Artificial Electron Donors Research Articles

Metal-induced inhibition of photosynthetic electron transport chain of the cyanobacteriumNostoc muscorum

This study presents information on the mechanism of inhibition of the photosynthetic electron transport of Nostoc muscorum by chromium (Cr) and lead (Pb). Photosystem II (PS II) was found to be more sensitive both to low and high concentrations of test metals used. A considerable inhibition of photosystem I (PS I) was, however, observed at high concentrations only. Although Cr-induced inhibition of DCPIP photoreduction and lowering of chlorophyll a (Chl a) fluorescence intensity (F685) could not be reversed by artificial electron donors (diphenyl-carbazide (DPC), NH2OH, MnCl2 and benzidine) of PS II, these electron donors did substantially reverse the Pb-induced inhibition of DCPIP photoreduction as well as the lowering of Chl a fluorescence. Nevertheless, an increase in Chl a fluorescence at high concentrations of Pb suggested that this metal also arrests electron flow on the reducing side of the PS II reaction centre. Besides this, the suppression of fluorescence intensity of phycocyanin at low concentrations of both metals points to the involvement of phycobilisomes in the inhibition of PS II activity. The present study demonstrates that the modes of action of Cr and Pb on PS II are quite different.

Enzyme localization and orientation of the active site of dissimilatory nitrite reductase from Bacillus firmus

The location of the dissimilatory nitrite reductase and orientation of its reducing site of the Grampositive denitrifier, Bacillus firmus NIAS 237 were examined. Approximately 90% of the total dissimilatory nitrite reductase activity with ascorbate-reduced phenazine methosulfate (PMS) as the electron donor was on the protoplast membrane. Nitrite induced with intact Bacillus cells an alkalinization in the external medium, followed by acidification. The electron transfer inhibitor, 2-heptyl-4-hydroxyquinoline-N-oxide, which blocked nitrite reduction with endogenous substrates, inhibited the acidification, but not the alkalinization. Alkalinization was not affected with ascorbate-reduced PMS as the artificial electron donor. This indicated that the alkalinization is not associated with proton consumption outside the cytoplasmic membrane by the extracellular nitrite reduction. The dissimilatory nitrite reductase of B. firmus NIAS 237 was located on the cytoplasmic membrane, and its reducing site is suggested to be on the inner side of this membrane.

Secondary electron transfer reactions of the isolated Photosystem II reaction centre after reconstitution with plastoquinone-9 and diacylglycerolipids

A Photosystem II (PS II) reaction centre preparation isolated with Triton X-100 and stabilised by transfer to dodecylmaltoside has been used in a reconstitution procedure with diacyl lipids and the naturally occurring PS II quinone, plastoquinone-9 (PQ9). As shown previously, this isolated protein complex consists of the D1, D2, b -559 α and β polypeptides, plus the psb I gene product. Before reconstitution there was only about 0.004 mol PQ9 and 0.2 mol diacyl lipid per mole chlorophyll a associated with the isolated complex. After reconstitution and precipitation by centrifugation the complex was found to have an associated lipid matrix of 1.3 mol PQ9 and 3.7 mol diacyl lipid for each mole of chlorophyll a . After reconstitution, the presence of a functional quinone in the PS II reaction centre was indicated by a strong thermoluminescence signal with a peak emission at −60°C. This was recognised as the previously characterised Z v signal on the basis of the relationship between emission and excitation temperatures and a complete quenching by ethanol. This signal was essentially unaltered by addition of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and was the same after reconstitution with phosphatidylcholine or a thylakoid lipid extract. Calculation of the activation free energy for the charge separation reaction which gave rise to this signal suggested that this was 0.48 ± 0.01 eV and was the same as for a smaller Z v signal detected in the isolated complex before reconstitution with PQ9. When duroquinone was used this value was not significantly different, although a much smaller signal was observed. With decylplastoquinone the thermoluminescence peak was as large as with PQ9 but the difference in shape indicated that the activation free energy was significantly higher at 0.55 ± 0.02 eV. Photosynthetic electron transfer in the reconstituted complex was assayed by reduction of 2,6-dichlorophenolindophenol (DCPIP) with diphenylcarbazide (DPC) as an artificial electron donor. A low residual rate was strongly enhanced by reconstitution with PQ9. This activity was greater when reconstitution was with a thylakoid lipid extract than with phosphatidylcholine and was inhibited by addition of DCMU ( I 50 = 0.2 mM).

