Purification,electrophoretic behavior,and kinetics of iron release of liver ferritin of Dasyatis akajei

From the liver of fish Dasyatis akajei, ferritin has been isolated by thermal denaturation and ammonium sulfate fractionation and then further purified by anion exchange chromatography and gel exclusion chromatography. The molecular weight of the liver ferritin of D. akajei (DALF) was measured to be 400 kDa by PAGE. Moreover, SDS-PAGE experimentation indicates that protein shell of DALF consists of the H and L subunits with molecular weight of 18 and 13 kDa, respectively. Using isoelectric focusing with pH ranging from 5.0 to 6.0, the ferritin purified by the PAGE exhibited three bands with different pI values in the gel slab. Diameters of the protein shell and iron core were also investigated by transmission electron microscope and determined to be 10–12 nm and 5–8 nm, respectively. A kinetic study of DALF reveals that the rate of self-regulation of the protein shell rather than the complex surface of the iron core plays an important role in forming a process for iron release with mixed orders.     

Heart tissue contains small and large aggregates of ferritin subunits

Ferritin purified from horse heart and applied to nondenaturing polyacrylamide gel electrophoresis migrated as a single band that stained for both iron and protein. This ferritin contained almost equal amounts of fast- and slow-sedimenting components of 58 S and 3-7 S, which could be separated on sucrose density gradients. Iron removal reduced the sedimentation coefficient of the fast-sedimenting ferritin to 18 S, and sedimentation equilibrium gave a molecular weight 650,000, with some preparations containing ferritin of 500,000 molecular weight as well. Sedimentation rates of the 3 S and 7 S ferritins were not affected by iron removal, and sedimentation equilibrium data were consistent with Mr's 40,000 and 180,000, respectively. Preparations of ferritin extracted from horse spleen contained only 67 S (holo) or 16 S (apo) ferritin and no slow-sedimenting species. When examined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, all of the ferritins contained the usual H and L subunits (23 and 20 kDa, respectively), but the slow-sedimenting (3 S and 7 S) heart apoferritins also contained appreciable quantities (ca 25%) of three larger subunits of 42, 55, and 65 kDa. All the subunits reacted positively in Western blots to polyclonal antibodies made against specially purified large heart or spleen ferritins containing only 20- and 23-kDa subunits. Similar results were obtained for ferritins from rat heart. The results indicate that mammalian heart tissue is peculiar not just in having an abnormally large iron-rich ferritin but also in having iron-poor ferritins of much lower molecular weight, partly composed of larger subunits.     

Characteristics of structure, composition, mass spectra, and iron release from the ferritin of shark liver (Sphyrna zygaena)

The ferritin consists of a protein shell constructed of 24 subunits and an iron core. The liver ferritin of Sphyrna zygaena (SZLF) purified by column chromatography is a protein composed of eight ferritins containing varying iron numbers ranging from 400+/-20 Fe3+/SZLF to 1890+/-20 Fe3+/SZLF within the protein shell. Nature SZLF (SZLFN) consisting of holoSZLF and SZLF with unsaturated iron (SZLFUI) to have been purified with polyacrylamide gel electrophoresis (PAGE) exhibited five ferritin bands with different pI values ranging from 4.0 to 7.0 in the gel slab of isoelectric focusing (IEF). HoloSZLF purified by PAGE (SZLFE) not only had 1890+/-20 Fe3+/SZLFE but also showed an identical size of iron core observed by transmission electron microscopy (TEM). Molecular weight of approximately 21 kDa for SZLFE subunit was determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Four peaks of molecular ions at mass/charge (m/z) ratios of 10611.07, 21066.52, 41993.16, and 63555.64 that come from the SZLFE were determined by matrix-assisted laser desorption ionization/time-of-flight mass spectrometry (MALDI-TOF MS), which were identified as molecular ions of the ferritin subunit (M+) and its polymers, namely, [M]2+, [M]+, [2M]+, and [3M]+, respectively. Both SZLFE and a crude extract from shark liver of S. zygaena showed similar kinetic characteristics of complete iron release with biphasic behavior. In addition, a combined technique of visible spectrometry and column chromatography was used for studying ratio of phosphate to Fe3+ within the SZLFE core. Interestingly, this ratio maintained invariable even after the iron release, which differed from that of other mammal ferritins.     

