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PubMed1992

Reactivity of hypervalent iron with biological compounds.

Although it is well established that the ferryl (Fe==O2+) and perferryl (Fe==O3+) oxidation states of iron have a beneficial role in enzymatic oxidation and hydroxylation processes, as small complexes that are suspected to be formed in Fenton reactions, they may have deleterious effects on living cells. I discuss the basic chemistry of small hypervalent iron complexes. Their reactivity with biological compounds is illustrated by considering ascorbic acid and essential amino acids. A method is proposed for the determination of the ratio of [.OH]/[ferryl] in Fenton-type reactions.

Amino Acids↗
PubMed1986

Coordination structures and reactivities of compound II in iron and manganese horseradish peroxidases. A resonance Raman study.

Resonance Raman investigations on compound II of native, diacetyldeuteroheme-, and manganese-substituted horseradish peroxidase (isozyme C) revealed that the metal-oxygen linkage in the compound differed from one another in its bond strength and/or structure. Fe(IV) = O stretching frequency for compound II of native enzyme was pH sensitive, giving the Raman line at 772 and 789 cm-1 at pH 7 and 10, respectively. The results confirmed the presence of a hydrogen bond between the oxo-ligand and a nearby amino acid residue (Sitter, A. J., Reczek, C. M., and Terner, J. (1985) J. Biol. Chem. 260, 7515-7522). The Fe(IV) = O stretch for compound II of diacetylheme-enzyme was located at 781 cm-1 at pH 7 which was 9 cm-1 higher than that of native enzyme compound II. At pH 10, however, the Fe(IV) = O stretch was found at 790 cm-1, essentially the same frequency as that of native enzyme compound II. The pK value for the pH transition, 8.5, was also the same as that of native compound II. Unlike in native enzyme, D2O-H2O exchange did not cause a shift of the Fe(IV) = O frequency of diacetylheme-enzyme. Thus, the metal-oxygen bond at pH 7 was stronger in diacetylheme-enzyme due to a weaker hydrogen bonding to the oxo-ligand, while the Fe(IV) = O bond strength became essentially the same between both enzymes at alkaline pH upon disruption of the hydrogen bond. A much lower reactivity of the diacetylheme-enzyme compound II was accounted to be due to the weaker hydrogen bond. Compound II of manganese-substituted enzyme exhibited Mn(IV)-oxygen stretch about 630 cm-1, which was pH insensitive but down-shifted by 18 cm-1 upon the D2O-H2O exchange. The finding indicates that its structure is in Mn(IV)-OH, where the proton is exchangeable with a water proton. These results establish that the structure of native enzyme compound II is Fe(IV) = O but not Fe(IV)-OH.

Horseradish Peroxidase↗
PubMed1987

Metabolic utilization of 57Fe-labeled coprogen in Neurospora crassa. An in vivo Mössbauer study.

Mössbauer spectra of whole cells of Neurospora crassa arg-5 ota aga (a siderophore-free mutant) show that the siderophore coprogen is accumulated inside the cell as an entity. 57Fe from 57Fe-labeled coprogen is slowly removed from the complex (45% in 27 h). The rate of removal depends on the degree of iron starvation of the cells. The distribution of 55Fe from [55Fe]coprogen in vacuoles, membranes, and cytoplasm has been also determined. From this it is clear that coprogen is accumulated in the cytoplasm. In addition to its role as a siderophore, coprogen serves as an iron-storage compound. No holoferritins could be detected. We therefore conclude that this type of iron-storage protein is lacking in N. crassa. Metabolized iron was found predominantly to exist as an envelope of Fe(II) high-spin (delta = 1.2-1.3 mm s-1; delta EQ = 3.0-3.1 mm s-1 at 4.2 K) and fast-relaxing Fe(III) high-spin species (delta approximately equal to 0.25 mm s-1 and 0.45 mm s-1; delta EQ approximately equal to 0.6 mm s-1 and 0.55 mm s-1, respectively, at 4.2 K). An assignment of these major iron metabolites is difficult. The Mössbauer data of the Fe(II) species do not fit those reported for heme, cytochromes and ferredoxins. We therefore assume that this iron metabolite represents a novel internal iron compound. One of the Fe(III) species becomes the dominant component of the cell spectra after 65 h of metabolization and might correspond to an iron-storage compound with iron oxide cores similar to bacterioferritin. After 27 h of growth in mycelia supplied with 57Fe-labeled coprogen, the siderophore ferricrocin was observed in the cell spectra. This is unexpected, since N. crassa arg-5 ota aga is unable to synthesize ornithine. We assume that ferricrocin is synthesized by the use of coprogen degradation products.