Purification and characterization of a catalase-peroxidase and a typical catalase from the bacterium Klebsiella pneumoniae

The bacterium Klebsiella pneumoniae synthesizes three different types of catalase: a catalase-peroxidase, a typical catalase and an atypical catalase, designated KpCP, KpT and KpA, respectively (Goldberg, I. and Hochman, A. (1989) Arch. Biochem. Biophys. 268, 124–128). KpCP, but not the other two enzymes, in addition to the catalatic activity, catalyzes peroxidatic activities with artificial electron donors, as well as with NADH and NADPH. Both KpCP and KpT are tetramers, with heme IX as a prosthetic group, and they show a typical high-spin absorption spectrum which is converted to low-spin when a cyanide complex is formed. The addition of dithionite to KpCP causes a shift in the absorption maxima typical of ferrous heme IX. KpCP has a pH optimum of 6.3 for the catalatic activity and 5.2–5.7 for the peroxidatic activity, and relatively low ‘ K m’ values: 6.5 mM and 0.65 H 2O 2 for the catalatic and peroxidatic activities, respectively. The activity of the catalase-peroxidase is inhibited by azide and cyanide, but not by 3-amino-1,2,4-triazole. KpT has wide pH optimum: 5–10.5 and a ‘ K m’ of 50 mM H 2O 2, it is inhibited by incubation with 3-amino-1,2,4-triazole and by the acidic forms of cyanide and azide. A significant distinction between the typical catalase and the catalase-peroxidase is the stability of their proteins: KpT is more stable than KpCP to H 2O 2, temperature, pH and urea.

Evidence for the ultraviolet‐B (280–320 nm) radiation induced structural reorganization and damage of photosystem II polypeptides in isolated chloroplasts

Direct evidence for the possible loss of photosystem II (PS II) activity in chloroplasts of Vigna sinensis L. cv. Walp after ultraviolet‐B (UV‐B, 280–320 nm) radiation treatment was provided by polyacrylamide gel electrophoretic analysis of PS II polypeptides. A 30 min UV‐B treatment of chloroplasts caused a 50% loss of PS II activity. The artificial electron donor. Mn2+ failed to restore UV‐B radiation induced loss of PS II activity, while diphenyl carbazide (DPC) and NH2OH only partially restored activity. Such a loss in PS II activity was found to be primarily due to a loss of 23 and 33 kDa extrinsic polypeptides. UV‐B treatment induced the synthesis of a few polypeptides and a 29 kDa light‐harvesting chlorophyll protein.

Inhibition of photosynthetic electron transport in Nostoc muscorum by Ni2+ and Ag+.

This paper presents the inhibition of photosynthetic electron transport chain of Nostoc muscorum by divalent Ni2+ and monovalent Ag+. PS I (DCPIP/ASC→MV) and PS II (H2O→PBQ) activities were markedly inhibited by Ni2+ and Ag+ in a concentration-dependent fashion but PS II was more susceptible than PS I to Ni2+. Ag+ was more toxic to PS I than PS II. The greater sensitivity of PS II to Ni2+ was further confirmed by the inhibition of DCPIP photoreduction and Chl a fluorescence. Restoration of Ag+ -induced inhibition of DCPIP photoreduction and Chl a fluorescence by artificial electron donors (DPC, NH2OH, MnCl2) and their failure to restore Ni2+-induced inhibition suggests that Ni2+ inactivates PS II by causing alteration and destruction of photosynthetic membranes, but Ag+ inhibits the electron flow at the oxidizing side of PS II. Nevertheless, the suppression of the fluorescence intensity at low concentrations of both metal cations points to the involvement of phycobilisomes in the inhibition of PS II activity.

