Ferritin excretion and iron balance in humans

Meconium Proteins Involved in Iron Metabolism

The lack of specific biological materials and biomarkers limits our knowledge of the mechanisms underlying intrauterine regulation of iron supply to the fetus. Determining the meconium content of proteins commonly used in the laboratory to assess the transport, storage, and distribution of iron in the body may elucidate their roles in fetal development. Ferritin, transferrin, haptoglobin, ceruloplasmin, lactoferrin, myeloperoxidase (MPO), neutrophil gelatinase-associated lipocalin (NGAL), and calprotectin were determined by ELISA in meconium samples obtained from 122 neonates. There were strong correlations between the meconium concentrations of haptoglobin, transferrin, and NGAL (p < 0.05). Meconium concentrations of ferritin were several-fold higher than the concentrations of the other proteins, with the exception of calprotectin whose concentration was approximately three-fold higher than that of ferritin. Meconium ceruloplasmin concentration significantly correlated with the concentrations of MPO, NGAL, lactoferrin, and calprotectin. Correlations between the meconium concentrations of haptoglobin, transferrin, and NGAL may reflect their collaborative involvement in the storage and transport of iron in the intrauterine environment in line with their recognized biological properties. High meconium concentrations of ferritin may provide information about the demand for iron and its utilization by the fetus. The associations between ceruloplasmin and neutrophil proteins may indicate the involvement of ceruloplasmin in the regulation of neutrophil activity in the intrauterine environment.

Physiology and Inflammation Driven Pathophysiology of Iron Homeostasis-Mechanistic Insights into Anemia of Inflammation and Its Treatment

Anemia is very common in patients with inflammatory disorders. Its prevalence is associated with severity of the underlying disease, and it negatively affects quality of life and cardio-vascular performance of patients. Anemia of inflammation (AI) is caused by disturbances of iron metabolism resulting in iron retention within macrophages, a reduced erythrocyte half-life, and cytokine mediated inhibition of erythropoietin function and erythroid progenitor cell differentiation. AI is mostly mild to moderate, normochromic and normocytic, and characterized by low circulating iron, but normal and increased levels of the storage protein ferritin and the iron hormone hepcidin. The primary therapeutic approach for AI is treatment of the underlying inflammatory disease which mostly results in normalization of hemoglobin levels over time unless other pathologies such as vitamin deficiencies, true iron deficiency on the basis of bleeding episodes, or renal insufficiency are present. If the underlying disease and/or anemia are not resolved, iron supplementation therapy and/or treatment with erythropoietin stimulating agents may be considered whereas blood transfusions are an emergency treatment for life-threatening anemia. New treatments with hepcidin-modifying strategies and stabilizers of hypoxia inducible factors emerge but their therapeutic efficacy for treatment of AI in ill patients needs to be evaluated in clinical trials.

Gastrointestinal iron excretion and reversal of iron excess in a mouse model of inherited iron excess

The current paradigm in the field of mammalian iron biology states that body iron levels are determined by dietary iron absorption, not by iron excretion. Iron absorption is a highly regulated process influenced by iron levels and other factors. Iron excretion is believed to occur at a basal rate irrespective of iron levels and is associated with processes such as turnover of intestinal epithelium, blood loss, and exfoliation of dead skin. Here we explore iron excretion in a mouse model of iron excess due to inherited transferrin deficiency. Iron excess in this model is attributed to impaired regulation of iron absorption leading to excessive dietary iron uptake. Pharmacological correction of transferrin deficiency not only normalized iron absorption rates and halted progression of iron excess but also reversed body iron excess. Transferrin treatment did not alter the half-life of ⁵⁹Fe in mutant mice. ⁵⁹Fe-based studies indicated that most iron was excreted via the gastrointestinal tract and suggested that iron-loaded mutant mice had increased rates of iron excretion. Direct measurement of urinary iron levels agreed with ⁵⁹Fe-based predictions that urinary iron levels were increased in untreated mutant mice. Fecal ferritin levels were also increased in mutant mice relative to wild-type mice. Overall, these data suggest that mice have a significant capacity for iron excretion. We propose that further investigation into iron excretion is warranted in this and other models of perturbed iron homeostasis, as pharmacological targeting of iron excretion may represent a novel means of treatment for diseases of iron excess.

