Microheterogeneity of horse spleen ferritin and apoferritin

Ferritin microheterogeneity, subunit composition, functional, and physiological implications

Ferritin is a ubiquitous intracellular iron storage protein that plays a crucial role in iron homeostasis. Animal tissue ferritins consist of multiple isoforms (or isoferritins) with different proportions of H and L subunits that contribute to their structural and compositional heterogeneity, and thus physiological functions. Using size exclusion and anion exchange chromatography, capillary isoelectric focusing (cIEF), and SDS-capillary gel electrophoresis (SDS-CGE), we reveal for the first time a significant variation in ferritin subunit composition and isoelectric points, in both recombinant and native ferritins extracted from animal organs. Our results indicate that subunits composition is the main determinant of the mean pI of recombinant ferritin heteropolymers, and that ferritin microheterogeneity is a common property of both natural and recombinant proteins and appears to be an intrinsic feature of the cellular machinery during ferritin expression, regulation, post-translational modifications, and post-subunits assembly. The functional significance and physiological implications of ferritin heterogeneity in terms of iron metabolism, response to oxidative stress, tissue-specific functions, and pathological processes are discussed.

Vesicle aggregates as a model for primitive cellular assemblies

Primitive cell models help to understand the role that compartmentalization plays in origin of life scenarios.

Assembly process of 2D protein arrays in wetting films

We were successful in developing a technique to form protein array developed directly on solid surfaces. This promising array formation revealed two new concepts of the ordering mechanism: the lifetime of a secondary minimum and secondary films. These concepts are not limited to our film formation technique. Any electrolytic thin film on a solid surfaces or any free film may contain a secondary film. If the secondary film can be retained in an electrolytic thin film for a sufficient period, a large single domain of small colloidal particles, such as proteins, fine metal particles, fine semi-conductive particles, etc., can be produced by an Alder-type transition.

Lipid-vesicle-surface chromatography

Egg-yolk phospholipid vesicles (liposomes) containing stearylamine cations or phosphatidylserine anions, were formed and entrapped in agarose gel beads (Sepharose 6B) by a dialysis procedure. On a column of entrapped phospholipid-stearylamine (4:1) (cationic) vesicles, 0.36 mg of ferritin was bound per mumol lipids at 0.05 M ionic strength and pH 7. About 30% of the vesicle surface thus became covered with ferritin. Only 0.04 mg of citraconylated myoglobin was bound per mumol lipids, as myoglobin is much smaller than ferritin. Haeme groups were readily inserted into the lipid bilayers. An excess amount of bovine serum albumin (BSA) or ribonuclease A was applied to entrapped ionic vesicles and the bound proteins were eluted by increasing the ionic strength from 0.01 to 0.2 or 0.5 M. After three to five runs, 82-88% of the vesicles (the phospholipids) remained entrapped. The capacity of the cationic vesicle-column for BSA decreased more than did the amount of entrapped vesicles, which indicates a preferential loss of stearylamine. Ion-exchange experiments were done with human plasma and with BSA monomers and dimers on entrapped cationic vesicles. Plasma proteins could be separated. BSA dimers were eluted later than BSA monomers in a sodium chloride gradient and the separation was better than on DEAE-Sepharose. The contact area between the protein and the vesicle surface is important for the binding strength. Protein-vesicle surface interactions can be studied by chromatography on entrapped vesicles.

Human placental ferritin receptor

Brush-border membranes from human placenta were prepared and their purity was clarified by biochemical and morphological methods. Ferritin binding to these prepared membranes was examined using horse spleen 125I-apoferritin, and was found to be completed within 10 min at 37 degrees C and pH 7.5. The amount of ferritin bound to the membranes was found to be proportional to the amount of membrane added and saturable for a given amount of the membrane in the presence of excess ligand. The membranes exhibited specific ferritin binding with a Ka of 2.3 X 10(7) M-1 at pH 7.5. A competitive binding assay indicated that horse spleen 125I-apoferritin binding was inhibited by a 10-fold molar excess of horse spleen ferric ferritin and a 500-fold molar excess of human transferrin. These results suggest that human placental brush-border membranes have specific receptors for horse spleen apoferritin molecules.

Preparative isolation of distinct molecular forms of rabbit liver ferritin using high-performance liquid chromatography

Rabbit liver ferritin was separated to fractions of distinct molecular form using a chromatofocusing column coupled to high-performance liquid chromatography equipment. This purification method was fast, less than 1 h, and enabled preparation of fractions, highly enriched in particular subtypes of ferritin. Analytical isoelectrofocusing of these fractions demonstrated a gradual shift in the range of isoelectric points of these subtypes of ferritin. Gradient-pore polyacrylamide slab-gel electrophoresis showed a distinct shift in the subunit ratio of the ferritins, ranging from 87% low molecular weight (L) subunit in the first fraction eluting at a pH 5.4, to 28% L-subunit in the fraction eluting in the trailing edge of the protein peak at pH 4.0. The pI range of the fractions covered the complete range, from pH 4.9 to pH 5.4, of isoelectric points of the whole rabbit liver ferritin preparation.

