Structure and spatial conformation of the iron-binding sites of transferrins

Identification of altered plasma proteins by proteomic study in valvular heart diseases and the potential clinical significance

BACKGROUND

Little is known about genetic basis and proteomics in valvular heart disease (VHD) including rheumatic (RVD) and degenerative (DVD) valvular disease. The present proteomic study examined the hypothesis that certain proteins may be associated with the pathological changes in the plasma of VHD patients.

METHODS AND RESULTS

Differential protein analysis in the plasma identified 18 differentially expressed protein spots and 14 corresponding proteins or polypeptides by two-dimensional electrophoresis and mass spectrometry in 120 subjects. Two up-regulated (complement C4A and carbonic anhydrase 1) and three down-regulated proteins (serotransferrin, alpha-1-antichymotrypsin, and vitronectin) were validated by ELISA in enlarging samples. The plasma levels (n = 40 for each) of complement C4A in RVD (715.8±35.6 vs. 594.7±28.2 ng/ml, P = 0.009) and carbonic anhydrase 1 (237.70±15.7 vs. 184.7±10.8 U/L, P = 0.007) in DVD patients were significantly higher and that of serotransferrin (2.36±0.20 vs. 2.93±0.16 mg/ml, P = 0.025) and alpha-1-antichymotrypsin (370.0±13.7 vs. 413.0±11.6 µg/ml, P = 0.019) in RVD patients were significantly lower than those in controls. The plasma vitronectin level in both RVD (281.3±11.0 vs. 323.2±10.0 µg/ml, P = 0.006) and DVD (283.6±11.4 vs. 323.2±10.0 µg/ml, P = 0.011) was significantly lower than those in normal controls.

CONCLUSIONS

We have for the first time identified alterations of 14 differential proteins or polypeptides in the plasma of patients with various VHD. The elevation of plasma complement C4A in RVD and carbonic anhydrase 1 in DVD and the decrease of serotransferrin and alpha-1-antichymotrypsin in RVD patients may be useful biomarkers for these valvular diseases. The decreased plasma level of vitronectin - a protein related to the formation of valvular structure - in both RVD and DVD patients might indicate the possible genetic deficiency in these patients.

The toposome, essential for sea urchin cell adhesion and development, is a modified iron-less calcium-binding transferrin

We describe the structure and function of the toposome, a modified calcium-binding, iron-less transferrin, the first member of a new class of cell adhesion proteins. In addition to the amino acid sequence of the precursor, we determined by Edman degradation the N-terminal amino acid sequences of the mature hexameric glycoprotein present in the egg as well as that of its derived proteolytically modified fragments necessary for development beyond the blastula stage. The approximate C-termini of the fragments were determined by a combination of mass spectrometry and migration in reducing gels before and after deglycosylation. This new member of the transferrin family shows special features which explain its evolutionary adaptation to development and adhesive function in sea urchin embryos: (i) a protease-inhibiting WAP domain, (ii) a 280 amino acid cysteine-less insertion in the C-terminal lobe, and (iii) a 240 residue C-terminal extension with a modified cystine knot motif found in multisubunit external cell surface glycoproteins. Proteolytic removal of the N-terminal WAP domain generates the mature toposome present in the oocyte. The modified cystine knot motif stabilizes cell-bound trimers upon Ca-dependent dissociation of hexamer-linked cells. We determined the positions of the developmentally regulated cuts in the cysteine-less insertion, which produce the fragments observed previously. These fragments remain bound to the hexameric 22S particle in vivo and are released only after treatment of the purified toposome with reducing agents. In addition, some soluble smaller fragments with possible signal function are produced. Sequence comparison of five sea urchin species reveals the location of the cell-cell contact site targeted by the species-specific embryo dissociating antibodies. The evolutionary tree of 2-, 1-, and 0-ferric transferrins implies their evolution from a basic cation-activated allosteric design modified to serve multiple functions.

