Ferritin Mineral Research Articles

Distinguishing health-related parameters between metabolically healthy and metabolically unhealthy obesity in women.

Obesity represents a global health crisis, yet a dichotomy is emerging with classification according to the metabolic state into metabolically healthy obesity (MHO) and metabolically unhealthy obesity (MUO). This study aimed to identify distinctive systemic clinical/endocrinological parameters between MHO individuals, employing a comprehensive comparative analysis of 50 biomarkers. Our emphasis was on routine analytes, ensuring cost-effectiveness for widespread use in diagnosing metabolic health. The study included 182 women diagnosed with obesity referred for bariatric surgery at the Endocrinology, Diabetes, and Metabolism Service of São João Hospital and University Centre in Portugal. MUO was defined by the presence of at least one of the following metabolic disorders: diabetes, hypertension, or dyslipidemia. Patients were stratified based on the diagnosis of these pathologies. Significantly divergent health-related parameters were observed between MHO and MUO patients. Notable differences included: albumin (40.1 ± 2.2 vs 40,98 ± 2.6 g/L, p value = 0.017), triglycerides (110.7 ± 51.1 vs 137.57 ± 82.6 mg/dL, p value = 0.008), glucose (99.49 ± 13.0 vs 119.17 ± 38.9 mg/dL, p value < 0.001), glycated hemoglobin (5.58 ± 0.4 vs 6.15 ± 1.0%, p value < 0.001), urea (31.40 ± 10.0 vs 34.61 ± 10.2 mg/dL, p value = 0.014), total calcium (4.64 ± 0.15 vs 4.74 ± 0.17 mEq/L, 1 mEq/L = 1 mg/L, p value < 0.001), ferritin (100.04 ± 129.1 vs 128.55 ± 102.1 ng/mL, p value = 0.005), chloride (104.68 ± 1.5 vs 103.04 ± 2.6 mEq/L, p value < 0.001), prolactin (13.57 ± 6.3 vs 12.47 ± 7.1 ng/mL, p value = 0.041), insulin (20.36 ± 24.4 vs 23.87 ± 19.6 μU/mL, p value = 0.021), c peptide (3.78 ± 1.8 vs 4.28 ± 1.7 ng/mL, p value = 0.003), albumin/creatinine ratio (15.41 ± 31.0 vs 48.12 ± 158.7 mg/g creatinine, p value = 0.015), and whole-body mineral density (1.27 ± 0.1 vs 1.23 ± 0.1 g/cm2, p value = 0.016). Our findings highlight potential additional parameters that should be taken into consideration alongside the commonly used biomarkers for classifying metabolic health in women. These include albumin, urea, total calcium, ferritin, chloride, prolactin, c-peptide, albumin-creatinine ratio, and whole-body mineral density. Moreover, our results also suggest that MHO may represent a transitional phase preceding the development of the MUO phenotype.

Association between serum ferritin and bone mineral density in US adults

BackgroundThe association between serum ferritin and bone mineral density (BMD) is still controversial. This study aims to investigate the association of serum ferritin level with BMD in US adults.MethodsWe conducted a cross-sectional study consisting of 8445 participants from National Health and Nutrition Examination Survey. Serum ferritin and lumbar spine BMD were used as independent variables and dependent variables, respectively. We evaluated the association between serum ferritin and lumbar spine BMD through a weighted multivariable linear regression model. Subgroup and interaction analysis was also performed in this study.ResultsAfter adjusting for other confounding factors, serum ferritin was negatively correlated with lumbar spine BMD [β = − 0.090, 95% CI (− 0.135, − 0.045)]. Further subgroup analysis found that the strongest negative association mainly exists in females aged over 45 years [β = − 0.169, 95% CI (− 0.259, − 0.079)], and this association is not significant in other groups.ConclusionsThe results found that the association between serum ferritin and lumber spine BMD differed by gender and age. Increased level of serum ferritin may indicate a higher risk of osteoporosis or osteopenia in females aged over 45 years.

Key carboxylate residues for iron transit through the prokaryotic ferritin SynFtn.

