cucumber (Cucumis sativus L.) plant. The study was done aimed to characterize the optimum dose and application time of Mn fertilizer on disease suppression as well as to identify the defense mechanisms by which Mn-treated plants resist to fungal disease. In preliminary tests, Mn was applied at different concentrations (1.8, 4.5 and 7.2 mM) and various time points (three days before or two hours before inoculation, or three days after inoculation). Results showed that application of Mn either before or after inoculation suppressed the fungal infection in leaves and cotyledons, with a higher efficiency when applied three days prior to inoculation. However, all applied concentrations of Mn equally reduced the disease severity. Mn treatment in the absence of the pathogen promoted lignification and reactive oxygen species (ROS) accumulation. Also, pre-inoculation Mn treatment enhanced pathogen-induced lignification, callose or ROS production and reduced pathogen-induced cell death. The increase of lignin, callose and ROS induction by Mn application were 34, 30 and 31 % compared to control, respectively. Together, the results suggested the effectiveness of Mn treatments on anthracnose alleviation in cucumber plants. The findings here have a practical importance in plant physiology studies to identify the resistance-relevant mechanisms to pathogens and in sustainable agriculture to control the fungal diseases by a safe method.

The modifications of cell wall composition and water status of cucumber leaves induced by powdery mildew and manganese nutrition

Powdery mildew caused by Podosphaera fuliginea, is one of the most important diseases, damaging cucumber's yield worldwide. Among the different controlling ways, management of plant nutrition is an effective method. The present experiment was aimed to study the effects of foliar application of manganese (Mn) on disease suppression and to elucidate the possible mechanisms by which Mn-treated plants resist to fungal disease. Cucumber plants were hydroponically grown and sprayed by MnSO₄ alone and in combination with lysine (Lys) and methionine (Met) four days before pathogen inoculation. The results showed that the foliar application of Mn reduced the fungal disease severity by increasing of lignin, cellulose and pectin contents of cell wall (CW) and improving of leaf water status. The reduction of disease severity by application of MnSO₄, Mn + Lys and Mn + Met, were 40, 33, and 40% compared to control, respectively. The increases of lignin contents at the same treatments were 33, 27, and 36%, respectively. The leaf water potential (LWP) enhanced by foliar spray of above-mentioned fertilizers up to 15, 18, and 17%, respectively. The results of present study revealed that the Mn nutrition could control the cucumber powdery mildew by reinforcing of CW structure and reducing of water loss from infected leaves. The current findings are the first reports elucidating the CW-related defense mechanisms in which Mn has important role. The obtained results have a practical importance either in studies of plant physiology and biochemistry or in agricultural sciences for cost-effective control of powdery mildew in cucumber plants.

Assessment and attitude of university students about elderly: Preliminary Study

Aims: The aim of this study was to evaluate the attitude of dental students towards elderly patients. This approach might increase the responsiveness and need of the geriatric dental education within the undergraduate dental students curriculum, which is the persistent necessity for today communities. Methods & Materials: A cross-sectional study was conducted on 201 students who were randomly selected. The investigation was carried out in Qazvin University of Medical Sciences. The attitude of dental students towards elderly was measured with a self-administered questionnaire consisting of an Aging Semantic Differential scale (ASD), which was developed by Rozencranz and Mc Nevin. Results: According to the findings of this study, the students' attitude remained very positive towards the elderly patients as they showed a strong demand to work with elderly patients. This consisted of a 24 bipolar pair of adjectives that described the attributes of behavioral characteristics thought to be applicable to persons of all ages. Conclusion: According to our finding, the future geriatric dentistry is not towards a weak point in Iran as compared with the undesirable attitudes of dental students in the developed countries.

Determination of lead and cadmium content in sausages from Iran

The contents of lead and cadmium in five major brands of six types of cooked beef sausages consumed in Iran were determined by a graphite furnace atomic absorption spectrometer (GFAAS) after hydrogen peroxide/nitric acid digestion. The metal content in the samples, expressed in µg kg(-1) wet weight, varied from 24.0 to 158.7 with an average of 53.5 for lead and from 2.2 to 13.5 with an average of 5.7 for cadmium. The highest lead and cadmium concentrations were obtained from a German sausage (158.7 µg kg(-1); brand B) and hot dog (13.5 µg kg(-1); brand D), respectively. The results indicate that the sausages from Iran have concentrations below the permitted levels for these heavy metals. The daily dietary intakes and the percentage contribution of the two considered metals to the provisional tolerable weekly intake (PTWI) were calculated for sausages.