Cell Membrane↗
PubMed2006

Degradation of btex compounds under iron-reducing conditions in contaminated aquifer microcosms.

The potential for benzene, toluene, ethylbenzene, and xylenes (BTEX) degradation was investigated in microcosms inoculated with sediment and groundwater from a polluted iron-reducing aquifer. Benzene, toluene, and each of the three xylene isomers were degraded by the intrinsic microorganisms under iron-reducing conditions, but there was no removal of ethylbenzene. This work provides the first evidence for para-xylene degradation by dissimilatory iron-reducing bacteria. Microcosms adapted to benzene, toluene, or m-xylene were subsequently exposed to a different BTEX compound, which was degraded without lag phase, suggesting that the same group of bacteria could be involved in the removal of more than one BTEX compound. Furthermore, when microcosms were exposed to a mixture of BTEX, concurrent degradation of benzene and toluene, but not of meta-xylene and ethylbenzene, was observed. These results suggest that, under the influence of the plume of pollution, an iron-reducing microbial community able to degrade multiple aromatic compounds has developed.

Benzene↗
PubMed1975

Kinetic studies on the specificity of chelate-iron uptake in Aspergillus.

Three strains of the fungus Aspergillus, Aspergillus quadricinctus (E. Yuill), A. fumigatus (Fresenius), and A. melleus (Yukawa), each producing different iron-chelating compounds during iron-deficient cultivation, were used for 55Fe3+ uptake measurements. Iron from chelates of the ferrichrome-type family was taken up by young mycelia of all strains tested, irrespective of the ferrichrome-type compound these strains predominantly produce in low-iron cultures. Ferrichrysin-producing strains, however, seem to favor ferrichrysin iron uptake, whereas ferrichrome, ferricrocin, and even ferrirubin showed similar iron transport properties in all of these strains. Compared to iron uptake from ferrichrome-type compounds (Km approximately 4 uM) iron uptake from fusigen revealed completely different kinetic values (Km approximately 50 to 80 muM). Iron from exogenous chelates, e.g., from coprogen produced by Neurospora crassa for ferrioxamine B produced by Streptomyces pilosus, can obviously not be taken up by Aspergillus, confirming the pronounced specificity of chelate-iron transport in fungi.

Aspergillus↗
PubMed1998

Interaction of iron with polyphenolic compounds: application to antioxidant characterization.

Antioxidant action of flavonoids and polyphenolics was analyzed in relation to iron coordination. Flavonoids including baicalein, baicalin, quercetin, and rutin showed little or no reducing activity, and enhanced the autooxidation of Fe2+ ion. Ascorbate-mediated reduction of iron was rather inhibited by flavonoids. Nonflavonoid polyphenolics such as protocatechuic acid and chlorogenic acid, on the contrary, showed a potent iron-reducing ability, and protected Fe2+ ion from autooxidation completely. Both flavonoids and nonflavonoids effectively inhibited the formation of thiobarbituric acid-reactive substances as a marker of lipid peroxidation of microsomes from rat liver. Flavonoids act as an antioxidant by inhibiting the oxygen radical formation through the enhanced oxidation of Fe2+ ion as the prooxidant. Antioxidant properties of nonflavonoids can be explained by the formation of inactive Fe(2+)-polyphenol complex, which cannot react with oxygen, and/or can scavenge reactive oxygen species. Analysis of the action of polyphenolics on the iron redox state may be useful for characterization of the antioxidant effect.