Effects of selective destruction of iron-sulfur center B on electron transfer and charge recombination in Photosystem I

Incubation of spinach thylakoids with HgCl2 selectively destroys Fe-S center B (FB). The function of electron acceptors in FB-less PS I particles was studied by following the decay kinetics of P700(+) at room temperature after multiple flash excitation in the absence of a terminal electron acceptor. In untreated particles, the decay kinetics of the signal after the first and the second flashes were very similar (t 1/2∼2.5 ms), and were principally determined by the concentration of the artificial electron donor added. The decay after the third flash was fast (t 1/2∼0.25 ms). In FB-less particles, although the decay after the first flash was slow, fast decay was observed already after the second flash. We conclude that in FB-less particles, electron transfer can proceed normally at room temperature from FX to FA and that the charge recombination between P700(+) and FX (-)/A1 (-) predominated after the second excitation. The rate of this recombination process is not significantly affected by the destruction of FB. Even in the presence of 60% glycerol, FB-less particles can transfer electrons to FA at room temperature as efficiently as untreated particles.

Anaerobic metabolism of resorcyclic acids (m-dihydroxybenzoic acids) and resorcinol (1,3-benzenediol) in a fermenting and in a denitrifying bacterium

The anaerobic metabolism of 2,4- and 2,6-dihydroxybenzoic acid (beta- and gamma-resorcyclic acid) and 1,3-benzenediol (resorcinol) was investigated in a fermenting coculture of a Clostridium sp. with a Campylobacter sp. (Tschech A and Schink B (1985) Arch Microbiol 143: 52–59) and in a newly isolated denitrifying gram-negative bacterium. The enzymes of this pathway were searched for and partly characterized in vitro. It is shown that resorcyclic acids are decarboxylated in both organisms by specific enzymes, 2,4- or 2,6-dihydroxybenzoic acid decarboxylase. In the fermenting bacterium, the aromatic product, 1,3-benzenediol, is reduced by 1,3-benzenediol (resorcinol) reductase to the non-aromatic 1,3-cyclohexanedione; the novel enzyme which catalyzes the two-electron-reduction of the aromatic nucleus is oxygen-sensitive and uses reduced methyl viologen as artificial electron donor. The cyclic dione is then hydrolytically cleaved to 5-oxocaproic acid by 1,3-cyclohexanedione hydrolase. The denitrifying bacterium did not metabolize 1,3-cyclohexanedione, and the enzymes metabolizing 1,3-benzenediol or 1,3-cyclohexanedione were not detected. It is concluded that two different pathways of anaerobic 1,3-benzenediol metabolism exist.

Electron Transport through photosystem I Stimulates Light Activation of Ribulose Bisphosphate Carboxylase/Oxygenase (Rubisco) by Rubisco Activase

The activation state of ribulose bisphosphate carboxylase/oxygenase (rubisco) in a lysed chloroplast system is increased by light in the presence of a saturating concentration of ATP and a physiological concentration of CO(2) (10 micromolar). Electron transport inhibitors and artificial electron donors and acceptors were used to determine in which region of the photosynthetic electron transport chain this light-dependent reaction occurred. In the presence of DCMU and methyl viologen, the artificial donors durohydroquinone and 2,6-dichlorophenolindophenol (DCPIP) plus ascorbate both supported light activation of rubisco at saturating ATP concentrations. No light activation occurred when DCPIP was used as an acceptor with water as electron donor in the presence of ATP and dibromothymoquinone, even though photosynthetic electron transport was observed. Nigericin completely inhibited the light-dependent activation of rubisco. Based on these results, we conclude that stimulation of light activation of rubisco by rubisco activase requires electron transport through PSI but not PSII, and that this light requirement is not to supply the ATP needed by the rubisco activase reaction. Furthermore, a pH gradient across the thylakoid membrane appears necessary for maximum light activation of rubisco even when ATP is provided exogenously.

