A novel fluorescence-based assay for the transglycosylation activity of endo-β-N-acetylglucosaminidases

A fluorescence-based assay for the transglycosylation activity of endo-beta-N-acetylglucosaminidases (ENGases) was developed. The assay was based on the findings that a coupled chitinase can specifically capture and hydrolyze the fluorogenic intermediate that is formed by the ENGase-catalyzed transglycosylation to release a fluorophore, but does not hydrolyze the donor asparagine-linked N-glycan and the acceptor 4-methylumbelliferyl N-acetylglucosaminide. The assay method was verified by detecting the transglycosylation activities of the known ENGases. Its application for assessing the effects of organic solvents on transglycosylation activity was demonstrated. The novel coupled assay provides a highly sensitive, easy, and quantitative method for screening endo-beta-N-acetylglucosaminidases with transglycosylation activities useful for glycoconjugate synthesis.

[Laser induced dispersed fluorescence spectra of NO2 in the range of 550-740 nm]

Laser induced dispersed fluorescence spectra (LIDFS) of NO2 molecules, excited by second harmonic lines (the output wavelength is 532 nm) of a pulsed Nd: YAG laser at room temperature and at low and high NO2 pressures, were obtained and analyzed. The authors got vibronic progressions in the range of 550-740 nm at low pressure and ascribed them to the transitions from the first excitation electronic state A(2)B2 to the vibrational levels of the ground electronic state X(2)A1, and then calculated the harmonic frequencies of symmetry stretch and bond stretch: omega1 = 1300.72 cm(-1) and omega2 = 744.14 cm(-1), respectively. On the basis of what was above-mentioned, the authors compared the spectra at low pressure with those at high pressure and pointed out the difference between them. Then the authors also interpreted the obvious red shift of fluorescence spectra at high NO2 pressures in terms of a stepladder model of vibrational deactivation and obtained significative results.

Quantitative point source photoacoustic inversion formulas for scattering and absorbing media

We present here an expression for the photoacoustic contribution of an optical point source in a diffusive and absorbing medium. By using this measurement as a reference, we present a direct inversion formula that recovers the absorption map quantitatively, at the same time accounting for instrumental factors such as the source strength, the shape of the optical pulse, and the impulse response and finite size of the transducers. We further validate this expression through accurate numerical simulations showing that the absorption map is recovered quantitatively in the presence of a rotating geometry. We finally discuss how the presented solutions for point sources within the photoacoustic problem enable the use of concurrent fluorescence and ultrasound measurements as appropriate for a hybrid tomographic system. The proposed system could retrieve absorption information using photoacoustic measurements, and use these data to more accurately describe the fluorescence problem and improve reconstruction fidelity.

Multiplex single strand conformation polymorphism analysis by capillary electrophoresis with on-the-fly fluorescence lifetime detection

This paper describes the use of on-the-fly fluorescence lifetime detection (OFLD) for multiplex single strand conformation polymorphism (SSCP) analysis by capillary electrophoresis (CE). The dye labels studied for multiplex SSCP-OFLD-CE analyses included RG, NBD, and BODIPY-FL. The dyes were first investigated for a model system of "Wild Type" and "Mutant" 43-base fragments designed to vary by a single A/T substitution. Two dye pairs, BODIPY-FL/ RG and BODIPY-FL/NBD, were then used to detect the G20210A mutation in the human prothrombin gene. Mobility correction was required for the BODIPY-FL/RG system. Three "blind" analyses were performed of three mixtures that combined a control fragment (wild type-BODIPY-FL) with two "unknown" fragments selected among four possibilities (wild type or mutant labeled with NBD or RG). In each multiplex analysis, the "origin" of the unknown fragments was correctly identified on the basis of fluorescence lifetime of the dye label and the presence or absence of the mutation was correctly determined on the basis of conformation-induced differences in migration time.

A novel time resolved fluorometric assay of anoikis using Europium-labelled Annexin V in cultured adherent cells

BACKGROUND

Adherent cells undergo apoptosis when detached from their home ground, a process called anoikis (homelessness).

METHODS

We developed a new and sensitive method to analyse apoptosis and anoikis of adherent cell types using a time resolved fluorometric assay with Europium-labelled Annexin V. Anoikis was induced with tumor necrosis factor-alpha/cycloheximide and three cell fractions of the cell cultures were prepared and analysed. Fraction 1 consisted of adherent cells, analysed while growing on their support (without detachment by trypsinisation). Fraction 2 contained detached cells due to anoikis (floating cells) and fraction 3 contained apoptotic bodies. Both fractions 2 and 3 were present in the culture medium and were isolated by differential centrifugation.

