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.

TiO2 exposure alters transition metal ion quota in Rhodococcus ruber GIN-1.

After exposure to micron-sized TiO2 particles, anatase and/or rutile, Rhodococcus ruber GIN-1 accumulates an increased concentration (2.2 ± 0.2 mg kg-1) of mobilized Ti into its biomass with concomitant decreases in cellular biometals Fe, Zn, and possibly Mn, while levels of Cu and Al are unaffected.

Solution structure, glycan specificity and of phenol oxidase inhibitory activity of Anopheles C-type lectins CTL4 and CTLMA2.

Malaria, the world's most devastating parasitic disease, is transmitted between humans by mosquitoes of the Anopheles genus. An. gambiae is the principal malaria vector in Sub-Saharan Africa. The C-type lectins CTL4 and CTLMA2 cooperatively influence Plasmodium infection in the malaria vector Anopheles. Here we report the purification and biochemical characterization of CTL4 and CTLMA2 from An. gambiae and An. albimanus. CTL4 and CTLMA2 are known to form a disulfide-bridged heterodimer via an N-terminal tri-cysteine CXCXC motif. We demonstrate in vitro that CTL4 and CTLMA2 intermolecular disulfide formation is promiscuous within this motif. Furthermore, CTL4 and CTLMA2 form higher oligomeric states at physiological pH. Both lectins bind specific sugars, including glycosaminoglycan motifs with ß1-3/ß1-4 linkages between glucose, galactose and their respective hexosamines. Small-angle x-ray scattering data supports a compact heterodimer between the CTL domains. Recombinant CTL4/CTLMA2 is found to function in vivo, reversing the enhancement of phenol oxidase activity in dsCTL4-treated mosquitoes. We propose these molecular features underline a common function for CTL4/CTLMA2 in mosquitoes, with species and strain-specific variation in degrees of activity in response to Plasmodium infection.

Crystal structures of sodium-, lithium-, and ammonium 4,5-di-hydroxy-benzene-1,3-di-sulfonate (tiron) hydrates.

The solid-state structures of the Na+, Li+, and NH4+ salts of the 4,5-di-hydroxy-benzene-1,3-di-sulfonate (tiron) dianion are reported, namely disodium 4,5-di-hydroxy-benzene-1,3-di-sulfonate, 2Na+·C6H4O8S22-, µ-4,5-di-hydroxy-benzene-1,3-di-sulfonato-bis-[aqua-lithium(I)] hemihydrate, [Li2(C6H4O8S2)(H2O)2]·0.5H2O, and di-ammonium 4,5-di-hydroxy-benzene-1,3-di-sulfonate monohydrate, 2NH4+·C6H4O8S22-·H2O. Inter-molecular inter-actions vary with the size of the cation, and the asymmetric unit cell, and the macromolecular features are also affected. The sodium in Na2(tiron) is coordinated in a distorted octa-hedral environment through the sulfonate oxygen and hydroxyl oxygen donors on tiron, as well as an inter-stitial water mol-ecule. Lithium, with its smaller ionic radius, is coordinated in a distorted tetra-hedral environment by sulfonic and phenolic O atoms, as well as water in Li2(tiron). The surrounding tiron anions coordinating to sodium or lithium in Na2(tiron) and Li2(tiron), respectively, result in a three-dimensional network held together by the coordinate bonds to the alkali metal cations. The formation of such a three-dimensional network for tiron salts is relatively rare and has not been observed with monovalent cations. Finally, (NH4)2(tiron) exhibits extensive hydrogen-bonding arrays between NH4+ and the surrounding tiron anions and inter-stitial water mol-ecules. This series of structures may be valuable for understanding charge transfer in a putative solid-state fuel cell utilizing tiron.

Elucidating the inhibition of peptidoglycan biosynthesis in Staphylococcus aureus by albocycline, a macrolactone isolated from Streptomyces maizeus.

