Iron incorporation into ferritins: evidence for the transfer of monomeric Fe(III) between ferritin molecules and for the formation of an unusual mineral in the ferritin of Escherichia coli

Iron that has been oxidized by H-chain ferritin can be transferred into other ferritin molecules before it is incorporated into mature ferrihydrite iron cores. Iron(III) dimers are formed at the ferroxidase centres of ferritin H chains at an early stage of Fe(II) oxidation. Mössbauer spectroscopic data now show that the iron is transferred as monomeric species arising from dimer dissociation and that it binds to the iron core of the acceptor ferritin. Human H-chain ferritin variants containing altered threefold channels can act as acceptors, as can the ferritin of Escherichia coli (Ec-FTN). A human H-chain ferritin variant with a substituted tyrosine (rHuHF-Y34F) can act as a donor of Fe(III). Since an Fe(III)-tyrosinate (first identified in bullfrog H-chain ferritin) is absent from variant rHuHF-Y34F, the Fe(III) transferred is not derived from this tyrosinate complex. Mössbauer parameters of the small iron cores formed within Ec-FTN are significantly different from those of mammalian ferritins. Analysis of the spectra suggests that they are derived from both ferrihydrite and non-ferrihydrite components. This provides further evidence that the ferritin protein shell can influence the structure of its iron core.

Non-steroidal anti-inflammatory drugs: how do they damage the gut?

NSAIDs are widely prescribed for the treatment of musculoskeletal disorders. The gastrointestinal tract, predominantly the stomach, bears the brunt of their side-effects. The basis of this toxicity is certainly multifactorial, with a wide range of local effects and mucosal defences being implicated. This review will highlight: (1) the epidemiology of NSAID-induced gastrointestinal toxicity; (2) their effects on prostaglandins, and the phenomenon of cytoprotection; (3) effects on neutrophil function; (4) effects on mucosal blood flow; (5) responses of the mucosa to damage (restitution, adaptation, and regenerative repair); (6) the relevance of growth factors; (7) interactions with Helicobacter pylori in ulcerogenesis, and finally (8) the effects of NSAIDs on the small intestine and colon.

The role of the L-chain in ferritin iron incorporation. Studies of homo and heteropolymers

Mammalian ferritins are 24-meric proteins composed of variable proportions of H and L-subunits. The L-chain, in contrast to the H-chain, lacks detectable ferroxidase activity, and its role in ferritin iron incorporation is unclear. In this study, apoferritins were subjected to iron loading with large iron increments to favour spontaneous iron hydrolysis. The homopolymers of the wild-type H-chain, and of a mutant H-chain with an inactivated ferroxidase centre, formed massive protein aggregates, while the L-chain homopolymers remained mostly soluble. The difference between H and L-ferritins was not related to the rate of iron oxidation or to the presence of preformed iron cores. Heteropolymers were constructed in vitro by co-renaturing different proportions of the H-chain with the L-chain or mutant H-chain with an inactivated ferroxidase centre. After loading with high iron increments, protein aggregation of the heteropolymers was reduced when the L-chain content was above 70 to 80%, either in combination with the wild-type H-chain or with the inactivated mutant H-chain. Under acidic conditions (pH 5.5, 1000 Fe atoms per molecule) the heteropolymers with about 20% H and 80% L-chains incorporated three to fourfold more iron into soluble 24-mers than the homopolymers. The data indicate that ferritins with more than 18 L-chains per molecule have the capacity to lower non-specific iron hydrolysis in bulk solution. This property is possibly due to a specific attraction of the incoming oxidized iron into the cavity and may be related to an effect of the L-chain on the cavity microenvironment. It is concluded that under high iron increments the ferritins with high L:H-chain ratios are the most efficient in incorporating iron, and this goes some way to explain why iron storage tissues contain L-rich isoferritins.

Human ferritin H-chains can be obtained in non-assembled stable forms which have ferroxidase activity

We found conditions to obtain the Leu-169-->Arg mutant of human ferritin H chain in a stable and non-assembled state. The protein obtained is an oligomer of subunits with a high degree of structured conformation, and when concentrated it re-assembles into ferritin cages. Functional studies showed that (i) it promotes iron oxidation like the assembled ferritin, but at slower rate, (ii) it is readily precipitated by the oxidised iron unless apotransferrin or L-chain ferritin are added to sequester Fe(III). The results confirm that ferroxidase activity is located within the H-chain, and indicate that the cages of the fully assembled ferritins are important not only in maintaining iron in a soluble form, but also in eliciting the activity of the ferroxidase centres.