Protease cleavage of iron-transferrin augments pyocyanin-mediated endothelial cell injury via promotion of hydroxyl radical formation

Although a number of bacterium- and host-derived factors have been suggested to contribute to the pathogenesis of Pseudomonas aeruginosa-associated tissue injury, the mechanism remains unclear. We have previously shown that protease modification of iron (Fe)-transferrin generates new iron chelates capable of catalyzing hydroxyl radical (.OH) formation from superoxide and hydrogen peroxide. The latter two oxidants are generated during redox cycling of another P. aeruginosa secretory product, pyocyanin. The lung is a major site of P. aeruginosa infection, with damage to local endothelial cells contributing to the pathogenesis of such infections. Endothelial cells are highly susceptible to oxidant-mediated injury. Therefore, we examined whether pseudomonas elastase-cleaved Fe-transferrin and pyocyanin synergistically enhance pulmonary artery endothelial cell injury via .OH formation. By measuring 51Cr release from cultured endothelial cell monolayers, pseudomonas elastase-cleaved Fe-transferrin significantly augmented cell injury resulting from cellular exposure to sublethal concentrations of pyocyanin. This enhancement in injury was not protease specific, as similar results were obtained with pyocyanin in combination with trypsin- or porcine pancreatic elastase-cleaved Fe-transferrin. The association of iron with the transferrin appeared to be necessary in this process. Supporting the involvement of .OH generation via the Haber-Weiss reaction in augmenting cell injury, catalase, dimethyl thiourea, superoxide dismutase, deferoxamine, and dimethyl sulfoxide significantly inhibited cell injury resulting from exposure to pyocyanin and protease-cleaved Fe-transferrin. Furthermore, spin trapping demonstrated the production of .OH in this cellular system. We conclude that .OH formation resulting from the interaction of protease-cleaved Fe-transferrin and endothelial cell redox cycling of pyocyanin may contribute to P. aeruginosa-associated tissue injury via endothelial cell injury.

Protease-cleaved iron-transferrin augments oxidant-mediated endothelial cell injury via hydroxyl radical formation

Previous work has shown that the Pseudomonas-derived protease, pseudomonas elastase (PAE), can modify transferrin to form iron complexes capable of catalyzing the formation of hydroxyl radical (.OH) from neutrophil (PMN)-derived superoxide (.O2-) and hydrogen peroxide (H2O2). As the lung is a major site of Pseudomonas infection, the ability of these iron chelates to augment oxidant-mediated pulmonary artery endothelial cell injury via release of 51Cr from prelabeled cells was examined. Diferrictransferrin previously cleaved with PAE significantly enhanced porcine pulmonary artery endothelial cell monolayer injury from 2.3-6.3 to 15.8-17.0% of maximum, resulting from exposure to H2O2, products of the xanthine/xanthine oxidase reaction, or PMA-stimulated PMNs. Iron associated with transferrin appeared to be responsible for cell injury. Spin trapping and the formation of thiobarbituric acid-reactive 2-deoxyribose oxidation products demonstrated the production of .OH in this system. The addition of catalase, dimethyl thiourea, and the hydrophobic spin trap, alpha-phenyl-n-terbutyl-nitrone, offered significant protection from injury (27.8-58.2%). Since sites of Pseudomonas infection contain other proteases, the ability of porcine pancreatic elastase and trypsin to substitute for PAE was examined. Results were similar to those observed with PAE. We conclude .OH formation resulting from protease alteration of transferrin may serve as a mechanism of tissue injury at sites of bacterial infection and other processes characterized by increased proteolytic activity.

Transferrin C2 and Alzheimer's disease: another piece of the puzzle found?

A significant increase in the occurrence of the transferrin C2 genetic subtype has been found in patients with Alzheimer's disease. This variant has previously been linked to diseases thought to be associated with free radical damage. We hypothesize that Alzheimer's disease is caused by free radical damage to membranes of endocytic vesicles due to defective binding of iron and aluminium by Tf C2. The aluminium binds to the membranes, creating pores, while the iron reacts with H2O2 and superoxide radicals produced by activated microglia (brain phagocytes), to produce hydroxyl radicals (oxidative toxins), which attack the fatty acids in the membranes through these pores. In order to treat the disease successfully, it would be necessary to alleviate the multiple deficiencies caused by these toxins by constantly providing the cells with antioxidants and other essential nutrients. In addition, a drug that would stimulate the regrowth of neurons is needed.

