Human serotransferrin: structure and function

Proteomics identification of overexpressed serum proteins in dogs with Babesiacanis infection

Background and Aim

Canine babesiosis, caused by the protozoan parasite Babesia canis, is characterized by clinical manifestations, including hemolytic anemia, thrombocytopenia, multiple organ failure, and may result in death. This disease is detected using conventional blood smears, which are time-consuming and have low sensitivity. This study aimed to investigate a more rapid and sensitive method for detecting B. canis infection in dogs by examining the expressed serum protein profiles using proteomics.

Materials and Methods

We collected six sera samples from three healthy and three B. canis-infected dogs diagnosed using blood smear and polymerase chain reaction. We analyzed the proteins using two-dimensional gel electrophoresis. The candidate spots from the gel were subjected to protein identification using a nano-liquid chromatography system coupled to an ion-trap mass spectrometer equipped with an electrospray ionization nano-sprayer.

Results

We found that 10 protein spots were overexpressed in the serum samples from infected dogs compared with healthy dogs, which corresponded to three proteins: serotransferrin, serotransferrin isoforms X1, and hemopexin. Furthermore, analysis of the protein-protein interaction network confirmed that they strongly interacted with each other.

Conclusion

This study suggests that high levels of serotransferrin and hemopexin are related to B. canis infection, making these proteins potential candidates for the development of diagnostic molecules or vaccines.

Diagnostic potential of site-specific serotransferrin N-glycosylation in discriminating different liver diseases

BACKGROUND

Altered glycosylation modulates the structure and function of disease-related proteins. The associations between serotransferrin (STF) N-glycosylation and liver diseases (LDs) have been revealed. However, how intact N-glycopeptides vary among different types of liver diseases remains unclear.

METHODS

Intact STF N-glycopeptides from patients with chronic liver disease (CLD, n = 92), primary liver cancer (PLC, n = 123), metastatic liver cancer (MLC, n = 57), and healthy controls (HCs, n = 59) were determined using high-resolution mass spectrometry.

RESULTS

Significant changes were displayed in STF glycosylation among 4 groups. The LD screening model, including Asn432 G1S/G2S, Asn432 G2S/G2S2, and Asn630 G2NS2/G2FNS2, was constructed to differentiate LDs from HCs, with a AUC of 0.92. The liver cancer (LC) diagnostic model, a combination of Asn432 G1-N/G1S-N, Asn432 G1/G2, Asn432 G2FS/G2FS2, and Asn630 G1S-N /G1S, showed good performance in discriminating LC from CLD (AUC = 0.93). Moreover, AFP-negative LC patients (93 %) were successfully predicted by the LC diagnostic model. Furthermore, the MLC triage model, composed of Asn432 G1/G2, Asn432 G3F/G3FS, Asn630 G2/G2S, Asn630 G2S2/G2NS2, and Asn630 G3FS/G3FS2, yielded an AUC of 0.98 between PLC and MLC.

CONCLUSIONS

STF N-glycosylation is a potential biomarker for the accurate classification of different LDs.

Serum α2,6-sialylated glycoform of serotransferrin as a glycobiomarker for diagnosis and prediction of clinical severity in cholangiocarcinoma

BACKGROUND

Glycoprotein sialylation changes are associated with severe development of various cancers. We previously discovered the sialylation of serotransferrin (TF) in cholangiocarcinoma (CCA) using glycoproteomics approach. However, a simple and reliable method for validating sialylation of a specific glycobiomarker is urgently needed.

METHODS

We identified the altered glycosylation in CCA tissues by glycoproteomics approach using mass spectrometry. An enzyme-linked lectin assay (ELLA) was developed for determining the serum levels of sialylated TF in CCA, hepatocellular carcinoma (HCC) and healthy controls in training and validation cohorts.

RESULTS

The nine highly sialylated glycoforms of TF were markedly abundant in CCA tumor tissues than in control. Serum SNA-TF and MAL1-TF were significantly higher in CCA patients. Under receiver operating characteristic curve, serum SNA-TF concentrations significantly differentiated CCA from healthy control. Higher SNA-TF were significantly correlated with severe tumor stages and lymph node metastasis. The combined SNA-TF, MAL1-TF, and CA19-9 as a novel glycobiomarkers panel demonstrated the highest specificity (96.2%) for distinguishing CCA from HCC patients. In CCA patients with low CA19-9 levels, SNA-TF in combination with CA19-9 achieved in 97% diagnostic accuracy.

CONCLUSIONS

Sialylated serotransferrin glycoforms could be used as a novel glycobiomarker for diagnosis and prediction of clinical severity in CCA patients.

Validation of Serotransferrin in the Serum as Candidate Biomarkers for the Diagnosis of Pulmonary Tuberculosis by Label-Free LC/MS

