Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Conductometry”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 325 records · Page 18Linked to original sources
PubMed1991

Determination of urea in serum by using naturally immobilized urease in a flow injection conductimetric system.

A flow injection method was developed, aimed at the determination of urea in human serum. The system makes use of the naturally immobilized urease present in Canavalia ensiformis DC (jack bean). A column is filled with small pieces of this bean, and the sample (50 microliters) containing urea passes through it carried by a 1% NaCl solution. On leaving the column the stream is merged with an alkaline reagent (0.5 mol dm-3 NaOH; 0.5% disodium dihydrogen ethylenediaminetetraacetate). The ammonium ions, arising from the enzymatic reaction that occurs inside the column, are changed into the molecular form, which permeates a polytetrafluoroethylene membrane and is received in a de-ionized water acceptor stream. The ammonia ionizes causing an increase in the conductance, which is proportional to the urea content of the sample. About 40 samples can be processed in 1 h with negligible carry-over and with a relative standard deviation of 1% or less. The results are in agreement with those obtained by a standard spectrophotometric method.

Conductometry↗
PubMed2002

An indirect conductimetric screening method for the detection of antibiotic residues in bovine kidneys.

An indirect conductimetric screening method using three test bacterium-medium combinations was developed for rapid detection of antibiotic residues in bovine carcasses. The detection time (DT), i.e. the point when the growth of the test bacterium was detected, was determined by observing the rate of change in the conductance plotted against time. This detection time averaged half of the reference time recorded by the instrument software. Total change in conductance (TC) was used as a further measure of growth. Threshold values for DT and TC were determined with inhibitor-free kidney samples. The presence of a residue was indicated if the DT exceeded the respective threshold value and was confirmed if the TC remained below the TC threshold value. The limits of detection (LODs) determined with fortified samples were at about or below the MRLs for cephalexin, chlortetracycline, ciprofloxacin, dihydrostreptomycin, enrofloxacin, oxytetracycline and penicillin G. The LODs for penicillin G, oxytetracycline and the sum of enrofloxacin and ciprofloxacin were also estimated with incurred samples; these samples were also analysed using liquid chromatography. The LODs determined with fortified and incurred samples were in close agreement. Given its rapid detection, good sensitivity to a wide range of antibiotics and ease of performance, the indirect conductimetric method developed here would seem to offer an appealing alternative to agar diffusion tests.

Animals↗
PubMed1969

Reactions of vitamin A with acceptors of electrons. Formation of radical anions from 7,7,8,8-tetracyanoquinodimethane and tetrachloro-1,4-benzoquinone.

1. The interactions of retinol and retinoic acid with two electron acceptors, 7,7,8,8-tetracyanoquinodimethane (TCNQ) and tetrachloro-1,4-benzoquinone (chloranil), were studied in an investigation on the ability of vitamin A to behave as a donor of electrons. 2. Retinol reacts with TCNQ in polar organic solvents with the formation, as judged by spectral studies, of the radical anion of TCNQ. 3. Addition of the products of this reaction to water is accompanied by a rapid consumption of OH(-) ions. 4. Consumption of OH(-) ions is also a feature of the reactions between retinol and chloranil, but the spectrum of the radical anion of chloranil is observed only when retinol and chloranil are suspended in aqueous salt solutions. 5. Retinoic acid behaves similarly to retinol in its reactions with TCNQ and chloranil, but it appears to be a weaker electron donor than retinol. 6. The reaction products that may be formed from retinol in its reactions with TCNQ and chloranil are discussed. 7. It is suggested that the ability of vitamin A to behave as a donor of electrons may be an important aspect of its biochemical mode of action.

Chemical Phenomena↗
PubMed1969

Reactions of vitamin A with acceptors of electrons. Interactions with iodine and the formation of iodide.

1. The reactions of retinol and retinoic acid with iodine were investigated since knowledge of the chemical reactions of vitamin A with acceptors of electrons may shed light on its biochemical mode of action. 2. Colloidal retinol, but not retinoic acid, reacts with iodine to yield a blue-green complex that rapidly decomposes, giving iodide and an unknown species with lambda(max.) at 870mmu. 3. In addition, both retinol and retinoic acid reduce iodine to iodide by a reaction that does not involve an intermediate coloured complex; this reaction appears to yield unstable carbonium ion derivatives of the vitamin. 4. The presence of water greatly facilitates the production of iodide from vitamin A and iodine. 5. Possible chemical pathways involved in these reactions are discussed. 6. It is suggested that the chemical properties of retinol and retinoic acid that underlie their biochemical behaviour might be apparent only when the molecules are at a lipid-water interface, and that vitamin A might be expected to react with a number of different electron acceptors in vivo.

Chemical Phenomena↗
PubMed1969

Evidence for the presence of several lipases n cow's milk.

