Second derivative tryptophan fluorescence spectroscopy as a tool to characterize partially unfolded intermediates of proteins

Enhanced Bioactivity of Enzyme/MOF Biocomposite via Host Framework Engineering

This study reports the successful development of a sustainable synthesis protocol for a phase-pure metal azolate framework (MAF-6) and its application in enzyme immobilization. An esterase@MAF-6 biocomposite was synthesized, and its catalytic performance was compared with that of esterase@ZIF-8 and esterase@ZIF-90 in transesterification reactions. Esterase@MAF-6, with its large pore aperture, showed superior enzymatic performance compared to esterase@ZIF-8 and esterase@ZIF-90 in catalyzing transesterification reactions using both n-propanol and benzyl alcohol as reactants. The hydrophobic nature of the MAF-6 platform was shown to activate the immobilized esterase into its open-lid conformation, which exhibited a 1.5- and 4-times enzymatic activity as compared to free esterase in catalyzing transesterification reaction using n-propanol and benzyl alcohol, respectively. The present work offers insights into the potential of MAF-6 as a promising matrix for enzyme immobilization and highlights the need to explore MOF matrices with expanded pore apertures to broaden their practical applications in biocatalysis.

Biophysical evaluation of the oligomerization and conformational properties of the N-terminal domain of TDP-43

TDP-43 is an RNA-binding protein that presents four domains comprising an N-terminal region, two RNA recognition motifs and a C-terminal region. The N-terminal domain (NTD) has a relevant role in the oligomerization and splicing activity of TDP-43. In this work, we have expressed, purified and biophysically characterized the region that includes residues 1 to 102 that contains the nuclear localization signal (residues 80-102, NLS). Furthermore, we have evaluated the oligomerization equilibrium for this protein fragment. Also, we have determined changes in the tertiary structure and its stability in a broad range of pH values by means of different spectroscopic methods. Additionally, we compared this fragment with the one that lacks the NLS employing experimental and computational methods. Finally, we evaluated the motion of dimeric forms to get insights into the conformational flexibility of this TDP-43 module in an oligomeric state. Our results suggest that this domain has a conformational plasticity in the vicinity of the single tryptophan of this domain (Trp68), which is enhanced by the presence of the nuclear localization signal. All these results help to understand the molecular features of the NTD of TDP-43.

The Transthyretin/Oleuropein Aglycone Complex: A New Tool against TTR Amyloidosis

The release of monomers from the homotetrameric protein transthyretin (TTR) is the first event of a cascade, eventually leading to sporadic or familial TTR amyloidoses. Thus, ligands able to stabilize TTR and inhibit monomer release are subject of intense scrutiny as potential treatments against these pathologies. Here, we investigated the interaction between TTR and a non-glycated derivative of the main olive polyphenol, oleuropein (OleA), known to interfere with TTR aggregation. We coupled fluorescence studies with molecular docking to investigate the OleA/TTR interaction using wild-type TTR, a monomeric variant, and the L55P cardiotoxic mutant. We characterized a fluorescence band emitted by OleA upon formation of the OleA/TTR complex. Exploiting this signal, we found that a poorly specific non-stoichiometric interaction occurs on the surface of the protein and a more specific stabilizing interaction takes place in the ligand binding pocket of TTR, exhibiting a K_D of 3.23 ± 0.32 µM, with two distinct binding sites. OleA interacts with TTR in different modes, stabilizing it and preventing its dissociation into monomers, with subsequent misfolding. This result paves the way to the possible use of OleA to prevent degenerative diseases associated with TTR misfolding.

Effect of pulsed light treatment on enzymes and protein allergens associated with their structural changes: a review