Novel properties of a mouse gamma-aminobutyric acid transporter (GAT4)

We expressed the mouse gamma-aminobutyric acid (GABA) transporter GAT4 (homologous to rat/ human GAT-3) in Xenopus laevis oocytes and examined its functional and pharmacological properties by using electrophysiological and tracer uptake methods. In the coupled mode of transport (Na+/ Cl-/GABA cotransport), there was tight coupling between charge flux and GABA flux across the plasma membrane (2 charges/GABA). Transport was highly temperature-dependent with a temperature coefficient (Q10) of 4.3. The GAT4 turnover rate (1.5 s(-l); -50 mV, 21 degrees C) and temperature dependence suggest physiological turnover rates of 15-20 s(-1). No uncoupled current was observed in the presence of Na+. In the absence of external Na+, GAT4 exhibited two distinct uncoupled currents. (i) A Cl- leak current (ICl(leak)) was observed when Na+ was replaced with choline or tetraethylammonium. The reversal potential of (ICl(leak)) followed the Cl- Nernst potential. (ii) A Li+ leak current (ILi(leak)) was observed when Na+ was replaced with Li+. Both leak currents were inhibited by Na+, and both were temperature-independent (Q10 approximately 1). The two leak modes appeared not to coexist, as Li+ inhibited (ICl(leak)). The results suggest the existence of cation- and anion-selective channel-like pathways in GAT4. Flufenamic acid inhibited GAT4 Na+/Cl-/GABA cotransport, ILi(leak), and ICl(leak), (Ki approximately 30 microM), and the voltage-induced presteady-state charge movements (Ki approximately 440 microM). Flufenamic acid exhibited little or no selectivity for GAT1, GAT2, or GAT3. Sodium and GABA concentration jicroumps revealed that slow Na+ binding to the transporter is followed by rapid GABA-induced translocation of the ligands across the plasma membrane. Thus, Na+ binding and associated conformational changes constitute the rate-limiting steps in the transport cycle.

Kinetics of the reverse mode of the Na+/glucose cotransporter

This study investigates the reverse mode of the Na(+)/glucose cotransporter (SGLT1). In giant excised inside-out membrane patches from Xenopus laevis oocytes expressing rabbit SGLT1, application of alpha-methyl-D: -glucopyranoside (alphaMDG) to the cytoplasmic solution induced an outward current from cytosolic to external membrane surface. The outward current was Na(+)- and sugar-dependent, and was blocked by phlorizin, a specific inhibitor of SGLT1. The current-voltage relationship saturated at positive membrane voltages (30-50 mV), and approached zero at -150 mV. The half-maximal concentration for alphaMDG-evoked outward current (K(0.5) (alphaMDG)) was 35 mM (at 0 mV). In comparison, K(0.5) (alphaMDG) for forward sugar transport was 0.15 mM (at 0 mV). K(0.5) (Na) was similar for forward and reverse transport ( approximately 35 mM at 0 mV). Specificity of SGLT1 for reverse transport was: alphaMDG (1.0) > D: -galactose (0.84) > 3-O-methyl-glucose (0.55) > D: -glucose (0.38), whereas for forward transport, specificity was: alphaMDG approximately D: -glucose approximately D: -galactose > 3-O-methyl-glucose. Thus there is an asymmetry in sugar kinetics and specificity between forward and reverse modes. Computer simulations showed that a 6-state kinetic model for SGLT1 can account for Na(+)/sugar cotransport and its voltage dependence in both the forward and reverse modes at saturating sodium concentrations. Our data indicate that under physiological conditions, the transporter is poised to accumulate sugar efficiently in the enterocyte.

Presteady-state and steady-state kinetics and turnover rate of the mouse gamma-aminobutyric acid transporter (mGAT3)

We expressed mouse gamma-aminobutyric acid (GABA) transporter (mGAT3) in Xenopus laevis oocytes and examined its steady-state and presteady-state kinetics and turnover rate by using tracer flux and electrophysiological methods. In oocytes expressing mGAT3, GABA evoked a Na+-dependent and Cl(-)-facilitated inward current. The dependence on Na+ was absolute, whereas that for Cl(-) was not. At a membrane potential of -50 mV, the half-maximal concentrations for Na+, Cl(-), and GABA were 14 mM, 5 mM, and 3 microM. The Hill coefficient for GABA activation and Cl(-) enhancement of the inward current was 1, and that for Na+ activation was > or =2. The GABA-evoked inward current was directly proportional to GABA influx (2.2 +/- 0.1 charges/GABA) into cells, indicating that under these conditions, there is tight ion/GABA coupling in the transport cycle. In response to step changes in the membrane voltage and in the absence of GABA, mGAT3 exhibited presteady-state current transients (charge movements). The charge-voltage (Q-V) relation was fitted with a single Boltzmann function. The voltage at half-maximal charge (V(0.5)) was +25 mV, and the effective valence of the moveable charge (zdelta) was 1.6. In contrast to the ON transients, which relaxed with a time constant of < or =30 msec, the OFF transients had a time constant of 1.1 sec. Reduction in external Na+ ([Na+]o) and Cl(-) ([Cl(-)]o) concentrations shifted the Q-V relationship to negative membrane potentials. At zero [Na+]o (106 mM Cl(-)), no mGAT3-mediated transients were observed, and at zero [Cl(-)]o (100 mM Na+), the charge movements decreased to approximately 30% of the maximal charge (Q(max)). GABA led to the elimination of charge movements. The half-maximal concentrations for Na+ activation, Cl(-) enhancement, and GABA elimination of the charge movements were 48 mM, 19 mM, and 5 mM, respectively. Q(max) and I(max) obtained in the same cells yielded the mGAT3 turnover rate, 1.7 sec(-1) at -50 mV. The low turnover rate of mGAT3 may be due to the slow return of the empty transporter from the internal to the external membrane surface.

