Citrate binding of Al3+ and Fe3+

Citrate occurs at about 0.1 mM in blood plasma and is the most likely small molecule plasma binder of both Al3+ and Fe3+. This paper assesses published stability constants for citrate binding to each metal ion. From pH 2 to 5 Al3+ forms a neutral complex with citrate that may be absorbed into the body in the upper regions of the gastrointestinal tract. It is especially dangerous to ingest aluminum-containing antacids with citrus fruit or juices. Ignoring the likely occurrence of a competing 2:1 citrate-Fe3+ complex necessitates adjustments in reported stability constants for Fe3+ binding to transferrin. In the blood, plasma transferrin steals both Fe3+ and Al3+ from citrate.     

Regulation of Fe3+-oxide Formation Among Fe2+-oxidizing Bacteria

Helical stalks (resembling Gallionella ferruginea, Mariprofundus ferrooxydans) and filamentous sheaths (resembling Leptothrix ochracea) of Fe²⁺-oxidizing bacteria (FeOB) are mineralized by hydrous ferric oxides (HFO). To perform both inter-species and inter-site size comparisons of HFO particles on stalks and sheaths we measured HFO particles in samples of natural bacteriogenic iron oxides (BIOS) from 3 contrasting field sites: the Loihi Seamount (southern Hawaii); Äspö Hard Rock Laboratory (eastern Sweden); and Chalk River Laboratories (northern Canada) representing seafloor saline, underground brackish, and surface freshwater aqueous conditions. Ambient temperatures were in the psychrophilic range and pHs measured for Loihi, CRL, and Äspö were 5.6, 6.9 and 7.4, respectively. Dissolved Fe was lowest for CRL (0.2 mg · L^?1) followed by Äspö (1.5 mg · L^?1), then Loihi (4.5–14.9 mg · L^?1). L. ochraceasheaths appear to have surface properties that restrict HFO particle growth in comparison to G.ferruginea-M.ferrooxydans stalks in the same environment, which we attribute to interfacial surface energy (γ). An inverse relationship between particle size and stalk/sheath length due to restrictions in reactive surface area was also observed, which may provide insight into FeOB survival strategies to alleviate oxidative stress arising from Fe³⁺ production.     

Rice plants take up iron as an Fe3+-phytosiderophore and as Fe2+

Only graminaceous monocots possess the Strategy II iron (Fe)-uptake system in which Fe is absorbed by roots as an Fe3+-phytosiderophore. In spite of being a Strategy II plant, however, rice (Oryza sativa) contains the previously identified Fe2+ transporter OsIRT1. In this study, we isolated the OsIRT2 gene from rice, which is highly homologous to OsIRT1. Real-time PCR analysis revealed that OsIRT1 and OsIRT2 are expressed predominantly in roots, and these transporters are induced by low-Fe conditions. When expressed in yeast (Saccharomyces cerevisiae) cells, OsIRT2 cDNA reversed the growth defects of a yeast Fe-uptake mutant. This was similar to the effect of OsIRT1 cDNA. OsIRT1- and OsIRT2-green fluorescent protein fusion proteins localized to the plasma membrane when transiently expressed in onion (Allium cepa L.) epidermal cells. OsIRT1 promoter-GUS analysis revealed that OsIRT1 is expressed in the epidermis and exodermis of the elongating zone and in the inner layer of the cortex of the mature zone of Fe-deficient roots. OsIRT1 expression was also detected in the ccompanion cells. Analysis using the positron-emitting tracer imaging system showed that rice plants are able to take up both an Fe3+-phytosiderophore and Fe2+. This result indicates that, in addition to absorbing an Fe3+-phytosiderophore, rice possesses a novel Fe-uptake system that directly absorbs the Fe2+, a strategy that is advantageous for growth in submerged conditions.     

高岭土对Fe~(2+)的吸附及其对高岭土中Fe~(3+)生物还原的影响研究

利用异化铁还原微生物可将高岭土中不溶性的Fe3+还原成可溶性的Fe2+,但是此过程中产生的Fe2+能够被高岭土以及异化铁还原微生物吸附,从而影响高岭土中铁的异化还原。本文研究了pH、高岭土量、Fe2+浓度、温度4个因素对高岭土吸附Fe2+的影响;并采用Logistic方程拟合,研究Fe2+及温度对高岭土中Fe3+的生物还原特征。结果表明:pH、高岭土量、浓度、温度4个因素均会影响高岭土吸附Fe2+,当Fe2+吸附在高岭土和微生物菌体表面时,微生物的活性下降,同时高岭土表面Fe3+的生物可利用性也降低,Fe3+生物还原的最大速率减小。     

Mouse intestinal Fe3+ uptake kinetics in vivo. The significance of brush-border membrane vesicle transport in the mechanism of mucosal Fe3+ uptake

Initial rates of mucosal uptake of Fe3+ from luminal Fe3+-nitrilotriacetate solutions by tied segments of mouse intestine in vivo have been measured. Duodenal uptake showed an approximately hyperbolic dependence of uptake on Fe3+ complex concentration (Km(app) 66 microM, Vmax 6.2 pmol/min per mg intestine) with little dependence on nitrilotriacetate:Fe3+ ratio or on added Ca2+. Duodenal uptake was greatly stimulated by hypoxic treatment of mice. Uptake rates by distal ileum were lower than by duodenum and more sensitive to added Ca2+. These results show that isolated duodenal brush-border membrane Fe3+ transport characteristics (Simpson, R.J. and Peters, T.J. (1984) Biochim. Biophys. Acta 772, 220-226) are inadequate to explain duodenal Fe3+ uptake in vivo. However, ileal uptake can be explained by the properties of isolated ileal brush-border membrane (Simpson, R.J., Raja, K.B. and Peters, T.J. (1985) Biochim. Biophys. Acta 814, 8-12).     

