Exploring Serum Transferrin Regulation of Nonferric Metal Therapeutic Function and Toxicity

Reinforcing Protein Biochemistry: A Two-Week Experiment Studying Iron(III) Binding by the Transferrin Protein through Stoichiometric Determination, Stability Analysis, and Visualization of the Binding Site

The two-week protein biochemistry experience described herein focuses on reinforcing key biochemical concepts and achieving significant learning domain accomplishments for students (Content Knowledge, Logical Mathematical Reasoning, Visualization, Information Literacy, and Knowledge Integration) and valuable teaching opportunities for instructors. The experience encompasses an exploration of the transport protein serum transferrin as an important regulator of Fe(III) biochemistry and incorporates techniques to assess protein-metal stoichiometry and protein stability and to perform molecular visualization. Students gain practical experience in utilizing spectrophotometric analysis for constructing stoichiometric curves, in performing urea-PAGE, and in applying the PyMOL program to evaluate metal coordination at a protein binding site and the associated protein structural change. The learning and teaching accomplishments provide valuable skills that can be extended into research and translated to other teaching formats.

Analysis of Transferrin in the Urine of Patients with Bladder Cancer Using Nanobodies

It is known that the saturation ratio of transferrin (Tf) with iron in human blood is an important clinical parameter. Specific antibodies can be used to analyze subtle changes in the relative abundance of different forms of transferrin potentially associated with a pathological process. Recently, the authors of this study were able to obtain and characterize highly specific single-domain antibodies (nanobodies) that predominantly recognize the iron-saturated (holo-Tf) or iron-unsaturated (apo-Tf) form of transferrin. In this work, under conditions closer to physiological than in the previous experiments, we further demonstrated that these unique nanobodies have extremely high differential binding specificity for different forms of Tf in different human biological fluids. Using these nanobodies, we were able to analyze for the first time relative abundance of the transferrin forms in urine samples from the patients with bladder cancer (BC). We have shown that increase in the concentration of total Tf in the urine samples normalized for creatinine is associated with the degree of progress and growth of malignancy of BC. In the samples of healthy donors and in the early stages of BC (G1), Tf is detected in much smaller amounts (compared to the later stages) and only with additional concentration of the studied samples. For most of the studied urine samples from the BC patients, it is expected (as previously shown in the case of Tf in the blood of terminal ovarian cancer patients) that the concentration of apo-Tf is clearly higher than holo-Tf, especially in the case of the most advanced muscle-invasive BC. It was a surprise for us that approximately equal amounts of apo-Tf and holo-Tf were found in the urine samples of some patients with BC. We hypothesized that the holo-Tf fraction in this case could be largely represented by the "secondary complexes" formed by apo-Tf in combination with ions other than Fe3+, which accumulate in the urine of some cancer patients and are able to bind to apo-Tf, changing its conformation towards holo-Tf. By using inductively coupled plasma mass spectroscopy (ICP-MS), we obtained first results confirming our hypothesis. Preparation of the holo-Tf in these urine samples was found to be highly enriched in zinc and nickel. Also, relative enrichment in cadmium has been observed in this preparation, but at much lower concentrations. The obtained data indicate that the used nanobody, while recognizing predominantly the iron-saturated form of transferrin (holo-Tf), is also capable of binding transferrin in association with other metal ions that are different from iron. This ability could potentially open up new possibilities for investigation of relative abundance of various metal ions in association with transferrin in human biological fluids in normal and pathological conditions.

A Possible Way to Relate the Effects of SARS-CoV-2-Induced Changes in Transferrin to Severe COVID-19-Associated Diseases

SARS-CoV-2 infections are responsible for the COVID-19 pandemic. Transferrin has been found to explain the link between diseases associated with impaired iron transport and COVID-19 infection. The effect of SARS-CoV-2 on human whole blood was studied by differential scanning calorimetry. The analysis of the thermal transition curves showed that the melting temperature of the transferrin-related peak decreased in the presence of SARS-CoV-2. The ratio of the under-curve area of the two main peaks was greatly affected, while the total enthalpy of the heat denaturation remained nearly unchanged in the presence of the virus. These results indicate that SARS-CoV-2, through binding to transferrin, may influence its Fe³⁺ uptake by inducing thermodynamic changes. Therefore, transferrin may remain in an iron-free apo-conformational state, which depends on the SARS-CoV-2 concentration. SARS-CoV-2 can induce disturbance in erythropoiesis due to toxicity generated by free iron overload.