Inhibition of gap junction hemichannels by chloride channel blockers

Electrophysiological methods were used to assess the effect of chloride-channel blockers on the macroscopic and microscopic currents of mouse connexin50 (Cx50) and rat connexin46 (Cx46) hemichannels expressed in Xenopus laevis oocytes. Oocytes were voltage-clamped at -50 mV and hemichannel currents (ICx50 or ICx46) were activated by lowering the extracellular Ca2+ concentration ([Ca2+]o) from 5 mM to 10 microM. Ion-replacement experiments suggested that ICx50 is carried primarily (>95%) by monovalent cations (PK : PNa : PCl = 1.0 : 0.74 : 0.05). ICx50 was inhibited by 18beta-glycyrrhetinic acid (apparent Ki, 2 microM), gadolinium (3 microM), flufenamic acid (3 microM), niflumic acid (11 microM), NPPB (15 microM), diphenyl-2-carboxylate (26 microM), and octanol (177 microM). With the exception of octanol, niflumic acid, and diphenyl-2-carboxylate, the above agents also inhibited ICx46. Anthracene-9-carboxylate, furosemide, DIDS, SITS, IAA-94, and tamoxifen had no inhibitory effect on either ICx50 or ICx46. The kinetics of ICx50 inhibition were not altered at widely different [Ca2+]o (10-500 microM), suggesting that drug-hemichannel interaction does not involve the Ca2+ binding site. In excised membrane patches, application of flufenamic acid or octanol to the extracellular surface of Cx50 hemichannels reduced single channel-open probability without altering the single-channel conductance, but application to the cytoplasmic surface had no effect on the channels. We conclude that some chloride-channel blockers inhibit lens-connexin hemichannels by acting on a site accessible only from the extracellular space, and that drug-hemichannel interaction involves a high-affinity site other than the Ca2+ binding site.

Na+-to-sugar stoichiometry of SGLT3

Sodium-glucose cotransporters (SGLTs) mediate active transport of sugar across cell membranes coupled to Na+, by using the electrochemical gradient as a driving force. In the kidney, there is evidence for two kinds of cotransporters, a high-affinity, low-capacity system, and a low-affinity, high-capacity system, with differences in substrate specificity and kinetics. Three renal SGLT clones have been identified: SGLT1 corresponding to the high-affinity system, and SGLT2 and SGLT3 with properties reminiscent of the low-affinity system. We have determined the stoichiometry of pig SGLT3 (pSGLT3) by using a direct method, comparing the substrate-induced inward charge to 22Na or [14C]alpha-methyl-D-glucopyranoside uptake in the same oocyte. pSGLT3 stoichiometry is 2 Na+:1 sugar, the same as that for SGLT1, but different from SGLT2 (1:1). The Na+ Hill coefficient for SGLT3 is approximately 1.5, suggesting low cooperativity between Na+ binding sites. Thus SGLT3 has functional characteristics intermediate between SGLT1 and SGLT2, so, whereas SGLT3 stoichiometry is the same as that for SGLT1 (2:1), sugar affinity and specificity are similar to SGLT2.

Role of Cl- in electrogenic Na+-coupled cotransporters GAT1 and SGLT1

We have investigated the functional role of Cl(-) in the human Na(+)/Cl(-)/gamma-aminobutyric acid (GABA) and Na(+)/glucose cotransporters (GAT1 and SGLT1, respectively) expressed in Xenopus laevis oocytes. Substrate-evoked steady-state inward currents were examined in the presence and absence of external Cl(-). Replacement of Cl(-) by gluconate or 2-(N-morpholino)ethanesulfonic acid decreased the apparent affinity of GAT1 and SGLT1 for Na(+) and the organic substrate. In the absence of substrate, GAT1 and SGLT1 exhibited charge movements that manifested as pre-steady-state current transients. Removal of Cl(-) shifted the voltage dependence of charge movements to more negative potentials, with apparent affinity constants (K(0.5)) for Cl(-) of 21 and 115 mm for SGLT1 and GAT1, respectively. The maximum charge moved and the apparent valence were not altered. GAT1 stoichiometry was determined by measuring GABA-evoked currents and the unidirectional influx of (36)Cl(-), (22)Na(+), or [(3)H]GABA. Uptake of each GABA molecule was accompanied by inward movement of 2 positive charges, which was entirely accounted for by the influx of Na(+) in the presence or absence of Cl(-). Thus, the GAT1 stoichiometry was 2Na(+):1GABA. However, Cl(-) was transported by GAT1 because the inward movement of 2 positive charges was accompanied by the influx of one Cl(-) ion, suggesting unidirectional influx of 2Na(+):1Cl(-):1GABA per transport cycle. Activation of forward Na(+)/Cl(-)/GABA transport evoked (36)Cl(-) efflux and was blocked by the inhibitor SKF 89976A. These data suggest a Cl(-)/Cl(-) exchange mechanism during the GAT1 transport cycle. In contrast, Cl(-) was not transported by SGLT1. Thus, in both GAT1 and SGLT1, Cl(-) modulates the kinetics of cotransport by altering Na(+) affinity, but does not contribute to net charge transported per transport cycle. We conclude that Cl(-) dependence per se is not a useful criterion to classify Na(+) cotransporters.

