Quinone-enhanced ascorbate reduction of nitric oxide: role of quinone redox potential

Tolerant mechanisms to O2 deficiency under submergence conditions in plants

Wetland plants can tolerate long-term strict hypoxia and anoxic conditions and the subsequent re-oxidative stress compared to terrestrial plants. During O2 deficiency, both wetland and terrestrial plants use NAD(P)+ and ATP that are produced during ethanol fermentation, sucrose degradation, and major amino acid metabolisms. The oxidation of NADH by non-phosphorylating pathways in the mitochondrial respiratory chain is common in both terrestrial and wetland plants. As the wetland plants enhance and combine these traits especially in their roots, they can survive under long-term hypoxic and anoxic stresses. Wetland plants show two contrasting strategies, low O2 escape and low O2 quiescence strategies (LOES and LOQS, respectively). Differences between two strategies are ascribed to the different signaling networks related to phytohormones. During O2 deficiency, LOES-type plants show several unique traits such as shoot elongation, aerenchyma formation and leaf acclimation, whereas the LOQS-type plants cease their growth and save carbohydrate reserves. Many wetland plants utilize NH4+ as the nitrogen (N) source without NH4+-dependent respiratory increase, leading to efficient respiratory O2 consumption in roots. In contrast, some wetland plants with high O2 supply system efficiently use NO3- from the soil where nitrification occurs. The differences in the N utilization strategies relate to the different systems of anaerobic ATP production, the NO2--driven ATP production and fermentation. The different N utilization strategies are functionally related to the hypoxia or anoxia tolerance in the wetland plants.

Nitrous Oxide Emissions from Paddies: Understanding the Role of Rice Plants

: Paddies are a potential source of anthropogenic nitrous oxide (N2O) emission. In paddies, both the soil and the rice plants emit N2O into the atmosphere. The rice plant in the paddy is considered to act as a channel between the soil and the atmosphere for N2O emission. However, recent studies suggest that plants can also produce N2O, while the mechanism of N2O formation in plants is unknown. Consequently, the rice plant is only regarded as a channel for N2O produced by soil microorganisms. The emission of N2O by aseptically grown plants and the distinct dual isotopocule fingerprint of plant-emitted N2O, as reported by various studies, support the production of N2O in plants. Herein, we propose a potential pathway of N2O formation in the rice plant. In rice plants, N2O might be formed in the mitochondria via the nitrate-nitrite-nitric oxide (NO3-NO2-NO) pathway when the cells experience hypoxic or anoxic stress. The pathway is catalyzed by various enzymes, which have been described. So, N2O emitted from paddies might have two origins, namely soil microorganisms and rice plants. So, regarding rice plants only as a medium to transport the microorganism-produced N2O might be misleading in understanding the role of rice plants in the paddy. As rice cultivation is a major agricultural activity worldwide, not understanding the pathway of N2O formation in rice plants would create more uncertainties in the N2O budget.

Potential Pathway of Nitrous Oxide Formation in Plants

Plants can produce and emit nitrous oxide (N₂O), a potent greenhouse gas, into the atmosphere, and several field-based studies have concluded that this gas is emitted at substantial amounts. However, the exact mechanisms of N₂O production in plant cells are unknown. Several studies have hypothesised that plants might act as a medium to transport N₂O produced by soil-inhabiting microorganisms. Contrarily, aseptically grown plants and axenic algal cells supplied with nitrate (NO₃) are reported to emit N₂O, indicating that it is produced inside plant cells by some unknown physiological phenomena. In this study, the possible sites, mechanisms, and enzymes involved in N₂O production in plant cells are discussed. Based on the experimental evidence from various studies, we determined that N₂O can be produced from nitric oxide (NO) in the mitochondria of plants. NO, a signaling molecule, is produced through oxidative and reductive pathways in eukaryotic cells. During hypoxia and anoxia, NO₃ in the cytosol is metabolised to produce nitrite (NO₂), which is reduced to form NO via the reductive pathway in the mitochondria. Under low oxygen condition, NO formed in the mitochondria is further reduced to N₂O by the reduced form of cytochrome c oxidase (CcO). This pathway is active only when cells experience hypoxia or anoxia, and it may be involved in N₂O formation in plants and soil-dwelling animals, as reported previously by several studies. NO can be toxic at a high concentration. Therefore, the reduction of NO to N₂O in the mitochondria might protect the integrity of the mitochondria, and thus, protect the cell from the toxicity of NO accumulation under hypoxia and anoxia. As NO₃ is a major source of nitrogen for plants and all plants may experience hypoxic and anoxic conditions owing to soil environmental factors, a significant global biogenic source of N₂O may be its formation in plants via the proposed pathway.