Advantageous Reactivity of Unstable Metal Complexes: Potential Applications of Metal-Based Anticancer Drugs for Intratumoral Injections

Injections of highly cytotoxic or immunomodulating drugs directly into the inoperable tumor is a procedure that is increasingly applied in the clinic and uses established Pt-based drugs. It is advantageous for less stable anticancer metal complexes that fail administration by the standard intravenous route. Such hydrophobic metal-containing complexes are rapidly taken up into cancer cells and cause cell death, while the release of their relatively non-toxic decomposition products into the blood has low systemic toxicity and, in some cases, may even be beneficial. This concept was recently proposed for V(V) complexes with hydrophobic organic ligands, but it can potentially be applied to other metal complexes, such as Ti(IV), Ga(III) and Ru(III) complexes, some of which were previously unsuccessful in human clinical trials when administered via intravenous injections. The potential beneficial effects include antidiabetic, neuroprotective and tissue-regenerating activities for V(V/IV); antimicrobial activities for Ga(III); and antimetastatic and potentially immunogenic activities for Ru(III). Utilizing organic ligands with limited stability under biological conditions, such as Schiff bases, further enhances the tuning of the reactivities of the metal complexes under the conditions of intratumoral injections. However, nanocarrier formulations are likely to be required for the delivery of unstable metal complexes into the tumor.

Solution speciation and human serum protein binding of indium(III) complexes of 8-hydroxyquinoline, deferiprone and maltol

Solution speciation and serum protein binding of selected In(III) complexes bearing O,O and O,N donor sets were studied to provide comparative data for In(III) and analogous Ga(III) complexes. Aqueous stability of the In(III) complexes of maltol, deferiprone, 8-hydroxyquinoline (HQ) and 8-hydroxyquinoline-5-sulfonate (HQS) was characterized by a combined pH-potentiometric and UV–visible spectrophotometric approach. Formation of mono, bis and tris-ligand complexes was observed. The tris-ligand complexes of HQ (InQ₃) and deferiprone (InD₃) are present in solution in ca. 90% at 10 µM concentration at pH = 7.4, while the tris-maltolato complex (InM₃) displays insufficient stability under these conditions. Binding towards human serum albumin (HSA) and (apo)transferrin ((apo)Tf) of InQ₃, InD₃ and InM₃ complexes and Ga(III) analogue of InQ₃ (GaQ₃) together with InCl₃ was investigated by a panel of methods: steady-state and time-resolved spectrofluorometry, UV–visible spectrophotometry and membrane ultrafiltration. Moderate binding of InQ₃ to HSA was found (log K′ = 5.0–5.1). InD₃ binds to HSA to a much lower extent in comparison to InQ₃. ApoTf is able to displace HQ, deferiprone and maltol effectively from their In(III) complexes. Protein binding of non-dissociated InQ₃ was also observed at high complex-to-apoTf ratios. Studies conducted with the InQ₃/GaQ₃ – HSA – Tf ternary systems revealed the more pronounced Tf binding of In(III) via ligand release, while the original GaQ₃ scaffold is preferably retained upon protein interactions and significant albumin binding occurs. Significant dissociation of InQ₃ was detected in human blood serum as well.

Urea Gel Electrophoresis in Studies of Conformational Changes of Transferrin on Binding and Transport of Non-Ferric Metal Ions

Transferrin (Tf) is a crucial transporter protein for Fe(III), but its biological role in binding other metal ions and their delivery into cells remain highly controversial. The first systematic exploration of the effect of non-Fe(III) metal ion binding on Tf conformation has been performed by urea-polyacrylamide gel electrophoresis (urea-PAGE), which is commonly used for nucleic acids but rarely for proteins. Closed Tf conformation, similar to that caused by Fe(III)-Tf binding, was formed for In(III), V(III) or Cr(III) binding to Tf. In all these cases, metal distribution between Tf lobes and/or the rate of metal release under acidic conditions differed from that of Fe(III)-Tf. By contrast, Ga(III) and V(IV) did not form closed Tf conformation under urea-PAGE conditions. Apart from Fe(III), only In(III) was able to increase the proportion of closed Tf conformation in whole serum. These results suggest that Tf is unlikely to act as a natural carrier of any metal ion, except Fe(III), into cells but can reduce toxicity of exogenous metal ions by binding them in serum and preventing their entry into cells.

New titanocene derivative with improved stability and binding ability to albumin exhibits high anticancer activity

Titanium-based therapies have emerged as a promising alternative for the treatment of cancer patients, particularly those with cisplatin resistant tumors. Unfortunately, some titanium compounds show stability and solubility problems that have hindered their use in clinical practice. Here, we designed and synthesized a new titanium complex containing a titanocene fragment, a tridentate ligand to improve its stability in water, and a long aliphatic chain, designed to facilitate a non-covalent interaction with albumin, the most abundant protein in human serum. The stability and human serum albumin affinity of the resulting titanium complex was investigated by UV-Vis absorption and fluorescence spectroscopy techniques. Complex [TiCp₂{(OOC)₂py-O-myr}] (3) (myr = C₁₄H₂₉, py = pyridine) and its analogous [TiCp₂{(OOC)₂py-OH}] (4), lacking the aliphatic chain, showed improved stability in phosphate saline buffer compared with [TiCp₂Cl₂] (1). 3 showed a strong interaction with human serum albumin in a 1:1 stoichiometry. The cytotoxic effect of 3 was higher compared to [TiCp₂Cl₂] in tumor cell lines and showed potential tumor selectivity when assayed in non-tumor human epithelial cells. Finally, 3 showed an antiproliferative effect on cancer cells, decreasing the population in the S phase, and increasing apoptotic cells in a significant manner. All this makes the novel Ti(IV) compound 3 a firm candidate to continue further studies of its therapeutic potential in vitro and in vivo.