Interaction of transferrin and its iron-binding fragments with heparin

The interaction of heparin with transferrin (Tf; bovine and rat) and the isolated iron-binding lobes of bovine Tf were investigated. Affinity chromatography of rat Tf on heparin-agarose showed that interaction depended on both the iron content of Tf and the pH of the medium. Both the iron-free and iron-saturated forms of Tf were strongly bound by the column at pH 5.6, but only the iron-free form revealed significant affinity at pH 7.4. Desialylation of Tf moderately promoted interaction, treatment with cyclohexanedione moderately reduced interaction, and succinylation abolished it altogether. In the presence of heparin, iron release from the N-terminal lobe of native bovine Tf was accelerated and from the C-terminal lobe it was slightly reduced. The heparin effect remained qualitatively the same on each lobe after their separation by tryptic digestion and DEAE-cellulose chromatography. The affinity of native bovine Tf for heparin was very close to that of its isolated N-terminal lobe, thus suggesting that it is this portion of the molecule that binds to the glycosaminoglycan. It is concluded that the consequences for iron-binding strength of the two transferrin lobes are diagonally opposite when Tf is bound to heparin as opposed to its natural cell-surface receptor.

Bovine mammary lactoferrin: implications from messenger ribonucleic acid (mRNA) sequence and regulation contrary to other milk proteins

The regulation of bovine mammary lactoferrin, an important component of the antimicrobial defenses of the mammary gland, is poorly understood compared with the other milk proteins. The complete sequence for bovine lactoferrin mRNA shows it to be highly homologous to other lactoferrins and transferrins. However, regional differences in the deduced AA sequence of bovine lactoferrin compared with human lactoferrin and transferrin imply functional differences between them. Steady-state levels of bovine lactoferrin mRNA (by Northern blot) in the bovine mammary gland indicate that bovine lactoferrin expression is minimal in the developing and lactating gland but is strongly induced by mammary involution. The overall regulation of bovine lactoferrin in the mammary gland appears to be contrary to that of the other milk proteins. Features identified in the mRNA of bovine mammary lactoferrin may contribute to the differences in regulation between lactoferrin and other bovine milk proteins and to differences in concentrations of lactoferrin in milk across species. Lactoferrin secretion by bovine mammary cells grown in vitro does not appear to be dependent on prolactin and shows regulation by substratum, serum, and cell population to be different from that for casein. In contrast to casein, efficient secretion of lactoferrin from the cell does not require detachment of collagen substratum.

Separation and characterization of the C-terminal half molecule of bovine lactoferrin

The C-terminal half molecule (C lobe) of bovine lactoferrin was isolated by mild tryptic hydrolysis of lactoferrin followed by gel filtration and ion-exchange chromatography. The identity of the fragment was established by determining its N-terminal and C-terminal amino acid sequences and comparing them with the amino acid sequence of intact lactoferrin. The isoelectric point of the C lobe ranged between pH 6.2 and 6.5 as measured by isoelectric focusing on polyacrylamide gels. The circular dichroic spectrum in the range of 250 to 350 nm of the C lobe differed slightly from that of intact lactoferrin. The pattern of lectin reactivity was similar for both the C lobe and intact lactoferrin. The C lobe showed partial antigenic identity with intact lactoferrin as demonstrated by the double immunodiffusion method, and pH dependence of iron binding of C lobe is the same as that of intact lactoferrin molecule.

Immunological studies of lactoferrin in human placentae

Lactoferrin (LF) and transferrin (Trf) are glycoproteins with strong affinities for ferric ions. Human syncytiotrophoblastic membranes analyzed by enzyme linked immunosorbent assay (ELISA) and immunoblotting were negative with monoclonal and polyclonal antibodies to LF. Immunohistological studies of 35 normal placentae showed that LF was absent from the trophoblast basement membranes, stroma and fetal stem vessel endothelium, but positive cells were occasionally noted in intervillous spaces and fetal stem vessels. In contrast, many LF-positive cells were identified within areas of immunopathology identified by the presence of T cells, HLA-DR-positive macrophages and platelets. Double-antibody experiments showed that the LF-positive cells in these areas reacted with CD15 and CD16 monoclonal antibodies (mAbs), indicating that the cells were polymorphonuclear neutrophils (PMN). PMN from peripheral blood analyzed by flow cytometry and immunocytology also showed reactivities with anti-LF, CD15 and CD16 and we consistently found that circulating PMN reacted better than placental PMN with antibodies to MHC class I antigens and gp 100, (CD67), which is a neutrophil activation marker. PMN adherent within placentae had no detectable MHC class I or CD67 antigens. These findings suggest PMN adherent to placental tissues down-regulate or alter plasma membrane markers. LF appears to play a role in placental inflammation, for LF-positive cells were significantly enriched in areas of immunopathology.

