Iron release from ferritin by flavin nucleotides

Background

Extensive in-vitro studies have focused on elucidating the mechanism of iron uptake and mineral core formation in ferritin. However, despite a plethora of studies attempting to characterize iron release under different experimental conditions, the in-vivo mobilization of iron from ferritin remains poorly understood.Several iron-reductive mobilization pathways have been proposed including, among others, flavin mononucleotides, ascorbate, glutathione, dithionite, and polyphenols. Here, we investigate the kinetics of iron release from ferritin by reduced flavin nucleotide, FMNH2, and discuss the physiological significance of this process in-vivo.

Methods

Iron release from horse spleen ferritin and recombinant human heteropolymer ferritin was followed by the change in optical density of the Fe(II)–bipyridine complex using a Cary 50 Bio UV–Vis spectrophotometer. Oxygen consumption curves were followed on a MI 730 Clark oxygen microelectrode.

Results

The reductive mobilization of iron from ferritin by the nonenzymatic FMN/NAD(P)H system is extremely slow in the presence of oxygen and might involve superoxide radicals, but not FMNH2. Under anaerobic conditions, a very rapid phase of iron mobilization by FMNH2 was observed.

Conclusions

Under normoxic conditions, FMNH2 alone might not be a physiologically significant contributor to iron release from ferritin.

General significance

There is no consensus on which iron release pathway is predominantly responsible for iron mobilization from ferritin under cellular conditions. While reduced flavin mononucleotide (FMNH2) is one likely candidate for in-vivo ferritin iron removal, its significance is confounded by the rapid oxidation of the latter by molecular oxygen.     

Tumor necrosis factor-alpha and interleukin 1-alpha regulate transferrin receptor in human diploid fibroblasts. Relationship to the induction of ferritin heavy chain

We have studied transferrin receptor expression in MRC5 human fibroblasts in response to tumor necrosis factor-alpha (TNF, cachectin) or interleukin 1-alpha (IL-1). Treatment of exponentially growing MRC5 cells with these cytokines led to a 3-4-fold increase in transferrin receptor mRNA and a coordinate increase in transferrin receptor protein by 24 h. Under these conditions, stimulation of [3H]thymidine incorporation was minimal, suggesting that the induction of transferrin receptor by TNF and IL-1 is mediated by a growth-independent regulatory mechanism. A study of the time course of this response showed that cytokine-mediated increases in transferrin receptor mRNA and protein proceeded after a lag of 12-24 h. A simultaneous analysis of the effects of TNF and IL-1 on ferritin in MRC5 cells was also performed. Ferritin L mRNA levels were unchanged. However, induction of ferritin H mRNA was seen within 4 h, preceding the induction of the transferrin receptor. The synthesis of ferritin H (but not ferritin L) protein peaked at 8 h after TNF or IL-1 treatment, followed by a rapid decrease in both ferritin H and L protein synthesis. As ferritin H synthesis declined, levels of transferrin receptor protein increased, reaching a maximum by 24 h. These results suggest that the cytokine-dependent induction of ferritin H and subsequent increase in the transferrin receptor are related and possibly interdependent events. This study demonstrates that the complex role of TNF and IL-1 in iron homeostasis includes modulation of the transferrin receptor.     

豌豆铁蛋白的纯化及其抗血清的制备

干豌豆种子粗提物经MgCl2 盐析、AcA2 2 凝胶过滤和DEAE 纤维素阴离子交换柱层析等方法进行纯化 ,以邻菲咯啉显色法检测铁蛋白 ,最后获得纯的铁蛋白 .纯化的铁蛋白在PAGE上显示一条带 ,SDS PAGE显示该蛋白仅含 2 8kD一条亚基 .纯化的豌豆铁蛋白免疫兔 7周后 ,琼脂糖双扩散法检测抗血清效价达 1∶32 .用分级盐析法纯化抗血清 ,纯化后的抗体用琼脂糖双扩散法对大豆铁蛋白粗提物有免疫交叉反应     

Modulation of cellular iron metabolism by hydrogen peroxide. Effects of H2O2 on the expression and function of iron-responsive element-containing mRNAs in B6 fibroblasts