RESULTS

TNF-alpha treatment of three different types of adherent cell cultures induced a significant increase of the amount of floating cells (anoikis) and apoptotic bodies compared to control cell cultures. Also in the adherent cell fractions a small amount of apoptosis was observed.

CONCLUSIONS

The novel time resolved assay provides the ability to analyse the cell death cascade in adherent cell cultures of the same sample at the same time in a sensitive and reproducible way.

Diagnostic plots for detecting outlying slides in a cDNA microarray experiment

Different sources of systematic and random error variations are often observed in cDNA microarray experiments. A simple scatter plot is commonly used to examine outlying slides that have unusual expression patterns or larger variability than other slides. These outlying slides tend to have large impacts on the subsequent analyses, such as identification of differentially expressed genes and clustering analysis. However, it is difficult to select outlying slides rigorously and consistently based on subjective human pattern recognition on their scatter plots. A graphical method and a rigorous diagnostic measure are proposed to detect outlying slides. The proposed graphical method is easy to implement and shown to be quite effective in detecting outlying slides in real microarray data sets. This diagnostic measure is also informative to compare variability among slides. Two cDNA microarray data sets are carefully examined to illustrate the proposed approach. A 3840-gene microarray experiment for neuronal differentiation of cortical stem cells and a 2076-gene microarray experiment for anticancer compound time-course expression of the NCI-60 cancer cell lines.

Electrically induced microflows probed by fluorescence correlation spectroscopy

We report on the experimental characterisation of electrically induced flows at the micrometer scale through Fluorescence Correlation Spectroscopy (FCS) measurements. We stress the potential of FCS as a useful characterisation technique in microfluidics devices for transport properties cartography. The experimental results obtained in a model situation are in agreement with previous calculations (F. Nadal, F. Argoul, P. Kestener, B. Pouligny, C. Ybert, A. Ajdari, Eur. Phys. J. E 9, 387 (2002)) predicting the structure and electric-field dependency of the induced flow. Additionally, the present study evidences a complex behaviour of the probe nanobeads under electric field whose precise understanding might prove relevant for situations where nano-objects interact with an external electric field.

lambda-Repressor oligomerization kinetics at high concentrations using fluorescence correlation spectroscopy in zero-mode waveguides

Fluorescence correlation spectroscopy (FCS) has demonstrated its utility for measuring transport properties and kinetics at low fluorophore concentrations. In this article, we demonstrate that simple optical nanostructures, known as zero-mode waveguides, can be used to significantly reduce the FCS observation volume. This, in turn, allows FCS to be applied to solutions with significantly higher fluorophore concentrations. We derive an empirical FCS model accounting for one-dimensional diffusion in a finite tube with a simple exponential observation profile. This technique is used to measure the oligomerization of the bacteriophage lambda repressor protein at micromolar concentrations. The results agree with previous studies utilizing conventional techniques. Additionally, we demonstrate that the zero-mode waveguides can be used to assay biological activity by measuring changes in diffusion constant as a result of ligand binding.

Highly sensitive simultaneous detection of cultured cellular thiols by laser induced fluorescence-capillary electrophoresis

We have recently described a new method to determine physiological thiols, in which the quantification of plasma homocysteine, cysteine, cysteinylglycine, glutathione, and glutamylcysteine was achieved after derivatization with 5-iodoacetamidofluorescein. Samples were separated and measured by capillary electrophoresis with laser-induced fluorescence in an uncoated fused-silica capillary, using a phosphate/borate run buffer and the organic base N-Methyl-D-glucamine as effective electrolyte addictive to obtain a baseline peak separation. In this paper, we propose an improvement of our method useful for the analysis of the intracellular thiols in different cultured cells. In particular, we studied run buffer and injection conditions in order to increase the sensitivity of the assay and we found that, by incrementing two times the injected volume and using the water plug before the sample injection, the sensitivity of our previous method was increased by about ten times. To maintain a good resolution between peaks, particularly between homocysteine and the internal standard d-penicillamine, we lengthened the run time by incrementing the concentration of the electrolyte buffer and the organic base d-glucamine and by decreasing the cartridge temperature from 40 to 25 degrees C. After these changes in electrophoretical parameters, cellular thiols were baseline-resolved in less than 14 min instead of 9 min as in our previous method, but the limit of quantification is increased from 50 to 1 nmol/L. This new procedure allows also to measure the intracellular thiols commonly found at low concentration, such as cysteinylglycine, glutamylcysteine, and homocysteine. The new analytical method performance was assessed by measuring the intracellular thiols in three different cell lines, i.e., HUVEC, ECV304, and R1 stem cells.