This study aimed to identify secreted protein biomarkers in serum from the label-free LC/MS proteomics of neutrophils in pulmonary tuberculosis (TB) patients for the diagnosis biomarkers of TB label-free LC/MS. The proteomic profiles of neutrophils from 15 active TB patients and 15 healthy controls (HCs) were analyzed using label-free LC/MS. We identified 358 differentially expressed proteins preliminarily, including 279 up-regulated proteins and 79 down-regulated proteins. Thirty-eight differentially expressed secreted proteins involved in the progress of platelet degranulation between TB patients and HCs were focused. Of these, serotransferrin (TRF), alpha-2-macroglobulin (AMG), alpha-1-antitrypsin (AAT), alpha-1-acid glycoprotein 1 (AAG), alpha-1-acid glycoprotein 2 (AGP2), and alpha-1B-glycoprotein (A1BG) were selected for further verification in the serum of additional 134 TB patients and 138 HCs by nephelometry and ELISA in the training set. Statistically significant differences of TRF (P < 0.0001), AAT (P < 0.0001), AAG (P < 0.0001), AGP2 (P < 0.0001), and A1BG (P = 0.0003) were observed. The serum concentration of TRF was down-regulated in TB patients compared with healthy controls, which was coincident with the proteomics results. An additional validation of TRF was performed in an independent cohort of patients with active TB (n = 46), patients with lung cancer (n = 37), 20 HCs, and patients with pneumonia (n = 35) in the test set by nephelometry. The serum expression levels of TRF in the TB patients showed lower levels compared with those in patients with pneumonia (P = 0.0125), lung cancer (P = 0.0005), HCs (P < 0.0001), and the non-TB controls (P < 0.0001). Furthermore, the AUC value of TRF was 0.647 with 90.22% sensitivity and 42.86% specificity in discriminating the TB group from the pneumonia group, 0.702 with 93.48% sensitivity and 47.16% specificity in discriminating the TB group from the lung cancer group, 0.894 with 91.30% sensitivity and 71.62% specificity in discriminating the TB group from all HCs, and 0.792 with 91.30% sensitivity and 58.90% specificity in discriminating the TB group from the non-TB controls. This study obtained the proteomic profiles of neutrophils in the TB patients and HCs, which contribute to a better understanding of the pathogenesis molecules existing in the neutrophils of pulmonary tuberculosis and provide candidate biomarkers for the diagnosis of pulmonary tuberculosis.

Superstoichiometric Binding of the Anticancer Agent Titanocene Dichloride by Human Serum Transferrin and the Accompanying Lobe Closure

Titanocene dichloride (TDC) is an anticancer agent that delivers Ti(IV) into each of the two Fe(III) binding sites of bilobal human serum transferrin (Tf). This protein has been implicated in the selective transport of Ti(IV) to cells. How Ti(IV) might be released from the Tf Fe(III) binding site has remained a question, and crystal structures have raised issues about lobe occupancy and lobe closure in Ti(IV)-loaded Tf, compared with the Fe(III)-loaded form. Here, inductively coupled plasma optical emission spectroscopy reveals that Tf can stabilize toward hydrolytic precipitation more than 2 equiv of Ti, implying superstoichiometric binding beyond the two Fe(III) binding sites. Further studies support the inability of TDC to induce a complete lobe closure of Tf. Fluorescence data for TDC binding at low equivalents of TDC support an initial protein conformational change and lobe closure upon Ti binding, whereas data at higher equivalents support an open lobe configuration. Spectroscopic titration reveals less intense protein-metal electronic transitions as TDC equivalents are increased. Denaturing urea-PAGE gels and small angle X-ray scattering studies support an open lobe conformation. The concentrations of bicarbonate used in some earlier studies are demonstrated here to cause a pH change over time, which may contribute to variation in the apparent molar absorptivity associated with Ti(IV) binding in the Fe binding site. Finally, Fe(III)-bound holo-Tf still stabilizes TDC toward hydrolytic precipitation, a finding that underscores the importance of the interactions of Tf and TDC outside the Fe(III) binding site and suggests possible new pathways of Ti introduction to cells.

Proteome profiling of human placenta reveals developmental stage-dependent alterations in protein signature

Introduction

Placenta is a complex organ that plays a significant role in the maintenance of pregnancy health. It is a dynamic organ that undergoes dramatic changes in growth and development at different stages of gestation. In the first-trimester, the conceptus develops in a low oxygen environment that favors organogenesis in the embryo and cell proliferation and angiogenesis in the placenta; later in pregnancy, higher oxygen concentration is required to support the rapid growth of the fetus. This oxygen transition, which appears unique to the human placenta, must be finely tuned through successive rounds of protein signature alterations. This study compares placental proteome in normal first-trimester (FT) and term human placentas (TP).

Methods

Normal human first-trimester and term placental samples were collected and differentially expressed proteins were identified using two-dimensional liquid chromatography-tandem mass spectrometry.

Results

Despite the overall similarities, 120 proteins were differently expressed in first and term placentas. Out of these, 72 were up-regulated and 48 were down-regulated in the first when compared with the full term placentas. Twenty out of 120 differently expressed proteins were sequenced, among them seven showed increased (GRP78, PDIA3, ENOA, ECH1, PRDX4, ERP29, ECHM), eleven decreased (TRFE, ALBU, K2C1, ACTG, CSH2, PRDX2, FABP5, HBG1, FABP4, K2C8, K1C9) expression in first-trimester compared to the full-term placentas and two proteins exclusively expressed in first-trimester placentas (MESD, MYDGF).

Conclusion

According to Reactome and PANTHER softwares, these proteins were mostly involved in response to chemical stimulus and stress, regulation of biological quality, programmed cell death, hemostatic and catabolic processes, protein folding, cellular oxidant detoxification, coagulation and retina homeostasis. Elucidation of alteration in protein signature during placental development would provide researchers with a better understanding of the critical biological processes of placentogenesis and delineate proteins involved in regulation of placental function during development.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12014-021-09324-y.

