Redox reactions parasitic

In normal battery operation several electrochemical reactions occur on the nickel hydroxide electrode. These are the redox reactions of the active material, oxygen evolution, and in the case of nickel-hydrogen and nickel-metal hydride batteries, hydrogen oxidation. In addition there are parasitic reactions such as the corrosion of nickel current collector materials and the oxidation of organic materials from separators. The initial reaction in the corrosion process is the conversion of Ni to Ni(OH)2. 

The mechanism of action for such peroxidic compounds involves a reductive activation by iron in haem, released as a result of hemoglobin digestion by Plasmodium. This irreversible redox reaction affords carbon-centered free radicals causing the alkylation of haem and of proteins. One such protein (the sarcoplasmic-endoplasmic reticulum ATPase PfATP6) appears to be critical for parasite survival, and there is no indication for resistance by the parasite. However, treatment is expensive and recrudescence of malaria occurs often. Moreover, it was found that at high doses such compounds are neurotoxic. 

Under the specific conditions (acidic and oxidizing) mimicking the parasite DV, FQ shows a reversible one-electron redox reaction [161]. This leads to the formation of ferriquinium and generation of hydroxyl radicals (Fig. 30), with kinetics which are relevant for an antimalarial activity on P. falciparum. 

Related to the corrosion problems was a recent SECM study, which demonstrated the possibility of eliminating typical experimental problems encountered in the measurements of heterogeneous electron transfer at semiconductor electrodes (27). In this experiment, the redox reaction of interest (e.g., reduction of Ru(NH3)s+) is driven at a diffusion-controlled rate at the tip. The rate of reaction at the semiconductor substrate is probed by measuring the feedback current as a function of substrate potential. By holding the substrate at a potential where no other species than the tip-generated one would react at the substrate, most irreversible parasitic processes, such as corrosion, did not contribute to the tip current. Thus, separation of the redox reaction of interest from parallel processes at the semiconductor electrode was achieved. 

The photovoltage generated in a depletion layer of a semiconductor electrode can be used for direct generation of electric power. The principle was already shown in Fig. 1.14. The redox reaction at the counter electrode reverses the chemical change at the semiconductor electrode. Since such cells should operate for a very long period of time, even a very small side reaction can ruin the performance after some while. If the parasitic reaction occurs only within the redox electrolyte, the failure can easily be repaired by an exchange of the electrolyte. If however it is a corrosion of the semiconductor, the lifetime of the whole cell is limited. Therefore the redox reaction at the semiconductor should proceed very fast in order to prevent corrosion. 

Next to fumarate reduction, some organisms use specific reactions in lipid biosynthesis as an electron sink to maintain redox balance in anaerobically functioning mitochondria. In anaerobic mitochondria two variants are known the production of branched-chain fatty acids and the production of wax esters. The parasitic nematode Ascaris suum reduces fumarate in its anaerobic mitochondria, but instead of only producing acetate and succinate or propionate, like most other parasitic helminths, this organism also use the intermediates acetyl-CoA and propionyl-CoA to form branched-chain fatty acids (Komuniecki et al. 1989). This pathway is similar to reversal of P-oxidation and a complex mixture of the end products acetate, propionate, succinate and branched-chain fatty acids is excreted. In this pathway, the... 

Mode of action Metronidazole is selectively toxic not only for amebae but also for anaerobic organisms (including bacteria), and for anoxic or hypoxic cells. Some anaerobic protozoan parasites (including amebae) possess ferrodoxin-like, low-redox potential, electron transport proteins that participate in metabolic electron removal reactions. The nitro group of metronidazole is able to serve as an electron acceptor, forming reduced cytotoxic compounds that bind to proteins and DIMA to result in cell death. 

Parallel with normal (enzymatic) four electron reduction of O2 to H2O by cytochrome oxidase, non-enzymatic one electron reduction of O2 to superoxide (O2 ) takes place in mitochondria. This parasitic chemical reaction appears to be inevitable since the initial and middle steps of the respiratory chain contain very reactive electron carriers of negative redox potential (e.g., chemically component in the one electron reduction of oxygen). 

Lacking GSH-dependent peroxidases, Plasmodium spp. rely on a Prx-linked detoxification for hydroperoxides and reduced GSH acts primarily as the principal redox buffer. It is also important in detoxification reactions as a co-factor for GST and glyoxalase and has been shown to be involved in the breakdown of free ferriprotoporphyrin IX. The lack of two major antioxidants present in other cells (catalase and GSH peroxidase) suggests that malaria parasites would be vulnerable to disturbances in their anti-oxidant systems. As a consequence, pro-oxidant drugs such as the artemisinins, which increase the oxidative stress, are efficient antimalarials. It has been proposed that a novel approach to malaria chemotherapy would be to develop drugs that disrupt the anti-oxidant and redox system of Plasmodium (Muller, 2004 Nickel et al., 2006 Rahlfs and Becker, 2005). 

Another disturbing factor in the course of copper determination in plating baths is the presence of chloride ions in high concentrations. In its presence complicated redox and complexation reactions influence the electrode response and result in irreproducible and unstable potentials of the indicator electrode. The more comprehensive study of the electrode behaviour in the presence of thiosulphate has indicated that the parasitic effect of chloride can be completely eliminated. Therefore such conditions were selected for determination of copper in plating baths. Electrode potential in thiosulphate solution is given by the equation ... 

A Haber-Weiss reaction can take place between hydrogen peroxide and the superoxide radical and allows the formation of hydroxyl radicals. In red blood cells, these processes are balanced by antioxidant systems involved [48]. In infected red blood cells, however, a surplus of radicals is produced causing an imbalance between pro-and antioxidant species [46]. Detoxification systems present in the parasite and also in the host cells are accelerated to counteract these radical species and create a new redox balance. Many studies [49] focused on the understanding of the antioxidant mechanisms, allowing the identification... 

The discussed observations led to study the redox properties of the FQ. Indeed, the presence of H2O2 in the digestive vacuole of the parasite, estimated to be 15 mM [47,54] locally (this value is subject of controversy), is particularly interesting because of the presence of Fe(II) cation in the structure of FQ. Indeed, as for the Fe(II)/Fe(III) couple, it has been shown that under oxidative conditions, close to those of the digestive vacuole, FQ is capable of generating Fenton-like reactions leading to the production of hydroxyl radicals [55] ... 

---