Epithelial organization of the mammalian lens

To understand the structural organization responsible for lens function, we have studied the three-dimensional arrangement of cells in the lens, and the location and molecular composition of specialized junctions controlling the paracellular and transcellular pathways. The lens is formed by a single layer of polarized cells that elongate along their apical-basal axis from the anterior to the posterior pole to form the cortex, and fold inward at the posterior pole to form the nucleus. The basal surfaces of all cells of the cortex (approximately two thirds of all lens cells) are bathed by the aqueous and vitreous humors. Therefore, their metabolism is not limited by diffusion of nutrients into the avascular lens. The apical surfaces of all cortical fibers are directed toward the interior of the lens, where they form two distinct structures here referred to as the 'apical interface' and the 'modiolus'. The apical interface is located at a point close to the anterior pole, and is formed by the association of the apical surface of anterior cortical cells and the apical surface of cortical fibers extending from the posterior pole. The modiolus is located close to the equator at the lateral edge of the apical interface, and is formed by the tapered apical ends of equatorial cortical fibers. The plasma membrane of cortical cells at the anterior pole are connected through 'leaky' tight junctions and small gap junctions. Extensive gap junction plaques composed of connexin43 connect equatorial fibers at the modiolus and posterior cortical fibers at the apical interface. Single cell-to-cell channels composed of connexin46 and connexin50 connect the lateral surfaces of equatorial and posterior cortical fibers. The lateral surfaces of these fibers also contain extensive junctions composed of aquaporin-0. The nucleus is connected to the humors through the paracellular pathway represented by the anterior (apical) and posterior (basal) suture lines. Therefore, the metabolic needs of nuclear fibers cannot be fulfilled by simple diffusion and requires the cell-to-cell pathway formed by specialized junctions. The lateral surfaces of nuclear fibers contain extensive wavy junctions composed of aquaporin-0, probably for the control of the permeability of the paracellular pathway. We propose a simple epithelium model for the lens in which nutrients move into the nucleus through the paracellular pathway represented principally by the suture lines, and the transcellular pathway represented by an extensive network of gap junction plaques composed of connexin43 at the apical surface, and single or small plaques of cell-to-cell channels composed of connexin46 and connexin50 in the lateral surfaces.

Molecular characterization of Vibrio parahaemolyticus vSGLT: a model for sodium-coupled sugar cotransporters

The Na(+)/galactose cotransporter (vSGLT) of Vibrio parahaemolyticus, tagged with C-terminal hexahistidine, has been purified to apparent homogeneity by Ni(2+) affinity chromatography and gel filtration. Resequencing the vSGLT gene identified an important correction: the N terminus constitutes an additional 13 functionally essential residues. The mass of His-tagged vSGLT expressed under its native promoter, as determined by electrospray ionization-mass spectrometry (ESI-MS), verifies these 13 residues in wild-type vSGLT. A fusion protein of vSGLT and green fluorescent protein, comprising a mass of over 90 kDa, was also successfully analyzed by ESI-MS. Reconstitution of purified vSGLT yields proteoliposomes active in Na(+)-dependent galactose uptake, with sugar preferences (galactose > glucose > fucose) reflecting those of wild-type vSGLT in vivo. Substrates are transported with apparent 1:1 stoichiometry and apparent K(m) values of 129 mm (Na(+)) and 158 microm (galactose). Freeze-fracture electron microscopy of functional proteoliposomes shows intramembrane particles of a size consistent with vSGLT existing as a monomer. We conclude that vSGLT is a suitable model for the study of sugar cotransporter mechanisms and structure, with potential applicability to the larger SGLT family of important sodium:solute cotransporters. It is further demonstrated that ESI-MS is a powerful tool for the study of proteomics of membrane transporters.

