Rapid small intestinal permeability assay based on riboflavin and lactulose detected by bis-boronic acid appended benzyl viologens

Background

Although organoboronic acids are efficient high-throughput sugar sensors, they have not been pursued for gut permeability studies. A modification of the lactulose/mannitol assay is described by which small intestinal permeability is assessed at the time of urine collection using a lactulose/riboflavin ratio.

Methods

Volunteers ingested 50 mg riboflavin and either 5 g mannitol or 10 g lactulose. Urine was collected for 6 hrs. Riboflavin was assayed by autofluorescence. Riboflavin was removed by C18 solid phase extraction. Lactulose and mannitol were then assayed using 1,1′-bis(2-boronobenzyl)-4,4′-bipyridinium (4,4′oBBV) coupled to the fluorophore HPTS.

Results

The temporal profile over 6 hrs for riboflavin paralleled mannitol. Riboflavin recovery in urine was 11.1 ± 1.9 % (mean ± SEM, n = 7), similar to mannitol. There was selective binding of 4,4′oBBV to lactulose, likely involving cooperativity between the fructose and galactose moieties. Lower limits of detection and quantification were 90 and 364 μM. The lactulose assay was insensitive to other permeability probes (e.g., sucrose, sucralose) while tolerating glucose or lactose. This assay can be adapted to automated systems. Stability of 4,4′oBBV exceeds 4 years.

Conclusions

Riboflavin measured by autofluorescence combined with lactulose measured with 4,4′oBBV represents a useful new chemistry for rapid measurement of intestinal permeability with excellent stability, cost and throughput benefits.

Electro-fluorescence switching of bis-imidazolium ionic liquids

Room-temperature ionic liquids (RTILs) containing bis-imidazolium salts were explored to control their optical properties of them in an organic device. The neat bis-imidazolium salts showed ionic conductivity of 3.5 x 10(-4) S/cm at room-temperature and the electrochemical window was exhibited within +/-2.5 V in a two electrode cell. The bis-imidazolium salts were transparent yellow and showed fluorescence upon excitation with light in the range of 360 to 500 nm. A two electrode organic ionic liquid cell was fabricated using a mixture of PEO200Blm-TFSI and electroactive molecules to control the emission property of the ionic liquid by electrochemical methods. The first example of the ionic liquid containing electro-fluorescence switch was explored with an electroactive benzyl viologen (BzV) blend of bis-imidazolium ionic liquid.

Characterization of catalytic properties of hydrogenase isolated from the unicellular cyanobacterium Gloeocapsa alpicola CALU 743

The main catalytic properties of the Hox type hydrogenase isolated from the Gloeocapsa alpicola cells have been studied. The enzyme effectively catalyzes reactions of oxidation and evolution of H2 in the presence of methyl viologen (MV) and benzyl viologen (BV). The rates of these reactions in the interaction with the physiological electron donor/acceptor NADH/NAD+ are only 3-8% of the MV(BV)-dependent values. The enzyme interacts with NADP+ and NADPH, but is more specific to NAD+ and NADH. Purification of the hydrogenase was accompanied by destruction of its multimeric structure and the loss of ability to interact with pyridine nucleotides with retained activity of the hydrogenase component (HoxYH). To show the catalytic activity, the enzyme requires reductive activation, which occurs in the presence of H2, and NADH accelerates this process. The final hydrogenase activity depends on the redox potential of the activation medium (E(h)). At pH 7.0, the enzyme activity in the MV-dependent oxidation of H2 increased with a decrease in E(h) from -350 mV and reached the maximum at E(h) of about -390 mV. However, the rate of H2 oxidation in the presence of NAD+ in the E(h) range under study was virtually constant and equal to 7-8% of the maximal rate of H2 oxidation in the presence of MV.

Nanoparticle films as electrodes: voltammetric sensitivity to the nanoparticle energy gap

Electron-transfer reactions of redox solutes at electrode/solution interfaces are facilitated when their formal potentials match, or are close to, the energy of an electronic state of the electrode. Metal electrodes have a continuum of electronic levels, and redox reactions occur without restraint over a wide span of electrode potentials. This paper shows that reactions on electrodes composed of films of metal nanoparticles do have constraints when the nanoparticles are sufficiently small and molecule-like so as to exhibit energy gaps, and resist electron transfers with redox solutes at potentials within the energy gap. When solute formal potentials are near the electronic states of the nanoparticles in the film, electron-transfer reactions can occur. The electronic states of the nanoparticle film electrodes are reflected in the formal potentials of the electrochemical reactions of the dissolved nanoparticles at naked metal electrodes. These ideas are demonstrated by voltammetry of aqueous solutions of the redox solutes methyl viologen, ruthenium hexammine, and two ferrocene derivatives at films on electrodes of 1.1 nm core diameter Au nanoparticles coated with protecting monolayers of phenylethanethiolate ligands. The methyl viologen solute is unreactive at the nanoparticle film electrode, having a formal potential lying in the nanoparticle's energy gap. The other solutes exhibit electron transfers, albeit slowed by the electron hopping resistance of the nanoparticle film. The nanoparticles are not linked together, being insoluble in the aqueous medium; a small amount of an organic additive (acetonitrile) facilitates observing the redox solute voltammetry.