Ferritins are proteins forming 24meric rhombic dodecahedral cages that play a key role in iron storage and detoxification in all cell types. Their function requires the transport of Fe2+ from the exterior of the protein to buried di-iron catalytic sites, known as ferroxidase centres, where Fe2+ is oxidized to form Fe3+-oxo precursors of the ferritin mineral core. The route of iron transit through animal ferritins is well understood: the Fe2+ substrate enters the protein via channels at the threefold axes and conserved carboxylates on the inner surface of the protein cage have been shown to contribute to transient binding sites that guide Fe2+ to the ferroxidase centres. The routes of iron transit through prokaryotic ferritins are less well studied but for some, at least, there is evidence that channels at the twofold axes are the major route for Fe2+ uptake. SynFtn, isolated from the cyanobacterium Synechococcus CC9311, is an atypical prokaryotic ferritin that was recently shown to take up Fe2+ via its threefold channels. However, the transfer site carboxylate residues conserved in animal ferritins are absent, meaning that the route taken from the site of iron entry into SynFtn to the catalytic centre is yet to be defined. Here, we report the use of a combination of site-directed mutagenesis, absorbance-monitored activity assays and protein crystallography to probe the effect of substitution of two residues potentially involved in this pathway. Both Glu141 and Asp65 play a role in guiding the Fe2+ substrate to the ferroxidase centre. In the absence of Asp65, routes for Fe2+ to, and Fe3+ exit from, the ferroxidase centre are affected resulting in inefficient formation of the mineral core. These observations further define the iron transit route in what may be the first characterized example of a new class of ferritins peculiar to cyanobacteria.

Flavin-mediated reductive iron mobilization from frog M and Mycobacterial ferritins: impact of their size, charge and reactivities with NADH/O2.

In vitro, reductive mobilization of ferritin iron using suitable electron transfer mediators has emerged as a possible mechanism to mimic the iron release process, in vivo. Nature uses flavins as electron relay molecules for important biological oxidation and oxygenation reactions. Therefore, the current work utilizes three flavin analogues: riboflavin (RF), flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), which differ in size and charge but have similar redox potentials, to relay electron from nicotinamide adenine dinucleotide (NADH) to ferritin mineral core. Of these, the smallest/neutral analogue, RF, released more iron (~ threefold) in comparison to the larger and negatively charged FMN and FAD. Although iron mobilization got marred during the initial stages under aerobic conditions, but increased with a greater slope at the later stages of the reaction kinetics, which gets inhibited by superoxide dismutase, consistent with the generation of O2∙- in situ. The initial step, i.e., interaction of flavins with NADH played critical role in the iron release process. Overall, the flavin-mediated reductive iron mobilization from ferritins occurred via two competitive pathways, involving the reduced form of flavins either alone (anaerobic condition) or in combination with O2∙- intermediate (aerobic condition). Moreover, faster iron release was observed for ferritins from Mycobacterium tuberculosis than from bullfrog, indicating the importance of protein nanocage and the advantages they provide to the respective organisms. Therefore, these structure-reactivity studies of flavins with NADH/O2 holds significance in ferritin iron release, bioenergetics, O2-based cellular toxicity and may be potentially exploited in the treatment of methemoglobinemia. Smaller sized/neutral flavin analogue, riboflavin (RF) exhibits faster reactivity towards both NADH and O2 generating more amount of O2∙- and releases higher amount of iron from different ferritins, compared to its larger sized/negatively charged derivatives such as FMN and FAD.

Impact of Phosphate on Iron Mineralization and Mobilization in Nonheme Bacterioferritin B from Mycobacterium tuberculosis.