Size and charge heterogeneity of rat tissue ferritins.

Ferritins purified from horse spleen and from rat liver, kidney, heart and hepatoma were analyzed by quantitative polyacrylamide gel electrophoresis. From the migration characteristics of these ferritins at several gel concentrations, Ferguson plots were constructed and the molecular sizes and charges (apparent valences) together with their statistical variability were obtained by applying Rodbard computer programs to the data. Finally, ellipses were drawn describing the 95% confidence limits of these data for size and charge and were used to identify those ferritins that differed in size and/or charge. By these criteria, many of the tissue ferritins were differentiated from one another in terms of their molecular size and/or charge. Among the various tissue ferritin monomers, the molecular sizes were essentially similar (420 000-490 000) except for the two heart ferritins which were larger (530 000 and 626 000, respectively). However, the estimated charges on rat liver, kidney and hepatoma monomers (30-38 net protons per molecule) differed from that of spleen monomer (51 net protons per molecule) while the larger rat heart ferritin also had a greater charge (83 net protons) than the smaller (40 net protons). Apoferritins prepared chemically by removal of iron from the holoferritins had migration properties indistinguishable from the parent holoferritins. The migration properties of minor (dimeric) ferritin bands on the gels were compared with those of the monomer bands. The molecular sizes of the minor bands were larger than those of the major bands, and were not inconsistent with a doubling in size. However, charge differences varied, being either similar for major and minor forms (spleen ferritin), approximately twice for the minor form (rat hepatoma ferritin) or five times greater for the minor form (rat liver ferritin). These differences in behavior were confirmed by using minimally sieving gels, on which the major bands of horse spleen ferritin failed to separate whereas those of rat liver ferritin were readily separable. It is concluded that dimers of ferritins from different tissues may associate in different ways.     

A chemical and physical comparison of ferritin subunit species fractionated by high-performance liquid chromatography

Selected chemical and physical properties were measured for different forms of ferritin subunits which had been separated by reverse-phase high-performance liquid chromatography. Ferritin subunits from porcine spleen behaved, on sodium dodecyl sulfatepolyacrylamide gel electrophoresis, as though they were ~ M_r 2000 larger than equine spleen ferritin, whereas no difference in size was observed by gel chromatography in 6 m guanidinium chloride. All subunit species exhibited similar isoelectric focusing properties. In contrast to previous reports, no carbohydrate could be found associated with any of the isolated subunit species. Thus, the aberrant behavior of the porcine ferritin subunits between the two empirical molecular weight estimation methods appears to be the result of factor(s) other than protein intrinsic charge or covalently attached carbohydrate.     

Characterization of ferritin from human placenta. Implications for analysis of tissue specificity and microheterogeneity of ferritins

Mammalian ferritins can be resolved into multiple components by isoelectric focusing, and each tissue contains a characteristic subset of isoferritins. Ferritin isolated from human liver was compared to acidic ferritin isolated from mid-gestational human placenta to define a structural basis for ferritin heterogeneity. Placenta ferritin contained several major bands with isoelectric points in the range of pI = 4.7-5.0 which were more acidic than the predominant isoferritins of human liver. Ferritin from each tissue was resistant to denaturation by 10 M urea and appeared to be identical by electron microscopy. Circular dichroism measurements revealed that placenta ferritin had substantially less ordered secondary structure than liver ferritin. Both types of ferritin contained only two subunits when analyzed by electrophoresis in sodium dodecyl sulfate gels, but isoelectric focusing of dissociated subunits in urea revealed 6-7 different components. In this system, placenta ferritin was enriched in the more acidic subunits and it completely lacked the most basic subunits noted in liver ferritin; placental ferritin had no unique components. Differences in isoelectric points among assembled ferritins from these two tissues appear to result from different proportions of these acidic and basic subunits.     