Skim milks containing sodium chloride (0.75m) were centrifuged at 80000g for 2hr. and portions of the supernatants were submitted to gel filtration on columns of Sephadex G-200. Enzymes in the effluent fractions were assayed titrimetrically for their hydrolytic activities towards tributyrin, triolein and milk-fat emulsions, and triacetin solution. Summation of the measurements gave ratios of activities towards the various substrates similar to those of the original skim milks. Although only partial separation was obtained, five enzymes appeared to be present. They showed some differences in substrate specificity, but all appeared to be lipases in that they hydrolysed the emulsified substrates more rapidly than the dissolved triacetin.

Animals↗
PubMed2003

Measurement of fungi by an indirect conductimetric assay.

AIMS: To investigate the growth of fungi using an indirect conductimetric assay and derive, experimentally and theoretically, the relationship between microbial concentration and electrical conductivity change. METHODS AND RESULTS: The indirect assay, in which change in electrical conductivity of an alkaline solution (NaOH) is produced by absorption of CO2 from microbial metabolism, was conducted with the Bactometer (bioMerieux, Marcy-l'Etoile, France) for the enumeration of fungi. A linear relationship was obtained between detection time and logarithmic initial microbial concentration. This indirect assay used growth media, which could not be used in the direct conductimetric assay, to monitor fungal growth. CONCLUSIONS: The indirect assay does not depend on the growth media and the turbidity of sample and could offer a simple and rapid assay for the measurement of fungal growth under various conditions. SIGNIFICANCE AND IMPACT OF THE STUDY: The indirect assay is applicable for rapid detection of fungi, estimation of the growth rate and evaluation of antifungal activity.

Colony Count, Microbial↗
PubMed1972

A molecular theory of ion-conductng channels: a field-dependent transition between conducting and nonconducting conformations.

Structural and conformational requirements for an electric field-dependent transition between conducting and nonconducting macromolecular systems are: two kinetically interconvertible and energetically similar conformations, one conducting and the other nonconducting, which have axes spanning the lipid layer of biological membranes, but which have different net dipole moments along those axes. Two examples are described. A previously defined helix, the pi(6)LD-helix now termed the beta(6) (3,3)-helix, is proposed as the conducting species, and the linear peptide correlate of the cyclic hexapeptide conformation containing two beta-turns and an inversion element of symmetry is proposed as a nonconducting species. The latter is termed an anti-beta(6) (2)-spiral and contains little or no net dipole moment per turn, whereas the beta(6) (3,3)-helix contains a net dipole moment along the helix axis of about 0.5 Debye per dipeptide unit. A related conducting and nonconducting pair with large net dipole moments of opposite sign, termed syn-beta(6) (2)-spiral and beta(6) (2,4)-helix, are also described. The spiral conformations are stabilized in a lipid layer by intermolecular hydrogen bonds, leading to a linear association of transmembrane structures. A conformational transition in one member of the array could lead to destabilization of an adjacent member of the array. The conformational analysis uses a concept of cyclic conformations with linear conformational correlates. The anti-beta(6) (2)-spiral and beta(6) (3,3)-helix are derivable from the conformations of the cyclic structure [unk], whereas the syn-beta(2)-spiral and beta(6) (2,4)-helix may be derived from the cyclic structure [unk].The conformational analysis leads to the expectation that N-formyl-(L-Ala-L-Ala-Gly)(n) would form conducting channels.

Amino Acid Sequence↗
PubMed1981

Diphtheria toxin fragment forms large pores in phospholipid bilayer membranes.

The cytotoxic effect of diphtheria toxin requires the entry of its enzymatic A fragment (Mr approximately 21,000) into the cytosol of sensitive cells. We show that the B45 fragment (Mr approximately 24,000) forms, in lipid bilayers, pores that are large enough (diameter greater than or equal to 18 A) to allow the passage of extended fragment A. Pore formation is maximal when the B45-containing side is at low pH (4.7) and the opposite side is at high pH (7.4). These conditions resemble the pH gradient existing across lysosomal membranes. We suggest that fragment A passes through these pores from acidic endocytotic vesicles (lysosomes?) to the cytosol.

Conductometry↗
PubMed1981

Single K+ channel currents of anomalous rectification in cultured rat myotubes.

The currents through single K+ channels of the anomalous (or inward) rectifier were recorded in tissue cultured rat myotubes by using the "gigohm seal" patch clamp technique developed by Sigworth and Neher. These unitary currents were detected as current fluctuations due to the blocking and unblocking of channels by Ba2+. The single-channel conductance was obtained from the slope of the linear relationship between unitary current amplitude and membrane potential. When the external solution contained 155 mM K+, the single-channel conductance was 10.4 +/- 2.6 pS (+/- SD; n = 6). This value was independent of the the concentration of blocking ions but increased with increasing external K+ concentration. The behavior of the unitary current agreed with that expected from the blocking kinetics of Ba2+ on the macroscopic K+ current of the anomalous rectifier. The density of the channel is likely to be small and may even be less than 1/micrometers 2.