Animals↗
PubMed2006

Abiotic reduction of nitroaromatic compounds by aqueous iron(ll)-catechol complexes.

Complexation of iron(ll) by catechol and thiol ligands leads to the formation of aqueous species that are capable of reducing substituted nitroaromatic compounds (NACs) to the corresponding anilines. No reactions of NACs are observed in FelI-only or ligand-only solutions. In solutions containing FeII and tiron, a model catechol, rates of NAC reduction are heavily dependent on pH, ligand concentration, and ionic strength. Observed pseudo-first-order rate constants (k(obs)) for 4-chloronitrobenzene reduction vary by more than 6 orders of magnitude, and the variability is well described by the expression k(obs) = k(FeL2)(6-) [FeL2(6-)], where [FeL2(6-)] is the concentration of the 1:2 FeII-tiron complex and kFeL2(6-) is the bimolecular rate constant for 4-chloronitrobenzene reaction with this species. The high reactivity of this FeII species is attributed to the low standard one-electron reduction potential of the corresponding FeIII/FeII redox couple (EH0 = -0.509 V vs NHE). The relative reactivity of different NACs can be described by a linear free-energy relationship (LFER) with the one-electron reduction potentials of the NACs, EH1'(ArNO2). The experimentally derived slope of the LFER indicates that electron transfer is rate determining. These findings suggest that FeII-organic complexes may play an important, previously unrecognized, role in the reductive transformation of persistent organic contaminants.

1,2-Dihydroxybenzene-3,5-Disulfonic Acid Disodium ↗
PubMed1993

Iron-regulated salicylate synthesis by Pseudomonas spp.

Two iron-regulated compounds have been found in acidified ethyl acetate extracts from culture supernatants of Pseudomonas aeruginosa and Pseudomonas cepacia type-strains. Synthesis of both compounds paralleled iron-deficient growth, and was repressed in the presence of 100 microM-FeCl3. Yields of these substances varied among different strains and attained maximum levels during stationary phase. Thin layer chromatographic analysis in five different solvent systems revealed that the slower-moving compound chromatographed as two distinct bands, and showed RF values and spectral properties similar to pyochelin. The faster-moving compound co-migrated as a single band with a standard of commercial salicylic acid in each of the chromatographic systems tested. Moreover, a molecule with an identical RF was also produced by Pseudomonas fluorescens CHA401, which is known to synthesize salicylic acid as the only siderophore during iron-limited growth. Spectrophotometric and spectrofluorometric titrations led to the identification of this iron-regulated compound as salicylic acid, in agreement with the structure deduced from 1H-NMR and mass spectroscopy. The identity of the P. cepacia siderophore azurechelin as salicylic acid was also conclusively demonstrated. Salicylic acid, like pyochelin and pyoverdin, promoted P. aeruginosa growth in an iron-depleted medium. These results are consistent with a putative siderophore activity for salicylic acid, i.e. azurechelin, as has been demonstrated for P. aeruginosa, P. fluorescens and P. cepacia. Thus, salicylic acid is likely to act as a siderophore in more than one species belonging to the genus Pseudomonas.

Burkholderia cepacia↗
PubMed1995

Novel 21-aminosteroidlike compounds prevent iron-induced free radical-mediated injury to vascular endothelial cells.