Properties and role of ferritin in the hemolymph of the chiton Clavarizona hirtosa

The major iron-binding protein found in the hemolymph of the chiton Clavarizona hirtosa has been purified for the first time and identified as ferritin. This ferritin, which is present at a concentration of approx. 400 μg·ml⁻¹, has a M_r of 28 000 and 25 500, exhibits microheterogeneity with isoelectric values in the range 5.3–6.0, binds 1500–2500 Fe atoms·mol⁻¹ and is immunologically distinct from horse spleen ferritin. The initial rate of iron accumulation by ferritin molecules was determined to be markedly higher than that exhibited by horse spleen ferritin. Taken together, these data suggest that ferritin found in the hemolymph serves as a key component of the high-capacity transport system necessary to deliver iron to the rapidly mineralizing tissue of the radula in these molluscs.     

Purification and characterization of rat liver glycosylasparaginase.

1. Rat liver glycosylasparaginase [N4-(beta-N-acetylglucosaminyl)-L-asparaginase, EC 3.5.1.26] was purified to homogeneity by using salt fractionation, CM-cellulose and DEAE-cellulose chromatography, gel filtration on Ultrogel AcA-54, concanavalin A-Sepharose affinity chromatography, heat treatment at 70 degrees C and preparative SDS/polyacrylamide-gel electrophoresis. The purified enzyme had a specific activity of 3.8 mumol of N-acetylglucosamine/min per mg with N4-(beta-N-acetylglucosaminyl)-L-asparagine as substrate. 2. The native enzyme had a molecular mass of 49 kDa and was composed of two non-identical subunits joined by strong non-covalent forces and having molecular masses of 24 and 20 kDa as determined by SDS/polyacrylamide-gel electrophoresis. 3. The 20 kDa subunit contained one high-mannose-type oligosaccharide chain, and the 24 kDa subunit had one high-mannose-type and one complex-type oligosaccharide chain. 4. N-Terminal sequence analysis of each subunit revealed a frayed N-terminus of the 24 kDa subunit and an apparent N-glycosylation of Asn-15 in the same subunit. 5. The enzyme exhibited a broad pH maximum above 7. Two major isoelectric forms were found at pH 6.4 and 6.6. 6. Glycosylasparaginase was stable at 75 degrees C and in 5% (w/v) SDS at pH 7.0.     

Properties of cytosolic epoxide hydrolase purified from the liver of untreated and clofibrate-treated mice. Purification procedure and physiochemical characterization of the pure enzymes

Cytosolic epoxide hydrolase was purified from the liver of untreated and clofibrate-treated male C57Bl/6 mice. The purification procedure involves chromatography on DEAE-cellulose, phenyl-Sepharose and hydroxyapatite, takes two days to perform and results in a 120-fold purification and approximately 35% yield of the enzyme from untreated mice. The purified enzyme is a dimer with a molecular mass of 120 kDa, a Stokes' radius of 4.2 nm, a frictional ratio of 1.0 and an isoelectric point of 5.5. The subunits behave identically upon isoelectric focusing in 8 M urea and only one band with a molecular mass of 60 kDa is seen after sodium dodecyl sulfate/polyacrylamide gel electrophoresis. The form purified from clofibrate-treated mice had very similar properties and was apparently identical to the control form as judged by amino acid analysis and peptide mapping as well. These analyses also demonstrated that the cytosolic enzyme is clearly different from microsomal epoxide hydrolase isolated from rat liver. Furthermore, Ouchterlony immunodiffusion using antibodies raised in rabbits towards the control form of cytosolic epoxide hydrolase revealed identity between the two forms of cytosolic epoxide hydrolase, but no reaction with the microsomal epoxide hydrolase was observed. These findings indicate large structural differences between the cytosolic and microsomal forms of epoxide hydrolase in the liver.     

