Reductive Mobilization Of Iron Research Articles

Reductive Mobilization of Iron from Intact Ferritin: Mechanisms and Physiological Implication.

Ferritins are highly conserved supramolecular protein nanostructures composed of two different subunit types, H (heavy) and L (light). The two subunits co-assemble into a 24-subunit heteropolymer, with tissue specific distributions, to form shell-like protein structures within which thousands of iron atoms are stored as a soluble inorganic ferric iron core. In-vitro (or in cell free systems), the mechanisms of iron(II) oxidation and formation of the mineral core have been extensively investigated, although it is still unclear how iron is loaded into the protein in-vivo. In contrast, there is a wide spread belief that the major pathway of iron mobilization from ferritin involves a lysosomal proteolytic degradation of ferritin, and the dissolution of the iron mineral core. However, it is still unclear whether other auxiliary iron mobilization mechanisms, involving physiological reducing agents and/or cellular reductases, contribute to the release of iron from ferritin. In vitro iron mobilization from ferritin can be achieved using different reducing agents, capable of easily reducing the ferritin iron core, to produce soluble ferrous ions that are subsequently chelated by strong iron(II)-chelating agents. Here, we review our current understanding of iron mobilization from ferritin by various reducing agents, and report on recent results from our laboratory, in support of a mechanism that involves a one-electron transfer through the protein shell to the iron mineral core. The physiological significance of the iron reductive mobilization from ferritin by the non-enzymatic FMN/NAD(P)H system is also discussed.

Model Study of the Effect of Gleyzation on Radionuclide Migration in Waterlogged Soils

A model study is made of joint leaching of iron and other macro- and microelements, among them radionuclides, from radioactively contaminated floodplain (aquic moisture regime). In developing anaerobic conditions, the redox potential Eh decreased to +140 mV, causing partial reduction of iron and solubilization of iron hydrous oxide and iron-containing organomineral films of the soil. Leaching of stable elements and radionuclides with water proceeds parallel to that of iron. To accelerate reductive mobilization of iron, 0.4% aqueous ascorbic acid was used instead of water as the liquid phase in the model system. In this case, Eh approached -60 mV, and the leaching of stable and radioactive elements considerably increased, the effect being most pronounced for plutonium.

Effects of citrinin on iron-redox cycle.

The ability of the mycotoxin citrinin to act as an inhibitor of iron-induced lipoperoxidation of biological membranes prompted us to determine whether it could act as an iron chelating agent, interfering with iron redox reactions or acting as a free radical scavenger. The addition of Fe3+ to citrinin rapidly produced a chromogen, indicating the formation of citrinin-Fe3+ complexes. An EPR study confirms that citrinin acts as a ligand of Fe3+, the complexation depending on the [Fe3+]:[citrinin] ratios. Effects of citrinin on the iron redox cycle were evaluated by oxygen consumption or the o-phenanthroline test. No effect on EDTA-Fe2+-->EDTA-Fe3+ oxidation was observed in the presence of citrinin, but the mycotoxin inhibited, in a dose-dependent manner, the oxidation of Fe2+ to Fe3+ by hydrogen peroxide. Reducing agents such as ascorbic acid and DTT reduced the Fe3+-citrinin complex, but DTT did not cause reduction of Fe3+-EDTA, indicating that the redox potentials of Fe3+-citrinin and Fe3+-EDTA are not the same. The Fe2+ formed from the reduction of Fe3+-citrinin by reducing agents was not rapidly reoxidized to Fe3+ by atmospheric oxygen. Citrinin has no radical scavenger ability as demonstrated by the absence of DPPH reduction. However, a reaction between citrinin and hydrogen peroxide was observed by UV spectrum changes of citrinin after incubation with hydrogen peroxide. It was also observed that citrinin did not induce direct or reductive mobilization of iron from ferritin. These results indicate that the protective effect on iron-induced lipid peroxidation by citrinin occurs due to the formation of a redox inactive Fe3+-citrinin complex, as well as from the reaction of citrinin and hydrogen peroxide.

Dihydrolipoamide-Mediated Redox Cycling of Quinones

The nonenzymatic reactions of dihydrolipoamide with a number of low-potential quinones, possessing either a fully or a partially substituted quinone ring at pH 7.0 were accompanied by consumption of oxygen in a significant excess of the quinone concentration, thus establishing their redox cycling. Contrary to this, only partially substituted quinones caused the consumption of oxygen in the presence of reduced glutathione due to reoxidation of reduced quinone-glutathione conjugates. Among compounds tested, 9,10-phenanthrene quinone catalyzed the most rapid consumption of oxygen in the presence of dihydrolipoamide with subsequent formation of lipoamide and H 2O 2. The rate constant of anaerobic reduction of phenanthrene quinone by dihydrolipoamide was 8.6 ± 1.6 × 10 3 M−1 s−1 (pH 7.0, 0.1 M phosphate, 20% ethanol, 25°C). The consumption of oxygen and formation of lipoamide were inhibited by superoxide dismutase, indicating that the redox cycling involves the autooxidation of 9,10-dihydroxy phenanthrene, mediated by superoxide. The reaction was accompanied by the reduction of added cytochrome c, which was insignificantly inhibited by superoxide dismutase, and the reductive mobilization of iron from ferritin, activated by superoxide disniutase. These data raise the possibility that dihydrolipoamide, usually regarded as an antioxidant, under certain conditions may exert moderate prooxidant activity, initiating the formation of radicals and activated forms of oxygen.