Cellular iron uptake and storage are coordinately controlled by binding of iron-regulatory proteins (IRP), IRP1 and IRP2, to iron-responsive elements (IREs) within the mRNAs encoding transferrin receptor (TfR) and ferritin. Under conditions of iron starvation, both IRP1 and IRP2 bind with high affinity to cognate IREs, thus stabilizing TfR and inhibiting translation of ferritin mRNAs. The IRE/IRP regulatory system receives additional input by oxidative stress in the form of H(2)O(2) that leads to rapid activation of IRP1. Here we show that treating murine B6 fibroblasts with a pulse of 100 microm H(2)O(2) for 1 h is sufficient to alter critical parameters of iron homeostasis in a time-dependent manner. First, this stimulus inhibits ferritin synthesis for at least 8 h, leading to a significant (50%) reduction of cellular ferritin content. Second, treatment with H(2)O(2) induces a approximately 4-fold increase in TfR mRNA levels within 2-6 h, and subsequent accumulation of newly synthesized protein after 4 h. This is associated with a profound increase in the cell surface expression of TfR, enhanced binding to fluorescein-tagged transferrin, and stimulation of transferrin-mediated iron uptake into cells. Under these conditions, no significant alterations are observed in the levels of mitochondrial aconitase and the Divalent Metal Transporter DMT1, although both are encoded by two as yet lesser characterized IRE-containing mRNAs. Finally, H(2)O(2)-treated cells display an increased capacity to sequester (59)Fe in ferritin, despite a reduction in the ferritin pool, which results in a rearrangement of (59)Fe intracellular distribution. Our data suggest that H(2)O(2) regulates cellular iron acquisition and intracellular iron distribution by both IRP1-dependent and -independent mechanisms.     

Iron release from ferritin by alloxan radical

Alloxan in the presence of reduced glutathione released iron from ferritin which is the major intracellular iron storage protein. Superoxide dismutase inhibited by only about 30% the alloxan-dependent iron release from ferritin but completely inhibited the iron release from ferritin induced by hypoxanthine-xanthine oxidase. Under anaerobic conditions, the ESR spectrum of alloxan radical was obtained and interaction with ferritin resulted in a marked diminution of the alloxan radical signal. These results indicate that alloxan radical rapidly releases iron from ferritin.     

Direct visualization of the binding and internalization of a ferritin conjugate of epidermal growth factor in human carcinoma cells A-431

We have prepared a conjugate of epidermal growth factor (EGF) and ferritin that retains substantial binding affinity for cell receptors and is biologically active. Glutaraldehyde-activated EGF was covalently linked to ferritin to produce a conjugate that contained EGF and ferritin in a 1:1 molar ratio. The conjugate was separated from free ferritin by affinity chromatography using antibodies to EGF. Monolayers of human epithelioid carcinoma cells (A-431) were incubated with EGF:ferritin at 4 degrees C and processed for transmission electron microscopy. Under these conditions, approximately 6 X 10(5) molecules of EGF:ferritin bound to the plasma membrane of each cell. In the presence of excess native EGF, the number of bound ferritin particles was reduced by 99%, indicating that EGF:ferritin binds specifically to cellular EGF receptors. At 37 degrees C, cell-bound EGF:ferritin rapidly redistributed in the plane of the plasma membrane to form small groups that were subsequently internalized into pinocytic vesicles. By 2.5 min at 37 degrees C, 32% of the cell-bound EGF:ferritin was localized in vesicles. After 2.5 min, there was a decrease in the proportion of conjugate in vesicles with a concomitant accumulation of EGF:ferritin in multivesicular bodies. By 30 min, 84% of the conjugate was located in structures morphologically identified as multivesicular bodies or lysosomes. These results are consistent with other morphological and biochemical studies utilizing 125I-EGF and fluorescein-conjugated EGF.     

Iron transfer from transferrin to ferritin mediated by pyrophosphate

There is no significant iron exchange from transferrin to ferritin in the absence of reducing and chelating agents. Pyrophosphate can release iron from transferrin and can be isolated as a ferric pyrophosphate complex by ion exchange chromatography. We have established that pyrophosphate alone can mediate iron exchange from transferrin to ferritin. Under these conditions, iron is incorporated directly into ferritin as Fe(III).     