Influence of heat treatment on rabbit liver ferritin

Ferritin was purified from rabbit livers either by heat treatment and immunoaffinity chromatography, or by immunoaffinity chromatography alone. The immunoreactivity of ferritin with antibodies raised against heat-treated ferritin was significantly higher for heat-treated preparations than for non-heated preparations. The amount of ferritin protein could be estimated with equal reliability by the assay according to Lowry et al. and by nitrogen determination. Heat treatment favoured the L-subunit-rich ferritin fraction, as measured by densitometric scanning of SDS gradient-pore polyacrylamide gels. Amino acid analysis showed small changes in the amounts of valine, isoleucine and histidine in the heat-treated ferritin, possibly due to selective partial degradation of H-subunit-rich forms of ferritin. These results illustrate that heat treatment, which is a commonly used step in most purification procedures, induces partial denaturation of the ferritin molecules.

Biological activity of an angiotensin II--ferritin conjugate on rabbit aortic smooth muscle cells

Specific binding sites for [Asp1,Ile5]angiotensin II (angiotensin) have been demonstrated in homogenates and subcellular fractions of aortic medial smooth muscle cells, but the localization of the angiotensin receptor responsible for contraction has not been determined [Devynck, M. A., & Meyer, P. (1976) Am. J. Med. 61, 758-767]. To establish the location of this receptor, we have prepared a membrane-impermeable analogue of angiotensin by acylating its N-terminal amino group with the N-hydroxysuccinimide ester of succinylated ferritin. This angiotensin-ferritin conjugate possessed the same intrinsic activity as angiotensin but was approximately 200 times less potent in inducing contraction in rabbit aortic strips. The stability of the conjugate was investigated, and approximately 5% of the contractile activity of the angiotensin-ferritin conjugate was attributable to low molecular weight components that were present before or after exposure to aortic strips. The time required for aortic strips to reach a plateau of contraction in response to angiotensin-ferritin was significantly longer than that required by free angiotensin to produce the same level of contraction. With enzymatically dispersed aortic smooth muscle cells, however, the time taken to produce contractions by both angiotensin and angiotensin-ferritin was indistinguishable. [Sar1,-Ala8]angiotensin II, a competitive inhibitor of angiotensin, completely suppressed contractions induced by angiotensin or angiotensin-ferritin in aortic strips or dispersed aortic smooth muscle cells. These results suggest that angiotensin need not directly penetrate the plasma membrane to cause contraction and imply that the angiotensin receptor responsible for initiating contraction of aortic smooth muscle cells is located on the plasma membrane.

Evidence for post-translational changes in rat liver ferritin

Analysis of rat liver isoferritins labelled by NaH14CO3 injection shows that acidic isoferritins decay exponentially, while activity in more basic isoferritins rises over 2-3 days. This suggests that isoferritins of low pI become more basic by post-translational modification, occurring over the whole life time of the protein.

Correlations between subunit distribution, microheterogeneity, and iron content of human liver ferritin

Subunit heterogeneity of human liver ferritin was investigated by two-dimensional electrophoretic methods. The protein which ordinarily remains assembled in 10 M urea solution was dissociated into subunits in acid-urea or sodium dodecyl sulfate solutions. In agreement with earlier studies, the subunits migrated as two bands in sodium dodecyl sulfate or acid-urea gel electrophoresis systems or in two-dimensional combinations of these systems. Isoelectric focusing methods, however, resolved four major subunit bands and three to five minor bands. Each of these components migrated as either a 22 000 or a 19 000 molecular weight component in sodium dodecyl sulfate gel electrophoresis in the second dimension. The multiple subunit model, which is contrary to currently accepted representations of ferritin structure, is compatible with certain inherent properties of the protein. Thus, ferritin was fractionated on the basis of iron content to show that the relative amounts of individual subunit types were directly dependent upon the iron composition of the protein. Iron-loaded molecules were deficient in the most basic subunit types, and apoferritin was enriched in these components. Aspects of microheterogeneity of assembled ferritin molecules were correlated to subunit heterogeneity, and discrete differences in subunit populations among purified isoferritin components were demonstrated.

Serum ferritin in haemochromatosis: changes in the isoferritin composition during venesection therapy

The isoferritin composition of serum ferritin in 13 patients with untreated idiopathic haemochromatosis (IHC) has been shown to differ from normal in exhibiting an increase in isoferritins in the pH range 5.54-5.62. A similar change was observed in four patients with gross iron overload secondary to haemolytic anaemia. During the course of venesection therapy there was a progressive rise in isoferritins of pI 5.02-5.06 relative to the more basic isoferritins. These observations are consistent with previous studies showing alterations in tissue isoferritins in untreated IHC before and after venesection therapy and they are compatible with the hypothesis that the more basic isoferritins correspond to a 'storage' ferritin and the more acidic to a 'secretory' ferritin. The studies also provide further evidence for a possible biological role of the individual isoferritins.