Pentameric assembly of a neuronal glutamate transporter

Freeze-fracture electron microscopy was used to study the structure of a human neuronal glutamate transporter (EAAT3). EAAT3 was expressed in Xenopus laevis oocytes, and its function was correlated with the total number of transporters in the plasma membrane of the same cells. Function was assayed as the maximum charge moved in response to a series of transmembrane voltage pulses. The number of transporters in the plasma membrane was determined from the density of a distinct 10-nm freeze-fracture particle, which appeared in the protoplasmic face only after EAAT3 expression. The linear correlation between EAAT3 maximum carrier-mediated charge and the total number of the 10-nm particles suggested that this particle represented functional EAAT3 in the plasma membrane. The cross-sectional area of EAAT3 in the plasma membrane (48 +/- 5 nm(2)) predicted 35 +/- 3 transmembrane alpha-helices in the transporter complex. This information along with secondary structure models (6-10 transmembrane alpha-helices) suggested an oligomeric state for EAAT3. EAAT3 particles were pentagonal in shape in which five domains could be identified. They exhibited fivefold symmetry because they appeared as equilateral pentagons and the angle at the vertices was 110 degrees. Each domain appeared to contribute to an extracellular mass that projects approximately 3 nm into the extracellular space. Projections from all five domains taper toward an axis passing through the center of the pentagon, giving the transporter complex the appearance of a penton-based pyramid. The pentameric structure of EAAT3 offers new insights into its function as both a glutamate transporter and a glutamate-gated chloride channel.

Acid resistance of Helicobacter pylori depends on the UreI membrane protein and an inner membrane proton barrier

ureI encodes an inner membrane protein of Helicobacter pylori. The role of the bacterial inner membrane and UreI in acid protection and regulation of cytoplasmic urease activity in the gastric microorganism was studied. The irreversible inhibition of urease when the organism was exposed to a protonophore (3,3',4', 5-tetrachlorsalicylanide; TCS) at acidic pH showed that the inner membrane protected urease from acid. Isogenic ureI knockout mutants of several H. pylori strains were constructed by replacing the ureI gene of the urease gene cluster with a promoterless kanamycin resistance marker gene (kanR). Mutants carrying the modified ureAB-kanR-EFGH operon all showed wild-type levels of urease activity at neutral pH in vitro. The mutants resisted media of pH > 4.0 but not of pH < 4.0. Whereas wild-type bacteria showed high levels of urease activity below pH 4.0, this ability was not retained in the ureI mutants, resulting in inhibition of metabolism and cell death. Gene complementation experiments with plasmid-derived H. pylori ureI restored wild-type properties. The activation of urease activity found in structurally intact but permeabilized bacteria treated with 0.01% detergent (polyoxy-ethylene-8-laurylether; C12E8), suggested a membrane-limited access of urea to internal urease at neutral pH. Measurement of 14C-urea uptake into Xenopus oocytes injected with ureI cRNA showed acid activation of uptake only in injected oocytes. Acceleration of urea uptake by UreI therefore mediates the increase of intracellular urease activity seen under acidic conditions. This increase of urea permeability is essential for H. pylori survival in environments below pH 4.0. ureI-independent urease activity may be sufficient for maintenance of bacterial viability above pH 4.0.

A H+-gated urea channel: the link between Helicobacter pylori urease and gastric colonization

Acidic media trigger cytoplasmic urease activity of the unique human gastric pathogen Helicobacter pylori. Deletion of ureI prevents this activation of cytoplasmic urease that is essential for bacterial acid resistance. UreI is an inner membrane protein with six transmembrane segments as shown by in vitro transcription/translation and membrane separation. Expression of UreI in Xenopus oocytes results in acid-stimulated urea uptake, with a pH profile similar to activation of cytoplasmic urease. Mutation of periplasmic histidine 123 abolishes stimulation. UreI-mediated transport is urea specific, passive, nonsaturable, nonelectrogenic, and temperature independent. UreI functions as a H+-gated urea channel regulating cytoplasmic urease that is essential for gastric survival and colonization.

Number of subunits comprising the epithelial sodium channel

The human epithelial sodium channel (hENaC) is a hetero-oligomeric complex composed of three subunits, alpha, beta, and gamma. Understanding the structure and function of this channel and its abnormal behavior in disease requires knowledge of the number of subunits that comprise the channel complex. We used freeze-fracture electron microscopy and electrophysiological methods to evaluate the number of subunits in the ENaC complex expressed in Xenopus laevis oocytes. In oocytes expressing wild-type hENaC (alpha, beta, and gamma subunits), clusters of particles appeared in the protoplasmic face of the plasma membrane. The total number of particles in the clusters was consistent with the whole-cell amiloride-sensitive current measured in the same cells. The size frequency histogram for the particles in the clusters suggested the presence of an integral membrane protein complex composed of 17 +/- 2 transmembrane alpha-helices. Because each ENaC subunit has two putative transmembrane helices, these data suggest that in the oocyte plasma membrane, the ENaC complex is composed of eight or nine subunits. At high magnification, individual ENaC particles exhibited a near-square geometry. Functional studies using wild-type alphabeta-hENaC coexpressed with gamma-hENaC mutants, which rendered the functional channel differentially sensitive to methanethiosulfonate reagents and cadmium, suggested that the functional channel complex contains more than one gamma subunit. These data suggest that functional ENaC consists of eight or nine subunits of which a minimum of two are gamma subunits.