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

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

Subunit dimers in sheep spleen apoferritin. The effect on iron storage

Ferritin with high and low iron content, 2000 and 790 iron atoms/molecule, was isolated from the spleens of copper-poisoned and control lambs, respectively. Differences in the iron content in vivo were reflected in the properties of the apoferritin protein shells, since the apoprotein from the low iron ferritin took up iron relatively more slowly (0.52 +/- 0.09) and released it more rapidly (1.68 +/- 0.06) in vitro. Although the two types of apoferritin were indistinguishable in terms of surface charge (pI range 4.98-5.43) and in consisting of both heavy and light subunits, the subunit interactions differed markedly; 40-50% of the subunits of low iron ferritin were in dimers stable to reduction and carboxylmethylation, 4% mercaptoethanol, 8% sodium dodecyl sulfate, and 100 degrees C for 30 min, 70% formic acid, and 30% methanol. Subunit dimers were also observed in liver ferritin from mouse and neonatal pig and were enriched in a low iron fraction of horse spleen ferritin. Based on cyanogen bromide fragmentation and NH2-terminal analysis, the natural and chemically cross-linked subunit dimers had two peptides in common; natural subunit dimers also appeared to have a second region cross-linked, suggesting the possibility of both intra- and intersubunit links in the natural dimers. In sheep spleen ferritin, both heavy and light subunits appeared to participate in subunit dimerization. Natural subunit dimers were enriched in low iron ferritin fractions of all ferritin preparations tested (linear correlation = 0.94) and can explain, at least in part, the previously observed effects of iron core size on the apoferritin shell. Whether the subunit cross-links represent part of the subunit assembly process subsequently cleaved by iron (or copper) or whether the cross-links form after iron core formation in vivo has yet to determined. In either case, it is clear that such post-translational variations can affect iron uptake and release and emphasize the importance of the protein shell in determining the iron storage properties of ferritin.     

Isolation and characterization of ferritin from the hepatopancreas of the musselMytilus edulis

Summary The main iron-binding protein in the hepatopancreas of the musselMytilus edulis, which had been previously iron-loaded by exposure to carbonyl iron (spheres of elemental iron less than 5 m diameter), has been isolated to electrophoretic purity and identified as ferritin. This ferritin hasM _r, of 480000, pI of 4.7–5.0 and is composed of two subunits,M _r 18500 andM _r 24600. Under the electron microscope, it appears as electron-dense iron cores of average diameter 5 nm surrounded by a polypeptide shell to a final average overall diameter of 11 nm. The purified protein contains, on average, 200 iron atoms/molecule protein. On immunodiffusion,M. edulis hepatopancreas ferritin gives a partial cross-reaction with antiserum to horse spleen ferritin and lamprey (Geotria australis) liver ferritin but does not react with antiserum to chiton (Acanthopleura hirtosa) haemolymph ferritin.     

Mobilization of iron from ferritin by isolated mitochondria effects of species compatibility between ferritin and mitochondria and iron content of ferritin

Mitochondria mobilize iron from ferritin by a mechanism that depends on external FMN. With rat liver mitochondria, the rate of mobilization of iron is higher from rat liver ferritin than from horse spleen ferritin. With horse liver mitochondria, the rate of iron mobilization is higher from horse spleen ferritin than from rat liver ferritin. The results are explained by a higher affinity between mitochondria and ferritins of the same species. The mobilization of iron increases with the iron content of the ferritin and then levels off. A maximum is reached with ferritins containing about 1 200 iron atoms per molecule. The results represent further evidence that ferritin may function as a direct iron donor to the mitochondria.     