Spectroscopy of single- and double-wall carbon nanotubes in different environments

Individual single-wall carbon nanotubes (SWNTs) and double-wall carbon nanotubes (DWNTs) were suspended in water for optical studies using sodium-cholate and other surfactants. We used time-resolved photoluminescence (PL) spectroscopy to study the influence of tube chirality and diameter as well as of the environment on nonradiative decay in small diameter tubes. The studies provide evidence for PL from small diameter core tubes in DWNTs and for a correlation of nonradiative decay with tube diameter and exciton red shift as induced by interaction with the environment.

Analysis of serotonin in brain microdialysates using capillary electrophoresis and native laser-induced fluorescence detection

Serotonin or 5-hydroxytryptamine (5-HT) is a major neurotransmitter in the central nervous system. In this work, a method for analyzing 5-HT in brain microdialysis samples using a commercially available capillary electrophoresis (CE) system has been developed. A pH-mediated in-capillary preconcentration of samples was performed, and after separation by capillary zone electrophoresis, native fluorescence of 5-HT was detected by a 266 nm solid-state laser. The separation conditions for the analysis of 5-HT in standard solutions and microdialysates have been optimized, and this method has been validated on both pharmacological and analytical bases. Separation of 5-HT was performed using a 80 mmol/L citrate buffer, pH 2.5, containing 20 mmol/L hydroxypropyl-beta-cyclodextrin (HP-beta-CD) and +30 kV voltage. The detection limit was 2.5 x 10(-10) mol/L. This method allows the in vivo brain monitoring of 5-HT using a simple, accurate CE measurement in underivatized microdialysis samples.

Purification and in vitro functional analyses of RGS12 and RGS14 GoLoco motif peptides

The GoLoco motif is a short polypeptide sequence that binds to heterotrimeric G-protein alpha subunits of the adenylyl cyclase-inhibitory (Galpha(i/o)) subclass in a nucleotide-dependent manner (i.e., solely to the GDP-bound ground state). This article describes methods used for the expression, purification, and in vitro evaluation of membrane-permeant tag fusion peptides derived from the GoLoco motif regions of "regulator of G-protein signaling" proteins type 12 (RGS12) and 14 (RGS14) and a consensus GoLoco sequence from the multiple GoLoco motif protein AGS3. Three different fluorescence-based assays are described for evaluating the in vitro function of these GoLoco peptides as guanine nucleotide dissociation inhibitors, including measurements of GTPgammaS binding and Galpha subunit activation by the planar ion aluminum tetrafluoride.

pH-dependent quenching of the fluorescence of tryptophan residues in class A beta-lactamase from E. coli (TEM-1)

We performed an investigation of the pH-dependent quenching of the fluorescence of tryptophan residues of TEM-1 beta-lactamase from E. coli by uncharged and charged quenchers. pH-dependent Stern-Volmer constants (Ksv/pH) of tryptophan residues allowed us to determine subtle but discrete structurally and functionally important processes.

Determination of single-cell gene expression in Arabidopsis by capillary electrophoresis with laser induced fluorescence detection

We report a new method for the measurement of gene expression in single cells of Arabidopsis using capillary electrophoresis with laser-induced fluorescence (CE-LIF) detection. Initially, the quantitative analysis of APETALA2 (AP2) and LEAFY (LFY) was performed by CE-LIF method. The detection limits of AP2 and LFY can reach 0.08 and 0.04 ng/ml (signal-to-noise ratio = 3), respectively. This protocol coupling with single-cell reverse transcriptase-polymerase chain reaction (SC-RT-PCR) has been used to monitor LFY and AP2 expression in individual cells from the shoot apical meristem, leaf, root, and stem of Arabidopsis, simultaneously. The effect of PCR cycle number on PCR product concentrations has been discussed. The changes of LFY expression were determined at single-cell level in different Arabidopsis tissues. The relationship between gibberallic acid (GA) and LFY expression was also revealed by this method. It was shown that the combination between CE-LIF and SC-RT-PCR could provide a highly sensitive and selective tool for the determination of different gene expression at single-cell level in specific tiny plant tissues.