An autocatalytic step in the reaction cycle of hydrogenase from Thiocapsa roseopersicina can explain the special characteristics of the enzyme reaction

A moving front has been observed as a special pattern during the hydrogenase-catalyzed reaction of hydrogen uptake with benzyl viologen as electron acceptor in a thin-layer reaction chamber. Such fronts start spontaneously and at random times at different points of the reaction chamber; blue spheres are seen expanding at constant speed and amplitude. The number of observable starting points depends on the hydrogenase concentration. Fronts can be initiated by injecting either a small amount of completed reaction mixture or activated hydrogenase, but not by injecting a low concentration of reduced benzyl viologen. These characteristics are consistent with an autocatalytic reaction step in the enzyme reaction. The special characteristics of the hydrogen-uptake reaction in the bulk reaction (a long lag phase, and the enzyme concentration dependence of the lag phase) support the autocatalytic nature. We conclude that there is at least one autocatalytic reaction step in the hydrogenase-catalyzed reaction. The two possible autocatalytic schemes for hydrogenase are prion-type autocatalysis, in which two enzyme forms interact, and product-activation autocatalysis, where a reduced electron acceptor and an inactive enzyme form interact. The experimental results strongly support the occurrence of prion-type autocatalysis.

Toxicity of redox cycling pesticides in primary mesencephalic cultures

A loss of nigrostriatal dopaminergic neurons is the primary neurodegenerative feature of Parkinson's disease. Paraquat, a known redox cycling herbicide, has recently been shown to kill selectively nigrostriatal dopaminergic cells in the mouse model. The purpose of this study was to test the ability of paraquat and other redox cycling pesticides to damage dopaminergic neurons in primary mesencephalic cultures. Addition of paraquat, diquat, or benzyl viologen to mesencephalic cultures induced morphological changes (e.g., dystrophic neuronal processes) consistent with dopaminergic cell injury. The three pesticides also caused cell death as assessed by a reduction of the number of tyrosine hydroxylase-immunoreactive neurons and a dose-dependent decrease in [(3)H]dopamine uptake. Quite interestingly, diquat and benzyl viologen were significantly more toxic than paraquat, probably reflecting their more pronounced ability to trigger redox cycling reactions. The data support a role of redox cycling as a mechanism of dopaminergic cell degeneration and suggest that the property of redox cycling should be taken into consideration when evaluating putative environmental risk factors for Parkinson's disease.

Reactions of H2, CO, and O2 with active [NiFe]-hydrogenase from Allochromatium vinosum. A stopped-flow infrared study

The Ni-Fe site in the active membrane-bound [NiFe]-hydrogenase from Allochromatium vinosum can exist in three different redox states. In the most oxidized state (Ni(a)-S) the nickel is divalent. The most reduced state (Ni(a)-SR) likewise has Ni(2+), while the intermediate state (Ni(a)-C) has Ni(3+). The transitions between these states have been studied by stopped-flow Fourier transform infrared spectroscopy. It is inferred from the data that the Ni(a)-S --> Ni(a)-C* and Ni(a)-C* --> Ni(a)-SR transitions induced by dihydrogen require one of the [4Fe-4S] clusters to be oxidized. Enzyme in the Ni(a)-S* state with all of the iron-sulfur clusters reduced reacts with dihydrogen to form the Ni(a)-SR state in milliseconds. By contrast, when one of the cubane clusters is oxidized, the Ni(a)-S state reacts with dihydrogen to form the Ni(a)-C state with all of the iron-sulfur clusters reduced. The competition between dihydrogen and carbon monoxide for binding to the active site was dependent on the redox state of the nickel ion. Formation of the Ni(a)-S.CO state (Ni(2+)) by reacting CO with enzyme in the Ni(a)-SR and Ni(a)-S states (Ni(2+)) is considerably faster than its formation from enzyme in the Ni(a)-C* (Ni(3+)) state. Excess oxygen converted hydrogen-reduced enzyme to the inactive Ni(r)* state within 158 ms, suggesting a direct reaction at the Ni-Fe site. With lower O(2) concentrations the formation of intermediate states was observed. The results are discussed in the light of the present knowledge of the structure and mechanism of action of the A. vinosum enzyme.