The Relationship Between Salivary Redox, Diet, and Food Flavor Perception

The mouth is the gateway for entrance of food and microorganisms into the organism. The oral cavity is bathed by saliva, which is thus the first fluid that food and microorganisms will face after their entrance. As a result, saliva plays different functions, including lubrication, predigestion, protection, detoxification, and even transport of taste compounds to chemoreceptors located in the taste buds. To ensure its function of protection, saliva contains reactive harmful compounds such as reactive oxygen species that are controlled and neutralized by the antioxidant activity of saliva. Several antioxidant molecules control the production of molecules such as reactive oxygen compounds, neutralize them and/or repair the damage they have caused. Therefore, a balance between reactive oxidant species and antioxidant compounds exists. At the same time, food can also contain antioxidant compounds, which can participate in the equilibrium of this balance. Numerous studies have investigated the effects of different food components on the antioxidant capacity of saliva that correspond to the ability of saliva to neutralize reactive oxygen species. Contradictory results have sometimes been obtained. Moreover, some antioxidant compounds are also cofactors of enzymatic reactions that affect flavor compounds. Recent studies have considered the salivary antioxidant capacity to explain the release of flavor compounds ex vivo or in vivo. This article aims to review the effect of food on the antioxidant capacity of saliva and the impact of salivary antioxidant capacity on flavor perception after a brief presentation of the different molecules involved.

Molecular dynamics simulations of plutonium binding and its decorporation from the binding-cleft of human serum transferrin

The possibility of plutonium (Pu) intake by radiation workers can not be ruled out. Transportation of Pu(IV) to various organs/cells is mainly carried through iron-carrying protein, serum transferrin (sTf), by receptor-mediated endocytosis. Understanding the Pu-sTf interaction is a primary step toward future design of its decorporating agents. We report MD simulations of Pu(IV) binding with sTf and look out for its decorporation at extracellular pH using suitable ligands. MD simulations were carried out in polarizable water environment at different protonation states of the protein. Results unravel the binding motif of Pu(IV): (1) sTf binds the ion in closed conformation at extracellular serum pH with carbonate as synergistic anions, (2) change in protonation state of dilysine (K206 and K296)-trigger and that of the carbonate ion at acidic endosomal pH is found to cause conformational changes of protein, conducive for the heavy ion to be released, although; (3) strong electrostatic interaction between D63 in the binding-cleft and Pu(IV) is found not to ever set free the ion. In an endeavour to decorporate Pu(IV), fragmented molecular form of hydroxypyridinone (HOPO) and catechol (CAM)-based ligands are docked at the binding site (BS) of the protein and metadynamics simulations are conducted. Pu(IV) binding at BS is found to be so strong that it was not detached from BS with the docked HOPO. However, for the identical set of simulation parameters, CAM is found to facilitate dislodging the heavy ion from the protein's binding influence. Differential behaviour of the two chelators is further explored. Fragmented molecular form of hydroxy-pyridinone (HOPO) and catecholamide (CAM) ligands were docked at the binding-site (BS) of human serum transferrin (sTf) to explore their feasibility as plausible Pu(IV) decorporating agents by employing metadynamics method. CAM was found to dislodge Pu from the sTf BS, while HOPO could not.

Surface roughness influences the protein corona formation of glycosylated nanoparticles and alter their cellular uptake

Recently the role of protein absorption in nanoparticle drug delivery has gathered significant attention as the protein corona can significantly decide on the fate of nanoparticles in the body. Although it is known that the surface chemistry will significantly influence the amount and type of bound protein, there is little known about the effect of surface roughness and surface topography on the interaction. In this work, we show how patchy nanoparticles can noticeably reduce the adsorption of proteins compared to spherical nanoparticles with a smooth surface as demonstrated using six ABC triblock terpolymers based on glucose, mannose and galactose. To obtain patchy nanoparticles, poly(2-d-sugar ethyl acrylate)-b-poly (n-butyl acrylate)-b-poly(4-vinyl pyridine) (PSugEA-b-PBuA-b-P4VP) was prepared by reversible addition-fragmentation chain-transfer (RAFT) polymerization and assembled into nanoparticles with a patch-like appearance and a hydrodynamic diameter of around 130-160 nm. As control, smooth nanoparticles were prepared from poly(2-d-sugar ethyl acrylate)-b-poly (n-butyl acrylate)-b-polystyrene (PSugEA-b-PBuA-b-PS). The patchy nanoparticles displayed significantly reduced protein absorption when exposed to serum-supplemented cell culture media, as observed using dynamic light scattering. The smooth particles, however, supported the formation of a large protein corona. Additionally, an enrichment of haemoglobin was observed in the corona compared to the serum protein in solution. The amount of albumin on the surface was observed to be dependent on the type of sugar with glucose resulting in the highest absorption. The protein corona led to cellular uptake that was unrelated to the underlying sugar, which was supposed to help targeting specific cell lines. This example demonstrated how the protein corona can override any attempts to target receptor expressing cells.

Binding of Cm(III) and Th(IV) with Human Transferrin at Serum pH: Combined QM and MD Investigations

Human serum transferrin (sTf) can also function as a noniron metal transporter since only 30% of it is typically saturated with a ferric ion. While this function of sTf can be fruitfully utilized for targeted delivery of certain metal therapeutics, it also runs the risk of trafficking the lethal radionuclides into cells. A large number of actinide (An) ions are known to bind to the iron sites of sTf although molecular-level understanding of their binding is unclear. Understanding the radionuclide interaction with sTf is a primary step toward future design of their decorporating agents since irrespective of the means of contamination, the radionuclides are absorbed and transported by blood before depositing into target organs. Here, we report an extensive multiscale modeling approach of two An (curium(III) and thorium(IV)) ions' binding with sTf at serum physiological pH. We find that sTf binds both the heavy ions in a closed conformation with carbonate as synergistic anions and the An-loaded sTf maintains its closed conformation even after 100 ns of equilibrium molecular dynamics (MD) simulations. MD simulations are performed in a polarizable water environment, which also incorporates electronic continuum corrections for ions via charge rescaling. The molecular details of the An coordination and An exchange free energies with iron in the interdomain cleft of the protein are evaluated through a combination of quantum mechanical (QM) and MD studies. In line with reported experimental observations, well-tempered metadynamics results of the ions' binding energetics show that An-sTf complexes are less stable than Fe-sTf. Additionally, curium(III) is found to bind more weakly than thorium(IV). The latter result might suggest relative attenuation of thorium(IV) cytotoxicity when compared with curium(III).