Cloning, expression, and immunological characterization of recombinant Lolium perenne allergen Lol p II

The molecular cloning of the cDNA encoding for an isoallergenic form of Lol p II, a major rye grass (Lolium perenne) pollen allergen, was performed by polymerase chain reaction amplification on mRNA extracted from pollen. The amino acid sequence derived from the cDNA was truncated by 4 and 5 residues at the NH2- and COOH-terminal ends, respectively, and differed only in one position from that previously reported. This cDNA was expressed in Escherichia coli by fusion to the carboxyl terminus of the human ferritin H-chain. The molecule was produced in high yields as a soluble protein and was easily purified. The protein retains the multimeric quaternary structure of ferritin, and it exposes on the surface the allergenic moiety, which can be recognized in Western blotting and in enzyme-linked immunosorbent assay experiments by specific IgE from allergic patients. The recombinant allergen was used to analyze the sera of 26 patients allergic to L. perenne compared with control sera. The results were in good agreement with the values obtained with the radioallergosorbent test assay. In addition, histamine release experiments in whole blood from an allergic patient and skin prick tests showed that the recombinant allergen retains some of the biological properties of the natural compound. These findings indicate that the availability of homogeneous recombinant allergens may be useful for the development of more specific diagnostic and therapeutic procedures. Moreover, this expression system may be of more general interest for producing large amounts of soluble protein domains in E. coli.

Ferroxidase kinetics of human liver apoferritin, recombinant H-chain apoferritin, and site-directed mutants

A detailed study of the kinetics of iron(II) oxidation by molecular oxygen in natural and recombinant human apoferritins has been carried out using electrode oximetry to better understand the ferroxidase activity of the protein shell. A comparative study of recombinant L-chain ferritin (rLF), recombinant H-chain ferritin (rHF), and variants has shown that (1) rLF lacks a ferroxidase activity, confirming the results of previous studies; (2) the ferroxidase site of rHF involves Glu-62 and His-65, presumably as Fe2+ ligands, since mutation of these residues abolishes most of the oxidase activity, in agreement with previous studies; and (3) mutation of both the putative ferroxidase and nucleation site ligands in rHF renders the protein totally incapable of catalyzing the oxidation of Fe2+ whereas mutation of nucleation site ligands alone (Glu-61, Glu-64, and Glu-67) decreases the activity only slightly. Analysis of the kinetics of rHF and natural human liver ferritin (HLF) (4% H-chain, 96% L-chain) gave the following apparent parameters at pH 7: Km,O2 = 6 +/- 2 microM, Km,Fe = 80 +/- 10 microM, and kcat = 201 +/- 14 min-1 for rHF and Km,O2 = 60 +/- 12 microM, Km,Fe = 50 +/- 10 microM, and kcat = 31.2 +/- 0.6 min-1 for HLF. Furthermore, Zn2+ was shown to be a noncompetitive inhibitor of Fe2+ oxidation in rHF but a mixed inhibitor in HLF. These different forms of Zn2+ inhibition in the two proteins and the higher activity of HLF than expected, based on its H-chain composition as well as differences in their enzyme kinetic parameters, suggest that H- and L-chains cooperate in modulating the ferroxidase activity of the apoferritin even though the L-subunit lacks a ferroxidase site itself.

Production and characterization of recombinant heteropolymers of human ferritin H and L chains

Vertebrate ferritins are iron storage proteins composed by 24 subunits of one or more types. The recombinant homopolymers of human ferritin H- and L-type chains differ in iron uptake and in physical stability, but the properties of heteropolymers with various proportions of H- and L-type chains cannot be predicted. Present study shows that unfolded human ferritin H- and L- type chains renature under similar conditions to form homopolymers indistinguishable from the native ones and that, when mixed, the unfolded H and L chains renature to form heteropolymers with restricted heterogeneity and with the expected H:L ratios. Seven of these ferritins with different H:L ratios were analyzed; electrophoretic mobility, immunological reactivity, and stability to guanidine denaturation varied as predicted, based on the homopolymers. In contrast, the rate of iron uptake, monitored by the variation of absorbance at 310 nm, increased in the ferritins that ranged in H chain content from 0 to 35%; further increments in H chains had no additional effect. This finding indicates that, under the present conditions, only a limited number of H chains are needed for the maximum rate of ferritin iron uptake. Variations of L- and H-type chains in vivo may thus have biological relevance.