Vascular Accessibility of Endothelial Targeted Ferritin Nanoparticles

Targeting nanocarriers to the endothelium, using affinity ligands to cell adhesion molecules such as ICAM-1 and PECAM-1, holds promise to improve the pharmacotherapy of many disease conditions. This approach capitalizes on the observation that antibody-targeted carriers of 100 nm and above accumulate in the pulmonary vasculature more effectively than free antibodies. Targeting of prospective nanocarriers in the 10-50 nm range, however, has not been studied. To address this intriguing issue, we conjugated monoclonal antibodies (Ab) to ICAM-1 and PECAM-1 or their single chain antigen-binding fragments (scFv) to ferritin nanoparticles (FNPs, size 12 nm), thereby producing Ab/FNPs and scFv/FNPs. Targeted FNPs retained their typical symmetric core-shell structure with sizes of 20-25 nm and ∼4-5 Ab (or ∼7-9 scFv) per particle. Ab/FNPs and scFv/FNPs, but not control IgG/FNPs, bound specifically to cells expressing target molecules and accumulated in the lungs after intravenous injection, with pulmonary targeting an order of magnitude higher than free Ab. Most intriguing, the targeting of Ab/FNPs to ICAM-1, but not PECAM-1, surpassed that of larger Ab/carriers targeted by the same ligand. These results indicate that (i) FNPs may provide a platform for targeting endothelial adhesion molecules with carriers in the 20 nm size range, which has not been previously reported; and (ii) ICAM-1 and PECAM-1 (known to localize in different domains of endothelial plasmalemma) differ in their accessibility to circulating objects of this size, common for blood components and nanocarriers.

Mathematical modeling of the dynamic storage of iron in ferritin

BACKGROUND

Iron is essential for the maintenance of basic cellular processes. In the regulation of its cellular levels, ferritin acts as the main intracellular iron storage protein. In this work we present a mathematical model for the dynamics of iron storage in ferritin during the process of intestinal iron absorption. A set of differential equations were established considering kinetic expressions for the main reactions and mass balances for ferritin, iron and a discrete population of ferritin species defined by their respective iron content.

RESULTS

Simulation results showing the evolution of ferritin iron content following a pulse of iron were compared with experimental data for ferritin iron distribution obtained with purified ferritin incubated in vitro with different iron levels. Distinctive features observed experimentally were successfully captured by the model, namely the distribution pattern of iron into ferritin protein nanocages with different iron content and the role of ferritin as a controller of the cytosolic labile iron pool (cLIP). Ferritin stabilizes the cLIP for a wide range of total intracellular iron concentrations, but the model predicts an exponential increment of the cLIP at an iron content > 2,500 Fe/ferritin protein cage, when the storage capacity of ferritin is exceeded.

CONCLUSIONS

The results presented support the role of ferritin as an iron buffer in a cellular system. Moreover, the model predicts desirable characteristics for a buffer protein such as effective removal of excess iron, which keeps intracellular cLIP levels approximately constant even when large perturbations are introduced, and a freely available source of iron under iron starvation. In addition, the simulated dynamics of the iron removal process are extremely fast, with ferritin acting as a first defense against dangerous iron fluctuations and providing the time required by the cell to activate slower transcriptional regulation mechanisms and adapt to iron stress conditions. In summary, the model captures the complexity of the iron-ferritin equilibrium, and can be used for further theoretical exploration of the role of ferritin in the regulation of intracellular labile iron levels and, in particular, as a relevant regulator of transepithelial iron transport during the process of intestinal iron absorption.

High-, but not low-bioavailability diets enable substantial control of women's iron absorption in relation to body iron stores, with minimal adaptation within several weeks

BACKGROUND

Adaptation of iron absorption in response to dietary iron bioavailability is less likely in premenopausal women, who generally have lower iron stores, than in men.