Simultaneous measurement of rat brain cortex PtO2using EPR oximetry and a fluorescence fiber-optic sensor during normoxia and hyperoxia

Electron paramagnetic resonance (EPR) oximetry is a promising, relatively non-invasive method of monitoring tissue partial pressure of oxygen (PtO(2)) that has proven useful in following changes in PtO(2) under various physiologic and pathophysiologic conditions. Optimal utilization of the method will be facilitated by systematic comparisons with other available methods. Here, we report on the absolute values and changes of rat brain PtO(2) using EPR oximetry and the OxyLite, an oxygen monitor based on fluorescence quenching, at adjacent locations in the same brain. EPR oximetry utilizes an implanted oxygen-sensitive material and reports tissue PtO(2) at the surface of the material. OxyLite measures PtO(2) using the fluorescence lifetime of a chromophore fixed to the tip of an optical fiber that is inserted into tissue. Measurements were made at a depth of 2-3 mm into the cortex during normoxia and during breathing of carbogen (95% O(2):5% CO(2)) followed by a return to normoxia. We conclude that in this study (1) PtO(2) values reported by the two methods are similar but not exactly the same, (2) both methods can record a baseline and rapid changes in PtO(2), (3) changes in PtO(2) induced by increasing FiO(2) from 0.26 to 0.95 (carbogen) were similar by the two methods and (4) in some rats breathing carbogen, absolute values of PtO(2) were above the sensitive range of the OxyLite method.

Labeling of capsid proteins and genomic RNA of human rhinovirus with two different fluorescent dyes for selective detection by capillary electrophoresis

During uncoating of human rhinoviruses, the innermost capsid protein VP4 and the genomic RNA are released from the viral protein shell. This process gives rise to subviral particles that are composed of the remaining three capsid proteins VP1, VP2, and VP3. The process is believed to take place in a sequential manner in that first VP4 is expelled resulting in A-particles sedimenting at 135S followed by the RNA resulting in B-particles sedimenting at 80S. Aiming at ultimately analyzing this process in vivo, we introduced two different fluorophores into the RNA and the viral capsid proteins, respectively. Incubation of the virus with RiboGreen resulted in formation of a RNA-dye complex with lambda(ex)/lambda(em) = 500/525 nm, whereas subsequent derivatization of the viral protein shell in the same sample with AMCA-S introduced a label with lambda(ex)/lambda(em) = 345-350/440-460 nm. In this way, both viral components could be selectively detected via fluorescence in a capillary electrophoresis system. The intact virus delivers two superimposed signals in the electropherogram. Derivatization of the free amino groups of the capsid proteins partially preserved the bioaffinity of the virus toward a synthetic receptor fragment, an artificial recombinant concatemer of repeat number 3 of the very low density lipoprotein receptor. Between 10 and 20% of the infectivity were recovered after labeling when compared to native virus. In addition to analysis of factors influencing the stability of the virus by CE, double-labeled virions might be useful for the investigation of the uncoating process by real-time confocal fluorescence microscopy.

Fluorescent sensor for imidazole derivatives based on monomer-dimer equilibrium of a zinc porphyrin complex in a polymeric film

A new zinc(II) porphyrin conjugate with an appended pyrene subunit has been synthesized and shown to exhibit significant and analytical usefulness for fluorescence sensing toward imidazole derivatives. The molecular recognition was based on the bridging interaction of the imidazole ring of analyte with the zinc(II) center of the porphyrin, while the transduction signal for the recognition process was the pyrene excimer fluorescence. The sensor was constructed and applied for fluorescence assay of histidine in aqueous solution by immobilizing the sensing material in a plasticized PVC membrane. When the membrane was bathed in an alkaline solution void of histidine, zinc(II) porphyrin was present in the monomer form, and pyrene emitted monomer fluorescence at 378 and 397 nm. With the presence of histidine in the sample solution, histidine was extracted into the membrane phase and bridged with the Zn(II) center of the porphyrin, causing the monomer porphyrin to be converted to its dimeric species. Since the formation of porphyrin dimer was accompanied by the enhancement of pyrene excimer emission at 454 nm, the chemical recognition process could be directly translated into a fluorescent signal. With the optode membrane M1 described, histidine in sample solution from 6.76 x 10(-7) to 5.01 x 10(-3) M can be determined. The limit of detection was 1.34 x 10(-7) M. The optical selectivity coefficient obtained for histidine over biologically relevant amino acids and anions met the selectivity requirements for the determination of histidine in biological samples. Serum histidine values obtained by the optode membrane fell in the normal range of the content reported in the literature and were in good agreement with those obtained by HPLC.