Characterization of hepatic L-threonine dehydrogenase of chicken

The L-threonine dehydrogenase (TDH) was purified approximately 1300-fold to a specific activity of approximately 18000 unit mg(-1) from chicken (Gallus domesticus) liver mitochondria. Purification was obtained by sequential chromatography on DEAE Cellulose, Phenyl Sepharose High Performance hydrophobic interaction, Affi-Gel Blue affinity and Matrex Gel Red A columns. The molecular weight of the subunit was estimated to be 36 kDa by sodium dodecyl-polyacrylamide gel electrophoresis. An apparent molecular mass of native protein between 62 and 74 kDa was obtained by gel filtration chromatography, suggesting a dimeric structure of TDH. The isoelectric point of TDH was determined by isoelectric focusing to be 5.3. Partial amino-terminal sequence analyses, carried out on two purified preparations of TDH, revealed a high degree of homology to the reported sequence of porcine TDH. The Michaelis constants for L-threonine and NAD for partially purified chicken hepatic TDH are 5.38 and 0.19 mM, respectively.     

Evidence that H-enriched human placental ferritin is structurally similar to L-enriched ferritins of other tissues

Ferritin was purified from normal full-term placenta, and the native structure and subunit composition were characterized. Reversed-phase high-performance liquid chromatographic analysis of the placental ferritin subunits suggested the presence of three subunit types. Using acid urea gel electrophoresis and amino acid analysis, these subunits were tentatively identified as two H-type and one L-type. The relative proportions of the subunit types were approx. 23% H-1, 33% H-2 and 44% L. The native structure of placental ferritin as judged by circular dichroism and fluorescence spectroscopy was quite similar to that of ferritin isolated from horse spleen, a source that is composed predominantly of L subunits. These results are consistent with a ferritin tetracosameric structure whose H and L subunits fit into 24 equivalent sites interchangeably because the secondary and tertiary structures of the two subunit types are very similar.     

Purification and characterization of avian liver mevalonate-5-pyrophosphate decarboxylase

Mevalonate-5-pyrophosphate decarboxylase [ATP:5-diphosphomevalonate carboxy-lyase (dehydrating), EC 4.1.1.33] has been purified 5800 times from chicken liver and obtained in a stable and highly purified form. The protein is a dimer of molecular weight 85400 +/- 1941, and its subunits were not resolved by gel electrophoresis in denaturing conditions. The purified enzyme does not require the presence of SH-containing reagents for either activity or stability. The enzyme shows a high specificity for adenosine 5'-triphosphate (ATP) and requires for activity a divalent metal cation, Mg2+ being most effective. The optimum pH for the enzyme ranges from 4.0 to 6.5. Inhibitory effects for the enzyme activity were detected by citrate, phthalate, and phosphate. The isoelectric point, as determined by column chromatofocusing, is 4.8. The kinetics are hyperbolic for both substrates, showing a sequential mechanism; true Km values of 0.0141 mM and 0.504 mM have been obtained for mevalonate-5-pyrophosphate and ATP, respectively.     

Characterization of cytochrome c3 from the thermophilic sulfate reducer Thermodesulfobacterium commune

A c3 type cytochrome has been purified from the thermophilic, non-spore-forming, sulfate-reducing bacterium Thermodesulfobacterium commune. The purified protein was homogeneous as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, gel filtration, and isoelectric focusing. A pI of 6.83 was observed. The molecular weight of the cytochrome was estimated to be ca. 13,000 from both gel filtration and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The hemoprotein exhibited absorption maxima at 530, 408.5, and 351 nm in the oxidized form and 551.5 (alpha band), 522.5 (beta band), and 418.5 nm (gamma band) in the reduced form. The extinction coefficients of T. commune cytochrome c3 were 130,000, 74,120, and 975,000 M-1 cm-1 at 551.5, 522.5, and 418.5 nm, respectively. It contains four hemes per molecule, on the basis of both the iron estimation and the extinction coefficient value of its pyridine hemochrome. The amino acid composition showed the presence of eight cysteine residues involved in heme binding. T. commune cytochrome c3 had low threonine, serine, and glycine contents and high glutamic acid and hydrophobic residue contents. The electrochemical study of T. commune cytochrome c3 by cyclic voltammetry and differential pulse polarography has shown that the cytochrome system behaves like a reversible system. Four redox potential values at Eh1 = -0.140 +/- 0.010 V, Eh2 = Eh3 = Eh4 = -0.280 +/- 0.010 V have been determined. T. commune cytochrome c3, which acts as the physiological electron carrier of hydrogenase, is similar in most respects to the multiheme low-potential cytochrome c3 which is characteristic of the genus Desulfovibrio.     