Diphenylene iodonium as an inhibitor for the hydrogenase complex of Rhodobacter capsulatus. Evidence for two distinct electron donor sites

The photosynthetic bacterium Rhodobacter capsulatus synthesises a membrane-bound [NiFe] hydrogenase encoded by the H2 uptake hydrogenase (hup)SLC structural operon. The hupS and hupL genes encode the small and large subunits of hydrogenase, respectively; hupC encodes a membrane electron carrier protein which may be considered as the third subunit of the uptake hydrogenase. In Wolinella succinogenes, the hydC gene, homologous to hupC, has been shown to encode a low potential cytochrome b which mediates electron transfer from H2 to the quinone pool of the bacterial membrane. In whole cells of R. capsulatus or intact membrane preparation of the wild type strain B10, methylene blue but not benzyl viologen can be used as acceptor of the electrons donated by H2 to hydrogenase; on the other hand, membranes of B10 treated with Triton X-100 or whole cells of a HupC- mutant exhibit both benzyl viologen and methylene blue reductase activities. We report the effect of diphenylene iodonium (Ph2I), a known inhibitor of mitochondrial complex I and of various monooxygenases on R. capsulatus hydrogenase activity. With H2 as electron donor, Ph2I inhibited partially the methylene blue reductase activity in an uncompetitive manner, and totally benzyl viologen reductase activity in a competitive manner. Furthermore, with benzyl viologen as electron acceptor, Ph2I increased dramatically the observed lagtime for dye reduction. These results suggest that two different sites exist on the electron donor side of the membrane-bound [NiFe] hydrogenase of R. capsulatus, both located on the small subunit. A low redox potential site which reduces benzyl viologen, binds Ph2I and could be located on the distal [Fe4S4] cluster. A higher redox potential site which can reduce methylene blue in vitro could be connected to the high potential [Fe3S4] cluster and freely accessible from the periplasm.

Evidence for a proposed intermediate redox state in the CO/CO(2) active site of acetyl-CoA synthase (Carbon monoxide dehydrogenase) from Clostridium thermoaceticum

When samples of the enzyme in the C(red1) state were reduced with Ti(3+) citrate, the C-cluster stabilized in an EPR-silent state. Subsequent treatment with CO or dithionite yielded C(red2). The EPR-silent state formed within 1 min of adding Ti(3+) citrate, while C(red2) formed after 60 min. Ti(3+) citrate appeared to slow the rate by which C(red2) formed from C(red1) and stabilize the C-cluster in the previously proposed C(int) state. This is the first strong evidence for C(int), and it supports the catalytic mechanism that required its existence. This mechanism is analogous to those used by flavins and hydrogenases to convert between n = 2 and n = 1 processes. Ti(3+) citrate had a different effect on enzyme in a CO(2) atmosphere; it shifted reduction potentials of metal centers (relative to those obtained using CO) and did not stabilize C(int). Different redox behavior was also observed when methyl viologen and benzyl viologen were used as reductants. This variability was exploited to prepare enzyme samples in which EPR from C(red2) was present without interfering signals from B(red). The saturation properties of B(red) depended upon the redox state of the enzyme. Three saturation "modes", called Sat1-Sat3, were observed. Sat1 was characterized by a sharp g = 1.94 resonance and low-intensity g = 2. 04 and 1.90 resonances, and was observed in samples poised at slightly negative potentials. Sat2 was characterized by weak intensity from all three resonances, and was strictly associated with intermediate redox states and the presence of CO(2). Sat3 was characterized by strong broad resonances with normalized intensities essentially unchanged relative to nonsaturating conditions, and was observed at the most negative potentials. Each mode probably reflects different spatial relationships among magnetic components in the enzyme.

Picosecond transient absorption spectroscopy in the blue spectral region of photosystem I