Ferritins are supramolecular nanocage proteins, which synthesize hydrated ferric oxyhydroxide mineral via protein mediated rapid Fe2+ sequestration and ferroxidase reactions. Ferritin minerals are also associated with a significant amount of phosphate, which contribute toward their structure and reactivity. Like iron, phosphate also regulates the pathogenesis of Mycobacterium tuberculosis (Mtb), which expresses two types of ferritin: heme binding bacterioferritin A (BfrA) and nonheme binding bacterioferritin B (BfrB). Unlike Mtb BfrA, the rapid kinetics and mechanism of ferroxidase activity, and the mineral core formation/dissolution in Mtb BfrB are not well explored. Moreover, the effect of physiological levels of phosphate (0-10 mM) on the synthesis, structure, and reactivity of ferritin mineral core also require investigation in detail. Therefore, the stopped-flow rapid kinetics of ferroxidase activity (ΔA650/Δt) of Mtb BfrB was carried out, which detected a transient intermediate similar to diferric peroxo species as observed in frog and human ferritins. Increasing phosphate concentration increased the initial rate of iron mineralization (ΔA350/Δt) and dissolved O2 consumption (both ∼1.5-2-fold). Phosphate not only decreased the amount of iron loading and size of the BfrB mineral core (both up to 2-fold) but also decreased its crystallinity, resembling the variations observed in the core morphology of different native ferritins. In addition, phosphate inhibited the kinetics of reductive iron mobilization (∼6-8-fold) indicating its influence on the stability of the iron mineral core. Hence, the current work provides the kinetic/mechanistic insight toward the ferroxidase activity in Mtb BfrB, apart from demonstrating the role of phosphate toward the structure/reactivity of its iron mineral.

On the structure and chemistry of iron oxide cores in human heart and human spleen ferritins using graphene liquid cell electron microscopy.

Ferritin is a protein that regulates the iron ions in humans by storing them in the form of iron oxides. Despite extensive efforts to understand the ferritin iron oxide structures, it is still not clear how ferritin proteins with a distinct light (L) and heavy (H) chain subunit ratio impact the biomineralization process. In situ graphene liquid cell-transmission electron microscopy (GLC-TEM) provides an indispensable platform to study the atomic structure of ferritin mineral cores in their native liquid environment. In this study, we report differences in the iron oxide formation in human spleen ferritins (HSFs) and human heart ferritins (HHFs) using in situ GLC-TEM. Scanning transmission electron microscopy (STEM) along with selected area electron diffraction (SAED) of the mineral core and electron energy loss spectroscopy (EELS) analyses enabled the visualization of morphologies, crystal structures and the chemistry of iron oxide cores in HSFs and HHFs. Our study revealed the presence of metastable ferrihydrite (5Fe2O3·9H2O) as a dominant phase in hydrated HSFs and HHFs, while a stable hematite (α-Fe2O3) phase predominated in non-hydrated HSFs and HHFs. In addition, a higher Fe3+/Fe2+ ratio was found in HHFs in comparison with HSFs. This study provides new understanding on iron-oxide phases that exist in hydrated ferritin proteins from different human organs. Such new insights are needed to map ferritin biomineralization pathways and possible correlations with various iron-related disorders in humans.

Tuning Ferritin’s band gap through mixed metal oxide nanoparticle formation

This study uses the formation of a mixed metal oxide inside ferritin to tune the band gap energy of the ferritin mineral. The mixed metal oxide is composed of both Co and Mn, and is formed by reacting aqueous Co2+ with in the presence of apoferritin. Altering the ratio between the two reactants allowed for controlled tuning of the band gap energies. All minerals formed were indirect band gap materials, with indirect band gap energies ranging from 0.52 to 1.30 eV. The direct transitions were also measured, with energy values ranging from 2.71 to 3.11 eV. Tuning the band gap energies of these samples changes the wavelengths absorbed by each mineral, increasing ferritin’s potential in solar-energy harvesting. Additionally, the success of using in ferritin mineral formation opens the possibility for new mixed metal oxide cores inside ferritin.

Ferritin Mineral Core Composition in Health and Disease

Ferritin Mineral Core Composition in Health and Disease

Solving Biology's Iron Chemistry Problem with Ferritin Protein Nanocages.