Free radicals appear to play a major role in the reperfusion injury of the myocardium. Iron chelators and antioxidants can prevent the reperfusion injury. The cellular source of free radicals during reperfusion is not clearly established. However, an important source may be the endothelial cells (EC). The protective effect of iron chelators and antioxidants on reperfusion injury of the myocardium may be partially mediated by their effect on vascular EC, which may attenuate the formation of hydroxyl radicals. We examined the protective effect of deferoxamine and antioxidants on iron-dependent free radical-mediated damage to EC and membrane lipid peroxidation and compared it with those of a new class of compounds (21-aminosteroids: U-74389F, U-74500A, U-78517F) which have both antioxidant and iron chelating properties. We examined viability of the EC and membrane lipid peroxidation using primary aortic EC. First, the effect of increasing iron concentration (0-3.8 microM) (added as ferric ammonium citrate) was characterized. We then examined the effect of deferoxamine (1-10 microM), butylated hydroxytoluene (BHT 1-10 mM), probucol (0.5-5.0 mM), allopurinol (1-1,000 microM), U-74389F (2-20 microM), U-74500A (1-5 microM), and U-78517F (0.1-1.0 microM) on reversing the effect of iron. With increasing iron concentration, there was a significant decrease in the viability of dog aortic or bovine pulmonary arterial EC as compared with NIH 3T3 or human fibroblasts. Equally, there was also a significant increase in lipid peroxidation of the cellular membranes. There were significant differences between the compounds with respect to their ability to maintain the viability of EC and prevent membrane lipid peroxidation.(ABSTRACT TRUNCATED AT 250 WORDS)

Allopurinol↗
PubMed1995

Degradation of Monochlorinated and Nonchlorinated Aromatic Compounds under Iron-Reducing Conditions.

The capacity for Fe(sup3+) to serve as an electron acceptor in the microbial degradation of monochlorinated and nonchlorinated aromatic compounds was investigated in anoxic sediment enrichments. The substrates tested included phenol, benzoate, aniline, their respective monochlorinated isomers, o-, m-, and p-cresol, and all six dimethylphenol isomers. Phenol and 2-, 3-, and 4-chlorophenol were utilized by anaerobic microorganisms, with the concomitant reduction of Fe(sup3+) to Fe(sup2+). The amount of Fe(sup2+) produced in the enrichments was 89 to 138% of that expected for the stoichiometric degradation of these substrates to CO(inf2), suggesting complete mineralization at the expense of Fe reduction. Under Fe-reducing conditions, there was initial loss of benzoate and 3-chlorobenzoate but not of 2- or 4-chlorobenzoate. In addition, there was initial microbial utilization of aniline but not of the chloroaniline isomers. There was also initial loss of o-, m-, and p-cresol in our enrichments. None of the dimethylphenol isomers, however, was degraded within 300 days. Furthermore, we tested the capacity of an Fe-reducing, benzoate-grown culture of Geobacter metallireducens GS-15 to utilize monochlorinated benzoates and phenols. G. metallireducens was able to degrade benzoate and phenol but none of their chlorinated isomers, suggesting that the degradation of chlorophenols in our sediment enrichments may be due to novel Fe-reducing organisms that have yet to be isolated.

Journal Article↗
PubMed1991

Dose-dependent inhibitory effect of phenolic compounds in foods on nonheme-iron absorption in men.

Nonheme-iron absorption from a typical Southeast Asian meal was studied to examine the effect of a common vegetable, Yod Kratin, which contains a considerable amount of iron-binding phenolic groups. Yod Kratin (leaves of the lead tree) is a very popular vegetable in Thailand. It is consumed at least once a week year round, sometimes every day, together with the main meal. With a common portion size of the vegetable (20 g), iron absorption was reduced by almost 90%. As little as 5 g inhibited iron absorption by 75%. Addition of ascorbic acid partly counteracted inhibition. Adding 100 mg ascorbic acid reduced inhibition of iron absorption from 5 g Yod Kratin by half and the inhibition from 10 g Yod Kratin by a quarter. The study illustrates the marked effect of iron-binding phenolic compounds on iron nutrition and, thus, the importance of acquiring knowledge of the content of such compounds in different foods.

Absorption↗