Purification and properties of a NADH‐dependent 5,10‐methylenetetrahydrofolate reductase from Peptostreptococcus productus

The methylenetetrahydrofolate reductase from the carbon-monoxide-utilizing homoacetogen Peptostreptococcus productus (strain Marburg) has been purified to apparent homogeneity. The purified enzyme catalyzed the oxidation of NADH with methylenetetrahydrofolate as the electron acceptor at a specific activity of 380 mumols.min-1 mg protein-1 (37 degrees C; pH 5.5). The apparent Km for NADH was near 10 microM. The apparent molecular mass of the enzyme was determined by gel filtration to be approximately 250.0 kDa. The enzyme consists of eight identical subunits with a molecular mass of 32 kDa. It contains 4 FAD/mol octamer which were reduced by the enzyme with NADH as the electron donor; iron could not be detected. Oxygen had no effect on the enzyme. Ultracentrifugation of cell extracts revealed that about 40% of the enzyme activity was recovered in the particulate fraction, suggesting that the enzyme is associated with the membrane. The enzyme also catalyzed the methylenetetrahydrofolate reduction with methylene blue as an artificial electron donor. The oxidation of methyltetrahydrofolate was mediated with methylene blue as the electron acceptor; neither NAD+ nor viologen dyes could replace methylene blue in this reaction. NADP(H) or FAD(H2) were not used to substrates for the reaction in either direction. The activity of the purified enzyme, which was proposed to be involved in sodium translocation across the cytoplasmic membrane, was not affected by the absence or presence of added sodium. The properties of the enzyme differ from those of the ferredoxin-dependent methylenetetrahydrofolate reductase of the homoacetogen Clostridium formicoaceticum and of the NADP(+)-dependent reductase of eucaryotes investigated so far.

Bicarbonate effects in leaf discs from spinach

In this paper, we show the unique role of bicarbonate ion in stimulating the electron transfer of photosystem II (PS II) in formate-treated leaf discs from spinach. This is referred to as the "bicarbonate effect" and is independent of the role of CO2 in CO2 fixation. It is shown to have two sites of action: (1) the first, described here for the first time, stimulates the electron flow between the hydroxylamine donation site ("Z" or "D") and QA, the first plastoquinone electron acceptor and (2) the other accelerates the electron flow beyond QA, perhaps at the QA QB complex, where QB is the second plastoquinone electron acceptor. The first site of inhibition by formate-treatment is detected by the decrease of the rate of oxygen evolution and the simultaneous quenching of the variable chlorophyll a (Chl a) fluorescence of leaf discs infiltrated with 100 mM formate for about 10 s followed by storage for 10 min in dark. This is referred to as short-term formate treatment. Addition of bicarbonate reverses this short-term formate effect and restores fully both Chl a fluorescence and oxygen evolution rate. Reversible quenching of variable Chl a fluorescence of heated and short-term formate treated leaf discs, in the presence of hydroxylamine as an artificial electron donor to PS II, is also observed. This suggests that the first site of action of the anion effect is indeed between the site of donation of hydroxylamine to PS II (i.e. "Z" or "D") and QA. The second site of the effect, where bicarbonate depletion has its most dramatic effect, as well known in thylakoids, is shown by an increase of Chl a fluorescence of leaf discs infiltrated with 100 mM formate for about 10 min followed by storage for 10 min in dark. This is referred to as the long-term formate treatment. Addition of bicarbonate fully restores the variable Chl a fluorescence of these leaf discs. Chl a fluorescence transient of DCMU-infiltrated (10 min) leaf discs is similar to that of long-term formate-treated one suggesting that the absence of bicarbonate, like the presence of DCMU, inhibits the electron flow beyond QA.

Biochemical and Immunological Characterization of Nitrate Reductase Deficient nia Mutants of Nicotiana plumbaginifolia