Ferritins reversibly synthesize iron-oxy(ferrihydrite) biominerals inside large, hollow protein nanocages (10-12 nm, ∼480 000 g/mol); the iron biominerals are metabolic iron concentrates for iron protein biosyntheses. Protein cages of 12- or 24-folded ferritin subunits (4-α-helix polypeptide bundles) self-assemble, experimentally. Ferritin biomineral structures differ among animals and plants or bacteria. The basic ferritin mineral structure is ferrihydrite (Fe2O3·H2O) with either low phosphate in the highly ordered animal ferritin biominerals, Fe/PO4 ∼ 8:1, or Fe/PO4 ∼ 1:1 in the more amorphous ferritin biominerals of plants and bacteria. While different ferritin environments, plant bacterial-like plastid organelles and animal cytoplasm, might explain ferritin biomineral differences, investigation is required. Currently, the physiological significance of plant-specific and animal-specific ferritin iron minerals is unknown. The iron content of ferritin in living tissues ranges from zero in "apoferritin" to as high as ∼4500 iron atoms. Ferritin biomineralization begins with the reaction of Fe(2+) with O2 at ferritin enzyme (Fe(2+)/O oxidoreductase) sites. The product of ferritin enzyme activity, diferric oxy complexes, is also the precursor of ferritin biomineral. Concentrations of Fe(3+) equivalent to 2.0 × 10(-1) M are maintained in ferritin solutions, contrasting with the Fe(3+) Ks ∼ 10(-18) M. Iron ions move into, through, and out of ferritin protein cages in structural subdomains containing conserved amino acids. Cage subdomains include (1) ion channels for Fe(2+) entry/exit, (2) enzyme (oxidoreductase) site for coupling Fe(2+) and O yielding diferric oxy biomineral precursors, and (3) ferric oxy nucleation channels, where diferric oxy products from up to three enzyme sites interact while moving toward the central, biomineral growth cavity (12 nm diameter) where ferric oxy species, now 48-mers, grow in ferric oxy biomineral. High ferritin protein cage symmetry (3-fold and 4-fold axes) and amino acid conservation coincide with function, shown by amino acid substitution effects. 3-Fold symmetry axes control Fe(2+) entry (enzyme catalysis of Fe(2+)/O2 oxidoreduction) and Fe(2+) exit (reductive ferritin mineral dissolution); 3-fold symmetry axes influence Fe(2+)exit from dissolved mineral; bacterial ferritins diverge slightly in Fe/O2 reaction mechanisms and intracage paths of iron-oxy complexes. Biosynthesis rates of ferritin protein change with Fe(2+) and O2 concentrations, dependent on DNA-binding, and heme binding protein, Bach 1. Increased cellular O2 indirectly stabilizes ferritin DNA/Bach 1 interactions. Heme, Fe-protoporphyrin IX, decreases ferritin DNA-Bach 1 binding, causing increased ferritin mRNA biosynthesis (transcription). Direct Fe(2+) binding to ferritin mRNA decreases binding of an inhibitory protein, IRP, causing increased ferritin mRNA translation (protein biosynthesis). Newly synthesized ferritin protein consumes Fe(2+) in biomineral, decreasing Fe(2)(+) and creating a regulatory feedback loop. Ferritin without iron is "apoferritin". Iron removal from ferritin, experimentally, uses biological reductants, for example, NADH + FMN, or chemical reductants, for example, thioglycolic acid, with Fe(2+) chelators; physiological mechanism(s) are murky. Clear, however, is the necessity of ferritin for terrestrial life by conferring oxidant protection (plants, animals, and bacteria), virulence (bacteria), and embryonic survival (mammals). Future studies of ferritin structure/function and Fe(2+)/O2 chemistry will lead to new ferritin uses in medicine, nutrition, and nanochemistry.