OBJECTIVE

The objective of the study was to ascertain whether iron absorption in women adapts to dietary iron bioavailability and whether adaptation reflects altered absorptive efficiency or adjustment to specific inhibitors or enhancers of absorption.

DESIGN

Heme- and nonheme-iron absorption from either high- or low-bioavailability diets was measured at 0 and 10 wk in premenopausal women as they consumed one of the diets for 12 wk (randomized 2 x 2 factorial design). The high- and low-bioavailability diets contained similar amounts of total iron, as 13.1 and 14.8 mg/d nonheme and 2.0 and 0.3 mg/d heme iron, respectively, and they differed in contents of meat, ascorbate, whole grains, legumes, and tea.

RESULTS

In premenopausal women, the efficiency of nonheme-iron absorption (P = 0.06, two-tailed test), but not of heme-iron absorption, tended to adapt in response to a 12-wk difference in dietary iron bioavailability, whether absorption was tested with high- or low-bioavailability menus. Bioavailability, but not adaptation, substantially influenced total iron absorption (approximately 6-fold). In contrast with iron absorption from the low-bioavailability diet, that from the high-bioavailability diet consistently was inversely associated with serum ferritin.

CONCLUSION

Only the high-bioavailability diet enabled women to absorb more iron in relation to their low iron stores. Women consuming the high-bioavailability diet absorbed up to 4.5 mg (30-35%) dietary iron with minimal influence of the diet consumed during the previous 10 wk.

Increased duodenal DMT-1 expression and unchanged HFE mRNA levels in HFE-associated hereditary hemochromatosis and iron deficiency

HFE-associated hereditary hemochromatosis is characterized by imbalances of iron homeostasis and alterations in intestinal iron absorption. The identification of the HFE gene and the apical iron transporter divalent metal transporter-1, DMT-1, provide a direct method to address the mechanisms of iron overload in this disease. The aim of this study was to evaluate the regulation of duodenal HFE and DMT-1 gene expression in HFE-associated hereditary hemochromatosis. Small bowel biopsies and serum iron indices were obtained from a total of 33 patients. The study population comprised 13 patients with hereditary hemochromatosis (C282Y homozygous), 10 patients with iron deficiency anemia, and 10 apparently healthy controls, all of whom were genotyped for the two common mutations in the HFE gene (C282Y and H63D). Total RNA was isolated from tissue and amplified via RT-PCR for HFE, DMT-1, and the internal control GAPDH. DMT-1 protein expression was additionally assessed by immunohistochemistry. Levels of HFE mRNA did not differ significantly between patient groups (P = 0.09), specifically between C282Y homozygotes and iron deficiency anemic patients, when compared to controls (P = 0.09, P = 0.9, respectively). In contrast, DMT-1 mRNA levels were at least twofold greater in patients with hereditary hemochromatosis and iron deficiency anemia when compared to controls (P = 0.02, P = 0.01, respectively). Heightened DMT-1 protein expression correlated with mRNA levels in all patients. Loss of HFE function in hereditary hemochromatosis is not derived from inhibition of its gene expression. DMT-1 expression in C282Y homozygote subjects is consistent with the hypothesis of a "paradoxical" duodenal iron deficiency in hereditary hemochromatosis. The observed twofold upregulation of the DMT-1 is consistent with the slow but steady increase in body iron stores observed in those presenting with clinical features of hereditary hemochromatosis.

Initial uptake and absorption of nonheme iron and absorption of heme iron in humans are unaffected by the addition of calcium as cheese to a meal with high iron bioavailability

BACKGROUND

Quantitative data on the mucosal uptake and serosal transfer of nonheme-iron absorption in humans and the effects of calcium on these components are limited.

OBJECTIVE

Our objective was to measure the initial mucosal uptake and the subsequent serosal transfer of nonheme iron and to determine the effects of adding calcium to a meal on both heme- and nonheme-iron retention.