Picosecond transient absorption difference spectroscopy in the blue wavelength region (380-500 nm) was used to study the early electron acceptors in photosystem I. Samples were photosystem I core particles with about 100 chlorophylls per reaction center isolated from the cyanobacterium Synechocystis sp. PCC 6803. After excitation at 590 nm at room temperature, decay-associated spectra (DAS) were determined from global analysis in the blue region, yielding two transient components and one nondecaying component. A 3 ps decay phase is interpreted as primarily due to antenna excited-state redistribution. A 28 ps decay phase is interpreted as due to overall excited-state decay by electron transfer. The nondecaying component is ascribed to the difference spectrum of P(700) and the quinone or A(1) electron acceptor (P(700)(+)A(1)(-) - P(700)A(1)). Decay curves on the millisecond time scale at different wavelengths were measured with an autoxidizable artificial electron acceptor, benzyl viologen, and the (P(700)(+) - P(700)) difference spectrum was constructed. The (A(1)(-) - A(1)) difference spectrum was obtained by taking the difference between the above two difference spectra. A parallel picosecond experiment under strongly reducing conditions was also done as a control experiment. These conditions stabilize the electron on an earlier acceptor, A(0). The nondecaying component of the DAS at low potential was assigned to (P(700)(+)A(0)(-) - P(700)A(0)) since the electron-transfer pathway from A(0) to A(1) was blocked. The [(P(700)(+)A(0)(-) - P(700)A(0)) - (P(700)(+) - P(700))] subtraction gives a spectrum, interpreted as the (A(0)(-) - A(0)) difference spectrum of a chlorophyll a molecule, consistent with previous studies. The (A(1)(-) - A(1)) spectrum resolved on the picosecond time scale shows significant differences with similar spectra measured on longer time scales. These differences may be due to electrochromic effects and spectral evolution.

Reactions of dimethylsulfoxide reductase from Rhodobacter capsulatus with dimethyl sulfide and with dimethyl sulfoxide: complexities revealed by conventional and stopped-flow spectrophotometry

Improved assays for the molybdenum enzyme dimethylsulfoxide reductase (DMSOR) with dimethyl sulfoxide (DMSO) and with dimethyl sulfide (DMS) as substrates are described. Maximum activity was observed at pH 6.5 and below and at 8.3, respectively. Rapid-scan stopped-flow spectrophotometry has been used to investigate the reduction of the enzyme by DMS to a species previously characterized by its UV-visible spectrum [McAlpine, A. S., McEwan, A. G., and Bailey, S. (1998) J. Mol. Biol. 275, 613-623], and its subsequent reoxidation by DMSO. Both these two-electron reactions were faster than enzyme turnover under steady-state conditions, indicating that one-electron reactions with artificial dyes were rate-limiting. Second-order rate constants for the two-electron reduction and reoxidation reactions at pH 5.5 were (1.9 +/- 0.1) x 10(5) and (4.3 +/- 0.3) x 10(2) M-1 s-1, respectively, while at pH 8.0, the catalytic step was rate-limiting (62 s-1). Kinetically, for the two-electron reactions, the enzyme is more effective in DMS oxidation than in DMSO reduction. Reduction of DMSOR by DMS was incomplete below approximately 1 mM DMS but complete at higher concentrations, implying that the enzyme's redox potential is slightly higher than that of the DMS-DMSO couple. In contrast, reoxidation of the DMS-reduced state by DMSO was always incomplete, regardless of the DMSO concentration. Evidence for the existence of a spectroscopically indistinguishable reduced state, which could not be reoxidized by DMSO, was obtained. Brief reaction (less than approximately 15 min) of DMS with DMSOR was fully reversible on removal of the DMS. However, in the presence of excess DMS, a further slow reaction occurred aerobically, but not anaerobically, to yield a stable enzyme form having a lambdamax at 660 mn. This state (DMSORmod) retained full activity in steady-state assays with DMSO, but was inactive toward DMS. It could however be reconverted to the original resting state by reduction with methyl viologen radical and reoxidation with DMSO. We suggest that in this enzyme form two of the dithiolene ligands of the molybdenum have dissociated and formed a disulfide. The implications of this new species are discussed in relation both to conflicting published information for DMSOR from X-ray crystallography and to previous spectroscopic data for its reduced forms.

Pre-steady-state kinetics of the reactions of [NiFe]-hydrogenase from Chromatium vinosum with H2 and CO

Results are presented of the first rapid-mixing/rapid-freezing studies with a [NiFe]-hydrogenase. The enzyme from Chromatium vinosum was used. In particular the reactions of active enzyme with H2 and CO were monitored. The conversion from fully reduced, active hydrogenase (Nia-SR state) to the Nia-C* state was completed in less than 8 ms, a rate consistent with the H2-evolution activity of the enzyme. The reaction of CO with fully reduced enzyme was followed from 8 to 200 ms. The Nia-SR state did not react with CO. It was discovered, contrary to expectations, that the Nia-C* state did not react with CO when reactions were performed in the dark. When H2 was replaced by CO, a Nia-C* EPR signal appeared within 11 ms; this was also the case when H2 was replaced by Ar. With CO, however, the Nia-C* state decayed within 40 ms, due to the generation of the Nia-S.CO state (the EPR-silent state of the enzyme with bound CO). The Nia-C* state, induced after 11 ms by replacing H2 by CO in the dark, could be converted, in the frozen enzyme, into the EPR-detectable state with CO bound to nickel (Nia*.CO) by illumination at 30 K (evoking the Nia-L* state), followed by dark adaptation at 200 K. This can be explained by assuming that the Nia-C* state represents a formally trivalent state of nickel, which is unable to bind CO, whereas nickel in the Nia-L* and the Nia*.CO states is formally monovalent.