The pulsed light (PL) technique is used for food and surface decontamination widely. The sterilization effect of PL is well known and identified as the photo-chemical effect. Besides, PL is used to inactivate enzymes, reduce the immunoreactivity of proteins, and change protein function properties at a laboratory level. The current study aims to review the effect of PL on proteins by highlighting the differences between proteins in buffer solutions or food systems. Although PL is known as a non-thermal technique, most studies done on food systems, food temperature raised considerably. Therefore, PL inactivated many enzymes in buffer solution non-thermally, while mostly with a high increase in temperature of a food system. PL reduced food allergens several folds in some foods. However, immunoreactivity responses of some protein were increased after PL treatment. Also, the current study covers the conformational changes of proteins that occur because of PL treatment. Therefore, some techniques used to follow proteins structural changes such as polyacrylamide gel electrophoresis (SDS-PAGE), high-performance liquid chromatography (HPLC), Fourier-transform infrared spectroscopy (FTIR), etc. were defined. Studies reported that PL altered proteins structure differently. For example, some studies reported that PL degraded some proteins, while other studies suggested that PL aggregated proteins. Also, there were contrary results regarding α-helix and ß-sheet concentration for the treated proteins. In conclusion, some techniques, such as amino acid sequencing, specially when some small new fragments proteins appeared on SDS-PAGE, should be used to detect the effect of PL on proteins precisely.

Pepsin Digest of Gliadin Forms Spontaneously Amyloid-Like Nanostructures Influencing the Expression of Selected Pro-Inflammatory, Chemoattractant, and Apoptotic Genes in Caco-2 Cells: Implications for Gluten-Related Disorders

SCOPE

Proteolysis-resistant gliadin peptides are intensely investigated in biomedical research relates to celiac disease and gluten-related disorders. Herein, the first integrated supramolecular investigation of pepsin-digested gliadin peptides (p-gliadin) is presented in combination with its functional behavior in the Caco-2 cell line.

METHODS AND RESULTS

First, gliadins are degraded by pepsin at pH 3, and the physicochemical properties of p-gliadin are compared with gliadin. An integrated approach using interfacial, spectroscopic, and microscopic techniques reveals that the p-gliadin forms spontaneously soluble large supramolecular structures, mainly oligomers and fibrils, capable of binding amyloid-sensitive dyes. The self-assembly of p-gliadin starts at a concentration of 0.40 µg mL^-1 . Second, the stimulation of Caco-2 cells with the p-gliadin supramolecular system is performed, and the mRNA expression levels of a panel of genes are tested. The experiments show that p-gliadin composed of supramolecular structures triggers significant mRNA up-regulation (p < 0.05) of pro-apoptotic biomarkers (ratio Bcl2/Bak-1), chemokines (CCL2, CCL3, CCL4, CCL5, CXCL8), and the chemokine receptor CXCR3.

CONCLUSIONS

This work demonstrates that p-gliadin is interfacial active, forming spontaneously amyloid-type structures that trigger genes in the Caco-2 cell line involved in recruiting specialized immune cells.

Probing conformational changes of monomeric transthyretin with second derivative fluorescence

We have studied the intrinsic fluorescence spectra of a monomeric variant of human transthyretin (M-TTR), a protein involved in the transport of the thyroid hormone and retinol and associated with various forms of amyloidosis, extending our analysis to the second order derivative of the spectra. This procedure allowed to identify three peaks readily assigned to Trp41, as the three peaks were also visible in a mutant lacking the other tryptophan (Trp79) and had similar FRET efficiency values with an acceptor molecule positioned at position 10. The wavelength values of the three peaks and their susceptibility to acrylamide quenching revealed that the three corresponding conformers experience different solvent-exposure, polarity of the environment and flexibility. We could monitor the three peaks individually in urea-unfolding and pH-unfolding curves. This revealed changes in the distribution of the corresponding conformers, indicating conformational changes and alterations of the dynamics of the microenvironment that surrounds the associated tryptophan residue in such transitions, but also native-like conformers of such residues in unfolded states. We also found that the amyloidogenic state adopted by M-TTR at mildly low pH has a structural and dynamical microenvironment surrounding Trp41 indistinguishable from that of the fully folded and soluble state at neutral pH.