Single-step method for purification of human transferrin from a by-product of chromatographic fractionation of plasma

A rapid, simple and convenient method is described for the isolation, on a pilot scale, of pure and functional human transferrin from an unexploited by-product of chromatographic fractionation of plasma. In a single chromatographic step on DEAE-Spherodex, 97% pure transferrin was obtained in 75% yield. A virus inactivation treatment was included in the preparative process in order to guarantee the safety of the final product, which could be used in culture media.

Pseudomonas and neutrophil products modify transferrin and lactoferrin to create conditions that favor hydroxyl radical formation

In vivo most extracellular iron is bound to transferrin or lactoferrin in such a way as to be unable to catalyze the formation of hydroxyl radical from superoxide (.O2-) and hydrogen peroxide (H2O2). At sites of Pseudomonas aeruginosa infection bacterial and neutrophil products could possibly modify transferrin and/or lactoferrin forming catalytic iron complexes. To examine this possibility, diferrictransferrin and diferriclactoferrin which had been incubated with pseudomonas elastase, pseudomonas alkaline protease, human neutrophil elastase, trypsin, or the myeloperoxidase product HOCl were added to a hypoxanthine/xanthine oxidase .O2-/H2O2 generating system. Hydroxyl radical formation was only detected with pseudomonas elastase treated diferrictransferrin and, to a much lesser extent, diferriclactoferrin. This effect was enhanced by the combination of pseudomonas elastase with other proteases, most prominently neutrophil elastase. Addition of pseudomonas elastase-treated diferrictransferrin to stimulated neutrophils also resulted in hydroxyl radical generation. Incubation of pseudomonas elastase with transferrin which had been selectively iron loaded at either the NH2- or COOH-terminal binding site yielded iron chelates with similar efficacy for hydroxyl radical catalysis. Pseudomonas elastase and HOCl treatment also decreased the ability of apotransferrin to inhibit hydroxyl radical formation by a Fe-NTA supplemented hypoxanthine/xanthine oxidase system. However, apotransferrin could be protected from the effects of HOCl if bicarbonate anion was present during the incubation. Apolactoferrin inhibition of hydroxyl radical generation was unaffected by any of the four proteases or HOCl. Alteration of transferrin by enzymes and oxidants present at sites of pseudomonas and other bacterial infections may increase the potential for local hydroxyl radical generation thereby contributing to tissue injury.

A transferrinlike (hemiferrin) mRNA is expressed in the germ cells of rat testis

In the testis, germ cells which are separated from the serum by the blood-testis barrier rely primarily on the Sertoli cell to obtain nutrients. For example, transferrin synthesized by the Sertoli cell is important in delivering iron from the serum to the developing germ cells. Because of its role in the testis, Sertoli cell transferrin protein and mRNA have been extensively studied. By using RNA blot analysis of rat testicular tissue, we detected a transcript of 2.6 kb which is attributed to transferrin. In addition, we detected a novel mRNA of 0.9 kb which had sequence similarity to the 3' end of transferrin. This 0.9-kb mRNA was present in germ cells, but not Sertoli cells, liver, or brain. The primary source of this mRNA in the testis was round spermatids. Sequence analysis of a cDNA clone showed that this mRNA encoded a protein with sequence similarity to the carboxy terminus of transferrin. Polysome analysis indicated that this transcript was translated and may therefore have importance in the iron metabolism of germ cells. The evolutionary implications between the transferrinlike mRNA germ cells and the gene duplication event which resulted in the diferric binding of transferrin are discussed.