Pcbp1 and Ncoa4 Regulate the Flux of Iron through Ferritin in Developing Erythroid Cells

In general human cell models, Poly(rC)-binding proteins (Pcbp) mediate the delivery and integration of iron into the iron storage protein. Conversely, nuclear receptor coactivator 4 (Ncoa4) initiates the liberation of ferritin iron by localizing the protein to autophagosomes. The erythron is the major consumer of body iron. Whether these regulators of ferritin iron are functional in developing red cells remains unclear, and thus was investigated. Both Pcbp1 and Ncoa4 were readily detectable in immunoprecipitates of erythroid ferritin. The peak of Pcbp1-binding to ferritin preceded the stage of maximal Ncoa4-ferritin interaction, which coincided with the induction of β-globin, during erythroid maturation. Both Pcbp1 and Ncoa4 deficiency resulted in lower iron incorporation into heme and hemoglobin by differentiation, without compromised Alas2 gene expression. In Pcbp1-deficent cells, less iron loading into ferritin accompanied the erythropoietic defects. Ncoa4-lacking cells exhibited a massive accumulation of iron in ferritin. No apparent interaction between Pcbp2 and ferritin was detectable in erythroid cells. However, Pcbp2 depletion affected ferritin mineralization in a manner opposite to Pcbp1 deficiency. This was associated with an increase in Pcbp1-ferritin complex formation detected by coimmunoprecipitation. Supplemental iron led to decreases in the amount of both Pcbp1 and Ncoa4 interacting with ferritin. The iron-responsiveness of Ncoa4's ferritin-binding activity was primarily due to post-transcriptional changes in protein abundance. The down-regulation of Ncoa4 protein by high iron required intact lysosome activity, and the expression of an E3 ubiquitin protein ligase, Herc2. Collectively, the present studies provide evidence of the metallochaperone activity of Pcbp1 for ferritin mineralization, and the involvement of Ncoa4 in the mobilization process of stored iron in erythroid cells. Furthermore, the impaired hemoglobinization by insufficiency of these genes identifies ferritin as an essential source of iron for heme production during terminal erythroid differentiation. DisclosuresNo relevant conflicts of interest to declare.

Association between levels of serum ferritin and bone mineral density in Korean premenopausal and postmenopausal women: KNHANES 2008-2010.

BackgroundAs women go through menopause, serum estrogen decreases and ferritin increases. Decreased serum estrogen is well known to cause detrimental effects on bone health; however, data on the associations of serum ferritin with BMD before and after menopause are still lacking. Therefore, this study aimed to investigate the association between serum ferritin levels and BMD in premenopausal and postmenopausal Korean women.MethodsThis study was performed using data from the 2008–2010 Korean National Health and Nutrition Examination Survey, including 7300 women (4229 premenopausal and 3071 postmenopausal). BMD was measured using dual X-ray absorptiometry at the femur and the lumbar spine, and serum ferritin levels were measured by chemiluminescent immunoassay.ResultsMedian serum ferritin levels in postmenopausal women were higher than those in premenopausal women despite the same age ranges. Serum ferritin levels were only significantly correlated with BMD on the lumbar spine (β = −0.189, p-value = 0.005) in premenopausal women after adjusting confounding factors. Additionally, BMD on the lumbar spine had tended to decrease as serum ferritin quartiles increase (P for trend = 0.035) in premenopausal women after adjusting confounding factors. On the other hand, there were no significant associations between serum ferritin levels and BMD on the total femur and, femur neck in premenopausal women, and BMD on the total femur, femur neck, and lumbar spine in postmenopausal women.ConclusionIncreased serum ferritin levels were significantly associated with BMD in premenopausal women, particularly on the lumbar spine, but not in postmenopausal women.

Ion and plasma based treatments for enhanced chemical speciation of metals in ferritin

Ferritin is the main protein devoted to iron storage in living systems. Several evidences show that ferritin can bind in vivo metals other than iron, such as aluminium. Moreover, recent studies demonstrated that ferritin iron core composition differs between physiological and pathological brain ferritins. The precise evaluation of chemical species present within the ferritin mineral core and their chemical coordination is therefore of paramount importance. X-ray photoelectron spectroscopy and energy dispersive X-ray spectroscopy are powerful tools to perform this chemical analysis: however the protein shell surrounding the mineral core reduces significantly the amount of electrons/photons detected, thus reducing the potentiality of these techniques.In this framework we investigated the use of physical treatments, namely focused ion beam milling and oxygen plasma etching, in order to overcome the limitations imposed by the presence of the protein shell. We demonstrated that the combined use of focused ion beam milling and energy dispersive X-ray spectroscopy provides an enhancement of iron detection by a factor of about two. As for the X-ray photoelectron spectroscopy technique, it has an intrinsic low capability of probing the core metal in ferritin due to the electron escape depth value that is of the order of the protein shell thickness. Using a pre-treatment in oxygen plasma the detection of iron was enhanced by a factor of more than 15. Therefore, the disruption of the protein shell by plasma etching or ion milling emerges as a fundamental prerequisite to perform an accurate chemical characterization of the ferritin core.