Exploring oxidovanadium(iv) homoleptic complexes with 8-hydroxyquinoline derivatives as prospective antitrypanosomal agents

[V^IVO(L-H)₂] and [V^VO(OCH₃)(L-H)₂] compounds of 8-hydroxyquinoline derivatives L showed activity againstTrypanosoma cruziandLeishmania infantumand high selectivities. Metallomics and interaction with BSA, apo-HTF and DNA were studied.

Real-Time Binding Monitoring between Human Blood Proteins and Heavy Metal Ions in Nanoporous Anodic Alumina Photonic Crystals

This study reports on the real-time binding assessment between heavy metal ions and blood proteins immobilized onto nanoporous anodic alumina photonic crystals (NAA-PCs) by reflectometric interference spectroscopy (RIfS). The surface of NAA-PCs is chemically functionalized with γ-globulin (GG), transferrin (TFN), and serum albumin (HSA), the major proteins present in human blood plasma. Protein-modified NAA-PC platforms are exposed to analytical solutions of mercury ions of different concentrations. Dynamic changes in the effective optical thickness of protein-modified NAA-PCs in response to heavy metal ions are assessed in real time to evaluate the binding kinetics, affinity, and mechanism. Protein molecules undergo conformational changes upon exposure to mercury ions, with HSA exhibiting the strongest affinity. The combination of protein-modified NAA-PCs with RIfS allows real-time monitoring of protein-heavy metal ions interactions under dynamic flow conditions. This system is capable of detecting dynamic conformational changes in these proteins upon exposure to heavy metal ions. Our results provide new insights into these binding events, which could enable new methodologies to study the toxicity of heavy metal ions and other biomolecular interactions.

Incorporation of transuranium elements: coordination of Cm(iii) to human serum transferrin

Structure determination of Cm(iii)-transferrin by a combined spectroscopic and theoretical approach gives insight into the biochemical behaviour of incorporated actinides.

Interaction of [VIV O(acac)2 ] with Human Serum Transferrin and Albumin

[VO(acac)₂ ] is a remarkable vanadium compound and has potential as a therapeutic drug. It is important to clarify how it is transported in blood, but the reports addressing its binding to serum proteins have been contradictory. We use several spectroscopic and mass spectrometric techniques (ESI and MALDI-TOF), small-angle X-ray scattering and size exclusion chromatography (SEC) to characterize solutions containing [VO(acac)₂ ] and either human serum apotransferrin (apoHTF) or albumin (HSA). DFT and modeling protein calculations are carried out to disclose the type of binding to apoHTF. The measured circular dichroism spectra, SEC and MALDI-TOF data clearly prove that at least two VO-acac moieties may bind to apoHTF, most probably forming [V^IV O(acac)(apoHTF)] complexes with residues of the HTF binding sites. No indication of binding of [VO(acac)₂ ] to HSA is obtained. We conclude that V^IV O-acac species may be transported in blood by transferrin. At very low complex concentrations speciation calculations suggest that [(VO)(apoHTF)] species form.

A Spectroscopy Approach for the Study of the Interaction of Oxovanadium(IV)-Salen Complexes with Proteins

Oxovanadium(IV)-salen complexes bind with bovine serum albumin (BSA) and ovalbumin (OVA) strongly with binding constant in the range 10(4)-10(7) M(-1) at physiological pH (7.4) confirmed using UV-visible absorption, fluorescence spectral and circular dichroism (CD) study. CD results show that the binding of oxovanadium(IV) complexes induces the conformational change with the loss of α-helicity in the proteins. Docking studies indicate that mode of binding of oxovanadium(IV)-salen complexes with proteins is hydrophobic in nature.

Influence of carbonate on the complexation of Cm(iii) with human serum transferrin studied by time-resolved laser fluorescence spectroscopy (TRLFS)

The influence of carbonate on the complexation of Cm(iii) with transferrin is investigated using TRLFS. The results prove directly that carbonate acts as a synergistic anion for Cm(iii) complexation with transferrin.