Sixty-five Nicotiana plumbaginifolia mutants affected in the nitrate reductase structural gene (nia mutants) have been analyzed and classified. The properties evaluated were: (a) enzyme-linked immunosorbent assay (two-site ELISA) using a monoclonal antibody as coating reagent and (b) presence of partial catalytic activities, namely nitrate reduction with artificial electron donors (reduced methyl viologen, reduced flavin mononucleotide, or reduced bromphenol blue), and cytochrome c (Cyt c) reduction with NADH. Four classes have been defined: 40 mutants fall within class 1 which includes all mutants that have no protein detectable in ELISA and no partial activities; mutants of classes 2 and 3 exhibit an ELISA-detectable nitrate reductase protein and lack either Cyt c reductase activity (class 2: fourteen mutants) or the terminal nitrate reductase activities (class 3: eight mutants) of the enzyme. Three mutants (class 4) are negative in the ELISA test, lack Cyt c reductase activity, and lack or have a very low level of reduced methyl viologen or reduced flavin mononucleotide-nitrate reductase activities; however, they retain the reduced bromphenol blue nitrate reductase activity. Variations in the degrees of terminal nitrate reductase activities among the mutants indicated that the flavin mononucleotide and methyl viologen-dependent activities were linked while the bromphenol blue-dependent activity was independent of the other two. The putative positions of the lesions in the mutant proteins and the nature of structural domains of nitrate reductase involved in each partial activity are discussed.

Functional Structure of the Oxygen-Evolving Unit of Photosystem II as Determined by Radiation Inactivation

By the analysis of the sizes of complexes required for evolution of oxygen in oxygen evolving PS2 core complexes, for DPC-supported PS2 activity, for the induction of Chl fluorescence in thylakoids and for photochemical reaction-center activity we found that the function of LHC, CP47 and CP43 are not necessary in the oxygen evolving 125kDa PS2 unit and that one electron oxidation of an artificial electron donor is catalyzed by a unit composed of a single subunit of the D 1 -D 2 heterodimer

Isolation and sequence of ctaA, a gene required for cytochrome aa3 biosynthesis and sporulation in Bacillus subtilis

Cytochrome aa3 is one of two terminal oxidase complexes in the Bacillus subtilis electron transport chain. A novel genetic strategy was devised which permitted the isolation of B. subtilis mutants lacking cytochrome aa3 by selection for streptomycin-resistant clones which failed to oxidize the artificial electron donor N,N,N',N'-tetramethyl-p-phenylenediamine. Two mutations were studied intensively. Spectroscopic examination showed that each mutant lacked cytochrome aa3; they were also asporogenous and unable to grow on lactate as the sole carbon and energy source. These mutations were mapped to a locus designated ctaA, located at 127 degrees between pyrD and metC on the B. subtilis chromosome. Both ctaA mutations were closely linked by transformation to the pycA locus. The ctaA locus and a portion of the pycA locus were cloned from a B. subtilis integration library constructed in Escherichia coli. A recombinant plasmid containing a 4.0-kilobase insert of B. subtilis DNA could transform both ctaA mutants to CtaA+. Gene disruption and complementation experiments with subcloned fragments revealed that the ctaA locus consisted of a single transcriptional unit about 1.35 kilobase pairs in size. The nucleotide sequence of the ctaA transcriptional unit contains a single open reading frame capable of coding for a protein with a predicted molecular weight of 34,065. The predicted protein is extremely hydrophobic, with several probable membrane-spanning domains. No sequence similiarity was found between ctaA and the highly conserved procaryotic and mitochondrial oxidase polypeptides. Cloning and sequence analysis of two ctaA mutations revealed that one allele is a nonsense mutation in the carboxy terminus and the other is a missense mutation in the amino terminus; this indicates that the pleiotropic phenotype conferred by each mutation was caused by loss of CtaA or of its activity. Genetic evidence suggests that the ctaA gene product is required as an accessory protein in the genetic expression, posttranslational biogenesis, or both, of the cytochrome aa3 complex and during an early stage of sporogenesis.