Zero-field splitting of Fe3+ in horseradish peroxidase and of Fe4+ in horseradish peroxidase compound I from electron spin relaxation data.

From the temperature dependence of the Orbach relaxation rate of the paramagnetic center in horseradish peroxidase (HRP), we deduce an excited-state energy of 40.9 +/- 1.1 K. Similar studies on the broad EPR signal of HRP compound I indicate a much weaker Orbach relaxation process involving an excited state at 36.8 +/- 2.5 K. The strength of the Orbach process in HRP-I is weaker than one would normally estimate by 2-4 orders of magnitude. This fact lends support to the model of HRP-I involving a spin 1/2 free radical coupled to a spin 1 Fe4+ heme iron via a weak exchange interaction. Such a system should exhibit an Orbach relaxation process involving delta E, the excited state of the Fe4+ ion, but reduced in strength by (Jyy/delta E)2, where Jyy is related to the strength of the exchange interaction between the two spin systems.     

Fe3+ binding to ovotransferrin in the presence of alpha-amino acids.

The ability of L-alpha-amino acids as synergistic anions for iron binding to ovotransferrin was investigated through electronic spectroscopy. Glycine and glutamic acid were found to form by far the most stable ternary Fe(3+)-ovotransferrin-amino acid complexes. Less stable adducts were formed by amino acids with a hydroxy, amide or sulphur-containing group in the side chain, while the complexes with leucine, isoleucine, valine, lysine, arginine, tyrosine and tryptophan failed to form. Evidence is obtained that the synergistic effectiveness of the H2N-CH-COO- moiety is determined not only by the isoelectric point of the amino acid and the steric hindrance of its side chain, but a significant role is also played by interactions of the side chain itself with residues in the metal binding domains. Zn2+, Cd2+ and Co2+ are found to bind to ovotransferrin in the presence of glycine. 113Cd-NMR spectra on the Cd-derivative indicate that, according to the interlocking-sites model, the amino group of glycine directly binds to the metal ion.     

A selective colorimetric chemosensor for Fe3+

Two simple colorimetric receptors PS and PP (thiophene and pyridine appended derivative) were prepared and their cation sensing properties were investigated. Receptors PS and PP displayed a selective colorimetric change (from colorless to orange) upon binding to Fe³⁺ in MeOH solution. The association constants for receptors PS–F e³⁺ and PP–F e³⁺ in MeOH were determined to be 1.15 × 10⁶ and 4.31 × 10⁶ M^?1, respectively, using Hill plots. The detection limits of PS and PP were 490.7 ppm and 393.7 ppm, respectively.     

Inhibition of Fe2+- and Fe3+- induced hydroxyl radical production by the iron-chelating drug deferiprone

Deferiprone (L1) is an effective iron-chelating drug that is widely used for the treatment of iron-overload diseases. It is known that in aqueous solutions Fe²⁺ and Fe³⁺ ions can produce hydroxyl radicals via Fenton and photo-Fenton reactions. Although previous studies with Fe²⁺ have reported ferroxidase activity by L1 followed by the formation of Fe³⁺ chelate complexes and potential inhibition of Fenton reaction, no detailed data are available on the molecular antioxidant mechanisms involved. Similarly, in vitro studies have also shown that L1–Fe³⁺ complexes exhibit intense absorption bands up to 800 nm and might be potential sources of phototoxicity. In this study we have applied an EPR spin trapping technique to answer two questions: (1) does L1 inhibit the Fenton reaction catalyzed by Fe²⁺ and Fe³⁺ ions and (2) does UV–Vis irradiation of the L1–Fe³⁺ complex result in the formation of reactive oxygen species. PBN and TMIO spin traps were used for detection of oxygen free radicals, and TEMP was used to trap singlet oxygen if it was formed via energy transfer from L1 in the triplet excited state. It was demonstrated that irradiation of Fe³⁺ aqua complexes by UV and visible light in the presence of spin traps results in the appearance of an EPR signal of the OH spin adduct (TMIO–OH, a(N)=14.15 G, a(H)=16.25 G; PBN–OH, a(N)=16.0 G, a(H)=2.7 G). The presence of L1 completely inhibited the OH radical production. The mechanism of OH spin adduct formation was confirmed by the detection of methyl radicals in the presence of dimethyl sulfoxide. No formation of singlet oxygen was detected under irradiation of L1 or its iron complexes. Furthermore, the interaction of L1 with Fe²⁺ ions completely inhibited hydroxyl radical production in the presence of hydrogen peroxide. These findings confirm an antioxidant targeting potential of L1 in diseases related to oxidative damage.     