Comparative Fluorescence Spectroscopy Shows Differences in Noncavitated Enamel Lesions

The aim of this study was to compare emission spectra from noncavitated enamel caries with different degrees of discoloration under a wide range of excitation wavelengths. Freshly extracted human molars with white spot, light discolored and dark discolored brown spot enamel caries were selected (n = 4 each). Rectangular blocks (3.5 x 3.5 mm) were cut from the carious area and a corresponding sound area of the same tooth. Emission spectra were recorded from carious and the corresponding sound areas using a fluorescence spectrophotometer at excitation wavelengths from 360 nm up to 580 nm in steps of 20 nm. The specimens were submerged in water during measurement. The spectra were corrected and normalized to peak intensity for comparisons between spectra from sound and carious areas of each tooth. Excitation spectra were recorded for selected emission wavelengths that showed maximum intensity. Emission spectra of all types of carious lesions were shifted towards longer wavelengths (red shift) when compared to the spectra of the corresponding sound enamel. The red shift was highest for dark brown spot lesions and lowest for white spot lesions (p <or= 0.001). Distinct fluorescence bands within 600-700 nm typical for porphyrin compounds were strongest for excitation wavelengths from 400 to 420 nm and present in most of the lesions investigated.

Probing the binding of scutellarin to human serum albumin by circular dichroism, fluorescence spectroscopy, FTIR, and molecular modeling method

The binding of scutellarin with human serum albumin (HSA) was investigated at four temperatures, 296, 303, 310, and 318 K, by fluorescence, circular dichroism (CD), Fourier transform infrared spectroscopy (FT-IR), and molecular modeling study at pH 7.40. The binding parameters were determined by Scatchard's procedure, which are approximately consistent with the results of Stern-Volmer equation. The thermodynamic parameters were calculated according to the dependence of enthalpy change on the temperature as follows: DeltaH degrees is a small negative value (-8.55 kJ/mol), whereas DeltaS degrees is a positive value (65.15 J/mol K). Quenching of the fluorescence HSA in the presence of scutellarin was observed. Data obtained by fluorescence spectroscopy and CD experiment, FT-IR experiment, and molecular modeling method suggested that scutellarin can strongly bind to the HSA and the primary binding site of scutellarin is located in site I of HSA. It is considered that scutellarin binds to site I (subdomain II) mainly by a hydrophobic interaction and there are hydrogen bond interactions between the scutellarin and the residues Arg222 and Arg257.

Characterization and Matching of Oil Samples Using Fluorescence Spectroscopy and Parallel Factor Analysis

A novel approach for matching oil samples by fluorescence spectroscopy combined with three-way decomposition of spectra is presented. It offers an objective fingerprinting based on the relative composition of polycyclic aromatic compounds (PACs) in oils. The method is complementary to GC-FID for initial screening of oil samples but can also be used for prescreening in the field, onboard ships, using a portable fluorescence spectrometer. Parallel factor analysis (PARAFAC) was applied to fluorescence excitation-emission matrixes (EEMs) of heavy fuel oils (HFOs), light fuel oils, lubricating oils, crude oils, unknown oils, and a sample collected in the spill area two weeks after the Baltic Carrier oil spill (Denmark, 2001). A total of 112 EEMs were decomposed into a five-factor PARAFAC model using excitation wavelengths from 245 to 400 nm and emission wavelengths from 280 to 550 nm. The PARAFAC factors were compared to EEMs of PAC standards with two to five rings, and the comparisons indicate that each of the factors can be related to a mixture of PACs with similar fluorescence characteristics: a mixture of naphthalenes and dibenzothiophenes, fluorenes, phenanthrenes, chrysenes, and five-ring PACs, respectively. Oils were grouped in score plots according to oil type. Except for HFOs and crude oils, the method easily discriminated between the four oil types. Minor overlaps of HFOs and crude oils were observed along all five PARAFAC factors, and the variability of crude oils was large along factor 2 due to a varying content of five-ring PACs. The spill sample was correctly assigned as a HFO with similar PAC pattern as oil from the cargo tank of the Baltic Carrier by comparing the correlation coefficient of scores for the oil spill sample and possible source oils (i.e., oils in the database).