Stoichiometry and Na+ binding cooperativity of rat and flounder renal type II Na+-Pi cotransporters

The stoichiometry of the rat and flounder isoforms of the renal type II sodium-phosphate (Na+-Pi) cotransporter was determined directly by simultaneous measurements of phosphate (Pi)-induced inward current and uptake of radiolabeled Pi and Na+ in Xenopus laevis oocytes expressing the cotransporters. There was a direct correlation between the Pi-induced inward charge and Pi uptake into the oocytes; the slope indicated that one net inward charge was transported per Pi. There was also a direct correlation between the Pi-induced inward charge and Na+ influx; the slope indicated that the influx of three Na+ ions resulted in one net inward charge. This behavior was similar for both isoforms. We conclude that for both Na+-Pi cotransporter isoforms the Na+:Pi stoichiometry is 3:1 and that divalent Pi is the transported substrate. Steady-state activation of the currents showed that the Hill coefficients for Pi were unity for both isoforms, whereas for Na+, they were 1.8 (flounder) and 2.5 (rat). Therefore, despite significant differences in the apparent Na+ binding cooperativity, the estimated Na+:Pi stoichiometry was the same for both isoforms.

Functional and morphological correlates of connexin50 expressed in Xenopus laevis oocytes

Electrophysiological and morphological methods were used to study connexin50 (Cx50) expressed in Xenopus laevis oocytes. Oocytes expressing Cx50 exhibited a new population of intramembrane particles (9.0 nm in diameter) in the plasma membrane. The particles represented hemichannels (connexin hexamers) because (a) their cross-sectional area could accommodate 24 +/- 3 helices, (b) when their density reached 300-400/microm2, they formed complete channels (dodecamers) in single oocytes, and assembled into plaques, and (c) their appearance in the plasma membrane was associated with a whole-cell current, which was activated at low external Ca2+ concentration ([Ca2+]o), and was blocked by octanol and by intracellular acidification. The Cx50 hemichannel density was directly proportional to the magnitude of the Cx50 Ca2+-sensitive current. Measurements of hemichannel density and the Ca2+-sensitive current in the same oocytes suggested that at physiological [Ca2+]o (1-2 mM), hemichannels rarely open. In the cytoplasm, hemichannels were present in approximately 0.1-microm diameter "coated" and in larger 0.2-0.5-microm diameter vesicles. The smaller coated vesicles contained endogenous plasma membrane proteins of the oocyte intermingled with 5-40 Cx50 hemichannels, and were observed to fuse with the plasma membrane. The larger vesicles, which contained Cx50 hemichannels, gap junction channels, and endogenous membrane proteins, originated from invaginations of the plasma membrane, as their lumen was labeled with the extracellular marker peroxidase. The insertion rate of hemichannels into the plasma membrane (80, 000/s), suggested that an average of 4,000 small coated vesicles were inserted every second. However, insertion of hemichannels occurred at a constant plasma membrane area, indicating that insertion by vesicle exocytosis (60-500 microm2 membranes/s) was balanced by plasma membrane endocytosis. These exocytotic and endocytotic rates suggest that the entire plasma membrane of the oocyte is replaced in approximately 24 h.

Substance P induces brief, localized increase in [Ca2+]i in dorsal horn neurons

We determined the spatial and temporal dynamics of the increase in intracellular Ca2+ levels [Ca2+]i produced by substance P (SP) in dorsal horn neurons. A microinjection technique was used to apply minute amounts of SP to small areas of cultured neurons loaded with the Ca2+ indicator fura-2. Five successive applications of SP to the soma produced short-lasting (< 50 s) increases in [Ca2+]i that became gradually smaller, indicating receptor desensitization. Focal application of SP to a distal locus in a neurite produced a brief (12 s) increase in [Ca2+]i that travelled down the dendrite but did not spread into cell soma. Prolonged application of SP to these neurons caused the appearance of varicosities in their dendrites.

Structural analysis of cloned plasma membrane proteins by freeze-fracture electron microscopy

We have used freeze-fracture electron microscopy to examine the oligomeric structure and molecular asymmetry of integral plasma membrane proteins. Recombinant plasma membrane proteins were functionally expressed in Xenopus laevis oocytes, and the dimensions of their freeze-fracture particles were analyzed. To characterize the freeze-fracture particles, we compared the particle cross-sectional area of proteins with alpha-helical transmembrane domains (opsin, aquaporin 1, and a connexin) with their area obtained from existing maps calculated from two-dimensional crystals. We show that the cross-sectional area of the freeze-fracture particles corresponds to the area of the transmembrane domain of the protein, and that the protein cross-sectional area varies linearly with the number membrane-spanning helices. On average, each helix occupies 1.40 +/- 0.03 nm2. By using this information, we examined members from three classes of plasma membrane proteins: two ion channels, the cystic fibrosis transmembrane conductance regulator and connexin 50 hemi-channel; a water channel, the major intrinsic protein (the aquaporin 0); and a cotransporter, the Na+/glucose cotransporter. Our results suggest that the cystic fibrosis transmembrane conductance regulator is a dimer containing 25 +/- 2 transmembrane helices, connexin 50 is a hexamer containing 24 +/- 3 helices, the major intrinsic protein is a tetramer containing 24 +/- 3 helices, and the Na+/glucose cotransporter is an asymmetrical monomer containing 15 +/- 2 helices.