An acetylcholinesterase purified from the greenbug (Schizaphis graminum) with some unique enzymological and pharmacological characteristics

An acetylcholinesterase (AChE, EC 3.1.1.7) was purified from the greenbug, Schizaphis graminum (Rondani). The maximum velocities (Vmax) for hydrolyzing acetylthiocholine (ATC), acetyl-(beta-methyl) thiocholine (AbetaMTC), propionylthiocholine, and S-butyrylthiocholine were 78.0, 67.0, 37.4, and 2.3 micromol/min/mg, and the Michaelis constants (Km) were 57.6, 60.6, 31.3, and 33.4 microM, respectively. More than 98% of AChE activity was inhibited by 10 microM eserine or BW284C51, but only 7% of the activity was inhibited by ethopropazine at the same concentration. Based on the substrate and inhibitor specificities, the purified enzyme appeared to be a true AChE. Nondenaturing polyacrylamide gel electrophoresis (PAGE) and isoelectric focusing of the purified AChE revealed three molecular forms. The isoelectric points were 7.3 for the major form and 6.3 and 7.1 for two minor forms. The major form of purified AChE showed molecular masses of 129 kDa for its native protein and 72 kDa for its subunits on SDS-PAGE. However, the purified AChE exhibited some distinctive characteristics including: (1) lack of affinity to the affinity ligand 3-(carboxyphenyl) ethyldimethyl ammonium, which has been used widely in purification of AChE from various insect species; and (2) 20-200-fold higher substrate-inhibition thresholds for ATC and AbetaMTC than AChE from other insect species. These biochemical properties may reflect structural differences of AChE purified from the greenbug compared with that from other insect species.     

Zn2+-dependent acid p-nitrophenylphosphatase from chicken liver. Purification, characterization and subcellular localization

1. Zn2+-dependent acid p-nitrophenylphosphatase from chicken liver was purified to homogeneity. 2. The purified enzyme moves as a single electrophoretic band at pH 8.3 in 7.5% acrylamide and was coincident with the enzyme activity. 3. Gel filtration on Sephadex G-200 gave an apparent molecular weight of 110,000 with two apparent identical subunits of 54,000-56,000 as determined by sodium dodecyl sulphate gel electrophoresis. 4. The maximum of enzyme activity was obtained in the presence of 3-5 mM ZnCl2 at pH 6-6.2, however, higher concentrations of metal are inhibitory. The enzyme hydrolyses p-nitrophenylphosphate, o-carboxyphenylphosphate and phenylphosphate, was insensitive to NaF and was inhibited by phosphate and ATP. The Km for p-nitrophenylphosphate was 0.28 x 10(-3)M at pH 6 in 50 mM sodium acetate/100 mM NaCl. 5. Phosphate is a competitive inhibitor (Ki = 0.5 x 10(-3)M) whereas ATP seems to be a non-competitive inhibitor (Ki = 0.35 x 10(-3)M). The isoelectric point determined by isoelectric focusing on polyacrylamide gel is 7.5. 6. Cell fractionation studies indicate that the Zn2+-dependent acid p-nitrophenylphosphatase of chicken liver is a soluble enzyme form.     

Structural homology between mouse liver and horse spleen ferritins

Mouse liver ferritin is composed almost exclusively of polypeptide chains similar in molecular mass (22 kDa) to that characteristic of the major chain (H) found in heart ferritin isolated from human, horse or rat. In these species the predominant polypeptide of liver (L) is smaller (about 20 kDa). Here we show that mouse liver and horse spleen ferritins and apoferritins exhibit extensive structural homology as judged by the similarity in the diffraction patterns of their crystals grown from cadmium sulphate solutions. Implications of this finding are discussed.