Investigating variations of fluorescent dissolved organic matter in wastewater treatment using synchronous fluorescence spectroscopy combined with principal component analysis and two-dimensional correlation

Synchronous fluorescence spectroscopy (SFS) combined with principal component analysis (PCA) and two-dimensional (2D) correlation was applied to investigate removal efficiencies and variations of dissolved organic matter (DOM) fractions in the wastewater treatment plant (WWTP) with an A2O craft. A decreasing order of total removal efficiencies was tyrosine-like fluorescence component (89.58%) > humic-like fluorescence (HLF) component (39.83%) > tryptophan-like fluorescence component (36.89%) > microbial humic-like fluorescence (HLF) component (12.47%) > fulvic-like fluorescence component (6.37%). The tyrosine-like, tryptophan-like and HLF components were deeply decomposed by anaerobic bacteria in the anaerobic zone. The tyrosine-like component was the preponderant fraction of DOM in the raw water and primary sediment tank. The tyrosine-like component was the dominant component of DOM too in the anaerobic and anoxic zones, but its proportion was slightly more than the tryptophan-like component. The tryptophan-like component was the dominant component in the facultative zone, the oxic zone and the secondary sediment tank. Based on the changing band order of 279 → 304 → 490 → 330 → 380 → 430 nm, the decreasing variation order was tyrosine-like > tryptophan-like > humic-like > microbial humic-like > fulvic-like component. Therefore, the SFS combined with PCA and 2D correlation is an effective tool for not only monitoring the removal of DOM components but also characterizing variations of DOM fractions in the WWTP.

In Vitro Investigation of the Interaction of Tolbutamide and Losartan with Human Serum Albumin in Hyperglycemia States

Serum albumin is exposed to numerous structural modifications which affect its stability and activity. Glycation is one of the processes leading to the loss of the original properties of the albumin and physiological function disorder. In terms of long lasting states of the hyperglycemia, Advanced Glycation End-products (AGEs) are formed. AGEs are responsible for cellular and tissue structure damage that cause the appearance of a number of health consequences and premature aging. The aim of the present study was to analyze the conformational changes of serum albumin by glycation—“fructation”—using multiple spectroscopic techniques, such as absorption (UV-Vis), fluorescence (SFM), circular dichroism (CD) and nuclear magnetic resonance (NMR) spectroscopy and evaluate of possible alteration of binding and competition between tolbutamide (TB, a first-generation sulfonylurea oral hypoglycemic drug) and losartan (LOS, an angiotensin II receptor (AT₁) blocker used in hypertension (1st line with a coexisting diabetes)) in binding to non-glycated (HSA) and glycated (gHSA_FRC) human serum albumin in high-affinity binding sites. The studies allowed us to indicate the structural alterations of human serum albumin as a result of fructose glycation. Changes in binding parameters, such as association (Ka) or Stern-Volmer (KSV) constants suggest that glycation increases the affinity of TB and LOS towards albumin and affects interactions between them. The process of albumin glycation influences the pharmacokinetics of drugs, thus monitored pharmacotherapy is reasonable in the case of diabetes and hypertension polypharmacy. This information may lead to the development of more effective drug treatments based on personalized medicine for patients with diabetes. Our studies suggest the validity of monitored polypharmacy of diabetes and coexisting diseases.

MERS-CoV papain-like protease (PLpro): expression, purification, and spectroscopic/thermodynamic characterization

Within a decade, MERS-CoV emerged with nearly four times higher case fatality rate than an earlier outbreak of SARS-CoV and spread out in 27 countries in short span of time. As an emerging virus, combating it requires an in-depth understanding of its molecular machinery. Therefore, conformational characterization studies of coronavirus proteins are necessary to advance our knowledge of the matter for the development of antiviral therapies. In this study, MERS-CoV papain-like protease (PL^pro) was recombinantly expressed and purified. Thermal folding pathway and thermodynamic properties were characterized using dynamic multimode spectroscopy (DMS) and thermal shift assay. DMS study showed that the PL^pro undergoes a single thermal transition and follows a pathway of two-state folding with T_m and van’t Hoff enthalpy values of 54.4 ± 0.1 °C and 317.1 ± 3.9 kJ/mol, respectively. An orthogonal technique based on intrinsic tryptophan fluorescence also showed that MERS-CoV PL^pro undergoes a single thermal transition and unfolds via a pathway of two-state folding with a T_m value of 51.4 °C. Our findings provide significant understandings of the thermodynamic and structural properties of MERS-CoV PL^pro.