Metal-independent reduction of hydrogen peroxide by semiquinones

The quinones 1,4-naphthoquinone (NQ), tetramethyl-1,4-benzoquinone (DQ), 2-methyl-1,4-naphthoquinone (MNQ), 2,3-dimethoxy-5-methyl-1,4-benzoquinone (UBQ-0), 2,6-dimethylbenzoquinone (DMBQ), 2,6-dimethoxybenzoquinone (DMOBQ), and 9,10-phenanthraquinone (PHQ) enhance the rate of H₂O₂ reduction by ascorbate, under anaerobic conditions, as detected from the amount of methane produced after hydroxyl radical reaction with dimethyl sulfoxide. The amount of methane produced increases with an increase in the quinone one-electron reduction potential. The most active quinone in this series, PHQ, is only 14% less active than the classic Fenton reagent cation, Fe²⁺, at the same concentration. Since PHQ is a common toxin present in diesel combustion smoke, the possibility that PHQ-mediated catalysis of hydroxyl radical formation is similar to that of Fe²⁺ adds another important pathway to the modes in which PHQ can execute its toxicity. Because quinones are known to enhance the antitumor activity of ascorbate and because ascorbate enhances the formation of H₂O₂ in tissues, the quinone-mediated reduction of H₂O₂ should be relevant to this type of antitumor activity, especially under hypoxic conditions.

Factors affecting current production in microbial fuel cells using different industrial wastewaters

This study evaluated how different types of industrial wastewaters (bakery, brewery, paper and dairy) affect the performance of identical microbial fuel cells (MFCs); and the microbial composition and electrochemistry of MFC anodes. MFCs fed with paper wastewater produced the highest current density (125 ± 2 mA/m(2)) at least five times higher than dairy (25 ± 1 mA/m(2)), brewery and bakery wastewaters (10 ± 1 mA/m(2)). Such high current production was independent of substrate degradability. A comprehensive study was conducted to determine the factor driving current production when using the paper effluent. The microbial composition of anodic biofilms differed according to the type of wastewater used, and only MFC anodes fed with paper wastewater showed redox activity at -134 ± 5 mV vs NHE. Electrochemical analysis of this redox activity indicated that anodic bacteria produced a putative electron shuttling compound that increased the electron transfer rate through diffusion, and as a result the overall MFC performance.

Role of quinones in the ascorbate reduction rates of S-nitrosoglutathione

Quinones are one of the largest classes of antitumor agents approved for clinical use, and several antitumor quinones are in various stages of clinical and preclinical development. Many of these are metabolites of, or are, environmental toxins. Because of their chemical structure they are known to enhance electron transfer processes such as ascorbate oxidation and NO reduction. The paraquinones 2,6-dimethyl-1,4-benzoquinone (DMBQ), 1,4-benzoquinone, methyl-1,4-benzoquinone, 2,6-dimethoxy-1,4-benzoquinone, 2-hydroxymethyl-6-methoxy-1,4-benzoquinone, trimethyl-1,4-benzoquinone, tetramethyl-1,4-benzoquinone, and 2,3-dimethoxy-5-methyl-1,4-benzoquinone; the paranaphthoquinones 1,4-naphthoquinone, menadione, 1,4-naphthoquinone-2-sulfonate, 2-ethylsulfanyl-3-methyl-1,4-naphthoquinone and juglone; and phenanthraquinone (PHQ) all enhance the anaerobic rate of ascorbate reduction of GSNO to produce NO and GSH. Rates of this reaction were much larger for p-benzoquinones and PHQ than for p-naphthoquinone derivatives with similar one-electron redox potentials. The quinone DMBQ also enhances the rate of NO production from S-nitrosylated bovine serum albumin upon ascorbate reduction. Density functional theory calculations suggest that stronger interactions between p-benzo- or phenanthrasemiquinones and GSNO than between p-naphthosemiquinones and GSNO are the major causes of these differences. Thus, quinones, and especially p-quinones and PHQ, could act as enhancers of NO release from GSNO in biomedical systems in the presence of ascorbate. Because quinones are exogenous toxins that could enter the human body via a chemotherapeutic application or as an environmental contaminant, they could boost the release of NO from S-nitrosothiol storages in the body in the presence of ascorbate and thus enhance the responses elicited by a sudden increase in NO levels.

Performance of mesophilic anaerobic granules for removal of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) from aqueous solution

The performance of mesophilic anaerobic granules to degrade octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) was investigated under various conditions. The results of batch experiments showed that anaerobic granules were capable of removing HMX from aqueous solution with high efficiency. Both biotic and abiotic mechanisms contributed to the removal of HMX by anaerobic granules under mesophilic conditions. Adsorption appeared to play a significant role in the abiotic process. Furthermore, HMX could be biodegraded by anaerobic granules as the sole substrate. After 16 days of incubation, 99.04% and 96.42% of total HMX could be removed by 1g VSS/L acclimated and unacclimated granules, respectively. Vancomycin, an inhibitor of acetogenic bacteria, caused a significant inhibition of HMX biotransformation, while 2-bromoethanesulfonic acid, an inhibitor of methanogenic bacteria, only resulted in a slight decrease of metabolic activity. The presence of the glucose, as a suitable electron donor and carbon source, was found to enhance the degradation of HMX by anaerobic granules. Our study showed that sulfate had little adverse effects on biotransformation of HMX by anaerobic granules. However, nitrate had significant inhibitory effect on the extent of HMX removal especially in the initial period. This study offered good prospects of using high-rate anaerobic technology in the treatment of munition wastewater.