Antibiotic resistance is a serious threat to global public health, and methicillin-resistant Staphylococcus aureus (MRSA) is a poignant example. The macrolactone natural product albocycline, derived from various Streptomyces strains, was recently identified as a promising antibiotic candidate for the treatment of both MRSA and vancomycin-resistant S. aureus (VRSA), which is another clinically relevant and antibiotic resistant strain. Moreover, it was hypothesized that albocycline's antimicrobial activity was derived from the inhibition of peptidoglycan (i.e., bacterial cell wall) biosynthesis. Herein, preliminary mechanistic studies are performed to test the hypothesis that albocycline inhibits MurA, the enzyme that catalyzes the first step of peptidoglycan biosynthesis, using a combination of biological assays alongside molecular modeling and simulation studies. Computational modeling suggests albocycline exists as two conformations in solution, and computational docking of these conformations to an ensemble of simulated receptor structures correctly predicted preferential binding to S. aureus MurA-the enzyme that catalyzes the first step of peptidoglycan biosynthesis-over Escherichia coli (E. coli) MurA. Albocycline isolated from the producing organism (Streptomyces maizeus) weakly inhibited S. aureus MurA (IC50 of 480 µM) but did not inhibit E. coli MurA. The antimicrobial activity of albocycline against resistant S. aureus strains was superior to that of vancomycin, preferentially inhibiting Gram-positive organisms. Albocycline was not toxic to human HepG2 cells in MTT assays. While these studies demonstrate that albocycline is a promising lead candidate against resistant S. aureus, taken together they suggest that MurA is not the primary target, and further work is necessary to identify the major biological target.

Ti(IV) and the Siderophore Desferrioxamine B: A Tight Complex Has Biological and Environmental Implications.

The siderophore desferrioxamine B (DFOB) binds Ti(IV) tightly and precludes its hydrolytic precipitation under biologically and environmentally relevant conditions. This interaction of DFOB with Ti(IV) is investigated by using spectro-potentiometric and spectro-photometric titrations, mass spectrometry, isothermal titration calorimetry (ITC), and computational modeling. The data from pH 2-10 suggest two one-proton equilibria among three species, with one species predominating below pH 3.5, a second from pH 3.5 to 8, and a third above pH 8. The latter species is prone to slow hydrolytic precipitation. Electrospray mass spectrometry allowed the detection of [Ti(IV) (HDFOB)]2+ and [Ti(DFOB)]+; these species were assigned as the pH < 3.5 and the 3.5 < pH < 8 species, respectively. The stability constant for Ti(IV)-DFOB was determined by using UV/vis-monitored competition with ethylenediaminetetraacetic acid (EDTA). Taking into consideration the available binding constant of Ti(IV) and EDTA, the data reveal values of log ß111 = 41.7, log ß110 = 38.1, and log ß11-1 = 30.1. The former value was supported by ITC, with the transfer of Ti(IV) from EDTA to DFOB determined to be both enthalpically and entropically favorable. Computational methods yielded a model of Ti-DFOB. The physiological and environmental implications of this tight interaction and the potential role of DFOB in solubilizing Ti(IV) are discussed.

Contemplating a role for titanium in organisms.

Titanium is the ninth most abundant element in the Earth's crust and some organisms sequester it avidly, though no essential biological role has yet been recognized. This Minireview addresses how the properties of titanium, especially in an oxic aqueous environment, might make a biological role difficult to recognize. It further considers how new -omic technologies might overcome the limitations of the past and help to reveal a specific role for this metal. While studies with well established model organisms have their rightful place, organisms that are known avid binders or sequesterers of titanium should be promising places to investigate a biological role.

Corona charge selective micelle degradation catalyzed by P. cepacia lipase isoforms.

Micelles derived from poly(ε-caprolactone) (PCL) poly(ethylene glycol)(PEG) diblock copolymers (HO-PCLn-b-PEG32-RX, R = -O(CH2)3S(CH2)2-; X = -CO2(-), -SO3(-), -NH3(+)) show corona charge selective degradation catalyzed by P. cepacia lipase isoforms.

Titanium mineralization in ferritin: a room temperature nonphotochemical preparation and biophysical characterization.

The incremental addition of titanium(III) citrate to H-chain homopolymers of human ferritin results in the formation of 1.5-6.5-nm particles of amorphous TiO(2) within the nanocage of the protein. The mineralization conditions are mild, featuring ambient temperature and no need for photochemical activation. Low ratios of titanium to protein favor intraprotein mineralization, and the products are characterized by stained and unstained transmission electron microscopy, UV-vis spectroscopy, dynamic light scattering, analytical ultracentrifugation, and metal analysis. With up to 1,000 equiv of metal, there is no change to the protein hydrodynamic radius or diffusion constant. There is, however, a systematic shift in the sedimentation coefficient, which confirms mineralization within the protein core.

Titanium(IV) and vitamin C: aqueous complexes of a bioactive form of Ti(IV).