Effect of Temperature on Tolbutamide Binding to Glycated Serum Albumin

Glycation process occurs in protein and becomes more pronounced in diabetes when an increased amount of reducing sugar is present in bloodstream. Glycation of protein may cause conformational changes resulting in the alterations of its binding properties even though they occur at a distance from the binding sites. The changes in protein properties could be related to several pathological consequences such as diabetic and nondiabetic cardiovascular diseases, cataract, renal dysfunction and Alzheimer's disease. The experiment was designed to test the impact of glycation process on sulfonylurea drug tolbutamide-albumin binding under physiological (T = 309 K) and inflammatory (T = 311 K and T = 313 K) states using fluorescence and UV-VIS spectroscopies. It was found in fluorescence analysis experiments that the modification of serum albumin in tryptophanyl and tyrosyl residues environment may affect the tolbutamide (TB) binding to albumin in subdomain IIA and/or IIIA (Sudlow's site I and/or II), and also in subdomains IB and IIB. We estimated the binding of tolbutamide to albumin described by a mixed nature of interaction (specific and nonspecific). The association constants Ka (L∙mol^-1) for tolbutamide at its high affinity sites on non-glycated albumin were in the range of 1.98-7.88 × 10⁴ L∙mol^-1 (λ_ex = 275 nm), 1.20-1.64 × 10⁴ L∙mol^-1 (λ_ex = 295 nm) and decreased to 1.24-0.42 × 10⁴ L∙mol^-1 at λ_ex = 275 nm (T = 309 K and T = 311 K) and increased to 2.79 × 10⁴ L∙mol^-1 at λ_ex = 275 nm (T = 313 K) and to 4.43-6.61 × 10⁴ L∙mol^-1 at λ_ex = 295 nm due to the glycation process. Temperature dependence suggests the important role of van der Waals forces and hydrogen bonding in hydrophobic interactions between tolbutamide and both glycated and non-glycated albumin. We concluded that the changes in the environment of TB binding of albumin in subdomain IIA and/or IIIA as well as in subdomains IB and IIB influence on therapeutic effect and therefore the studies of the binding of tolbutamide (in diabetes) to transporting protein under glycation that refers to the modification of a protein are of great importance in pharmacology and biochemistry. This information may lead to the development of more effective drug therapy in people with diabetes.

Spectroscopic Studies on the Molecular Ageing of Serum Albumin

Pathological states in the organism, e.g., renal or hepatic diseases, cataract, dysfunction of coronary artery, diabetes mellitus, and also intensive workout, induce the structural modification of proteins called molecular ageing or N-A isomerization. The aim of this study was to analyze the structural changes of serum albumin caused by alkaline ageing using absorption, spectrofluorescence, and circular dichroism spectroscopy. The N-A isomerization generates significant changes in bovine (BSA) and human (HSA) serum albumin subdomains-the greatest changes were observed close to the tryptophanyl (Trp) and tyrosyl (Tyr) residue regions while a smaller change was observed in phenyloalanine (Phe) environment. Moreover, the changes in the polarity of the Trp neighborhood as well as the impact of the ageing process on α-helix, β-sheet content, and albumin molecule rotation degree have been analyzed. Based on the spectrofluorescence study, the alterations in metoprolol binding affinity to the specific sites that increase the toxicity of the drug were investigated.

Self-organization of gliadin in aqueous media under physiological digestive pHs

Here we showed that gliadin, a complex protein system related to celiac disease and other human diseases, is spontaneously self-organized in a very dilute solution at pH 3.0 and 7.0 in water under low ionic strength (10mM NaCl). The spontaneous self-organization at pH 3.0 increases the apparent solubility due to the formation of finite sized aggregates, such as those formed in the micellization of amphiphilic molecules. Switching the pH from 3.0 to 7.0 lead to a phase separation, however part of the nano-particles are stable remaining disperse in water after centrifugation. Also, beside the pH change led to changes in protein composition and concentration, we determined that the secondary structure of both system is the same. Moreover, Tyrs are slightly more buried and Trps are slightly more exposed to water at pH 7.0 than those at pH 3.0. Electron microscopy techniques showed that both gliadin systems are composed of nanostructures and in the case of pH 7.0 amorphous microaggregates were found, too. Only nanostructures at pH 3.0 showed a micromolar binding affinity to Nile red probe, suggesting the presence of accessible hydrophobic patches which are not more accessible at pH 7.0. All our results suggest that gliadin is able to self-organized at pH 3.0 forming protein micelles type nanostructures (ζ=+13, 42 ± 1.55 mV), meanwhile at 7.0 the decrease of superficial charge to ζ of +4, 78 ± 0.48 mV led to the formation of stable colloidal nanoparticles, unable to interact with Nile red probe. Our findings may open new perspectives for the understanding of gliadin ability to avoid proteolysis, to reach and cross the intestinal lumen and to trigger different immunological disorders.