Chronic liver disease due to hepatitis C

The low concentration of hepatitis C virus in the blood of infected patients has made it difficult to detect. Infected patients can now be identified by using more sensitive immunoassays and amplification of viral RNA by the polymerase chain reaction. Nevertheless, the virus remains difficult to eliminate. We present the case of a woman with a history of autoimmune haemolytic anaemia, thrombocytopenia, and common variable immunodeficiency who developed chronic hepatitis.

Evidence of H- and L-chains have co-operative roles in the iron-uptake mechanism of human ferritin

The ability to incorporate iron in vitro was studied in homopolymers of human ferritin L-chain, human ferritin H-chain and its variants and in homopolymer mixtures. The H-chain variants carried amino acid substitutions in the ferroxidase centre and/or in carboxy residues on the cavity surface. Iron incorporation was examined by gel electrophoresis of the reaction products by staining for iron and protein. It was found that inactivation of the ferroxidase centre combined with the substitution of four carboxy groups on the cavity abolished the ability of H-chain ferritin to incorporate iron. Competition experiments with limited amounts of iron showed that, at neutral pH, L-chain ferritin is more efficient in forming iron cores than the H-chain variants altered at the ferroxidase activity or in the cavity. Competition experiments at pH 5.5 demonstrated that L-chain apoferritin is able to incorporate iron only when in the presence of H-chain variants with ferroxidase activity. The results indicate that L-chain apoferritin has a higher capacity than the H-chain apoferritin to induce iron-core nucleation, whereas H-chain ferritin is superior in promoting Fe(II) oxidation. The finding of cooperative roles of the H- and L-chains in ferritin iron uptake provides a clue to understanding the biological function of isoferritins.

Urinary excretion of glycosaminoglycans in patients with postmenopausal osteoporosis

The organic bone matrix contains glycosaminoglycans (GAG) of which the precise function and importance in bone mineralisation are still unclear. We examined 85 persons--35 healthy women (25 premenopausal [preMP] mean aged 40.7 years; 10 menopausal [MP] mean aged 59.3 years) and 50 patients with postmenopausal osteoporosis [PMOP] at a mean age 60.4 years. The dynamic of urinary excretion of GAG was measured in 24-hour collected urine by precipitation with cetylpyridinum chloride and spectrophotometry at 560 nm, corrected for the level of excretion of creatinine. There was a significant increase in GAG excretion in patients with PMOP compared with healthy persons (8.25 mg/g and 9.53 mg/g vs 24.11 mg/g; p < 0.0001). A significant positive correlation was established between GAG and calcium urinary excretion and a negative one between GAG and serum estradiol levels. During the treatment with calcitonin the excretion of GAG was decreased which can be used for monitoring the changes of bone metabolism.

Structure, function, and evolution of ferritins

The ferritins of animals and plants and the bacterioferritins (BFRs) have a common iron-storage function in spite of differences in cytological location and biosynthetic regulation. The plant ferritins and BFRs are more similar to the H chains of mammals than to mammalian L chains, with respect to primary structure and conservation of ferroxidase center residues. Hence they probably arose from a common H-type ancestor. The recent discovery in E. coli of a second type of iron-storage protein (FTN) resembling ferritin H chains raises the question of what the relative roles of these two proteins are in this organism. Mammalian L ferritins lack ferroxidase centers and form a distinct group. Comparison of the three-dimensional structures of mammalian and invertebrate ferritins, as well as computer modeling of plant ferritins and of BFR, indicate a well conserved molecular framework. The characterisation of numerous ferritin homopolymer variants has allowed the identification of some of the residues involved in iron uptake and an investigation of some of the functional differences between mammalian H and L chains.

Functional roles of the ferritin receptors of human liver, hepatoma, lymphoid and erythroid cells

Ferritin receptors are present on the membranes of many normal and malignant cells. The binding specificity of these receptors for H and L subunits was examined using recombinant human ferritin homopolymers. At least two different types of ferritin receptors were found, one derived from normal rat, pig, and human liver which shows similar binding of H- and L-ferritin. The second receptor type, specific for the H-chain ferritin, has been identified on membranes of hepatic and other transformed cells, and of normal lymphoblasts and erythroid precursors. These two receptor types may have different metabolic functions: the hepatic receptor acting as a scavenger for circulating ferritin and possibly for iron exchange between hepatocytes and macrophages; the H-ferritin receptor having a regulatory role which is not directly related to iron metabolism. The expression of the H-ferritin receptor is closely related to the activation and proliferation state of the cells. Addition of H-ferritin to the culture medium of cells expressing the H-ferritin receptor resulted in inhibition of cell proliferation and of colony formation.