Novel method for in vivo hydroxyl radical measurement by microdialysis in fetal sheep brain in utero

Hydroxyl radical (.OH) is a reactive oxygen species produced during severe hypoxia, asphyxia, or ischemia that can cause cell death resulting in brain damage. Generation of .OH may occur in the fetal brain during asphyxia in utero. The very short half-life of .OH requires use of trapping agents such as salicylic acid or phenylalanine for detection, but their hydroxylated derivatives are either unstable, produced endogenously, or difficult to measure in the small volume of microdialysis samples. In the present study, we used terephthalic acid (TA), hydroxylation of which yields a stable and highly fluorometric isomer (excitation, 326 nm; emission, 432 nm). In vitro studies using .OH generated by the Fenton reaction showed that hydroxylated TA formed quickly (<10 s), was resistant to bleaching (<5% change in fluorescence), and permitted detection of <0.5 pmol .OH. In vivo studies were performed in fetal sheep using microdialysis probes implanted into the parasagittal cortex. The probe was perfused at 2 mul/min with artificial cerebrospinal fluid containing 5 mM TA, and samples were collected every 30 min. Fluorescence measured in 10 mul of dialysate was significantly greater than in the efflux from probes perfused without TA. High-performance liquid chromotography analysis showed that the fluorescence in dialysis samples was entirely due to hydroxylation of TA. Thus this study shows that it is possible to use TA as a trapping agent for detecting low concentrations of .OH both in vitro and in vivo and that low concentrations of .OH are present in fetal brain tissue and fluctuate with time.

Spatial-temporal studies of membrane dynamics: scanning fluorescence correlation spectroscopy (SFCS)

Giant unilamellar vesicles (GUVs) have been widely used as a model membrane system to study membrane organization, dynamics, and protein-membrane interactions. Most recent studies have relied on imaging methods, which require good contrast for image resolution. Multiple sequential image processing only detects slow components of membrane dynamics. We have developed a new fluorescence correlation spectroscopy (FCS) technique, termed scanning FCS (i.e., SFCS), which performs multiple FCS measurements simultaneously by rapidly directing the excitation laser beam in a uniform (circular) scan across the bilayer of the GUVs in a repetitive fashion. The scan rate is fast compared to the diffusion of the membrane proteins and even small molecules in the GUVs. Scanning FCS outputs a "carpet" of timed fluorescence intensity fluctuations at specific points along the scan. In this study, GUVs were assembled from rat kidney brush border membranes, which included the integral membrane proteins. Scanning FCS measurements on GUVs allowed for a straightforward detection of spatial-temporal interactions between the protein and the membrane based on the diffusion rate of the protein. To test for protein incorporation into the bilayers of the GUVs, antibodies against one specific membrane protein (NaPi II cotransporter) were labeled with ALEXA-488. Fluorescence images of the GUVs in the presence of the labeled antibody showed marginal fluorescence enhancement on the GUV membrane bilayers (poor image contrast and resolution). With the application of scanning FCS, the binding of the antibody to the GUVs was detected directly from the analysis of diffusion rates of the fluorescent antibody. The diffusion coefficient of the antibody bound to NaPi II in the GUVs was approximately 200-fold smaller than that in solution. Scanning FCS provided a simple, quantitative, yet highly sensitive method to study protein-membrane interactions.