DESIGN

Whole-gut lavage and whole-body scintillation counting methods were applied to determine the 8-h uptake of nonheme iron and the 2-wk retention (absorption) of heme and nonheme iron in healthy adults (n = 17) after the consumption of meals of radiolabeled food.

RESULTS

The initial uptake and absorption of nonheme iron were 11% and 7%, respectively, and the absorption of heme iron was 15%. Two-thirds of the nonheme iron taken up by the mucosa within 8 h was retained by the body after 2 wk (serosal transfer index: 0.63). Serum ferritin correlated inversely with the initial uptake and absorption of nonheme iron, but not with the nonheme serosal transfer index or the absorption of heme iron. Adding calcium (127 mg in cheese) to the meal did not affect absorption.

CONCLUSIONS

On the basis of its association with serum ferritin, the initial mucosal uptake was the primary control point for nonheme-iron absorption. An apparent reduction in heme-iron absorption associated with the lavage procedure suggested that uptake of heme iron may take longer and proceed further through the intestine than that of nonheme iron. The absorption of both forms of iron was unaffected by the addition of cheese to this meal with high iron bioavailability.

Idiorrhythmic zinc dose-rate induction of intestinal metallothionein in rats depends upon their nutritional zinc status(dagger)

The idiorrhythmic dose-rate feeding experimental model was used to study the induction of intestinal metallothionein (iMT) by zinc (Zn) in the gastrointestinal (GIT) mucosa of young growing male rats relative to their nutritional Zn status. The idiorrhythmic approach requires that the average dietary Zn concentration, referred to as modulo (M), is kept constant across different groups over the whole experimental epoch (E). This is done by adjusting the Zn concentration of the supplemented diet to compensate for the reduction in the number of days on which this diet is fed, the latter being spread evenly over the whole experiment. Idiorrhythms (I) involve offering the diet with n times the overall Zn concentration (M) only every nth day with a Zn-deficient diet offered on other days. We studied three modulos (low-Zn, M3; adequate-Zn, M12; and high-Zn, M48), each M having 8 analogous idiorrhythms (I = Mx/1 to 8Mx/8); every I was fed over a 48-d idiorrhythmic E. Over the wide range of peak doses of dietary Zn (3-384 mg Zn/kg diet), the higher the modulo, the greater the capacity for iMT to be induced (M3 < M12 < M48; P < 0.05). Also, the ability of Zn to induce iMT increased proportionally with the progression of the idiorrhythms from I = Mx/1 to 8Mx/8 (P < 0.001). When rats were fed M3, less Zn was required to induce iMT than when they were fed M12 or M48. Thus, within the M and E limits of this study, the better the nutritional Zn status of the animal, the more Zn is required to induce iMT and vice versa. The fact that iMT was increased means that the amount of available Zn was not proportional with the actual steady state of its metabolism. This indicates that for any Zn supplementation program to be effective, it should progress gradually from a lower to a higher Zn dose relative to the given nutritional Zn status.

Adaptation in iron absorption: iron supplementation reduces nonheme-iron but not heme-iron absorption from food

BACKGROUND

Results of cross-sectional studies suggest that in healthy people, iron absorption adapts to meet physiologic needs and stabilize iron stores, but this has not been adequately tested in longitudinal studies.

OBJECTIVE

We tested whether heme- and nonheme-iron absorption decrease in response to increased iron intake and whether iron stores reach a steady state.

DESIGN

In a randomized, placebo-controlled trial, heme- and nonheme-iron absorption by healthy men and women (n = 57) were measured before and after 12 wk of supplementation with 50 mg Fe/d as ferrous sulfate. Serum and fecal ferritin were measured during supplementation and for 6 mo thereafter.

RESULTS

Initially, both heme- and nonheme-iron absorption were inversely associated with serum ferritin concentration. Volunteers who took iron supplements, even those with serum ferritin <21 microg/L (n = 5), adapted to absorb less nonheme iron (3.2% at week 12 compared with 5.0% at week 0, P: < 0.001) but not less heme iron from a beef-based meal. Serum ferritin concentration was slightly but significantly higher after iron supplementation than after placebo (difference = 13 microg/L). This higher ferritin concentration persisted for >/=6 mo after supplementation, except in subjects with low iron stores, whose serum ferritin returned to baseline within 3 mo. Fecal ferritin excretion increased 2.5-fold (P: < 0.05) during supplementation.