Interaction of antidiabetic vanadium compounds with hemoglobin and red blood cells and their distribution between plasma and erythrocytes

The interaction of V(IV)O(2+) ion with hemoglobin (Hb) was studied with the combined application of spectroscopic (EPR), spectrophotometric (UV-vis), and computational (DFT methods) techniques. Binding of Hb to V(IV)O(2+) in vitro was proved, and three unspecific sites (named α, β, and γ) were characterized, with the probable coordination of His-N, Asp-O(-), and Glu-O(-) donors. The value of log β for (VO)Hb is 10.4, significantly lower than for human serum apo-transferrin (hTf). In the systems with V(IV)O potential antidiabetic compounds, mixed species cis-VOL2(Hb) (L = maltolate (ma), 1,2-dimethyl-3-hydroxy-4(1H)-pyridinonate (dhp)) are observed with equatorial binding of an accessible His residue, whereas no ternary complexes are observed with acetylacetonate (acac). The experiments of uptake of [VO(ma)2], [VO(dhp)2], and [VO(acac)2] by red blood cells indicate that the neutral compounds penetrate the erythrocyte membrane through passive diffusion, and percent amounts higher than 50% are found in the intracellular medium. The biotransformation of [VO(ma)2], [VO(dhp)2], and [VO(acac)2] inside the red blood cells was proved. [VO(dhp)2] transforms quantitatively in cis-VO(dhp)2(Hb), [VO(ma)2] in cis-VO(ma)2(Hb), and cis-VO(ma)2(Cys-S(-)), with the equatorial coordination of a thiolate-S(-) of GSH or of a membrane protein, and [VO(acac)2] in the binary species (VO)xHb and two V(IV)O complexes with formulation VO(L(1),L(2)) and VO(L(3),L(4)), where L(1), L(2), L(3), and L(4) are red blood cell bioligands. The results indicate that, in the studies on the transport of a potential pharmacologically active V species, the interaction with red blood cells and Hb cannot be neglected, that a distribution between the erythrocytes and plasma is achieved, and that these processes can significantly influence the effectiveness of a V drug.

Interaction of insulin-enhancing vanadium compounds with human serum holo-transferrin

The interaction of VO(2+) ion and four insulin-enhancing compounds, [VO(ma)2], [VO(dhp)2], [VO(acac)2], and cis-[VO(pic)2(H2O)], where Hma, Hdhp, Hacac, and Hpic are maltol, 1,2-dimethyl-3-hydroxy-4(1H)-pyridinone, acetylacetone, and picolinic acid, with holo-transferrin (holo-hTf) was studied through the combined application of electron paramagnetic resonance (EPR) and density functional theory (DFT) methods. Since in holo-hTf all of the specific binding sites of transferrin are saturated by Fe(3+) ions, VO(2+) can interact with surface sites (here named sites C), probably via the coordination of His-N, Asp-COO(-), and Glu-COO(-) donors. In the ternary systems with the insulin-enhancing compounds, mixed species are observed with Hma, Hdhp, and Hpic with the formation of VOL2(holo-hTf), explained through the interaction of cis-[VOL2(H2O)] (L = ma, dhp) or cis-[VOL2(OH)](-) (L = pic) with an accessible His residue that replaces the monodentate H2O or OH(-) ligand. The residues of His-289, His-349, His-473, and His-606 seem the most probable candidates for the complexation of the cis-VOL2 moiety. The lack of a ternary complex with Hacac was attributed to the square-pyramidal structure of [VO(acac)2], which does not possess equatorial sites that can be replaced by the surface His-N. Since holo-transferrin is recognized by the transferrin receptor, the formation of ternary complexes between VO(2+) ion, a ligand L(-), and holo-hTf may be a way to transport vanadium compounds inside the cells.

Interaction of vanadium(IV) with human serum apo-transferrin

The interaction of V(IV)O-salts as well as of a few V(IV)O(carrier)n complexes with human serum transferrin (hTF) is studied focusing on the determination of the nature and stoichiometry of the binding of V(IV)O(2+) to hTF, as well as whether the conformation of hTF upon binding to V(IV)O(2+) or to its complexes is changed. Circular dichroism (CD) spectra measured for solutions containing V(IV)O(2+) and apo-hTF, and V(IV)O-maltol and apo-hTF, clearly indicate that hTF-V(IV)O-maltol ternary species form with a V(IV)O:maltol stoichiometry of 1:1. For V(IV)O salts and several V(IV)O(carrier)n complexes (carrier ligand=maltolato, dhp, picolinato and dipicolinato) (Hdhp=1,2-dimethyl-3-hydroxy-4-pyridinone) the maximum number of V(IV)O(2+) bound per mole of hTF is determined to be ~2 or lower in all cases. The binding of V(IV)O to apo-hTF most certainly involves several amino acid residues of the Fe-binding site, and as concluded by urea gel electrophoresis experiments, the formation of (V(IV)O)2hTF species may occur with the closing of the hTF conformation as is the case in (Fe(III))2hTF, which is an essential feature for the transferrin receptor recognition.

Transport of the anti-diabetic VO2+ complexes formed by pyrone derivatives in the blood serum

The biotransformation in the blood serum of the two anti-diabetic agents [VO(ema)(2)] - or BEOV - and [VO(koj)(2)] formed by ethylmaltol (Hema) and kojic acid (Hkoj) was studied with EPR spectroscopy, pH-potentiometry and DFT calculations. For comparison, the behavior of the systems with tropolone (Htrop) was also analyzed. The interaction of [VO(ema)(2)] and [VO(koj)(2)] with the most important bioligands of the serum, lactic (Hlact) and citric acid (H(3)citr), human serum transferrin (hTf), human serum albumin (HSA) and immunoglobulin G (IgG) was examined and discussed. Among the several mixed species observed, cis-VO(carrier)(2)(hTf), cis-VO(carrier)(2)(HSA) and cis-VO(carrier)(2)(IgG), where carrier is ethylmaltolate or kojate, with a His-N of the protein coordinated in the equatorial position, are plausible candidates for the transport processes of the drug toward the target organs. The values of the logβ are in the range 19.6-19.8 for the species formed by ethylmaltol and 17.4-17.6 for those formed by kojic acid. The formation of such species was confirmed through pH-titrations of the model systems VO(2+)/carrier/1-MeIm and VO(2+)/carrier/Ac-his, where 1-MeIm and Ac-his are 1-methylimidazole and N-acetylhistamine, and DFT calculations of (51)V A(z) of the model species cis-[VO(carrier)(2)(1-MeIm)] and cis-[VO(carrier)(2)(Ac-his)]. The values of the stability constants for the mixed species observed were used to predict the biodistribution of VO(2+) ion between the blood serum components for concentrations of 1, 10 and 50 μM.