Low-molecular-weight chromium-binding substance (LMWCr) may bind and carry Cr(III) from the endosome

Trivalent chromium has been proposed to be transported in vivo from the bloodstream to the tissues via endocytosis by transferrin (Tf), the major iron transport protein in the blood. While Cr(III) loss from the Tf/Tf receptor complex after acidification to pH 5.5 has recently been shown to be sufficiently rapid to be physiologically relevant, the released Cr(III) still must exit the endosome during the time of the endocytosis cycle (circa 15 min). Cr(III) binds too slowly to small ligands such as citrate or ascorbate, or even EDTA, for such complexes to form and be transported from the endosome, while no trivalent ion transporters are known. However, the apo form of the peptide low-molecular-weight chromium-binding substance (LMWCr) can remove Cr(III) from Cr(III)₂-Tf at neutral pH, albeit slowly, and LMWCr is known to be transported from cells after binding Cr(III), although the transporter is not known. LMWCr subsequently carries Cr(III) to the bloodstream ultimately for removal from the body in the urine. The rate of binding of Cr(III) to apoLMWCr was significantly enhanced in the presence of the Tf/Tf receptor complex. These results suggest that apoLMWCr may function to bind Cr(III) released in the endosomes for ultimate removal from the body as part of a Cr(III) detoxification process.

Using X-ray Diffraction Techniques for Biomimetic Drug Development, Formulation, and Polymorphic Characterization

Drug development is a decades-long, multibillion dollar investment that often limits itself. To decrease the time to drug approval, efforts are focused on drug targets and drug formulation for optimal biocompatibility and efficacy. X-ray structural characterization approaches have catalyzed the drug discovery and design process. Single crystal X-ray diffraction (SCXRD) reveals important structural details and molecular interactions for the manifestation of a disease or for therapeutic effect. Powder X-ray diffraction (PXRD) has provided a method to determine the different phases, purity, and stability of biological drug compounds that possess crystallinity. Recently, synchrotron sources have enabled wider access to the study of noncrystalline or amorphous solids. One valuable technique employed to determine atomic arrangements and local atom ordering of amorphous materials is the pair distribution function (PDF). PDF has been used in the study of amorphous solid dispersions (ASDs). ASDs are made up of an active pharmaceutical ingredient (API) within a drug dispersed at the molecular level in an amorphous polymeric carrier. This information is vital for appropriate formulation of a drug for stability, administration, and efficacy purposes. Natural or biomimetic products are often used as the API or the formulation agent. This review profiles the deep insights that X-ray structural techniques and associated analytical methods can offer in the development of a drug.

Biomimetic activity of soluble, well-defined, aqueous Ti(IV)-citrate species toward adipogenesis. An in vitro study

Metal-organic complexes bearing physiological substrates have been the target of several investigations, probing into the potential of well-defined atoxic metalloforms to influence fundamental cellular processes overcoming insulin resistance in Diabetes mellitus 2. Outstanding cases of such metals include zinc and vanadium, both being the target of intense synthetic and biological studies toward insulin mimesis. Owing to the close proximity in the periodic table, an early transition metal, titanium, emerges as another potential candidate of biologically relevant complexation, reflecting species capable of promoting insulin mimetic activity. Driven by the so far explored aqueous synthetic chemistry of Ti(IV)-hydroxycaboxylato systems, a well-defined Ti(IV)-citrate compound was synthesized under physiological conditions, isolated, and characterized, followed by its introduction in biological assays, targeting adipogenic events linked to glucose uptake and catabolism. The mononuclear Ti(IV)-citrate complex was introduced to 3T3-L1 cells, thereby probing into its biological activity (toxicity, morphology, migration, and adipogenic capacity). The results project an atoxic profile for the Ti(IV)-citrate species, thus justifying further incorporation in cellular differentiation processes, leading to mature adipocytes in a time- and concentration-dependent fashion. The experiments suggest that Ti(IV)-citrate is a competent agent promoting fibroblast differentiation, as evidenced by key adipogenic biomarkers, while concurrently exhibiting synergistic/enhancing action with insulin. The collective results show, for the first time, that an appropriately configured soluble-bioavailable complex Ti(IV) form exhibits a distinctly unique bioprofile, thereby lending credence to the notion that titanium metallopharmaceuticals hold merit as competent agents in adipogenesis and insulin mimesis in Diabetes mellitus.