Ascorbic acid is among the biorelevant ligands that render Ti(IV) stable in aqueous solution. A series of pH-dependent titanium(IV) coordination complexes of L-ascorbic acid is described. Directed by spectropotentiometric methods, important aspects of the aqueous interactions in this system are investigated, including ligand binding mode, pH-dependent metal-ligand stoichiometry, and the importance of metal ion-promoted hydrolysis and the binding of hydroxide. Stability constants are determined for all metal ion-ligand-proton complexes by a process of model optimization and nonlinear least-squares fitting of the combined spectropotentiometric titration data to the log ß(MLH) values in the model. A speciation diagram is generated from the set of stability constants described in the model. In the range pH 3-10, the aqueous speciation is characterized by the sequential appearance of the following complexes as a function of added base: Ti(asc)(2)(0) → Ti(asc)(3)(2-) → Ti(asc)(2)(OH)(2)(2-) → Ti(asc)(OH)(4)(2-). These species dominate the speciation at pH < 3, pH 4-5, pH ~ 8, and pH > 10, respectively, with minimum log stability constants (ß values) of 25.70, 36.91, 16.43, and -6.91. Results from electrospray mass spectrometry, metal-ligand binding experiments, and kinetics measurements support the speciation, which is characterized by bidentate chelation of the ascorbate dianion to the titanium(IV) ion via proton displacement, and a pH-dependent metal-ligand binding motif of ligand addition followed by metal ion-promoted hydrolysis, stepwise ligand dissociation, and the concomitant binding of hydroxide ion. Additionally, the kinetics of ligand exchange of titanium ascorbate with citrate are reported to understand better the possible fate of titanium ascorbate under biologically relevant conditions.

Cytotoxicity of a Ti(IV) compound is independent of serum proteins.

Titanium(IV) compounds are excellent anticancer drug candidates, but they have yet to find success in clinical applications. A major limitation in developing further compounds has been a general lack of understanding of the mechanism governing their bioactivity. To determine factors necessary for bioactivity, we tested the cytotoxicity of different ligand compounds in conjunction with speciation studies and mass spectrometry bioavailability measurements. These studies demonstrated that the Ti(IV) compound of N,N'-di(o-hydroxybenzyl)ethylenediamine-N,N'-diacetic acid (HBED) is cytotoxic to A549 lung cancer cells, unlike those of citrate and naphthalene-2,3-diolate. Although serum proteins are implicated in the activity of Ti(IV) compounds, we found that these interactions do not play a role in [TiO(HBED)](-) activity. Subsequent compound characterization revealed ligand properties necessary for activity. These findings establish the importance of the ligand in the bioactivity of Ti(IV) compounds, provides insights for developing next-generation Ti(IV) anticancer compounds, and reveal [TiO(HBED)](-) as a unique candidate anticancer compound.

Beyond bilobal: transferrin homologs having unusual domain architectures.

BACKGROUND: Most transferrin family proteins have a familiar bilobal structure, the result of an ancient gene duplication, with an iron binding site in each of two homologous lobes. Scattered throughout the evolutionary tree from algae to mammals, though, are transferrin homologs having other kinds of domain architectures. SCOPE OF REVIEW: This review covers a variety of unusual transferrin forms, including monolobals, bilobals with one or both iron-binding sites abrogated, bilobals accessorized with long insertions or with membrane anchors, and even trilobals. The monolobal transferrin homologs from marine invertebrate ascidians are especially highlighted here. MAJOR CONCLUSIONS: Unusual transferrin homologs appear scattered through much of the evolutionary tree. For some of these proteins, iron binding and/or iron transport appear to be the primary roles; for others they clearly are not. Many are incompletely or not at all studied. GENERAL SIGNIFICANCE: Taken together, these proteins begin to offer a glimpse into how the transferrin architecture has been repurposed for a diversity of applications. This article is part of a Special Issue entitled Transferrins: Molecular mechanisms of iron transport and disorders.

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.

The challenges of trafficking hydrolysis prone metals and ascidians as an archetype.