Correlation between femoral head bone iron content, serum ferritin and bone mineral density of postmenopausal women with femoral neck fragility fracture

Objective To explore the correlation between femoral head bone iron content,serum ferritin (SF) and bone mineral density of postmenopausal women with femoral neck fragility fracture.Methods From June 2010 to March 2013,156 postmenopausal women with femoral neck fragility fractures aged from 56 to 92 years old,with an average age of 72.40±8.97 years,were divided into 5 groups based upon the stratification of 10-year age intervals (≤60,61-70,71-80,81-90 and ≥91).Blood samples were collected to determine iron metabolism and bone metabolic markers after overnight fasting.Bone specimens were obtained for detecting bone iron contents and performing iron stain respectively after hip arthroplasty surgery.BMD was measured by dual energy X-ray absorptiometry(DXA)at hip and lumbar spine from L1 to L4 ten days after surgery.Results There were significant differences in the values of bone iron content,SF,transferrin,total iron binding capacity,serum procollagen type Ⅰ N-terminal propeptide,β-carboxy terminal telopeptide of collagen type Ⅰ and BMD at hip and L1 to L4 in all groups.BMD at hip and L1 to L4 decreased while bone iron content and SF increased with age; the peaks of bone iron content and SF were both in 81-90 years group; the average of bone iron content and SF was 96.81 μg/g and 235.66 μg/L respectively.There were 100 patients (100/156,64.1％) whose SF was over 200 μg/L.Bone iron content combined with SF,age and BMI could be put into the regression models of BMD at various proximal femur sites (femoral neck R2=0.443,Wards triangle R2=0.397,greater trochanter R2=0.322 and total femur R2=0.379).After adjustment of age,weight,BMI and other factors,we found that bone iron content and SF were negatively correlated with BMD at proximal femur,but had no significant correlation with BMD at lumbar spine.Conclusion Iron accumulation is prevalent in postmenopausal women with hip fragility fractures.High bone iron content and SF are independent risk factors for low BMD at proximal femur.These results provide clinical evidences that iron accumulation is associated with postmenopausal osteoporosis. Key words: Osteoporosis, postmenopausal; Femur head; Ferritins; Bone density

Micronutrient deficiencies one year after sleeve gastrectomy

Obesity is one of the largest problems in public health worldwide today. For patients with severe obesity and associated comorbidities, surgical treatment is recommended. To analyze the frequency of micronutrient deficiencies and food intake at least 12 months after sleeve gastrectomy (SG). This is a cross sectional study carried out between October and December 2009 with measure of serum levels of vitamin B12, vitamin D, folate, calcium, ferritin, zinc, parathyroid hormone and bone mineral density on patients underwent SG at least 12 months before the study. A food intake questionnaire was also performed. For the statistical analysis, the Pearson or Spearman tests was used. Forty patients were evaluated with a mean age of 40 ± 10 years and mean time post surgery of 26 ± 6 months. Mean plasma level of 25 OH-vitamin D was 20.9 ± 10.5 ng/ml. Forty-three percent had low levels of vitamin D (< 20 ng/ml), and 68% showed low levels of calcium (< 1,1 mmol/L) without secondary hyperparathyroidism. Anemia was present in 28% and iron deficiency occurred in 38% of these patients. Deficit of folic acid and vitamin B12 were observed in 13% of the patients. Average daily food intake was 1,256 kcal, 54% of total calories as carbohydrates, 26% as fat and 21% as protein. Iron, calcium and vitamin D are the most prevalent micronutrient deficiencies after SG. Long-term vitamin and mineral supplementation should be considered on every patient. Additional long-term studies are needed to establish specific supplementation recommendations after SG.