Ferritin phenotypes: structure and metabolism

Recent advances in our understanding of the biochemistry of ferritin have provided new insights into its role in iron metabolism. Findings of multiple structural forms in many tissues may have important consequences for ferritin's function and metabolism. This article reviews the molecular basis of apoferritin heterogeneity and discusses mechanisms operating in the phenotypic expression of ferritin in normal and malignant cells.

Isolation and characterization of iron-binding proteins from rat intestinal mucosa

Two iron-binding proteins were isolated from rat intestinal mucosa. From determination of their molecular weights, their electrophoretic and iron-binding properties it was established that one was a mucosal ferritin and the other a mucosal transferrin. The mucosal ferritin is compared in its molecular weight, isoelectric point, amino acid composition and tryptic peptide pattern with the ferritins of rat spleen and liver. All three ferritins are distinctly different from one another. In addition the iron content of mucosal ferritin was found to be much lower than that of liver and spleen ferritins. Mucosal transferrin was separated into two components by isoelectric focusing, as was plasma transferrin. The plasma and mucosal transferrins differ in their isoelectric points and in their amino acid compositions. Differences were also found in vitro in the iron-binding of mucosal transferrin as compared with plasma transferrin. The role of these mucosal proteins in the absorption of iron is briefly discussed.

Alterations in tissue ferritins in iron storage disorders

Purified tissue ferritins isolated from Bantu subjects with gross haemosiderosis, from a patient with idiopathic haemochromatosis (HC) treated by phlebotomy, and from rats with experimental iron overload were studied in order to determine the significance of the abnormality previously demonstrated in tissue isoferritins in patients with IHC. The isoferrin profile of the tissues from the Bantu subjects and the iron-loaded rats showed a similar abnormality to that previously found in patients with untreated IHC--that is, an abnormally uniform distribution of iron-containing isoferritins with an increase in the more basic isoferritins and an apparent absence of the more acidic ones. In contrast, tissues from the patient with treated IHC, who was iron depleted at the time of death, showed the normal organ-specific isoferritin distribution. These findings strongly suggest that the abnormal distribution of tissue isoferritins in IHC is an acquired phenomenon and unlikely to be related to an underlying genetic defect in ferritin or iron metabolism.

Human isoferritins: organ specific iron and apoferritin distribution

Ferritins from human liver, spleen, heart, pancreas and kidney were compared by electrophoresis and isoelectric focusing in polyacrylamide gels, by immunodiffusion against antisera to homologous and heterologous ferritin, and in some cases by their cyanogen bromide peptides. All ferritins appeared to consist of a single species on gel electrophoresis with the exception of heart ferritin which separated into two major components. Small differences in electrophoretic mobility were found in all tissue ferritins. By contrast, all tissue ferritins were found to consist of multiple forms when analysed by gel electrofocusing. At least five isoferritins were found in most tissues, several of which were common to most tissues. At least two were common to all tissues. Those ferritins which were most easily distinguishable electrophoretically, e.g. spleen and heart ferritin, showed the greatest differences on gel electrofocusing. The ferritin profile was characteristic of each organ and was reproducible both within individuals and between individual tissues. There were striking differences in the iron content of the various isoferritins within a tissue. Further, the iron content of isoferritins common to more than one tissue varied with the tissue of origin. Some isoferritins in several organs and all of the isoferritins in pancreas appeared to contain little, if any, iron. All five tissue ferritins contained antigenic determinants in common with liver ferritin. However, an additional antigenic determinant was found in liver ferritin which was not detectable in the ferritins from the other organs.

Evidence against the occurrence of artifacts due to carrier ampholyte-protein binding during isoelectric focusing

The formation of irreversible complexes between carrier ampholyte components and proteins was investigated by gel filtration of mixtures of proteins and radioactively labelled ampholytes. Experiments were performed both with purified proteins (albumin, ferritin, beta-glucuronidase) and with a complex mixture of proteins (serum); in no case was binding of ampholytes to proteins detected. Thus the results argue against the occurrence in isoelectric focusing of proteins of artifacts due to such complex formation.

Heterogeneity in tissue ferritins displayed by gel electrofocusing

1. Horse spleen ferritin and human liver ferritin were examined by gel electrofocusing under conditions that demonstrated equilibrium focusing. Both ferritins were resolved into multiple isoferritins. Both families of isoferritins were separable from one another. 2. Horse spleen ferritin was also resolved into five components by ion-exchange chromatography on DEAE-Sephadex A-50. Each of the major chromatographic fractions contained only a few of the isoferritins seen on gel electrofocusing. Each chromatographic fraction corresponded to different portions of the isoferritin profile. 3. These results indicate that the heterogeneity seen in many ferritins by gel electrofocusing represents structural heterogeneity in the ferritin population as isolated from the tissues.