Characterization of feline serum ferritin-binding proteins: the presence of a novel ferritin-binding protein as an inhibitory factor in feline ferritin immunoassay

Ferritin-binding proteins (FBPs) such as anti-ferritin antibody, α-2-macroglobulin, apolipoprotein B are expected to interact with circulating ferritin to eliminate it from circulation. However, we found that feline serum more strongly inhibits the detection of canine liver ferritin by immunoassay than its apoferritin; putative FBPs probably conceal ferritin epitopes detected by anti-ferritin antibodies. After complex formation between affinity-purified FBPs and canine liver ferritin, co-immunoprecipitates of the complex by anti-bovine spleen ferritin antibody were found to contain autoantibodies (IgG, IgM, and IgA) to ferritin by immunoblot analysis with antibodies specific for feline IgG, IgM, and IgA. On the other hand, affinity-purified samples did not show any inhibitory effect in the ferritin immunoassay. This result shows that feline serum has another FBP, which inhibits ferritin immunoassays, but not anti-ferritin autoantibody. A feline FBP was partially purified from feline serum by (NH₄)₂SO₄ fractionation (33–50%), gel filtration chromatography, and anion exchange chromatography. After binding of the partially purified sample with canine liver ferritin coupled-Sepharose gel, the FBP was separated and purified from complexes formed in a native-PAGE gel. SDS–PAGE analysis showed that the purified FBP is a homomultimer composed of 31 kDa monomeric subunits connected by intermolecular disulfide bonds. Detection of feline liver ferritin by immunoassay was inhibited by FBP in a dose-dependent manner. The purified protein molecules appeared to be conglomerate of pentraxin-like molecules by its electron micrographic appearance. These results demonstrate that feline serum contains a novel FBP as inhibitory factor of ferritin immunoassay with different molecular properties from those of other mammalian FBPs, in addition to auto-antibodies (IgG, IgM, and IgA) to ferritin.     

Structure-guided discovery of cyclin-dependent kinase inhibitors

CDK2 inhibitors containing the related bicyclic heterocycles pyrazolopyrimidines and imidazopyrazines were discovered through high-throughput screening. Crystal structures of inhibitors with these bicyclic cores and two more related ones show that all but one have a common binding mode featuring two hydrogen bonds (H-bonds) to the backbone of the kinase hinge region. Even though ab initio computations indicated that the imidazopyrazine core would bind more tightly to the hinge, pyrazolopyrimidines gain an advantage in potency through participation of N4 in an H-bond network involving two catalytic residues and bridging water molecules. Further insight into inhibitor/CDK2 interactions was gained from analysis of additional crystal structures. Significant gains in potency were obtained by optimizing the fit of hydrophobic substituents to the gatekeeper region of the ATP binding site. The most potent inhibitors have good selectivity.     

Effect of actinomycin D, cycloheximide, and acute blood loss on ferritin synthesis in rat liver

1. The mechanism of the stimulation of ferritin synthesis by iron in vivo has been studied in rat liver. Ferritin synthesis and turnover was measured by [(14)C]leucine incorporation. 2. Actinomycin D had no inhibitory effect, after administration of iron, on [(14)C]leucine incorporation into ferritin but appeared to augment the effect of iron on ferritin synthesis. 3. Cycloheximide completely abolished the stimulation by iron of [(14)C]leucine into ferritin and was subsequently utilized to show that iron acts in vivo by translational induction of apoferritin synthesis, rather than by stabilization of apoferritin or its precursors. 4. This conclusion was confirmed by showing that 2 days after acute bleeding, when iron was in the process of being removed from hepatic ferritin stores, ferritin synthesis was decreased whereas breakdown rates were unchanged.     

Exploring the microbial metalloproteome using MIRAGE

The microbial metalloproteome has been largely unexplored. Using the metalloproteomics approach MIRAGE (Metal Isotope native RadioAutography in Gel Electrophoresis) we have been able to explore the soluble Fe and Zn metalloproteome of Escherichia coli. The protein identification by MS/MS typically resulted in several overlapping proteins for each metal containing spot. Using the E. coli genome annotation the proteins relevant to the iron and zinc proteome were selected. Superoxide dismutase (SodB) was found to be the major iron protein after cultivation with a normal iron concentration of 6 μM. Upon an elevated iron concentration of 40 μM, ferritin (FtnA) became dominant. Under both conditions 90% of the iron was associated with just three different proteins: superoxide dismutase (SodB), ferritin (FtnA) and bacterioferritin (Bfr). The uncharacterized proteins YgfK and XdhD were found to be significant iron containing proteins under elevated iron conditions. The zinc proteome of E. coli experiencing zinc stress was dominated by ZraP, a putative zinc storage protein.     