Crystallographic and biochemical analyses of the metal-free Haemophilus influenzae Fe3+-binding protein.

The crystal structure of the iron-free (apo) form of the Haemophilus influenzae Fe(3+)-binding protein (hFbp) has been determined to 1.75 A resolution. Information from this structure complements that derived from the holo structure with respect to the delineation of the process of iron binding and release. A 21 degrees rotation separates the two structural domains when the apo form is compared with the holo conformer, indicating that upon release of iron, the protein undergoes a change in conformation by bending about the central beta-sheet hinge. A surprising finding in the apo-hFbp structure was that the ternary binding site anion, observed in the crystals as phosphate, remained bound. In solution, apo-hFbp bound phosphate with an affinity K(d) of 2.3 x 10(-3) M. The presence of this ternary binding site anion appears to arrange the C-terminal iron-binding residues conducive to complementary binding to Fe(3+), while residues in the N-terminal binding domain must undergo induced fit to accommodate the Fe(3+) ligand. These observations suggest a binding process, the first step of which is the binding of a synergistic anion such as phosphate to the C-terminal domain. Next, iron binds to the preordered half-site on the C-terminal domain. Finally, the presence of iron organizes the N-terminal half-site and closes the interdomain hinge. The use of the synergistic anion and this iron binding process results in an extremely high affinity of the Fe(3+)-binding proteins for Fe(3+) (nFbp K'(eff) = 2.4 x 10(18) M(-1)). This high-affinity ligand binding process is unique among the family of bacterial periplasmic binding proteins and has interesting implications in the mechanism of iron removal from the Fe(3+)-binding proteins during FbpABC-mediated iron transport across the cytoplasmic membrane.     

Fe3+ coordination and redox properties of a bacterial transferrin

The Fe(3+) binding site of recombinant nFbp, a ferric-binding protein found in the periplasmic space of pathogenic Neisseria, has been characterized by physicochemical techniques. An effective Fe(3+) binding constant in the presence of 350 microm phosphate at pH 6.5 and 25 degrees C was determined as 2.4 x 10(18) m(-1). EPR spectra for the recombinant Fe(3+)nFbp gave g' = 4.3 and 9 signals characteristic of high spin Fe(3+) in a strong ligand field of low (orthorhombic) symmetry. (31)P NMR experiments demonstrated the presence of bound phosphate in the holo form of nFbp and showed that phosphate can be dialyzed away in the absence of Fe(3+) in apo-nFbp. Finally, an uncorrected Fe(3+/2+) redox potential for Fe-nFbp was determined to be -290 mV (NHE) at pH 6.5, 20 degrees C. Whereas our findings show that nFbp and mammalian transferrin have similar Fe(3+) binding constants and EPR spectra, they differ greatly in their redox potentials. This has implications for the mechanism of Fe transport across the periplasmic space of Gram-negative bacteria.     

Application of an M13 bacteriophage displaying tyrosine on the surface for detection of Fe3+ and Fe2+ ions

Ferric and ferrous ion plays critical roles in bioprocesses,their influences in many fields have not been fully explored due to the lack of methods for quantification of ferric and ferrous ions in biological system or complex matrix.In this study,an M13 bacteriophage(phage) was engineered for use as a sensor for ferric and ferrous ions via the display of a tyrosine residue on the P8 coat protein.The interaction between the specific phenol group of tyrosine and Fe~(3+)./ Fe~(2+).was used as the sensor.Transmission electron microscopy showed aggregation of the tyrosine-displaying phages after incubation with Fe~(3+) and Fe~(2+).The aggregated phages infected the host bacterium inefficiently.This phenomenon could be utilized for detection of ferric and ferrous ions.For ferric ions,a calibration curve ranging from 200 nmol/L to 8 μmol/L with a detection limit of 58 nmol/L was acquired.For ferrous ions,a calibration curve ranging from 800 nmol/L to 8μmol/L with a detection limit of 641.7 nmol/L was acquired.The assay was specific for Fe~((3+)) and Fe~((2+)) when tested against Ni~(2+),Pb~(2+),Zn~(2+),Mn~(2+),Co~(2+),Ca~(2+),Cu~(2+),Cr~(3+),Ba~(2+),and K~+.The tyrosine displaying phage to Fe~(3+) and Fe~(2+) interaction would have plenty of room in application to biomatenals and bionanotechnology.     

Competition of Mn2+ and Zn2+ with59Fe2+ and59Fe3+ for the plasma membrane receptors from lactating mouse mammary gland

Nonlabeled MnCl₂ and ZnSO₄ compete with⁵⁹Fe²⁺-ascorbate and⁵⁹Fe ₂ ³⁺ O₃ for transport binding sites situated on the plasma membranes of lactating mouse mammary gland cells. The binding was found to be a process reaching saturation. The heterologous competition used here ruled out the participation of transferrin and to propose that Fe, Mn, and Zn are transported from blood to milk by a mechanism involving one receptor during lactation. Further experiments are necessary to establish the details of the transport mechanism.