The association between higher serum ferritin level and lower bone mineral density is prominent in women ≥45 years of age (KNHANES 2008–2010)

Data gathered from a nationally representative cohort demonstrate that higher serum ferritin levels are significantly associated with lower bone mass at various skeletal sites and the increased prevalence of osteoporosis and fractures, especially in women ≥45 years of age. Despite extensive in vitro and in vivo studies showing the detrimental effects of iron on bone metabolism, the clinical studies relating to osteoporosis-related phenotypes have not been evaluated extensively. In the present study, we investigated and compared the association between serum ferritin and bone mineral density (BMD), depending on the stratified age groups in both genders. This is a population-based, cross-sectional study from the Korea National Health and Nutrition Examination Surveys, including 14,017 Koreans (6,817 men and 7,200 women) aged 10-80 years. BMD was measured using dual X-ray absorptiometry, and osteoporosis was diagnosed by the World Health Organization definition. Initially, we divided the subjects into three age groups, based on the patterns of age-related BMD changes in this national cohort (i.e., ≤24, 25-44, and ≥45 years old). Serum ferritin concentrations were inversely associated with BMD values at all measured sites after adjustment for confounders, only in women ≥45 years of age (P = 0.041 to <0.001). Furthermore, when we divided these women into serum ferritin quartiles, the odds for prevalent osteoporosis and fractures were 1.55-fold (95 % CI = 1.09-2.23) and 1.52-fold (95 % CI = 1.02-2.27) higher, respectively, in subjects in the highest quartile compared with those in the lowest quartile. These results provide the first clinical evidence that the associations between serum ferritin level and bone parameters could be the most prominent in women ≥45 years of age.

Ferritin iron mineralization proceeds by different mechanisms in MOPS and imidazole buffers

The buffer used during horse spleen ferritin iron loading significantly influences the mineralization process and the quantity of iron deposited in ferritin. Ferritin iron loading in imidazole shows a rapid hyperbolic curve in contrast to iron loading in 3-(N-morpholino)propanesulfonic acid (MOPS), which displays a slower sigmoidal curve. Ferritin iron loading in an equimolar mixture of imidazole and MOPS produces an iron-loading curve that is intermediate between the imidazole and MOPS curves indicating that one buffer does not dominate the reaction mechanism. The UV–visible spectrum of the ferritin mineral has a higher absorbance from 250 to 450 nm when prepared in imidazole buffer than in MOPS buffer. These results suggest that different mineral phases form in ferritin by different loading mechanisms in imidazole and MOPS buffered reactions. Samples of 1500 Fe/ferritin were prepared in MOPS or imidazole buffer and were analyzed for crystallinity and using the electron diffraction capabilities of the electron microscope. The sample prepared in imidazole was significantly more crystalline than the sample prepared in MOPS. X-ray powder diffraction studies showed that small cores (~ 500 Fe/ferritin) prepared in MOPS or imidazole possess a 2-line ferrihydrite spectrum. As the core size increases the mineral phase begins to change from 2-line to 6-line ferrihydrite with the imidazole sample favoring the 6-line ferrihydrite phase. Taken together, these results suggest that the iron deposition mechanism in ferritin can be controlled by properties of the buffer with samples prepared in imidazole forming a larger, more ordered crystalline mineral than samples prepared in MOPS.

A New Role for Heme, Facilitating Release of Iron from the Bacterioferritin Iron Biomineral

Bacterioferritin (BFR) from Escherichia coli is a member of the ferritin family of iron storage proteins and has the capacity to store very large amounts of iron as an Fe(3+) mineral inside its central cavity. The ability of organisms to tap into their cellular stores in times of iron deprivation requires that iron must be released from ferritin mineral stores. Currently, relatively little is known about the mechanisms by which this occurs, particularly in prokaryotic ferritins. Here we show that the bis-Met-coordinated heme groups of E. coli BFR, which are not found in other members of the ferritin family, play an important role in iron release from the BFR iron biomineral: kinetic iron release experiments revealed that the transfer of electrons into the internal cavity is the rate-limiting step of the release reaction and that the rate and extent of iron release were significantly increased in the presence of heme. Despite previous reports that a high affinity Fe(2+) chelator is required for iron release, we show that a large proportion of BFR core iron is released in the absence of such a chelator and further that chelators are not passive participants in iron release reactions. Finally, we show that the catalytic ferroxidase center, which is central to the mechanism of mineralization, is not involved in iron release; thus, core mineralization and release processes utilize distinct pathways.

