Methanogenesis from acetate: enrichment studies.
(17/29)
An acetate enrichment culture was initiated by inoculating anaerobic sludge from a mesophilic methane digestor into a mineral salts medium with calcium acetate as the sole carbon and energy source. This enrichment was maintained indefinitely by weekly transfer into medium of the same composition. A study of this enrichment disclosed an unexpected age-dependent inhibition of methanogenesis by H2 and formate which apparently differed from the inhibition by chloroform and benzyl viologen. This age-dependent inhibition indicated that microbial interactions of the mixed enrichment population may play a regulatory role in methane formation. Futhermore, stimulation of methanogenesis in the acetate enrichment by addition of yeast extract showed a nutrient limitation which indicated that syntrophic interactions leading to formation of growth factors may also occur. A model is presented to illustrate the possible interrelationships between methanogenic and nonmethanogenic bacteria in their growth and formation of methane and carbon dioxide from acetate. (+info)
Spectral properties of chlorophyll a monolayers in the presence of an exogenous electron donor and acceptor.
(18/29)
Chlorophyll a monolayers are studied at a nitrogen-water interface in the presence of a reducing or oxidizing agent: sodium ascorbate and benzyl viologen, respectively. Absorption spectra of the films are measured directly on the aqueous surface. With the aid of a computer, fourth derivative and difference spectra are determined. In the presence of ascorbate, a bathochromic shift of the absorption maximum to 693 nm can be induced as compared to 683 nm for a chlorophyll monolayer without any additives. In the presence of ascorbate, chlorophyll species at 676, 712 and 750 nm (present in a pure chlorophyll monolayer) are decreased or diminished. Illumination causes no change in the position of these absorption maxima; however, there is an increase of the absorbance of the main red absorption band. In the presence of benzyl viologen there is a hypsochromic shift of the red absorption maximum to 679 nm. Chlorophyll species at 670, 694, 712 and 740 nm (present in pure chlorophyll monolayers) are decreased or diminished upon addition of benzyl viologen. Upon illumination, there is a decrease in absorbance at 686 nm. It appears that the redox reagents induce the formation of specific chlorophyll aggregates, in the interfacial system, which might be analogous to the various chlorophyll species observed in green plant photosynthesis. (+info)
Resolution of distinct selenium-containing formate dehydrogenases from Escherichia coli.
(19/29)
Formate dehydrogenase, a component activity of two alternative electron transport pathways in anaerobic Escherichia coli, has been resolved as two distinguishable enzymes. One, which was induced with nitrate reductase as a component of the formate-nitrate reductase pathway, utilized phenazine methosulfate (PMS) in preference to benzyl viologen (BV) as an artificial electron acceptor and appeared to be exclusively membrane-bound. A second formate dehydrogenase, which was induced as a component of the formate hydrogenlyase pathway, appeared to exist both as a membrane-bound form and as a cytoplasmic enzyme; the cytoplasmic activity was resolved completely from the PMS-linked activity on a sucrose gradient. When E. coli was grown in the presence of 75Se-selenite, a 110,000-dalton selenopeptide, previously shown to be a component of the PMS-linked enzyme, was induced and repressed with this activity. In contrast, an 80,000-dalton selenopeptide was induced and repressed with the BV-linked activity and exhibited a distribution similar to the BV-linked formate dehydrogenase in cell fractions and in sucrose gradients. The results indicate that the two formate dehydrogenases are distinguishable on the basis of their artificial electron acceptor specificity, their cellular localization, and the size of their respective selenoprotein components. (+info)
Localization of hydrogenase in Thiocapsa roseopersicina photosynthetic membrane.
(20/29)
The photosynthetic cell membrane is impermeable to the oxidized redox dyes Methyl Viologen and Benzyl Viologen, whereas the reduced forms easily penetrate into the cells. By exploiting this permeability difference, the orientation of the membrane-bound hydrogenase has been determined. (+info)
Purification and characterization of a benzylviologen-linked, tungsten-containing aldehyde oxidoreductase from Desulfovibrio gigas.