Applications of a monoclonal antibody to human ferritin in various immunoassays

Monoclonal antibodies to human liver ferritin were generated by an improved hybridoma technique using a semisolid medium containing methylcellulose for initial cloning after the cell fusion. Out of more than 1000 hybrid clones, only 1 was shown to secrete high-affinity monoclonal antibodies to human liver ferritin. The immunoglobulin subclass of this antibody was determined to be IgG2. The association constant between liver ferritin and this antibody was determined to be greater than 1 X 10(10) M-1. Due to the oligomeric nature of ferritin, this antibody can be simultaneously utilized as the first and second antibody in solid-phase sandwich immunoradiometric and enzyme immunoassays. This immunoassay procedure can be performed within 30-45 min and has a sensitivity of about 1 ng/ml. Under identical assay conditions, ferritin isolated from human spleen and human heart gave 50 and 30% cross-reactivity, respectively.     

Studies on the utilization of ferritin iron in the ferrochelatase reaction of isolated rat liver mitochondria.

The utilization of ferritin as a source of iron for the ferrochelatase reaction has been studied in isolated rat liver mitochondria. 1. It was found that isolated rat liver mitochondria utilized ferritin as a source of iron for the ferrochelatase reaction in the presence of succinate plus FMN (or FAD). 2. Under optimal experimental conditions, i.e., approx. 50 micromol/1 FMN, 37 degrees C, pH 7.4 and 0.5 mmol/l Fe(III) (as ferritin iron), the release process, as shown by the formation of deuteroheme, amounted to approx. 0.5 nmol iron/min per mg protein. 3. The release process could not be elicited by ultrasonically treated mitochondria, lysosomes, microsomes or cytosol, i.e., the release of iron from ferritin was due to mitochondria and was a function of the in situ orientation of the mitochondrial inner membrane. 4. The release of iron from ferritin by the mitochrondria might be of relevance not only for the in situ synthesis of heme in the hepatocyte, but also with respect to the mechanism(s) by means of which iron is mobilized for transport to the erythroid tissue.     

APPEARANCE AND DISTRIBUTION OF FERRITIN IN MOUSE PERITONEAL MACROPHAGES IN VITRO AFTER UPTAKE OF HETEROLOGOUS ERYTHROCYTES

Mouse peritoneal macrophages have been studied in vitro after ingestion of treated rat, rabbit, or sheep erythrocytes. Under light microscopy, phagocytic vacuoles persist up to 24 h. Macrophages lose benzidine reactivity about 5 h after red cell ingestion, and they become prussian blue positive at 2 days. Ultrastructural studies show little or no ferritin in control macrophages not fed erythrocytes. In contrast, after red cell ingestion, ferritin is widely distributed in the cytoplasmic matrix and in some cytoplasmic granules by 48 h. The Golgi complex, pinocytic vacuoles, endoplasmic reticulum, nuclei, and mitochondria do not contain ferritin. Between 2 and 4 days, ferritin in cytoplasmic granules increases, concomitant with decrease in the ferritin in the cytoplasmic matrix. Evidence is presented suggesting that ferritin in the cytoplasmic matrix is translocated into cytoplasmic granules by autophagy. Polyacrylamide gel studies on macrophages after uptake of red blood cells labeled with radioiron confirm that macrophages produce radiolabeled ferritin by 4 days.     

Hydrogen ion interactions of horse spleen ferritin and apoferritin.

The interactions of horse spleen ferritin and its derivative apoferritin with H+ ions were studied by potentiometric and spectrophotometric titration; to aid in data analysis, heats of ionization over a limited pH range and amide content were also determined. Per apoferritin subunit, all tyrosine and cysteine side chains, two of the nine lysine side chains and at least three of the six histidine side chains were found not to titrate; a preliminary but self-consistent analysis of the titration data is proposed. The titration curve of ferritin was identical with that of apoferritin in the pH range 5.5 to 3. In addition, under the conditions used, the reactivities of ferritin histidines to bromoacetate and of ferritin lysines to formaldehyde were identical with those in apoferritin. Above pH 8, a time-dependent titration of the ferritin core occurs which prevents comparison of the titration curves of the two proteins in this region. However, in the pH regions 5.5 to 7.5, two extra groups per subunit titrate reversibly in ferritin relative to apoferritin. Moreover, although the isoionic points of ferritin and apoferritin are identical in water, the isoionic point of ferritin is 0.5 pH unit lower than that of apoferritin in 0.16 to 1 M KCl. The different effects of KCl and NaCl on the two proteins indicate the presence of cation binding sites in ferritin that are absent in apoferritin and possibly also the presence of anion binding sites in apoferritin that are occupied in ferritin by anions of the core. The difference between the isoionic points of the two proteins in KCl has been interpreted to indicate the presence of approximately 2 phosphate residues per ferritin subunit which serve as cation binding sites and which are negatively charged at the isoionic point in KCl. These phosphates may also represent the additional residues that titrate in ferritin between pH 5.5 and 7.5, or may interact with positively charged residues on the inner surface of the ferritin shell, or both.     