Animals↗
PubMed1993

Direct measurement of cyclic current-voltage responses of integral membrane proteins at a self-assembled lipid-bilayer-modified electrode: cytochrome f and cytochrome c oxidase.

Direct cyclic voltage-current responses, produced in the absence of redox mediators, for two detergent-solubilized integral membrane proteins, spinach cytochrome f and beef heart cytochrome c oxidase, have been obtained at an optically transparent indium oxide electrode modified with a self-assembled lipid-bilayer membrane. The results indicate that both proteins interact with the lipid membrane so as to support quasi-reversible electron transfer redox reactions at the semiconductor electrode. The redox potentials that were obtained from analysis of the cyclic "voltammograms," 365 mV for cytochrome f and 250 and 380 mV for cytochrome c oxidase (vs. normal hydrogen electrode), compare quite well with the values reported by using conventional titration methods. The ability to obtain direct electrochemical measurements opens up another approach to the investigation of the properties of integral membrane redox proteins.

Conductometry↗
PubMed1993

A single amino acid determines the subunit-specific spider toxin block of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate/kainate receptor channels.

Joro spider toxin (JSTX) is one of the most potent antagonists of glutamatergic AMPA/KA (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate/kainate) receptor channels in invertebrates and vertebrates. A differential blocking effect on certain types of glutamatergic synapses--e.g., parallel and climbing fiber synaptic inputs to rat cerebellar Purkinje neurons--has been shown by using a synthetic analog of the spider toxin. By investigating the molecular basis of the JSTX action on the recombinant AMPA/KA receptors GluR1-GluR4 and GluR6 expressed in Xenopus oocytes, we found that submicromolar concentrations of JSTX exert a subunit-specific block. Thus, receptor subunits forming a receptor channel with a linear current-voltage (I-V) relationship (GluR1/2, GluR2/3, and GluR6) were not affected, while receptor subunits with rectifying I-V relationships (GluR1, GluR3, GluR4, and GluR1/3) were reversibly blocked by JSTX. By using receptor-subunit mutants obtained by site-directed mutagenesis, we have identified a single amino acid position (glutamine in the proposed second transmembrane domain) that is critical for the JSTX block. Since this site has previously been shown to control the I-V relationship of the AMPA/KA receptor channel and to participate in the regulation of the channel's permeability for calcium ions, our findings suggest that JSTX binds close to the central pore region of the channel.

Amino Acid Sequence↗
PubMed1993

Expression of an atrial G-protein-activated potassium channel in Xenopus oocytes.

Injection of rat atrial RNA into Xenopus oocytes resulted in the expression of guanine nucleotide binding (G) protein-activated K+ channel. Current through the channel could be activated by acetylcholine or, if RNA encoding a neuronal 5HT1A receptor was coinjected with atrial RNA, by serotonin (5HT). A 5HT-evoked current (I5HT) was observed in oocytes injected with ventricle RNA fractions (of 2.5-5.5 kb) and 5HT1A receptor RNA. I5HT displayed strong inward rectification with very little conductance above the K+ equilibrium potential, was highly selective for K+ over Na+, and was blocked by 5-300 microM Ba2+. I5HT was suppressed by intracellular injection of the nonhydrolyzable analog of GDP, guanosine 5'-[beta-thio]diphosphate, but not by treatment with pertussis toxin (PTX), suggesting coupling of the receptor to the G-protein-activated K+ channel via a PTX-insensitive G protein, possibly endogenously present in the oocyte. Coexpression of the alpha subunit of a PTX-sensitive G protein, G(i2), rendered I5HT sensitive to PTX inhibition. Native oocytes displayed a constitutively active inwardly rectifying K+ current with a lower sensitivity to Ba2+ block; expression of a similar current was also directed by atrial or ventricle RNA of 1.5-3 kb. Xenopus oocytes may be employed for cloning of the G-protein-activated K+ channel cDNA and for studying the coupling between this channel and G proteins.

Acetylcholine↗
PubMed1995

Amperometric detection of stimulus-induced quantal release of catecholamines from cultured superior cervical ganglion neurons.

Amperometry has been used for real-time electrochemical detection of the quantal release of catecholamines and indolamines from secretory granules in chromaffin and mast cells. Using improved-sensitivity carbon fiber electrodes, we now report the detection of quantal catecholamine release at the surface of somas of neonatal superior cervical ganglion neurons that are studded with axon varicosities containing synaptic vesicles. Local application of a bath solution containing high K+ or black widow spider venom, each of which greatly enhances spontaneous quantal release of transmitter at synapses, evoked barrages of small-amplitude (2-20 pA), short-duration (0.5-2 ms) amperometric quantal "spikes". The median spike charge was calculated as 11.3 fC. This figure corresponds to 3.5 x 10(4) catecholamine molecules per quantum of release, or approximately 1% that evoked by the discharge of the contents of a chromaffin granule.

Animals↗