3D morphology of the human hepatic ferritin mineral core: New evidence for a subunit structure revealed by single particle analysis of HAADF-STEM images

Ferritin, the major iron storage protein, has dual functions; it sequesters redox activity of intracellular iron and facilitates iron turn-over. Here we present high angle annular dark field (HAADF) images from individual hepatic ferritin cores within tissue sections, these images were obtained using spherical aberration corrected scanning transmission electron microscopy (STEM) under controlled electron fluence. HAADF images of the cores suggest a cubic morphology and a polycrystalline (ferrihydrite) subunit structure that is not evident in equivalent bright field images. By calibrating contrast levels in the HAADF images using quantitative electron energy loss spectroscopy, we have estimated the absolute iron content in any one core, and produced a three dimensional reconstruction of the average core morphology. The core is composed of up to eight subunits, consistent with the eight channels in the protein shell that deliver iron to the central cavity. We find no evidence of a crystallographic orientation relationship between core subunits. Our results confirm that the ferritin protein shell acts as a template for core morphology and within the core, small (∼2 nm), surface-disordered ferrihydrite subunits connect to leave a low density centre and a high surface area that would allow rapid turn-over of iron in biological systems.

Ferritin Contains Less Iron (59Fe) in Cells When the Protein Pores Are Unfolded by Mutation

Ferric minerals in ferritins are protected from cytoplasmic reductants and Fe2+ release by the protein nanocage until iron need is signaled. Deletion of ferritin genes is lethal; two critical ferritin functions are concentrating iron and oxidant protection (consuming cytoplasmic iron and oxygen in the mineral). In solution, opening/closing (gating) of eight ferritin protein pores controls reactions between external reductant and the ferritin mineral; pore gating is altered by mutation, low heat, and physiological urea (1 mm) and monitored by CD spectroscopy, protein crystallography, and Fe2+ release rates. To study the effects of a ferritin pore gating mutation in living cells, we cloned/expressed human ferritin H and H L138P, homologous to the frog open pore model that was unexpressable in human cells. Human ferritin H L138P behaved like the open pore ferritin model in vitro as follows: (i) normal protein cage assembly and mineralization, (ii) increased iron release (t1/2) decreased 17-fold), and (iii) decreased alpha-helix (8%). Overexpression (> 4-fold), in HeLa cells, showed for ferritin H L138P equal protein expression and total cell 59Fe but increased chelatable iron, 16%, p < 0.01 (59Fe in the deferoxamine-containing medium), and decreased 59Fe in ferritin, 28%, p < 0.01, compared with wild type. The coincidence of decreased 59Fe in open pore ferritin with increased chelatable 59Fe in cells expressing the ferritin open pore mutation suggests that ferritin pore gating influences to the amount of iron (59Fe) in ferritin in vivo.

Structural characterisation of protein-caged iron minerals in biological systems

In this paper we have collected multiple high angle annular dark field (HAADF) images of purified horse spleen ferritin mineral cores with an aberration corrected STEM (SuperSTEM). The images of individual cores have been separated into groups to determine characteristic views, and averaged to reduce the noise. We have found that these isolated cores do not have the same clear substructure as ferritin cores in plastic-embedded human liver sections (from a patient with haemochromatosis). There is a clear discrepancy between the in situ human liver and purified horse spleen ferritin, but also between Massover's observation of regular substructure in purified horse spleen ferritin by cryo-electron microscopy [1] and these results. A possible basis for this discrepancy is the difference in purification methods used between the two horse spleen ferritin preparations, with the one here purified using CdSO4 precipitation, and the one used by Massover purified without the use of CdSO4.