Effect of Excipients on Liquid-Liquid Phase Separation and Aggregation in Dual Variable Domain Immunoglobulin Protein Solutions

Liquid-liquid phase separation (LLPS) and aggregation can reduce the physical stability of therapeutic protein formulations. On undergoing LLPS, the protein-rich phase can promote aggregation during storage due to high concentration of the protein. Effect of different excipients on aggregation in protein solution is well documented; however data on the effect of excipients on LLPS is scarce in the literature. In this study, the effect of four excipients (PEG 400, Tween 80, sucrose, and hydroxypropyl beta-cyclodextrin (HPβCD)) on liquid-liquid phase separation and aggregation in a dual variable domain immunoglobulin protein solution was investigated. Sucrose suppressed both LLPS and aggregation, Tween 80 had no effect on either, and PEG 400 increased LLPS and aggregation. Attractive protein-protein interactions and liquid-liquid phase separation decreased with increasing concentration of HPβCD, indicating its specific binding to the protein. However, HPβCD had no effect on the formation of soluble aggregates and fragments in this study. LLPS and aggregation are highly temperature dependent; at low temperature protein exhibits LLPS, at high temperature protein exhibits aggregation, and at an intermediate temperature both phenomena occur simultaneously depending on the solution conditions.

The polyphenol Oleuropein aglycone hinders the growth of toxic transthyretin amyloid assemblies

Transthyretin (TTR) is involved in a subset of familial or sporadic amyloid diseases including senile systemic amyloidosis (SSA), familial amyloid polyneuropathy and cardiomyopathy (FAP/FAC) for which no effective therapy has been found yet. These conditions are characterized by extracellular deposits primarily found in the heart parenchyma and in peripheral nerves whose main component are amyloid fibrils, presently considered the main culprits of cell sufferance. The latter are polymeric assemblies grown from misfolded TTR, either wt or carrying one out of many identified mutations. The recent introduction in the clinical practice of synthetic TTR-stabilizing molecules that reduce protein aggregation provides the rationale to search natural effective molecules able to interfere with TTR amyloid aggregation by hindering the appearance of toxic species or by favoring the growth of harmless aggregates. Here we carried out an in depth biophysical and morphological study on the molecular features of the aggregation of wt- and L55P-TTR involved in SSA or FAP/FAC, respectively, and on the interference with fibril aggregation, stability and toxicity to cardiac HL-1 cells to demonstrate the ability of Oleuropein aglycone (OleA), the main phenolic component of the extra virgin olive oil. We describe the molecular basis of such interference and the resulting reduction of TTR amyloid aggregate cytotoxicity. Our data offer the possibility to validate and optimize the use of OleA or its molecular scaffold to rationally design promising drugs against TTR-related pathologies that could enter a clinical experimental phase.

Heat-induced unfolding of apo-CP43 studied by fluorescence spectroscopy and CD spectroscopy

CP43 is a chlorophyll-binding protein, which acts as a conduit for the excitation energy transfer. The thermal stability of apo-CP43 was studied by intrinsic fluorescence, exogenous ANS fluorescence, and circular dichroism spectroscopy. Under heat treatment, the structure of apo-CP43 changed and existed transition state occurred between 56 and 62 °C by the intrinsic, exogenous ANS fluorescence and the analysis of hydrophobicity. Besides, the isosbestic point of the sigmoidal curve was 58.10 ± 1.02 °C by calculating α-helix transition and the Tm was 56.45 ± 0.52 and 55.59 ± 0.68 °C by calculating the unfolded fraction of tryptophan and tyrosine fluorescence, respectively. During the process of unfolding, the hydrophobic structure of C-terminal segment firstly started to expose at 40 °C, and then the hydrophobic cluster adjacent to the N-terminal segment also gradually exposed to hydrophilic environment with increasing temperature. Our results indicated that heat treatment, especially above 40 °C, has an important impact on the structural stability of apo-CP43.