Modulation of the substrate specificity of Escherichia coli dimethylsulfoxide reductase

X-ray crystallographic studies of the Rhodobacter sphaeroides dimethyl sulfoxide (Me2SO) reductase [Schindelin, H., Kisker, C., Hilton, J., Rajagopalan, K. V., & Rees, D. C. (1996) Science, 272, 1615-1620] indicated that the active site is at the bottom of a 25-A funnel. Substrates must travel to the bottom of the funnel for reduction to occur. The homologous DmsA subunit of the trimeric Escherichia coli Me2SO reductase, was subjected to site-directed mutagenesis of residues potentially lining the bottom of the funnel, based on sequence alignment of the E. coli and Rhodobacter Me2SO reductases. Sixteen E. coli DmsA mutants were characterized. Mutants G167N, A178Q, Q179I and R217Q showed functional impairment, as indicated by abnormal anaerobic growth with Me2SO as the sole terminal acceptor, in a recombinant strain deleted for chromosomal dmsABC. The kinetic parameters of the mutant enzymes were examined using the artificial electron donor benzyl viologen and the quinone analogue dimethylnaphthoquinone, with Me2SO and pyridine N-oxide as electron acceptors. Mutants A178Q and R217Q showed dramatic alterations of their electron-acceptor Km, with values at least 35-fold less or greater than wild-type values, respectively, for Me2SO and pyridine N-oxide. T148S showed altered kinetic parameters for pyridine N-oxide and Me2SO, with Km and k(cat) decreasing and increasing approximately fourfold, respectively. Other mutants showed less drastic alterations in kinetic parameters. This analysis has identified amino acids important in substrate binding and catalysis.

The one-electron reduction potential of 3-amino-1,2,4-benzotriazine 1,4-dioxide (tirapazamine): a hypoxia-selective bioreductive drug

The one-electron reduction potential of 3-amino-1,2,4-benzotriazine 1,4-dioxide, tirapazamine (SR 4233) in aqueous solution has been determined by pulse radiolysis. Reversible electron transfer was achieved between radiolytically-generated one-electron reduced radicals of tirapazamine (T), and quinones or benzyl viologen as redox standards. The reduction potential Em7(T/T-) was -0.45 +/- 0.01 V vs. NHE at pH 7 From the pH dependence of the reduction potential, pKa = 5.6 +/- 0.2 was estimated for the tirapazamine radical, a value similar to the pKa determined by other methods.

Temperature effects on the one-electron reduction potentials of nitroaryl compounds

Pulse radiolysis was used to establish one-electron transfer equilibria between radical cations of methyl or benzyl viologens (V2+) and nitroaryl compounds (ArNO2): a nitroimidazole (misonidazole or metronidazole), 4-nitrobenzoate or nitrofurazone. The equilibrium constants in water at pH 8 were estimated over the temperature range approximately 5 to 75 degrees C. The difference delta E in mid-point one-electron reduction potentials between the nitro compounds and the viologens varied with temperature T; increasing temperature made the nitro compounds apparently less electron-affinic compared to the effects of temperature on the viologen potential. Values of delta(delta E)/delta T were in the range -0.7 to -1.1 mV K-1 at 25 degrees C. If delta[E(V2+/V.+)]/delta T = -0.9 mV K-1 for methyl viologen then delta[E(ArNO2/ArNO2.-)]/delta T is about -2 mV K-1 for these compounds.

The reduction potential of benzyl viologen: an important reference compound for oxidant/radical redox couples

Bipyridinium compounds such as the viologens (V2+) have been used extensively as redox indicators, but radical dimerization leads to reduction potentials of the oxidant/radical one-electron couple, E(V2+/V2+) varying with radical concentration: a particular problem with benzyl viologen. Pulse radiolysis has been used to characterize the one-electron transfer equilibria between methyl (MV2+) or benzyl (BV2+) viologen and a 2-nitroimidazole (misonidazole), and between methyl and benzyl viologens directly. Total radical concentrations were typically approximately 0.4 mumol dm-3 to minimize dimerization. Based on E(MV2+/MV2+) = -450 mV, a value of E(BV2+/BV2+) = -374 mV is now recommended. The implications for other 'reference' redox couples are discussed.