CONCLUSIONS

Healthy individuals, even those with low iron stores, had reduced nonheme-iron absorption from food in response to iron supplementation. Despite this partial adaptation, iron stores were greater after iron supplementation than after placebo and this difference was sustained, except in individuals with the lowest iron stores.

Adaptation of iron absorption in men consuming diets with high or low iron bioavailability

BACKGROUND

Short-term measurements of iron absorption are substantially influenced by dietary bioavailability of iron, yet bioavailability negligibly affects serum ferritin in longer, controlled trials.

OBJECTIVE

Our objective was to test the hypothesis that in men fed diets with high or low iron bioavailability, iron absorption adapts to homeostatically maintain body iron stores.

DESIGN

Heme- and nonheme-iron absorption from whole diets were measured in 31 healthy men at 0 and 10 wk while the men consumed weighed, 2-d repeating diets with either high or low iron bioavailability for 12 wk. The diets with high and low iron bioavailability contained, respectively, 14.4 and 15.3 mg nonheme Fe/d and 1.8 and 0.1 mg heme Fe/d and had different contents of meat, ascorbic acid, whole grains, legumes, and tea.

RESULTS

Adaptation occurred with nonheme- but not with heme-iron absorption. Total iron absorption decreased from 0.96 to 0.69 mg/d (P < 0.05) and increased from 0.12 to 0.17 mg/d (P < 0.05) after 10 wk of the high- and low-bioavailability diets, respectively. This partial adaptation reduced the difference in iron bioavailability between the diets from 8- to 4-fold. Serum ferritin was insensitive to diet but fecal ferritin was substantially lower with the low- than the high-bioavailability diet. Erythrocyte incorporation of absorbed iron was inversely associated with serum ferritin.

CONCLUSIONS

Iron-replete men partially adapted to dietary iron bioavailability and iron absorption from a high-bioavailability diet was reduced to approximately 0.7 mg Fe/d. Short-term measurements of absorption overestimate differences in iron bioavailability between diets.

Long-term weekly iron supplementation improves and sustains nonpregnant women's iron status as well or better than currently recommended short-term daily supplementation

This 7-mo double-blind study compared the efficacy of two iron supplementation schemes in improving iron nutrition among 116 healthy fertile-age women. They were randomly distributed in three groups, receiving: Group 1, iron + folate (60 mg and 250 microg, respectively) daily for 3 mo (currently recommended scheme), and folate (250 microg) weekly the subsequent 4 mo. Group 2, folate daily, and 60 mg iron only once weekly for 3 mo, and then weekly iron + folate for 4 mo. Group 3, folate daily for 3 mo and then weekly for 4 mo. At baseline, 16% had depleted stores (plasma ferritin <15 microg/L) and 16% had hemoglobin levels <125 g/L. Eight percent had hemoglobin levels <120 g/L. In Group 1 hemoglobin and ferritin increased at 3 mo but returned to near basal conditions after 4 mo of weekly folate. In Group 2, hemoglobin and ferritin increased progressively throughout the 7 mo but mostly after 3 mo. Group 3 did not change. Side effects were highest with daily iron. Weekly iron supplementation over 7 mo (30 doses) improved and sustained iron nutrition at least as effectively and was better tolerated than 90 daily iron supplements consumed during 3 mo.