Pharmaceutical formulation affects titanocene transferrin interactions

Since the discovery of the anticancer activity of titanocene dichloride (TDC), many derivatives have been developed and evaluated. MKT4, a soluble, water-stable formulation of TDC, was used for both Phase I and Phase II human clinical trials. This formulation is investigated here by using (1)H and (13)C NMR, FT-ICR mass spectrometry, and UV/vis-detected pH-dependent speciation. DFT calculations are also utilized to assess the likelihood of proposed species. Human serum transferrin has been identified as a potential vehicle for the Ti anticancer drugs; these studies examine whether and how formulation of TDC as MKT4 may influence its interactions, both thermodynamic and kinetic, with human serum transferrin by using UV/vis absorption and fluorescence quenching. MKT4 binds differently than TDC to transferrin, showing different kinetics of binding as well as a different molar absorptivity of binding (7500 M(-1) cm(-1) per site). Malate, used in the buffer for MKT4 administration, acts as a synergistic anion for Ti binding, shifting the tyrosine to Ti charge transfer energy and decreasing the molar absorptivity to 5000 M(-1) cm(-1) per site. These differences may have had consequences after the change from TDC to MKT4 in human clinical trials.

Anion binding properties of the transferrins. Implications for function

BACKGROUND

Since the transferrins have been defined by the highly cooperative binding of Fe(3+) and a carbonate anion to form an Fe-CO(3)-Tf ternary complex, the focus has been on synergistic anion binding. However, there are other types of anion binding with both apotransferrin and diferric transferrin that affect metal binding and release.

SCOPE OF REVIEW

This review covers the binding of anions to the apoprotein, as well as the formation and structure of Fe-anion-transferrin ternary complexes. It also covers interactions between ferric transferrin and non-synergistic anions that appear to be important in vivo.

GENERAL SIGNIFICANCE

The interaction of anions with apotransferrin can alter the effective metal binding constants, which can affect the transport of metal ions in serum. These interactions also play a role in iron release under physiological conditions.

MAJOR CONCLUSIONS

Apotransferrin binds a variety of anions with no special selectivity for carbonate. The selectivity for carbonate as a synergistic anion is associated with the iron binding reaction. Conformational changes in the binding of the synergistic carbonate and competition from non-synergistic anions both play a role in intracellular iron release. Anion competition also occurs in serum and reduces the effective metal binding affinity of Tf. Lastly, anions bind to allosteric sites (KISAB sites) on diferric transferrin and alter the rates of iron release. The KISAB sites have not been well-characterized, but kinetic studies on iron release from mutant transferrins indicate that there are likely to be multiple KISAB sites for each lobe of transferrin. This article is part of a Special Issue entitled Transferrins: Molecular mechanisms of iron transport and disorders.

Quadrupolar central transition (QCT) and 13C NMR competition studies of metal ion binding to ovotransferrin

The transferrins are a family of relatively large bilobal proteins that play a major role in the transport of Fe3+, as well as several other physiological and nonphysiological metal ions. Transferrins can also act as antimicrobial agents, by tightly sequestering iron and making it unavailable for bacterial growth. Using a combination of quadrupolar central transition (QCT) metal ion NMR (27Al, 45Sc, 51V, and 71Ga) and 13C NMR, the binding and displacement of a variety of metal ions to ovotransferrin was studied through direct metal ion competition experiments. The metal ions investigated (Al3+, Co3+, Fe3+, Ga3+, In3+, Sc3+, Y3+, and VO2+) were of differing ionic radius and charge, thus allowing for an assessment of how these factors contribute to metal ion affinity. The competition for the N- and C-terminal metal ion binding sites on ovotransferrin was directly followed by metal ion QCT NMR. Moreover, 13C NMR was used to study the two protein-bound synergistic anions (13C-labeled carbonate), whose chemical shifts are distinct and dependent on the bound metal ion that is present in the binding sites. The observed order of decreasing affinity for the metal ions studied was Fe3+ ≈ In3+ ≥ Sc3+ ≥ Ga3+ > Al3+ > VO2+ > Y3+ ≥ Co3+. These results illustrate how a combination of multinuclear solution NMR methods can provide unique insights into the ligand binding properties of larger metalloproteins.

Allosteric effects of sulfonate anions on the rates of iron release from serum transferrin

Serum transferrin is the protein that transports ferric ion through the bloodstream and is thus a potential target for iron chelation therapy. However, the release of iron from transferrin to low-molecular-weight chelating agents is usually quite slow. Thus a better understanding of the mechanism for iron release is important to assist in the design of more effective agents for iron removal. This paper describes the effect of sulfonate anions on the rates of iron removal from C-terminal monoferric transferrin by acetohydroxamic acid, deferiprone, nitrilotriacetic acid (NTA), and diethylenetriaminepentaacetic acid at 25°C in 0.1M N-(2-hydroxyethyl)piperazine-N'-(2-ethanesulfonic acid) (Hepes) buffer at pH 7.4. These ligands remove iron via a combination of pathways that show saturation and first order dependence on the ligand concentration. The kinetic effects of the anions methanesulfonate, methylenedisulfonate, and ethylenedisulfonate were evaluated. All these anions increase the overall rates of iron release, presumably by binding to an allosteric anion binding site on the protein. The two disulfonates produce a larger acceleration in iron release than the monosulfonate. More detailed studies using methylenedisulfonate show that this anion accelerates the rate of iron release via the saturation pathway. The addition of methylenedisulfonate results in the appearance of a large saturation pathway for iron release by NTA, which otherwise removes iron by a simple first-order process. The sulfonate group was selected for these studies because it represents an anionic functional group that can be covalently linked to a therapeutic ligand to accelerate iron release in vivo. The current studies indicate that the binding of the sulfonates to the allosteric site on the protein is quite weak, so that one would not expect a significant acceleration in iron release at clinically relevant ligand concentrations.