Fumarate Reductase Mutants of Escherichia coli That Lack Covalently Bound Flavin

Menaquinol-fumarate oxidoreductase of Escherichia coli is a four-subunit membrane-bound complex that catalyzes the final step in anaerobic respiration when fumarate is the terminal electron acceptor. The catalytic domain of fumarate reductase consists of the FrdA subunit, which contains the active site, and a FAD prosthetic group covalently attached to His44, plus the FrdB subunit which contains at least two of the three nonidentical iron-sulfur clusters of the enzyme. To examine the role of covalently bound FAD in enzyme activity and electron transfer during anaerobic cell growth, site-directed mutagenesis was used to alter His44 of the FrdA subunit to a Ser, Cys, or Tyr residue. The resulting mutant enzyme complexes that were synthesized associated normally with the cytoplasmic membrane, but had decreased ability (greater than 70%) to reduce fumarate with reduced benzyl viologen, an artificial electron donor of low redox potential (Em = -359 mV; Clark, W. M. (1972) Oxidation-Reduction Potentials of Organic Systems, Robert E. Kreiger Publishing Co., Melbourne, FL). Even lower activities were measured when the higher potential, natural electron donor menaquinol was used, which, however, correlated with the slower growth rates of the different mutant complexes. In contrast to the normal enzyme, the mutant enzyme complexes were unable to oxidize succinate. Substitution of Arg for His44 produced a totally inactive enzyme complex that permitted no cell growth on nonfermentable substrates with fumarate as electron acceptor. All four mutant complexes contained noncovalently bound FAD in stoichiometric amounts. These data indicate a unique role of the 8 alpha-[N(3)-histidyl] FAD linkage in enzyme activity, by raising the redox potential of free FAD to permit reduction by both menaquinol and succinate.

Effect of localized and systemic tobacco mosaic virus infection on some photochemical and enzymatic activities of isolated tobacco chloroplasts

Certain photochemical and enzymic activities of isolated chloroplast preparations were evaluated in tobacco plants susceptible ( Nicotiana tabacum cv. Bright BC 60) and hypersensitive ( N. tabacum cv. Havana 425) to tobacco mosaic virus (TMV) infection. The main results were as follows: 1. (i) Chloroplasts isolated from inoculated (asymptomatic) and systemically infected (with mosaic symptoms) leaves of the susceptible plants did not exhibit any change in the rates of nicotinamide adenine dinucleotide phosphate (NADP) and ferricyanide photoreduction expressed as μmol mg −1 chlorophyll. In contrast, chloroplasts isolated from inoculated leaves of the hypersensitive plants, starting at the beginning of local lesion expression (collapsed areas, 2 days after inoculation; necrotic areas, 3 to 4 days after inoculation) exhibited a strong inhibition of electron transport reaching values about 60–70% of the control. This effect was observed when water was the electron donor, irrespective of whether NADP or ferricyanide was the electron acceptor; no differences were observed when NADP reduction occurred via Photosystem I after 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) inhibition, using ascorbate and 2,6-dichlorophenol indophenol (DCPIP) as an artificial electron donor. 2. (ii) The rate of oxygen photoreduction, expressed as μmol mg −1 chlorophyll, was slightly-increased in the chloroplasts from hypersensitive leaves 1 day after inoculation and then reduced to values about 40% of the control 2–4 days after inoculation. Less oxygen photoreduction was also observed in chloroplasts from systemically infected leaves, while no variation was observed in the chloroplasts from inoculated leaves of the susceptible plants. 3. (iii) Ribulose-1,5-bisphosphate carboxylase activity (measured in chloroplast extracts) was decreased in inoculated hypersensitive leaves during differentiation of local lesions (2 to 4 days after inoculation), reaching values of 30–40% of the control. A less intense decrease of this activity was observed in uninoculated leaves of the susceptible plants displaying mosaic symptoms which did not exhibit any significant variation in photochemical activity (NADP and ferricyanide photoreduction). 4. (iv) Other enzymatic activities measured in chloroplast extracts and/or total-leaf extracts, including malate dehydrogenase nicotinamide adenine dinucleotide-dependent (NADH-dependent) glutamate oxalacetate transaminase, superoxide dismutase, and phosphoenolpyruvate carboxylase, were either unchanged or little affected by TMV infection both in susceptible and hypersensitive combinations. These results are discussed with respect to the hypothesis that the hypofunctionality of the photosynthetic apparatus, characteristic of chloroplasts from hypersensitive leaves, may contribute, in part, to the localization of infection and thence to host resistance.