Evidence that a salt bridge in the light chain contributes to the physical stability difference between heavy and light human ferritins

Human ferritin, a multimeric iron storage protein, is composed by various proportions of two subunit types: the H- and L-chains. The biological functions of these two genic products have not been clarified, although differences in reactivity with iron have been shown. Starting from the hypothesis that the high stability typical of ferritin is an important property which may be relevant for its iron storage function, we studied ferritin homopolymers of H- and L-chains in different denaturing conditions. In addition we analyzed 13 H-chain variants with alterations in regions conserved within mammalian H-chains. In all the denaturation experiments H-chain ferritin showed lower stability than L-chain ferritin. The difference was greater in guanidine HCl denaturation experiments, where the end products are fully unfolded peptides, than in acidic denaturation experiments, where the end products are peptides with properties analogous to "molten globule." The study on H-chain variants showed: (i) ferritin stability was not affected by alterations of regions exposed to the inner or outer surface of the shell and not involved in intra- or inter-chain interactions; (ii) stability was reduced by alterations of sequences involved in inter-subunit interactions such as the deletion of the N-terminal extension or substitutions along the hydrophobic and hydrophilic channels; (iii) stability was increased by the substitution of 2 amino acids inside the four-helix bundle with those of the homologous L-chain. One of the residues is involved in a salt bridge in the L-chain, and we concluded that the stability difference between H- and L-ferritins is to a large extent due to the stabilizing effect of this salt bridge on the L-subunit fold.

Steroid ulcers

Images

Antibodies for denatured human H-ferritin stain only reticuloendothelial cells within the bone marrow

Human H-ferritin homopolymer was denatured in sodium dodecyl sulphate and injected in mice to obtain antibodies for dissociated H-subunit. The antisera and Moabs obtained were specific for the denatured H-chain with no cross-reactivity with assembled ferritins in immunoblotting experiments. In contrast the Moabs for native recombinant H-ferritin are specific for the assembled ferritin molecules with weak cross-reactivity with the denatured H-subunits. The epitope recognized by one of the anti-denatured H-chain Moabs was mapped on the C-terminal helix of ferritin. The antibodies were used to study H-ferritin conformation in cells. In immunocytochemistry experiments the antibodies for denatured H-ferritin stained HeLa and K562 cells weakly, with a different intensity and pattern to those obtained with anti-native H-ferritin antibody. In human bone marrow smears the anti-denatured ferritin antibodies stained only reticuloendothelial cells, and did not recognize the H-ferritin rich immature erythroblasts. It is concluded that assembled and denatured H-ferritins are immunogenically distinct, and that erythroid and reticuloendothelial cells within the bone marrow contain H-ferritin in different conformations.

Suppression of Helicobacter pylori reduces gastrin releasing peptide stimulated gastrin release in duodenal ulcer patients

Helicobacter pylori increases gastrin release in duodenal ulcer patients. This may be through disruption or changes in the mucus layer affecting the access of luminal stimulants to gastrin releasing cells. The effect of suppressing H pylori on gastrin release stimulated by a non-luminal stimulus, gastrin releasing peptide (GRP), was examined. Eleven patients with active duodenal ulcer disease and colonised with H pylori received an intravenous infusion of GRP (2.9 pmol/kg/minute for 30 minutes) and the plasma gastrin response was measured. Basal and peak pentagastrin stimulated acid output were also determined. Patients were treated with tripotassium dicitratobismuthate (De-Nol) and metronidazole to suppress H pylori and the tests were repeated. Suppression of H pylori decreased plasma gastrin concentrations during GRP infusion, but acid output was not affected. Chromatographic analysis of the forms of gastrin in plasma showed a significant fall in gastrin 17, the predominant form found in the gastric antrum. Gastrin 34 did not fall significantly. This study shows that suppression of H pylori decreases the hypergastrinaemia caused by the nonluminal stimulant, GRP, mainly via decreasing gastrin 17.