Interaction of VO2+ ion and some insulin-enhancing compounds with immunoglobulin G

Complexation of VO(2+) ion with the most abundant class of human immunoglobulins, immunoglobulin G (IgG), was studied using EPR spectroscopy. Differently from the data in the literature which report no interaction of IgG with vanadium, in the binary system VO(2+)/IgG at least three sites with comparable strength were revealed. These sites, named 1, 2, and 3, seem to be not specific, and the most probable candidates for metal ion coordination are histidine-N, aspartate-O or glutamate-O, and serinate-O or threoninate-O. The mean value for the association constant of (VO)(x)IgG, with x = 3-4, is log β = 10.3 ± 1.0. Examination of the ternary systems formed by VO(2+) with IgG and human serum transferrin (hTf) and human serum albumin (HSA) allows one to find that the order of complexing strength is hTf ≫ HSA ≈ IgG. The behavior of the ternary systems with IgG and one insulin-enhancing agent, like [VO(6-mepic)(2)], cis-[VO(pic)(2)(H(2)O)], [VO(acac)(2)], and [VO(dhp)(2)], where 6-mepic, pic, acac, and dhp indicate the deprotonated forms of 6-methylpicolinic and picolinic acids, acetylacetone, and 1,2-dimethyl-3-hydroxy-4(1H)-pyridinone, is very similar to the corresponding systems with albumin. In particular, at the physiological pH value, VO(6-mepic)(IgG)(OH), cis-VO(pic)(2)(IgG), and cis-VO(dhp)(2)(IgG) are formed. In such species, IgG coordinates nonspecifically VO(2+) through an imidazole-N belonging to a histidine residue exposed on the protein surface. For cis-VO(dhp)(2)(IgG), log β is 25.6 ± 0.6, comparable with that of the analogous species cis-VO(dhp)(2)(HSA) and cis-VO(dhp)(2)(hTf). Finally, with these new values of log β, the predicted percent distribution of an insulin-enhancing VO(2+) agent between the high molecular mass (hTf, HSA, and IgG) and low molecular mass (lactate) components of the blood serum at physiological conditions is calculated.

Absence of binding between the human transferrin receptor and the transferrin complex of biological toxic trace element, aluminum, because of an incomplete open/closed form of the complex

Human transferrin (Tf) very tightly binds two ferric ions to deliver iron to cells. Fe(III)(2)Tf (Fe(2)Tf) binds to the Tf receptor (TfR) at pH 7.4; however, iron-free Tf (apoTf) does not. Iron uptake is facilitated by endocytosis of the Fe(2)Tf-TfR complex. Tf can also bind aluminum ions, which cause toxic effects and are associated with many diseases. Since Al(III)(2)Tf (Al(2)Tf) does not bind to TfR, the uptake of aluminum by the cells does not occur through a TfR-mediated pathway. We have studied the absence of binding between Al(2)Tf and TfR by investigating the physicochemical characteristics of apoTf, Al(2)Tf, Fe(2)Tf, and TfR. The hydrodynamic radius of 38.8 A for Al(2)Tf obtained by dynamic light scattering was between that of 42.6 A for apoTf and 37.2 A for Fe(2)Tf. The zeta potential of -11.3 mV for Al(2)Tf measured by capillary electrophoresis was close to -11.2 mV for apoTf as compared to -11.9 mV for Fe(2)Tf, indicating that the Al(2)Tf surface had a relatively scarce negative charge as the apoTf surface had. These results demonstrated that the structure of Al(2)Tf was a trade-off between the closed and open forms of Fe(2)Tf and apoTf, respectively. Consequently, it is suggested that Al(2)Tf cannot form specific ionic interresidual interactions, such as those formed by Fe(2)Tf, to bind to TfR, resulting in impossible complex formation between Al(2)Tf and TfR.

On the transport of vanadium in blood serum

The complexation of the VO(2+) ion in several systems that can model the physiological conditions of its transport in blood serum was studied using electron paramagnetic resonance (EPR) spectroscopy. Particularly, the ternary systems formed by (i) VO(2+) and two high-molecular-mass components of blood serum, human serum apo-transferrin (hTf) and human serum albumin (HSA); (ii) VO(2+), hTf, and bL; and (iii) VO(2+), HSA, and bL, where bL is one of the six most important low-molecular-mass bioligands of the blood serum (bL = lactate, citrate, oxalate, phosphate, glycine, or histidine), were examined. The results indicate that, in aqueous solution, transferrin is a stronger binder than albumin, and at the physiological ratio, most of the VO(2+) ion is present as (VO)(2)hTf, and a small amount as (VO)(2)(d)HSA, the dinuclear species formed by albumin where the two metal ions are interacting and the spin state S is 1. Among the bL ligands, only lactate and citrate are able to bind VO(2+) in the presence of transferrin or albumin, the others not interacting at all. Finally, the quaternary systems formed by (i) VO(2+), hTf, HSA, and lactate and (ii) VO(2+), hTf, HSA, and citrate were studied. In these cases, the results suggest that the predominant species is (VO)(2)hTf, followed by the mixed complexes VO(2+)-hTf-lactate or VO(2+)-hTf-citrate, whereas (VO)(2)(d)HSA and [(VO)(2)(citrH(-1))(2)](4-) are minor components at physiological pH. The conclusions of this study give new insights on how the VO(2+) ion distributes among the blood serum components and is transported in the plasma toward the target sites in the organism.