Reciprocal regulation of HFE and NNamp2 gene expression by iron in human intestinal cells

The newly identified hemochromatosis gene, HFE, and a candidate iron transporter gene, Nramp2, have been proposed as key factors responsible for the regulation of intestinal iron absorption. Although the exact functions of these proteins in intestinal iron absorption are unknown, HFE may be required for the down-regulation of iron absorption that occurs with increasing iron status, and Nramp2 may up-regulate iron absorption when iron status is low. Thus, we examined whether the expression of the HFE and Nramp2 genes are regulated by iron status in the human intestinal cell line Caco-2. HFE mRNA and HFE protein were increased and Nramp2 mRNA was decreased by increasing cellular iron status in Caco-2 cells. This iron-mediated modulation of mRNA levels was specific to iron. Moreover, super-induction of HFE mRNA in the presence of cycloheximide suggests that HFE gene expression may be controlled by a short-lived repressor protein. HFE and Nramp2 mRNA levels also changed in opposite directions during cellular differentiation. This reciprocal modification of the HFE and Nramp2 gene expression during both iron treatment and cell differentiation in Caco-2 cells is consistent with an opposing role for these proteins in homeostatic regulation of human intestinal iron absorption.

Cellular expression and regulation of iron transport and storage proteins in genetic haemochromatosis

Genetic haemochromatosis is a common iron overload disorder of unknown aetiology. To characterize the defect of iron metabolism responsible for this disease, this study localized and semi-quantified the mRNA and protein expression of transferrin, transferrin receptor and ferritin in the liver and duodenum of patients with genetic haemochromatosis. Biopsies were obtained from iron-loaded non-cirrhotic patients with genetic haemochromatotic and control patients with normal iron stores. Additional duodenal biopsies were obtained from patients with iron deficiency. Immunohistochemical and in situ hybridization analysis for transferrin, transferrin receptor and ferritin was performed. Hepatic transferrin, transferrin receptor and ferritin protein expression was localized predominantly to hepatocytes and was increased in patients with genetic haemochromatosis when compared with normal controls. Interestingly, hepatic ferritin mRNA expression was not increased in these same patients. In the genetic haemochromatotic duodenum, ferritin mRNA and protein was localized mainly to crypt and villus epithelial cells and the level of expression was decreased compared with normal controls, but similar to iron deficiency. Duodenal transferrin receptor mRNA and protein levels colocalized to epithelial cells of the crypt and villus were similar to normal controls. Early in the course of genetic haemochromatosis and before the onset of hepatic fibrosis, transferrin receptor-mediated iron uptake by hepatocytes contributes to hepatic iron overload. Increased hepatic ferritin expression suggests this is the major iron storage protein. While persisting duodenal transferrin receptor expression may be a normal response to increased body iron stores in patients with genetic haemochromatosis, decreased duodenal ferritin levels suggest that duodenal mucosa is regulated as if the patient were iron deficient.

Iron supplementation for the control of iron deficiency in populations at risk

Iron supplementation, mostly with a therapeutic orientation, has been a key strategy for the short-term control of iron deficiency and ferropenic anemia. It has been used almost exclusively in antenatal clinics, but in spite of its confirmed efficacy in supervised trials, it has proven ineffective in practice in most developing countries. Poor effectiveness has been attributed to various factors including insufficient dose and time of supplementation and poor adherence. These problems have led to the administration of high iron doses, which have proven equally ineffective in practice. This paper introduces four concepts: (1) that iron supplementation targeted to pregnant women should cover the full reproductive cycle, from prepregnancy to at least the end of lactation instead of only the pregnant women; (2) that entering pregnancy with iron deficiency contributes to the failure of antenatal iron supplementation and that prepregnancy iron reserves increase the effectiveness of antenatal supplementation; (3) that medium- to long-term weekly ingestion of proper iron-folate supplements, with a preventive aim and directed to all risk groups, should be community based rather than health service based but supervised by the latter (in this sense, preventive supplementation is equal to targeted iron fortification); and (4) that preventive supplementation, based on weekly dosing, has proven efficacious. Problem-oriented research to evaluate the sustainability and medium- to long-term efficacy of these concepts is called for. The bases for the concepts and suggestions are summarized in this paper.