A transferrinlike (hemiferrin) mRNA is expressed in the germ cells of rat testis

In the testis, germ cells which are separated from the serum by the blood-testis barrier rely primarily on the Sertoli cell to obtain nutrients. For example, transferrin synthesized by the Sertoli cell is important in delivering iron from the serum to the developing germ cells. Because of its role in the testis, Sertoli cell transferrin protein and mRNA have been extensively studied. By using RNA blot analysis of rat testicular tissue, we detected a transcript of 2.6 kb which is attributed to transferrin. In addition, we detected a novel mRNA of 0.9 kb which had sequence similarity to the 3' end of transferrin. This 0.9-kb mRNA was present in germ cells, but not Sertoli cells, liver, or brain. The primary source of this mRNA in the testis was round spermatids. Sequence analysis of a cDNA clone showed that this mRNA encoded a protein with sequence similarity to the carboxy terminus of transferrin. Polysome analysis indicated that this transcript was translated and may therefore have importance in the iron metabolism of germ cells. The evolutionary implications between the transferrinlike mRNA germ cells and the gene duplication event which resulted in the diferric binding of transferrin are discussed.

Molecular cloning and sequence analysis of bovine lactotransferrin

The screening of a bovine submaxillary gland cDNA library yielded 25 clones coding for bovine lactotransferrin. The nucleotide sequence of the longest insert contained a protein-coding region of 2115 nucleotides and a 3' non-coding region of 194 nucleotides followed by a poly(A) tract of about 55 nucleotides. The predicted peptide sequence included a 16-amino-acid signal sequence upstream of the first amino acid of the native protein. The identity of the clone was confirmed by matching the amino acid sequence predicted from the cDNA with the N-terminal and tryptic peptide sequences derived from purified bovine milk lactotransferrin, and also by similarity with human and murine lactotransferrins. The cDNA described corresponds to a 705-amino-acid-long preprotein that lacks the start methionine. The sequence of the secreted protein is 689 amino acids long and contains five potential glycosylation sites. Bovine lactotransferrin is 69% and 64% identical to human and murine lactotransferrins, respectively.

Enzymic synthesis of steroid sulphates. XVII. On the structure of bovine estrogen sulphotransferase

Estrogen sulphotransferase plays a major role in controlling intracellular levels of 17 beta-estradiol in human mammary cancer cells and human endometrium. Bovine estrogen sulphotransferase c-DNA has recently been cloned; the encoded protein having a maximum Mr of 35,000 (Nash, A.R. et al. (1988) Aust. J. Biol. Sci. 41, 507-516). Enzyme of Mr 35,000 by SDS-PAGE has now been isolated and cyanogen bromide-cleaved peptides sequenced. The latter were identified in the c-DNA-predicted amino acid sequence which confirms that the active enzyme (Mr approximately 70,000) exists as a dimer of identical subunits. Sequence data on similar peptides isolated from an enzyme preparation containing a protein of Mr 74,000 as the major species on SDS-PAGE, which was previously thought to represent the enzyme, suggested that this protein was transferrin. This was confirmed by PAGE, SDS-PAGE, susceptibility to neuraminidase and reaction with bovine transferrin antibody. Isoelectric focusing experiments show that active enzyme exists in two or three polymorphic forms (pI values 5.3, 5.7 and possibly 5.9) having similar physicochemical properties of polymorphic forms of transferrin so that they overlap on ion-exchange chromatography and PAGE. The enzyme shows some homology to the amino acid sequence close to the Fe-binding site in lactoferrin and the question is raised as to the possible presence of a tightly bound metal in estrogen sulphotransferase involved in the binding of adenosine 3'-phosphate 5'-phosphosulphate.

An overview of iron metabolism at a molecular level

Over the last 10 years there has been steady progress in our understanding of the structure of the iron-binding proteins transferrin and ferritin, and the transferrin receptor. In the last few years there have been very rapid developments in understanding of the genetics of these proteins and the regulation of synthesis. This review includes a description of gene localization and structure, the regulation of protein synthesis and the structure of proteins of the transferrin family, the transferrin receptor and the iron storage protein ferritin.