Influence of methylamine on anaerobic rumen bacterial growth and plant fiber digestion

Anaerobic rumen bacteria were screened for growth inhibition by methylamine. Under nitrogen-limited growth conditions only Ruminobacter amylophilus, Prevotella ruminicola and Megasphaera elsdenii were significantly inhibited. Ruminobacter amylophilus was inhibited more than any other species. Inhibition of R. amylophilus growth occured at all increments in methylamine for all NH₄Cl concentrations tested (0·2–1·8 m ). Dimethylamine was less inhibitory, while trimethyl-, ethyl- and diethylamine were not inhibitory. When methylamine was added to in vitro mixed culture, ruminal fermentations containing alfalfa, neutral detergent fiber disappearance and ammonia production were not affected. This suggests that although methylamine may inhibit specific rumen bacteria, it does not alter overall mixed ruminal microorganism activities with alfalfa as a substrate.     

Ferritin and ferritin isoforms II: protection against uncontrolled cellular proliferation, oxidative damage and inflammatory processes

Ferritin is a major iron storage protein involved in the regulation of iron availability. Each ferritin molecule comprises 24 subunits. Various combinations of H-subunits and L-subunits make up the 24-subunit protein structure and these ferritin isoforms differ in their H-subunit to L-subunit ratio, as well as in their metabolic properties. Ferritin is an acute-phase protein and its expression is up-regulated in conditions such as uncontrolled cellular proliferation, in any condition marked by excessive production of toxic oxygen radicals, and by infectious and inflammatory processes. Under such conditions ferritin up-regulation is predominantly stimulated by increased reactive oxygen radical production and by cytokines. The major function of ferritin in these conditions is to reduce the bio-availability of iron in order to stem uncontrolled cellular proliferation and excessive production of reactive oxygen radicals. Ferritin is not, however, indiscriminately up-regulated in these conditions as a marked shift towards a predominance in H-subunit rich ferritins occurs. Preliminary indications are that, while the L-subunit primarily fulfils the conventional iron storage role, the H-subunit functions primarily as rapid regulator of iron availability, and perhaps indirectly as regulator of other cellular processes. It is suggested that the optimum differential expression of the two subunits differ for different cells and under different conditions and that the expression of appropriate isoferritins offers protection against uncontrolled cellular proliferation, oxidative stress and against side effects of infectious and inflammatory conditions.     

A Chelator is Required for Microsomal LIPID Peroxidation Following Reductive Ferritin-Iron Mobilisation

In the past, antioxidant and chelator studies have implicated a role for iron-dependent oxidative damage in tissues subjected to ischaemia followed by reperfusion. As ferritin is a major source of iron in non-muscular organs and therefore a potential source of the iron required for oxygen radical chemistry, we have determined conditions under which ferritin iron reduction leads to the formation of a pool of iron which is capable of catalysing lipid peroxidation. Under anaerobic conditions and in the presence of rat liver microsomes, flavin mononucleotide (FMN) catalysed the reduction of ferritin iron as shown by both continuous spectrophotometric measurements of tris ferrozine-Fe(II) complex formation and post-reaction Fe(II) determination. The presence of either ferrozine or citrate was not found to alter the time course or extent of ferritin reduction. In contrast, the addition of air to the reactants after a 20 min period of anaerobic reduction resulted in peroxidation of the microsome suspension (as determined with the 2-thiobarbituric acid test) only in the presence of a chelator such as citrate, ADP or nitrilotriacetic acid. These results support the concept that reduced ferritin iron can mediate oxidative damage during reperfusion of previously ischaemic tissues, provided that chelating agents such as citrate or ADP are present.     

Reliability of the Amplicor internal control to disclose false-negative Chlamydia trachomatis PCR results

Four possibly false-negative samples were detected when 514 male urine specimens were tested in the Amplicor Chlamydia trachomatis assay. In three of the four samples, the inhibition could be reduced by removal of urine supernatant. Under partially inhibitory conditions, after spiking with 50 C. trachomatis elementary bodies/ml specimen, a selective inhibition of the C. trachomatis target amplification and a preferential internal control amplification was observed. We conclude that a positive internal control signal might be misleading in inhibitory specimens with low amount of C. trachomatis.