Vanadate complexes in serum: a speciation modeling study

The speciations of two drug candidate ligands, 2-hydroxypyridine-N-oxide (Hhpno) and 2-mercaptopyridine-N-oxide (Hmpno), with vanadate (V(V)) were determined at 25.0 degrees C and 0.20 mol dm(-3) KCl by pH-metric and (51)V-NMR methods. At pH 7.4, the two predominant compounds with both ligands are the VO(2)L(2) and VO(2)L(OH). NH(4)[VO(2)(hpno)(2)] x 3 H(2)O was prepared in solid form, and its crystal structure was determined by X-ray diffraction. The stabilities of the complexes VO(2)L(2) of five drug candidate ligands were compared at pH 7.4. In view of the stability sequence hpno > maltol approximately hdp (Hhdp: 3-hydroxy-1,2-dimethyl-4-pyridinone) >> mpno > picolinic acid, the first two of these ligands were chosen for equilibrium studies with apotransferrin (apoTf) competition. The V(V)-apoTf stability constants (log K(1) = 6.03 +/- 0.10; log K(2) = 5.46 +/- 0.18) determined by (51)V-NMR spectroscopy were confirmed by ultrafiltration. Both methods proved that there seems to be no hydrogencarbonate-vanadate competition for the apoTf anion-binding positions. The other potential high molecular mass V(V) binder in the serum is human serum albumin (HSA). As no interaction was detected by (51)V-NMR spectroscopy or fluorimetry, the binding properties of HSA were quantified on the basis of literature data. As a final conclusion, speciation modeling calculations suggest that, under serum conditions, apoTf is probably the primary metal ion binder, even in the presence of the most stable V(V) carrier ligands hpno and maltol and HSA plays a negligible role in V(V) binding.

New vanadium-based magnetic resonance imaging probes: clinical potential for early detection of cancer

We have developed a magnetic resonance imaging (MRI) method for improved detection of cancer with a new class of cancer-specific contrast agents, containing vanadyl (VO(2+))-chelated organic ligands, specifically bis(acetylacetonato)oxovanadium(IV) [VO(acac)(2)]. Vanadyl compounds have been found to accumulate within cells, where they interact with intracellular glycolytic enzymes. Aggressive cancers are metabolically active and highly glycolytic; an MRI contrast agent that enters cells with high glycolytic activity could provide high-resolution functional images of tumor boundaries and internal structure, which cannot be achieved by conventional contrast agents. The present work demonstrates properties of VO(acac)(2) that may give it excellent specificity for cancer detection. A high dose of VO(acac)(2) did not cause any acute or short-term adverse reactions in murine subjects. Calorimetry and spectrofluorometric methods demonstrate that VO(acac)(2) is a blood pool agent that binds to serum albumin with a dissociation constant K (d) ~ 2.5 +/- 0.7 x 10(-7) M and a binding stoichiometry n = 1.03 +/- 0.04. Owing to its prolonged blood half-life and selective leakage from hyperpermeable tumor vasculature, a low dose of VO(acac)(2) (0.15 mmol/kg) selectively enhanced in vivo magnetic resonance images of tumors, providing high-resolution images of their interior structure. The kinetics of uptake and washout are consistent with the hypothesis that VO(acac)(2) preferentially accumulates in cancer cells. Although VO(acac)(2) has a lower relaxivity than gadolinium-based MRI contrast agents, its specificity for highly glycolytic cells may lead to an innovative approach to cancer detection since it has the potential to produce MRI contrast agents that are nontoxic and highly sensitive to cancer metabolism.

Apotransferrin is internalized and distributed in the same way as holotransferrin in K562 cells

Transferrin (Tf), a naturally existing protein, has received considerable attention in the area of drug targeting since it is biodegradable, non-toxic, and non-immunogenic. The efficient cellular uptake of Tf shows it has potential in the delivery of anti-cancer drugs, proteins, and therapeutic genes into proliferating malignant cells that overexpress transferrin receptor (TfR). In human serum, about 30% of Tf exists in the iron-saturated form (Fe(2)-Tf) and the remainder exists as apotransferrin (apo-Tf). Understanding the uptake of apo-Tf by cells will provide key insights into studies on Tf-mediated drug delivery. In the present study, we investigated visually the transport of apo-Tf into K562 cells and its intracellular localization by laser-scanning confocal microscopy (LSCM) and flow cytometry analysis (FCA). It was found that, like Fe(2)-Tf, apo-Tf can be taken up into the cells. The process is time- and temperature-dependent, competitively inhibited by Fe(2)-Tf, and significantly abolished by pronase pretreatment. Visual evidence showed that the transport of apo-Tf into K562 cells is a TfR-mediated process. Furthermore, the investigations using optical-slicing technique demonstrated that the distribution of apo-Tf is similar to that of Fe(2)-Tf, both appearing in the perinuclear region in ball-in-bowl shape.