Structure and function of the Na+/glucose cotransporter

Cotransporters are a major class of membrane transport proteins that are responsible for the accumulation of nutrients, neurotransmitters, osmolytes and ions in cells from bacteria to man. The energy for solute accumulation comes from the proton and/or sodium electrochemical gradients that exist across cell membranes. A major problem in biology is how transport is coupled to these electrochemical potential gradients. The primary example of this class of membrane proteins is the intestinal brush border Na+/glucose cotransporter (SGLT1), first described by Bob Crane in 1960. Over 35 members of the SGLT1 gene family have been identified in animal cells, yeast and bacteria, and all share a common core structure of 13 transmembrane (TM) helices. Electrophysiological techniques have been used to examine the function of several family members, chimeras and mutants expressed in heterologous systems such as Xenopus laevis oocytes. These have revealed that cotransporters are multi-functional proteins: they are responsible for 1). uncoupled passive Na+ transport (Na+ uniport); 2). down-hill water transport in the absence of substrate; 3). Na+/substrate cotransport; and 4). Na+/substrate/water cotransport. The sugar binding and translocation pathway is formed by 4 TM helices near the C-terminal of the protein, helices 10-13. We propose that the N-terminal domains of SGLT1 are responsible for Na+ binding and/or translocation, and that Na+/glucose cotransport results from interactions between the N- and C-terminal domains of the protein.

Thyroid Na+/I- symporter. Mechanism, stoichiometry, and specificity

The rat thyroid Na+/I- symporter (NIS) was expressed in Xenopus laevis oocytes and characterized using electrophysiological, tracer uptake, and electron microscopic methods. NIS activity was found to be electrogenic and Na+-dependent (Na+ >> Li+ >> H+). The apparent affinity constants for Na+ and I- were 28 +/- 3 mM and 33 +/- 9 microM, respectively. Stoichiometry of Na+/anion cotransport was 2:1. NIS was capable of transporting a wide variety of anions (I-, ClO3-, SCN-, SeCN-, NO3-, Br-, BF4-, IO4-, BrO3-, but perchlorate (ClO4-) was not transported. In the absence of anion substrate, NIS exhibited a Na+-dependent leak current (approximately 35% of maximum substrate-induced current) with an apparent Na+ affinity of 74 +/- 14 mM and a Hill coefficient (n) of 1. In response to step voltage changes, NIS exhibited current transients that relaxed with a time constant of 8-14 ms. Presteady-state charge movements (integral of the current transients) versus voltage relations obey a Boltzmann relation. The voltage for half-maximal charge translocation (V0.5) was -15 +/- 3 mV, and the apparent valence of the movable charge was 1. Total charge was insensitive to [Na+]o, but V0.5 shifted to more negative potentials as [Na+]o was reduced. NIS charge movements are attributed to the conformational changes of the empty transporter within the membrane electric field. The turnover rate of NIS was >/=22 s-1 in the Na+ uniport mode and >/=36 s-1 in the Na+/I- cotransport mode. Transporter density in the plasma membrane was determined using freeze-fracture electron microscopy. Expression of NIS in oocytes led to a approximately 2. 5-fold increase in the density of plasma membrane protoplasmic face intramembrane particles. On the basis of the kinetic results, we propose an ordered simultaneous transport mechanism in which the binding of Na+ to NIS occurs first.

Five transmembrane helices form the sugar pathway through the Na+/glucose cotransporter

To test the hypothesis that the C-terminal half of the Na+/glucose cotransporter (SGLT1) contains the sugar permeation pathway, a cDNA construct (C5) coding for rabbit SGLT1 amino acids 407-662, helices 10-14, was expressed in Xenopus oocytes. Expression and function of C5 was followed by Western blotting, electron microscopy, radioactive tracer, and electrophysiological methods. The C5 protein was synthesized in 20-fold higher levels than SGLT1. The particle density in the protoplasmic face of the oocyte plasma membrane increased 2-fold after C5-cRNA injection compared with noninjected oocytes. The diameters of the C5 particles were heterogeneous (4.8 +/- 0.3, 7.1 +/- 1.2, and 10.3 +/- 0.8 nm) in contrast to the endogenous particles (7.6 +/- 1.2 nm). C5 increased the alpha-methyl-D-glucopyranoside (alphaMDG) uptake up to 20-fold above that of noninjected oocytes and showed an apparent K0.5alphaMDG of 50 mM and a turnover of approximately 660 s-1. Influx was independent of Na+ with transport characteristics similar to those of SGLT1 in the absence of Na+: 1) selective (alphaMDG > D-glucose > D-galactose >> L-glucose approximately D-mannose), 2) inhibited by phloretin, KiPT = approximately 500 microM, and 3) insensitive to phlorizin. These results indicate that C5 behaves as a specific low affinity glucose uniporter. Preliminary studies with three additional constructs, hC5 (the human equivalent of C5), hC4 (human SGLT1 amino acids 407-648, helices 10-13), and hN13 (amino acids 1-648, helices 1-13), further suggest that helices 10-13 form the sugar permeation pathway for SGLT1.