The permeability of gap junction channels to probes of different size is dependent on connexin composition and permeant-pore affinities

Gap junctions have traditionally been characterized as nonspecific pores between cells passing molecules up to 1 kDa in molecular mass. Nonetheless, it has become increasingly evident that different members of the connexin (Cx) family mediate quite distinct physiological processes and are often not interchangeable. Consistent with this observation, differences in permeability to natural metabolites have been reported for different connexins, although the physical basis for selectivity has not been established. Comparative studies of different members of the connexin family have provided evidence for ionic charge selectivity, but surprisingly little is known about how connexin composition affects the size of the pore. We have employed a series of Alexa dyes, which share similar structural characteristics but range in size from molecular weight 350 to 760, to probe the permeabilities and size limits of different connexin channels expressed in Xenopus oocytes. Correlated dye transfer and electrical measurements on each cell pair, in conjunction with a three-dimensional mathematical model of dye diffusion in the oocyte system, allowed us to obtain single channel permeabilities for all three dyes in six homotypic and four heterotypic channels. Cx43 and Cx32 channels passed all three dyes with similar efficiency, whereas Cx26, Cx40, and Cx45 channels showed a significant drop-off in permeability with the largest dye. Cx37 channels only showed significant permeability for the smaller two dyes, but at two- to sixfold lower levels than other connexins tested. In the heterotypic cases studied (Cx26/Cx32 and Cx43/Cx37), permeability characteristics were found to resemble the more restrictive parental homotypic channel. The most surprising finding of the study was that the absolute permeabilities calculated for all gap junctional channels in this study are, with one exception, at least 2 orders of magnitude greater than predicted purely on the basis of hindered pore diffusion. Consequently, affinity between the probes and the pore creating an energetically favorable in-pore environment, which would elevate permeant concentration within the pore and hence the flux, is strongly implicated.

Uranyl precipitation by Pseudomonas aeruginosa via controlled polyphosphate metabolism

The polyphosphate kinase gene from Pseudomonas aeruginosa was overexpressed in its native host, resulting in the accumulation of 100 times the polyphosphate seen with control strains. Degradation of this polyphosphate was induced by carbon starvation conditions, resulting in phosphate release into the medium. The mechanism of polyphosphate degradation is not clearly understood, but it appears to be associated with glycogen degradation. Upon suspension of the cells in 1 mM uranyl nitrate, nearly all polyphosphate that had accumulated was degraded within 48 h, resulting in the removal of nearly 80% of the uranyl ion and >95% of lesser-concentrated solutions. Electron microscopy, energy-dispersive X-ray spectroscopy, and time-resolved laser-induced fluorescence spectroscopy (TRLFS) suggest that this removal was due to the precipitation of uranyl phosphate at the cell membrane. TRLFS also indicated that uranyl was initially sorbed to the cell as uranyl hydroxide and was then precipitated as uranyl phosphate as phosphate was released from the cell. Lethal doses of radiation did not halt phosphate secretion from polyphosphate-filled cells under carbon starvation conditions.

Purification and Fluorescent Labeling of the Human Serotonin Transporter

To establish a purification procedure for the human serotonin transporter (hSERT) we expressed in Sf9 insect cells an epitope-tagged version of the transporter containing a FLAG epitope at the N-terminus and a polyhistidine tail at the C-terminus (FLAG-hSERT-12H). For purification, the transporter was solubilized in digitonin followed by nickel affinity and subsequent concanavalin A chromatography. Using this procedure we were able to obtain an overall purification of 700-fold and a yield of approximately 0.1 mg/L of cell culture. The purified transporter displayed pharmacological properties similar to those of hSERT expressed in native tissues and in transfected cell lines. Fluorescent labeling of the purified transporter with the thiol-reactive fluorophore nitrobenxoxadiazol-iodoacetamide (IANBD) and Texas Red bromoacetamide preserved the pharmacological profile of FLAG-hSERT-12H. Collisional quenching experiments revealed that the aqueous quencher iodide was able to cause marked quenching of the fluorescence of the IANBD labeled transporter with a K(SV) of 3.4 +/- 0.10 M(-)(1). In a mutant transporter with five cysteines mutated (5CysKO) we observed a significant reduction in this quenching (K(SV) = 2.1 +/- 0.16 M(-)(1), p < 0.01). This reduction was most likely due to labeling of (109)Cys since mutation of this cysteine alone resulted in a reduction in collisional quenching that was similar to that observed with 5CysKO (K(SV) = 2.2 +/- 0.15 M(-)(1)). These data suggest that labeling of (109)Cys contributes substantially to the overall fluorescence of IANBD labeled FLAG-hSERT-12H. On the basis of these data we infer that (109)Cys is embedded in a mixed hydrophobic/hydrophilic environment at the external ends of transmembrane segments 1 and 2. Further use of fluorescent techniques on purified hSERT should prove useful in future studies aimed at understanding the molecular structure and function of Na(+)/Cl(-)-dependent neurotransmitter transporters.