Anoxic nitric oxide cycling in plants: participating reactions and possible mechanisms

At sufficiently low oxygen concentrations, hemeproteins are deoxygenated and become capable of reducing nitrite to nitric oxide (NO), in a reversal of the reaction in which NO is converted to nitrate or nitrite by oxygenated hemeproteins. The maximum rates of NO production depend on the oxygen avidity. The hemeproteins with the highest avidity, such as hexacoordinate hemoglobins, retain oxygen even under anoxic conditions resulting in their being extremely effective NO scavengers but essentially incapable of producing NO. Deoxyhemeprotein-related NO production can be observed in mitochondria (at the levels of cytochrome c oxidase, cytochrome c, complex III and possibly other sites), in plasma membrane, cytosol, endoplasmic reticulum and peroxisomes. In mitochondria, the use of nitrite as an alternative electron acceptor can contribute to a limited rate of ATP synthesis. Non-heme metal-containing proteins such as nitrate reductase and xanthine oxidase can also be involved in NO production. This will result in a strong anoxic redox flux of nitrogen through the hemoglobin-NO cycle involving nitrate reductase, nitrite: NO reductase, and NO dioxygenase. In normoxic conditions, NO is produced in very low quantities, mainly for signaling purposes and this nitrogen cycling is inoperative.

Quinone-enhanced reduction of nitric oxide by xanthine/xanthine oxidase

The quinones 1,4-naphthoquinone, methyl-1,4-naphthoquinone, tetramethyl-1,4-benzoquinone, 2,3-dimethoxy-5-methyl-1,4-benzoquinone, 2,6-dimethylbenzoquinone, 2,6-dimethoxybenzoquinone, and 9,10-phenanthraquinone enhance the rate of nitric oxide reduction by xanthine/xanthine oxidase in nitrogen-saturated phosphate buffer (pH 7.4). Maximum initial rates of NO reduction (V(max)) and the amount of nitrous oxide produced after 5 min of reaction increase with quinone one- and two-electron redox potentials measured in acetonitrile. One of the most active quinones of those studied is 9,10-phenanthraquinone with a V(max) value 10 times larger than that corresponding to the absence of quinone, under the conditions of this work. Because NO production is enhanced under hypoxia and under certain pathological conditions, the observations obtained in this work are very relevant to such conditions.

Plant mitochondrial function during anaerobiosis

BACKGROUND

Under hypoxic conditions, plant mitochondria preserve the capacity to oxidize external NADH, NADPH and tricarboxylic acid cycle substrates. Nitrite serves as an alternative electron acceptor at the level of cytochrome oxidase, with possibly complex III and the alternative oxidase also being involved. Nitric oxide is a significant product of the reaction, which has a high affinity for cytochrome c oxidase, inhibiting it. The excess NO is scavenged by hypoxically induced class 1 haemoglobin in the reaction involving ascorbate.

SCOPE

By using nitrite, mitochondria retain a limited capacity for ATP synthesis. NADH, produced from glycolysis during anaerobiosis and oxidized in the mitochondrial electron transport chain, should shift the composition of metabolites formed during anaerobiosis with increased conversion of pyruvate to alanine and greater involvement of other transamination reactions, such as those involving gamma-aminobutyric acid formation.

CONCLUSIONS

Anaerobic mitochondrial metabolism may have a more significant role than previously thought in alleviating the effects of anoxia on plant cells. There is a need to re-examine mitochondrial carbon and nitrogen metabolism under anoxia to establish the extent of this involvement.

The pharmacology of nitroxyl (HNO) and its therapeutic potential: not just the Janus face of NO

Nitroxyl (HNO), the 1-electron reduced and protonated congener of nitric oxide (NO), has received recent attention as a potential pharmacological agent for the treatment of heart failure and as a preconditioning agent for the mitigation of ischemia-reperfusion injury. Interest in the pharmacology and biology of HNO has prompted examination, or in some instances reexamination, of many of its chemical properties. Such studies have provided insight into the chemical basis for the biological effects of HNO, although the biochemical mechanisms for many of these effects remain to be established. In this review, a brief description of the biologically relevant chemistry of HNO is given, followed by a more detailed discussion of the pharmacology and potential toxicology of HNO.