Cutaneous transport of Ca2+ in the frog Rana pipiens: significance and specificity

Rana pipiens were divided into four groups: controls; hypocalcemic frogs, depleted of salts by acclimation to deionized water; hypercalcemic frogs, calcium-loaded by the introduction of 40 mumol calcium gluconate; and frogs exposed to the potential competing ions Mg2+, Sr2+, and Ba2+. All groups displayed calcium influx that was proportional to external [Ca2+]; however, the group acclimated to deionized water also displayed hypocalcemia (P < 0.025) and enhanced Ca2+ influx at higher (> 0.3 mM) external [Ca2+]. Ca2+ efflux was depressed in hypocalcemic frogs, and thus net Ca2+ flux shifted from net loss in control frogs to net uptake in hypocalcemic frogs. Hypocalcemia also resulted in increased skin Ca2+ deposits which may be related to a decreased Ca2+ (and other ions) permeability as a consequence of the acclimation to deionized water. Another group of frogs was Ca(2+)-loaded by injecting calcium gluconate: Sodium gluconate controls did not significantly alter Ca2+ fluxes. The frogs that received calcium gluconate treatments became hypercalcemic (P < 0.01) and did not display significant changes in calcium fluxes, nor did they show significant changes in skin calcium deposits. We conclude that hypocalcemia leads to regulatory responses that stimulate active Ca2+ transport in Rana pipiens skin and possibly inhibits cutaneous and renal efflux. We also conclude that hypercalcemia does not alter calcium fluxes across skin. The ions from Group IIA of the Periodic Table of Elements had little effect on Ca2+ fluxes at concentrations ranging from 0.5-4.0 mM; neither Sr2+ or Ba2+ affected Ca2+ influx. The only divalent ion tested that influenced Ca2+ was Mg2+, which significantly inhibited Ca2+ influx but only at 4.0 mM or eight times the external [Ca2+]. We conclude, therefore, that the Ca2+ transport mechanism is fairly specific for Ca2+ within Group IIA.

Interrenal function in larval Ambystoma tigrinum. IV. Acid-base balance and the renin-angiotensin system

Larval Ambystoma tigrinum were cannulated nonocclusively in the truncus arteriosus and allowed to recover for 20-24 hr. In one group of animals a peritoneal cannula was inserted in order to induce acidosis through the injection of lactic acid (2 micromol/g). Immediately following a control blood sample (hr 0), lactic acid was injected, and blood samples were collected at 1, 4, 8, and 24 hr and analyzed for pH, PCO2, PO2, [HCO3-], and aldosterone. These animals exhibited a significant metabolic acidosis, which was accompanied by a significant increase in plasma aldosterone, and recovered in approximately 24 hr. Additional groups of animals were subjected to the same acidosis and also received either saralasin (0.01 or 1 microg/g at 0, 1, and 4 hr) or captopril (0.01-0.1 or 1 microg/g at 0, 1, and 4 hr). The groups of animals whose renin-angiotensin system was blocked by saralasin or captopril did not show a significant change in their ability to recover from the metabolic acidosis. Furthermore, saralasin and captopril were ineffective in inhibiting the normal rise in circulating aldosterone in response to acidosis. In another group of animals, synthetic human angiotensin II (1 microg/g; Ang II) was infused immediately following the control blood sample (hr 0) and blood samples were collected at 2, 4, 6, and 8 hr and assayed for aldosterone. Plasma aldosterone levels increased significantly from 133 +/- 91 pg/ml at hr 0 to a maximum of 3288 +/- 519 pg/ml at hr 4. Sham-treated animals did not increase circulating aldosterone. When Ang II (1 microg/g) and saralasin (1 microg/g) were given simultaneously, however, the rise in plasma aldosterone was only about 35% that of animals which received Ang II alone. We conclude that administration of Ang II leads, either directly or indirectly, to synthesis and release of aldosterone from the interrenal tissues of larval Ambystoma tigrinum and that this rise can be significantly attenuated by saralasin. We furthermore conclude that although the renin-angiotensin system may be indirectly involved in recovery from an acid challenge, it does not appear to be the stimulus for the observed increase in plasma aldosterone in response to acidosis in these animals.