Photosynthetic Responses of Different Varieties of Wheat to High Temperature

The effect of heat stress on photosynthetic electron transport was investigated in thylakoids isolated from the wheat (Triticum aestivum L.) varieties APU (Finland) and K65 (India) grown under both cool (13 °C day, 10 °C night) and warm (30 °C day, 25 °C night) regimes which gave rise to varietal differences in photosynthetic temperature acclimation. The responses of the uncoupled activities of both whole-chain electron transport and photosystem II to heat stress were similar. Both activities exhibited higher rates in thylakoids isolated from warm-grown plants and were more resistant to high temperature pretreatment than in those isolated from cool-grown plants, but varietal differences were not observed. Uncoupled photosystem I activity driven by either reduced 2,6-dichlorophenol indophenol (DCPIPH2) or N,N,N',N' tetramethyl-p-phenylene diamine (TMPDH2) showed a stimulation following high temperature pretreatment which was more marked in thylakoids isolated from warm-grown plants, followed by inhibition at extreme high temperatures. This stimulation was due largely to an increase in Kmas but did not occur when reduced diaminodurene, which is highly lipophilic, was used as the electron donor. It appears that stimulation of PS I activity may involve increased accessibility of some artificial electron donors to the native acceptor sites within the thylakoid membrane in a process which is influenced by growth temperature.

Farnesylacetone, a sesquiterpenic hormone of crustacea, inhibits electron transport in isolated rat liver mitochondria

Farnesylacetone (C18 H30 0) is a male hormone extracted from the androgenic gland of crab, Carcinus maenas. Appropriate enzymatic assays, as well as spectrophotometric studies, indicate that micromolar concentrations of farnesylacetone interact with the electron transport pathway of rat liver mitochondria. By the use of artificial electron donors and electron acceptors, it is shown that farnesylacetone immediately inhibits the electron transfer within complex I (NADH ubiquinone reductase activity) and complex II (succinate ubiquinone reductase activity). It is proposed that farneylacetone could interact with these two complexes of the respiratory chain at the level of the iron-sulfur centers implicated in the dehydrogenase activities. These observations are compared with the results obtained with terpenic molecules which interact with mitochondrial respiration.

Farnesylacetone, a sesquiterpenic hormone of crustacea, inhibits electron transport in isolated rat liver mitochondria

Farnesylacetone (C18 H30 0) is a male hormone extracted from the androgenic gland of crab, Carcinus maenas. Appropriate enzymatic assays, as well as spectrophotometric studies, indicate that micromolar concentrations of farnesylacetone interact with the electron transport pathway of rat liver mitochondria. By the use of artificial electron donors and electron acceptors, it is shown that farnesylacetone immediately inhibits the electron transfer within complex I (NADH ubiquinone reductase activity) and complex II (succinate ubiquinone reductase activity). It is proposed that farnesylacetone could interact with these two complexes of the respiratory chain at the level of the iron-sulfur centers implicated in the dehydrogenase activities. These observations are compared with the results obtained with terpenic molecules which interact with mitochondrial respiration.

A simplified methylcoenzyme M methylreductase assay with artificial electron donors and different preparations of component C from Methanobacterium thermoautotrophicum delta H.

Different preparations of the methylreductase were tested in a simplified methylcoenzyme M methylreductase assay with artificial electron donors under a nitrogen atmosphere. ATP and Mg2+ stimulated the reaction. Tris(2,2'-bipyridine)ruthenium (II), chromous chloride, chromous acetate, titanium III citrate, 2,8-diaminoacridine, formamidinesulfinic acid, cob(I)alamin (B12s), and dithiothreitol were tested as electron donors; the most effective donor was titanium III citrate. Methylreductase (component C) was prepared by 80% ammonium sulfate precipitation, 70% ammonium sulfate precipitation, phenyl-Sepharose chromatography, Mono Q column chromatography, DEAE-cellulose column chromatography, or tetrahydromethanopterin affinity column chromatography. Methylreductase preparations which were able to catalyze methanogenesis in the simplified reaction mixture contained contaminating proteins. Homogeneous component C obtained from a tetrahydromethanopterin affinity column was not active in the simplified assay but was active in a methylreductase assay that contained additional protein components.