Assessing removal efficiency of dissolved organic matter in wastewater treatment using fluorescence excitation emission matrices with parallel factor analysis and second derivative synchronous fluorescence

To assess removal efficiency of dissolved organic matter (DOM) in the wastewater treatment plant (WWTP), fluorescence excitation emission matrices (EEM) with parallel factor analysis (PARAFAC) and second derivative synchronous fluorescence (SDSF) were used for the characterization of DOM in wastewater. In A(2)/O process, tryptophan-like and tyrosine-like materials were removed to the much greater extent than that of fulvic-like. The protein-like material might be mostly decomposed by anaerobic and aerobic bacteria in anaerobic/anoxic and oxic zones. C1, C2, I276, I286, AP1 and AP2 were much better in tracing variations of tryptophan-like and tyrosine-like materials than C3, I329 and AF1 in tracing fulvic-like. The number of sampling sites should be reduced, as concentration variations of DOM components were subtle among sampling sites in the oxic zone and secondary sedimentation tank. SDSF may be a useful tool as PARAFAC to monitor removal efficiency of DOM fractions from wastewater in the WWTP.

Use of spectroscopic, zeta potential and molecular dynamic techniques to study the interaction between human holo-transferrin and two antagonist drugs: comparison of binary and ternary systems

For the first time, the binding of ropinirole hydrochloride (ROP) and aspirin (ASA) to human holo-transferrin (hTf) has been investigated by spectroscopic approaches (fluorescence quenching, synchronous fluorescence, time-resolved fluorescence, three-dimensional fluorescence, UV-vis absorption, circular dichroism, resonance light scattering), as well as zeta potential and molecular modeling techniques, under simulated physiological conditions. Fluorescence analysis was used to estimate the effect of the ROP and ASA drugs on the fluorescence of hTf as well as to define the binding and quenching properties of binary and ternary complexes. The synchronized fluorescence and three-dimensional fluorescence spectra demonstrated some micro-environmental and conformational changes around the Trp and Tyr residues with a faint red shift. Thermodynamic analysis displayed the van der Waals forces and hydrogen bonds interactions are the major acting forces in stabilizing the complexes. Steady-state and time-resolved fluorescence data revealed that the fluorescence quenching of complexes are static mechanism. The effect of the drugs aggregating on the hTf resulted in an enhancement of the resonance light scattering (RLS) intensity. The average binding distance between were computed according to the forster non-radiation energy transfer theory. The circular dichroism (CD) spectral examinations indicated that the binding of the drugs induced a conformational change of hTf. Measurements of the zeta potential indicated that the combination of electrostatic and hydrophobic interactions between ROP, ASA and hTf formed micelle-like clusters. The molecular modeling confirmed the experimental results. This study is expected to provide important insight into the interaction of hTf with ROP and ASA to use in various toxicological and therapeutic processes.

Simultaneous detection and analysis of protein aggregation and protein unfolding by size exclusion chromatography with post column addition of the fluorescent dye BisANS

For development and optimization of protein formulation sensitive analytical tools are required to follow both aggregation and changes in protein structure. The latter can be seen as the beginning of physical instability leading to aggregation. The focus of this work laid on the development of a novel analysis simultaneously detecting changes in protein conformation and the formation of oligomers. By adding the extrinsic fluorescent dye 4,4'-dianilino-1,1'-binaphthyl-5,5'-disulfonic acid dipotassium salt (BisANS) after size exclusion chromatography (SEC) and UV detection, it was possible to separate protein monomer and oligomers by size, analyze the amount of formed oligomers quantitatively using UV detection, and observe changes in protein structure of different protein species by fluorescence detection. This enabled us to distinguish between native-like and denatured oligomers and monomers formed under different stress conditions. Correspondingly, increased fluorescence reflecting partial unfolding was assigned specifically to monomer, oligomer, or both. The unfolding of monomer is not traceable by commonly used detection methods, but its monitoring may provide important information about activity and long-term stability. By adding the dye after SEC and UV detection, interferences with prior detectors are precluded, excipients are separated avoiding interferences with the protein-dye interaction and, in addition, the dye-protein interaction cannot impact the aggregation formation, as added after the separation of monomer and aggregates.