Some of the metal ions that are required, exploited, or simply managed in biological systems are susceptible to hydrolysis and to hydrolytic precipitation in the aqueous, aerobic environment of much of biology. Organisms have evolved exquisite mechanisms for handling these metal ions, offering striking examples of biological control over inorganic coordination chemistry. This year marks the one hundredth anniversary of the discovery of remarkably high vanadium concentrations in the blood cells of the ascidian. In the ensuing years, these marine invertebrates were established as masters of the biological chemistry of very hydrolysis-prone metals, with various ascidian species accumulating high concentrations of iron, vanadium, and titanium, among others. These three metals have very different histories of biological relevance, and many questions remain about how, and ultimately why, these organisms sequester them. This Perspective addresses the aqueous coordination chemistry that organisms like ascidians must control if they are to manipulate hydrolysis-prone metal ions, and describes some of the ascidian biomolecules that have been implicated in this phenomenon. The recently available genome sequence for one ascidian species offers a glimpse into its metal-management arsenal. It offers the opportunity to map the relatively well-studied paradigm of iron management onto the genome of an organism that is intermediate in evolution between invertebrates and vertebrates. The ascidians have much to teach us about how to manage metals like iron, titanium, and vanadium and how that ability evolved.

Redox potentials of Ti(IV) and Fe(III) complexes provide insights into titanium biodistribution mechanisms.

Transferrin, the human iron transport protein, binds Ti(IV) even more tightly than it binds Fe(III). However, the fate of titanium bound to transferrin is not well understood. Here we present results which address the fate of titanium once bound to transferrin. We have determined the redox potentials for a series of Ti(IV) complexes and have used these data to develop a linear free energy relationship (LFER) correlating Ti(IV) <==> Ti(III) redox processes with Fe(III) <==> Fe(II) redox processes. This LFER enables us to compare the redox potentials of Fe(III) complexes and Ti(IV) complexes that mimic the active site of transferrin and allows us to predict the redox potential of titanium-transferrin. Using cyclic voltammetry and discontinuous metalloprotein spectroelectrochemistry (dSEC) in conjunction with the LFER, we report that the redox potential of titanium-transferrin is lower than -600 mV (lower than that of iron-transferrin) and is predicted to be ca. -900 mV vs. NHE (normal hydrogen electrode). We conclude that Ti(IV)/Ti(III) reduction in titanium-transferrin is not accessible by biological reducing agents. This observation is discussed in the context of current hypotheses concerning the role of reduction in transferrin mediated iron transport.

Contrasting synergistic anion effects in vanadium(V) binding to nicatransferrin versus human serum transferrin.

Some ascidians sequester vanadium and other metal ions that are bound and transported in higher organisms by transferrin. The ascidian Ciona intestinalis has a monolobal transferrin (nicatransferrin) in its plasma. The binding of vanadium(V) to nicatransferrin was investigated by using isothermal titration calorimetry and UV-vis spectroscopy, in the presence and absence of NaHCO(3), and was compared with human serum transferrin. Nicatransferrin and serum transferrin bind V(V) with similar strengths [K = (2.0 +/- 0.6) x 10(5) M(-1) for nicatransferrin]; however, nicatransferrin requires a synergistic anion for V(V) binding, whereas serum transferrin does not. Spectroscopy supports a different kind of coordination site.

Titanium(IV) complexes with N,N'-dialkyl-2,3-dihydroxyterephthalamides and 1-hydroxy-2(1H)-pyridinone as siderophore and tunichrome analogues.

The aqueous chemistry of Ti(IV) with biological ligands siderophores and tunichromes is modeled by using N,N'-dialkyl-2,3-dihydroxyterephthalamides (alTAMs), analogues of catecholamide-containing biomolecules, and 1-hydroxy-2(1H)-pyridinone (1,2-HOPO), an analogue of hydroxamate-containing biomolecules. Both types of ligands stabilize Ti(IV) with respect to hydrolytic precipitation, and afford tractable complexes. Complexes with the methyl derivative of alTAM, meTAM, are characterized by using mass spectrometry and UV/vis spectroscopy. Complexes with etTAM are characterized by the same techniques as well as X-ray crystallography, cyclic voltammetry, and spectropotentiomeric titration. The ESI mass spectra of these complexes in water show both 1:2 and 1:3 metal/ligand species. The X-ray crystal structure of a 1:2 complex, K(2)[Ti(etTAM)(2)(OCH(3))(2)].2CH(3)OH (1), is reported. The midpoint potential for reduction of 1 dissolved in solution is -0.98 V. A structure for a 1:3 Ti/etTAM species, Na(2)[Ti(etTAM)(3)] demonstrates the coordination and connectivity in that complex. Spectropotentiometric titrations in water reveal three metal-containing species in solution between pH 3 and 10. 1,2-HOPO supports Ti(IV) complexes that are stable and soluble in aqueous solution. The bis-HOPO complex [Ti(1,2-HOPO)(2)(OCH(3))(2)] (5) was characterized by X-ray crystallography and by mass spectrometry in solution, and the tris-HOPO dimer [(1,2-HOPO)(3)TiOTi(1,2-HOPO)(3)] (6) was characterized by X-ray crystallography. Taken together, these experiments explore the characteristics of complexes that may form between siderophores and tunichromes with Ti(IV) in biology and in the environment, and guide efforts toward new, well characterized aqueous Ti(IV) complexes. By revealing the identities and some characteristics of complexes that form under a variety of conditions, these studies further our understanding of the complicated nature of aqueous titanium coordination chemistry.