(21/29)
Desulfovibrio gigas NCIMB 9332 cells grown in ethanol-containing medium with 0.1 microM tungstate contained a benzylviologen-linked aldehyde oxidoreductase. The enzyme was purified to electrophoretic homogeneity and found to be a homodimer with a subunit M(r) of 62,000. It contained 0.68 +/- 0.08 W, 4.8 Fe, and 3.2 +/- 0.2 labile S per subunit. After acid iodine oxidation of the purified enzyme, a fluorescence spectrum typical for form A of molybdopterin was obtained. Acetaldehyde, propionaldehyde, and benzaldehyde were excellent substrates, with apparent Km values of 12.5, 10.8, and 20 microM, respectively. The natural electron acceptor is not yet known; benzylviologen was used as an artificial electron acceptor (apparent Km, 0.55 mM). The enzyme was activated by potassium ions and strongly inhibited by cyanide, arsenite, and iodoacetate. In the as-isolated enzyme, electron paramagnetic resonance studies readily detected W(V) as a complex signal with g values in the range of 1.84 to 1.97. The dithionite-reduced enzyme exhibited a broad signal at low temperature with g = 2.04 and 1.92; this is indicative of a [4Fe-4S]1+ cluster interacting with a second paramagnet, possibly the S = 1 system of W(IV). Until now W-containing aldehyde oxidoreductases had only been found in two Clostridium strains and two hyperthermophilic archaea. The D. gigas enzyme is the first example of such an enzyme in a gram-negative bacterium. (+info)
Is hydrogen peroxide involved in the benzyl viologen-mediated in-vivo inactivation of rat liver glutamine synthetase?
(22/29)
After benzyl viologen administration to rats, a decrease in the rat liver glutamine synthetase activity was observed. An increase in the rat liver catalase activity was found concomitantly. In combination with the catalase inhibitor aminotriazole, benzyl viologen again diminished, but markedly, the rat liver glutamine synthetase activity. Moreover, partially purified glutamine synthetase from rat liver underwent rapid inactivation upon aerobic incubation with NAD(P)H and benzyl viologen. This inactivation was prevented by catalase, which suggests that the NAD(P)H/BV2+/O2-dependent system has a role in H2O2 production. Our results suggest that H2O2 is involved in the benzyl viologen-mediated in-vivo inactivation of the rat liver glutamine synthetase. In contrast, benzyl viologen alone or in combination with aminotriazole produced a significant increase of brain glutamine synthetase. (+info)
Benzyl viologen inactivation of rat liver glutamine synthetase.
(23/29)
Partially purified glutamine synthetase from rat liver underwent rapid inactivation upon incubation with NADH and benzyl viologen, under aerobic conditions. This in vitro inactivation was prevented by catalase or chelating-agents, which suggests that hydrogen peroxide and metal ions are involved in the process. Similar inactivation was observed when the rat liver glutamine synthetase was preincubated, under anaerobic conditions, with NADH and benzyl viologen, and hydrogen peroxide was added to the reaction mixture. A radical scavenger, histidine, partially prevents the inactivation, while hydrogen peroxide shows a low inactivation capacity when incubated without NADH. Furthermore, the fact that the inactivation can also be catalyzed by a model consisting of ferrous ions and hydrogen peroxide leads to the conclusion that hydroxyl radicals, or something with similar reactivity, are most likely produced through a Fenton reaction. (+info)
Classification of fumarate reductases and succinate dehydrogenases based upon their contrasting behaviour in the reduced benzylviologen/fumarate assay.
(24/29)
Reduction of fumarate by soluble beef heart succinate dehydrogenase has been shown previously by voltammetry to become increasingly retarded as the potential is lowered below a threshold potential of -80 mV at pH 7.5. The behaviour resembles that of a tunnel diode, an electronic device exhibiting the property of negative resistance. The enzyme thus acts to oppose fumarate reduction under conditions of high thermodynamic driving force. We now provide independent evidence for this phenomenon from spectrophotometric kinetic assays. With reduced benzylviologen as electron donor, we have studied the reduction of fumarate catalysed by various enzymes classified either as succinate dehydrogenases or fumarate reductases. For succinate dehydrogenases, the rate increases as the concentration of reduced dye (driving force) decreases during the reaction. In contrast, authentic fumarate reductases of anaerobic cells (and 'succinate dehydrogenase' from Bacillus subtilis) neither exhibit the electrochemical effect nor deviate from simple kinetic behaviour in the cuvette assay. The 'tunnel-diode' effect may thus represent an evolutionary adaptation to aerobic metabolism. (+info)