Lomefloxacin promotes the interaction between human serum albumin and transferrin: a mechanistic insight into the emergence of antibiotic's side effects

Human serum albumin (HSA) and serum transferrin (TF) are two drug carrier proteins in vivo. In this study it was investigated how lomefloxacin (LMF) binding affected the HSA-TF interaction using different spectroscopic, calorimetric and molecular modeling techniques. Fluorescence, circular dichroism and synchronous fluorescence revealed that LMF could bind to both proteins, resulting in protein conformational changes. Moreover, HSA and TF could interact so that some fluorescence residues were positioned at the interface and were shielded from quenching by LMF. The interaction between HSA and TF was further confirmed by fluorescence resonance energy transfer. Quantitative analyses of the far-UV CD spectra of the HSA-TF interaction in the presence and absence of LMF revealed secondary structural changes in detail. Resonance light-scattering studies demonstrated that the HSA-TF interaction resulted in a new species with a larger size, and that the presence of LMF could further favor this reaction. Isothermal titration calorimetry revealed that electrostatic interaction was dominant in the absence of LMF, whereas van der Waals forces and hydrogen bonding become significant in its presence. On the other hand, it was found that the binding constant of TF bound to HSA was stronger in the presence of LMF. ANS fluorescence further indicated that hydrophobic interactions play a minor part in the HSA-TF system. Molecular modeling studies confirmed the presence of fluorophore residues, hydrogen bonding and electrostatic interactions at the interface of the HSA-TF complex. It also suggested that the binding sites of LMF were not located there. These data indicate that LMF can modify the interaction between HSA and TF as two model proteins present in serum. The relevance to drugs' side effects, pharmacokinetic of drugs and selection of diagnostic biomarker is discussed.

Selective modification of Trp19 in beta-lactoglobulin by a new diazo fluorescence probe

To obtain the local information on the tryptophan domain in a protein, the design and synthesis of a new fluorescent probe, 1,7-bis(4-hydroxy-3-methoxyphenyl)-4-diazo-1,6-heptadiene-3,5-dione, is reported for the selective modification of tryptophan residues. The probe comprises a curcumin fluorophore and a diazo labeling group, whose spectroscopic properties are characterized. The diazo group may be catalytically degraded by transition metal complexes such as Rh2(OAc)4, generating an active rhodium carbenoid intermediate, which can react selectively with tryptophan residues. By the use of the carbene's intermolecular reactions, the tryptophan residue (Trp19) of beta-lactoglobulin may be modified with the diazo curcumin probe. Furthermore, slight secondary but larger tertiary structural changes are detected after Trp19 is modified, and the Trp19 modification produces a great effect on the binding of 8-anilino-1-naphthalenesulfonic acid and retinol. These results indicate that the Trp19 residue plays an essential role in the structure and stability of beta-lactoglobulin, and the specific modification of this residue may have a potential use in further elucidating the relationship between the structure and function of the protein.

Antibody response to aggregated human interferon alpha2b in wild-type and transgenic immune tolerant mice depends on type and level of aggregation

The aim of this study was to determine the sensitivity of transgenic immune tolerant mice for the type and level of aggregation of recombinant human interferon alpha2b (rhIFNalpha2b). RhIFNalpha2b was aggregated by metal-catalyzed oxidation or by incubation at elevated temperature and various pHs. Native rhIFNalpha2b was mixed with oxidized rhIFNalpha2b at different ratios to obtain samples with different aggregation levels. The preparations were characterized by UV and fluorescence spectroscopy, gel permeation chromatography (GPC), dynamic light scattering (DLS), sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and Western blotting, and ELISA. The immunogenicity was evaluated in wild-type mice and transgenic mice immune tolerant for hIFNalpha2. Sera were analyzed by ELISA for the presence of rhIFNalpha2b-specific antibodies. The oxidized and aged preparations widely differed regarding the level and nature of aggregates. All preparations containing aggregates increased the immune response in the wild-type mice as compared to native rhIFNalpha2b and were able to break the tolerance of the transgenic mice. The more native-like the conformation of the aggregated proteins, the more immunogenic the preparations were in the transgenic mice. The native-like aggregates prepared via metal catalysis induced a dose-dependent loss of tolerance in the transgenic mice. In conclusion, the transgenic mouse model can be used to screen rhIFNalpha2b formulations for low levels of immunogenic aggregates obtained under accelerated storage conditions.