Effects of nonsteroidal anti-inflammatory drugs and misoprostol on gastroduodenal epithelial proliferation in arthritis

In this study, quantitation of the epithelial proliferation status of gastric glands and duodenal crypts was performed using endoscopic biopsy specimens from patients with rheumatological conditions requiring long-term anti-inflammatory and analgesic therapy, testing the hypothesis that long-term administration of nonsteroidal anti-inflammatory drugs (NSAIDs) may lead to enhanced epithelial cell proliferation in the stomach and duodenum. After a 1-week washout period from NSAID administration, specimens were taken from endoscopically normal gastric antrum and duodenum, and microdissection was used to quantitate mitoses in these sites. Endoscopy and biopsy were then repeated after 2 weeks of NSAID administration, during which patients received in addition either the prostaglandin E1 analogue misoprostol (10 patients) or a placebo (10 patients). Mitotic counts in gastric glands increased similarly and significantly in the two groups from (mean +/- SEM) 4.45 +/- 0.57 to 7.69 +/- 1.08 (P less than 0.001) in the NSAID+placebo-treated group and from 4.31 +/- 0.53 to 7.68 +/- 1.08 (P = 0.006) in the NSAID+misoprostol-treated group. Similar changes took place in the duodenal crypts, from 5.44 +/- 0.68 to 8.60 +/- 1.28 (P = 0.013) in the NSAID+placebo-treated group and from 4.68 +/- 0.56 to 6.35 +/- 0.63 (P = 0.011) in the NSAIDs+misoprostol-treated group. NSAIDs increased the proportion of glands and crypts with bifid or more complex architectural forms. In a small group of patients, misoprostol alone did not alter mitotic rate in glands or crypts over 4 weeks. Thus, NSAIDs increase the rate of proliferation in endoscopically normal gastric and duodenal epithelium of patients with arthritis. This may form one of the mechanisms underlying gastric and duodenal adaptation to NSAIDs.

Binding and suppressive activity of human recombinant ferritins on erythroid cells

We studied the relation between ferritin cellular binding and suppressive activity of recombinant H- and L-ferritin on human erythroid cells at different proliferation/differentiation phases. L-ferritin failed to show any suppressive activity or detectable binding to erythroblasts at any stage of maturation. In contrast, H-ferritin demonstrated binding to erythroblasts derived from peripheral BFU-E cells which increased steadily between 7-14 days of culture up to 15,000 molecules per cell. Reticulocytes and erythrocytes failed to bind either L- or H-ferritin. H-ferritin suppressed BFU-E colony formation and reduced K562 cell proliferation at nanomolar concentrations. This suggests that the expression of H-ferritin binding sites is modulated by cellular proliferation and differentiation, that cells expressing H-ferritin binding sites are sensitive to ferritin suppressive activity and that a causal relation exists between ferritin cellular binding and suppressive activity.

Sucralfate diminishes basal acid output without affecting gastrin, H. pylori or gastritis in duodenal ulcer patients

Twelve patients with active duodenal ulcer disease and Helicobacter pylori infection were treated with 1 g sucralfate q.d.s. for 1 month. Ulcers healed in 8 of the 12 patients without an alteration in the H. pylori-associated antral gastritis. Sucralfate produced a significant fall in basal acid output in all the patients, from a median of 4.8 (range 2.1-12.1) to 1.6 (0.4-8) mmol/h, P less than 0.01, whereas peak acid output was unchanged from 41 (21-59) before to 38 (24-55) mmol/h after treatment. Basal plasma gastrin concentrations and the meal-stimulated integrated gastrin response were not altered significantly by sucralfate: 8 (2-17) pmol/L and 732 (188-1045) pmol. min/L pre-treatment and 6 (2-17) pmol/L and 600 (140-1302) pmol. min/L post-treatment, respectively. The fall in basal acid output observed may contribute to prolonged duodenal ulcer remission after treatment with sucralfate.

Low frequency of ras oncogene mutations in Philadelphia-positive acute leukemia and report of a novel mutation H61 Leu in a single case

Activating ras mutations are frequent (25-60%) in chronic myelomonocytic leukemia (CMML) and in acute myeloid leukemia (AML) (30%), in contrast to chronic myeloid leukemia (CML) in which the incidence is very low (0-3%). This might reflect that the leukemic cell in CML is at a level of differentiation in which ras gene activation is not involved or, alternatively, might be due to the presence in CML of the bcrlabl fused gene. We have analyzed the presence of point mutations in codons 12, 13, 59, 61 and 63 of N-, K-, and H-ras genes, in 26 cases of Philadelphia-chromosome-positive, bcrlabl-positive acute leukemia (Ph+ AL), and in eight CMML cases by using the polymerase chain reaction. Aberrant ras genes were detected in a single Ph+ AL case, and in four out of eight CMML patients. The Ph+ AL showing altered ras allele had an unusual point mutation in H-ras gene, substituting leucine for glutamine. This mutation has not been previously found in any hematological disease. Our findings suggest that ras mutations are probably not involved in the pathogenesis of those leukemias in which blast cells contain bcrlabl oncogene activation.