Histopathological evaluation of liver, pancreas, spleen, and heart from iron-overloaded Sprague-Dawley rats

The effects of increasing dietary levels of Fe on the histopathology of liver, pancreas, spleen, and heart were examined in a rat model for iron overload. Sprague-Dawley rats were fed diets containing 35, 350, 3,500, or 20,000 micrograms Fe/g, and, after 12 wk, there was a direct correlation between increased liver nonheme Fe and lipid peroxidation measured by the lipid-conjugated diene assay. Histopathological examination of tissues revealed the following: (a) hepatocellular hemosiderosis in all groups of rats, with a dose-related accumulation of cytoplasmic Fe-positive material predominantly in hepatocytes located in the periportal region (Zone 1), (b) myocardial degeneration and necrosis (cardiomyopathy) with hemosiderin in interstitial macrophages or in myocardial fibers of animals with heart damage, (c) splenic lymphoid atrophy affecting the marginal zone of the white pulp and hemosiderin deposition in the sinusoidal macrophages, and (d) pancreatic atrophy with loss of both the endocrine and exocrine pancreatic tissue in those animals receiving 3,500 and 20,000 micrograms Fe/g of diet. The toxic effects of Fe overload in this rat model include cellular apoptosis or necrosis in heart, spleen, and pancreas and, when coupled with the findings on lipid peroxidation, suggests that oxidative stress is involved in the pathogenesis of the lesions.

The ferritins: molecular properties, iron storage function and cellular regulation

The iron storage protein, ferritin, plays a key role in iron metabolism. Its ability to sequester the element gives ferritin the dual functions of iron detoxification and iron reserve. The importance of these functions is emphasised by ferritin's ubiquitous distribution among living species. Ferritin's three-dimensional structure is highly conserved. All ferritins have 24 protein subunits arranged in 432 symmetry to give a hollow shell with an 80 A diameter cavity capable of storing up to 4500 Fe(III) atoms as an inorganic complex. Subunits are folded as 4-helix bundles each having a fifth short helix at roughly 60 degrees to the bundle axis. Structural features of ferritins from humans, horse, bullfrog and bacteria are described: all have essentially the same architecture in spite of large variations in primary structure (amino acid sequence identities can be as low as 14%) and the presence in some bacterial ferritins of haem groups. Ferritin molecules isolated from vertebrates are composed of two types of subunit (H and L), whereas those from plants and bacteria contain only H-type chains, where 'H-type' is associated with the presence of centres catalysing the oxidation of two Fe(II) atoms. The similarity between the dinuclear iron centres of ferritin H-chains and those of ribonucleotide reductase and other proteins suggests a possible wider evolutionary linkage. A great deal of research effort is now concentrated on two aspects of ferritin: its functional mechanisms and its regulation. These form the major part of the review. Steps in iron storage within ferritin molecules consist of Fe(II) oxidation, Fe(III) migration and the nucleation and growth of the iron core mineral. H-chains are important for Fe(II) oxidation and L-chains assist in core formation. Iron mobilisation, relevant to ferritin's role as iron reserve, is also discussed. Translational regulation of mammalian ferritin synthesis in response to iron and the apparent links between iron and citrate metabolism through a single molecule with dual function are described. The molecule, when binding a [4Fe-4S] cluster, is a functioning (cytoplasmic) aconitase. When cellular iron is low, loss of the [4Fe-4S] cluster allows the molecule to bind to the 5'-untranslated region (5'-UTR) of the ferritin m-RNA and thus to repress translation. In this form it is known as the iron regulatory protein (IRP) and the stem-loop RNA structure to which it binds is the iron regulatory element (IRE). IREs are found in the 3'-UTR of the transferrin receptor and in the 5'-UTR of erythroid aminolaevulinic acid synthase, enabling tight co-ordination between cellular iron uptake and the synthesis of ferritin and haem. Degradation of ferritin could potentially lead to an increase in toxicity due to uncontrolled release of iron. Degradation within membrane-encapsulated "secondary lysosomes' may avoid this problem and this seems to be the origin of another form of storage iron known as haemosiderin. However, in certain pathological states, massive deposits of "haemosiderin' are found which do not arise directly from ferritin breakdown. Understanding the numerous inter-relationships between the various intracellular iron complexes presents a major challenge.