Hydrolytic metal with a hydrophobic periphery: titanium(IV) complexes of naphthalene-2,3-diolate and interactions with serum albumin.

Serum albumin, the most abundant protein in human plasma (700 microM), binds diverse ligands at multiple sites. While studies have shown that serum albumin binds hard metals in chelate form, few have explored the trafficking of these metals by this protein. Recent work demonstrated that serum albumin may play a pivotal role in the transport and bioactivity of titanium(IV) complexes, including the anticancer drug candidate titanocene dichloride. The current work explores this interaction further by using a stable Ti(IV) complex that presents a hydrophobic surface to the protein. Ti(IV) chelation by 2,3-dihydroxynaphthalene (H2L1) and 2,3-dihydroxynaphthalene-6-sulfonate (H2L2) affords water soluble complexes that protect Ti(IV) from hydrolysis at pH 7.4 and bind to bovine serum albumin (BSA). The solution and solid Ti(IV) coordination chemistry were explored by aqueous spectropotentiometric titrations and X-ray crystallography, respectively, and with complementary electrochemistry, mass spectrometry, and IR and NMR spectroscopies. Four Ti(IV) species of L2, TiLH0, TiL2H0, TiL3H0, and TiL3H(-1), adequately represent the pH-dependent speciation. The isolation of Ti(C10H6O2)2 x 1.75 H2O at pH approximately 3 and K2[Ti(C10H6O2)3] x 3 H2O and Cs5[Ti(C10H5O5S)3] x 2.5 H2O at pH approximately 7 correlates well with the solution studies. At pH 7.4 and micromolar concentrations, the TiL3H0 species are favored. The Ti(naphthalene-2,3-diolate)3(2-) complex binds with moderate affinity to multiple sites of BSA. The primary site (K = 2.05 +/- 0.34 x 10(6) M(-1)) appears to be hydrophobic as indicated by competition studies with different ligands and a hydrophilic Ti(IV) complex. The Ti(naphthalene-2,3-diolate)3(2-) interaction with the Fe(III)-binding protein human serum transferrin (HsTf), a protein also important for Ti(IV) transport, and DNA was examined. The complex does not deliver Ti(IV) to HsTf and while it does bind to DNA, no cleavage promotion activity is observed. This investigation provides insight into the use of ligands to direct metal binding at different sites of albumin, which may facilitate transport to distinct targets.

Isolation and characterization of the iron-binding properties of a primitive monolobal transferrin from Ciona intestinalis.

Transferrins are bilobal glycoproteins responsible for iron binding, transport, and delivery in many higher organisms. The two homologous lobes of transferrins are thought to have evolved by gene duplication of an ancestral monolobal form. In the present study, a 37.7-kDa primitive monolobal transferrin (nicatransferrin, or nicaTf) from the serum of the model ascidian species Ciona intestinalis was isolated by using an immobilized iron-affinity column and characterized by using mass spectrometry and N-terminal sequencing. The protein binds one equivalent of iron(III) and exhibits an electron paramagnetic resonance spectrum that is anion-dependent. The UV/vis spectrum of nicaTf has a shoulder at 330 nm in both the iron-depleted and the iron-replete forms, but does not display the approximately 460 nm tyrosine-to-iron charge transfer band common to vertebrate serum transferrins under the conditions investigated. This result suggests that iron may adopt a different binding mode in nicaTf compared with the more highly evolved transferrin proteins. This difference in binding mode could have implications for the physiological role of the protein in the ascidian. The genome of C. intestinalis has genes for both a monolobal and a bilobal transferrin, and the sequences of both proteins are discussed in light of the known features of vertebrate serum transferrins as well as other transferrin homologs.