Amyloid fibril formation by β-lactoglobulin is inhibited by gold nanoparticles

Nanoparticle-assisted tubulin assembly is environment dependent

Nanomaterials acquire a biomolecular corona upon introduction to biological media, leading to biological transformations such as changes in protein function, unmasking of epitopes, and protein fibrilization. Ex vivo studies to investigate the effect of nanoparticles on protein-protein interactions are typically performed in buffer and are rarely measured quantitatively in live cells. Here, we measure the differential effect of silica nanoparticles on protein association in vitro vs. in mammalian cells. BtubA and BtubB are a pair of bacterial tubulin proteins identified in Prosthecobacter strains that self-assemble like eukaryotic tubulin, first into dimers and then into microtubules in vitro or in vivo. Förster resonance energy transfer labeling of each of the Btub monomers with a donor (mEGFP) and acceptor (mRuby3) fluorescent protein provides a quantitative tool to measure their binding interactions in the presence of unfunctionalized silica nanoparticles in buffer and in cells using fluorescence spectroscopy and microscopy. We show that silica nanoparticles enhance BtubAB dimerization in buffer due to protein corona formation. However, these nanoparticles have little effect on bacterial tubulin self-assembly in the complex mammalian cellular environment. Thus, the effect of nanomaterials on protein-protein interactions may not be readily translated from the test tube to the cell in the absence of particle surface functionalization that can enable targeted protein-nanoparticle interactions to withstand competitive binding in the nanoparticle corona from other biomolecules.

Enhancement in chaperone activity of human αA-crystallin by nanochaperone gold nanoparticles: Multispectroscopic studies on their molecular interactions

The chaperone activity of human αA-crystallin (HAA) against aggregation of human γD-crystallin (HGD) was enhanced by gold nanoparticles (AuNPs). Chaperone activity of HAA was almost doubled in the presence of 5.5 nM gold nanoparticles (AuNPs). To decipher this effect at molecular level, interactions between HAA and AuNPs were studied using fluorescence and circular dichroism spectroscopic techniques. The nanoparticles were synthesized and characterized by using TEM and dynamic light scattering (DLS). TEM and DLS studies revealed that bioconjugation of AuNPs with HAA did not cause any significant change in the size of the nanoparticles. AuNPs had caused static quenching of tryptophan (Trp) fluorescence, which was confirmed through determination of excited state lifetime of Trp residue of HAA in absence and the presence of AuNPs. The association and quenching constant for HAA-AuNPs conjugation were ∼ 10⁹ M^-1. Hydrogen bonding and van der Waals interactions were found to be involved in HAA-AuNPs complex. Polarity of Trp microenvironment in HAA was not perturbed by AuNPs as supported by synchronous and three-dimensional fluorescence spectroscopy. Far-UV CD spectra suggested that the secondary structure of HAA was not significantly affected by AuNPs.

Interactions of proteins with metal-based nanoparticles from a point of view of analytical chemistry - Challenges and opportunities

Interactions of proteins with nanomaterials draw attention of many research groups interested in fundamental phenomena. However, alongside with valuable information regarding physicochemical aspects of such processes and their mechanisms, they more and more often prove to be useful from a point of view of bioanalytics. Deliberate use of processes based on adsorption of proteins on nanoparticles (or vice versa) allows for a development of new analytical methods and improvement of the existing ones. It also leads to obtaining of nanoparticles of desired properties and functionalities, which can be used as elements of analytical tools for various applications. Due to interactions with nanoparticles, proteins can also gain new functionalities or lose their interfering potential, which from perspective of bioanalytics seems to be very inviting and attractive. In the framework of this article we will discuss the bioanalytical potential of interactions of proteins with a chosen group of nanoparticles, and implementation of so driven processes for biosensing. Moreover, we will show both positive and negative (opportunities and challenges) aspects resulting from the presence of proteins in media/samples containing metal-based nanoparticles or their precursors.

Inhibiting protein aggregation with nanomaterials: The underlying mechanisms and impact factors

Protein aggregation is correlated with the onset and progression of protein misfolding diseases (PMDs). Inhibiting the generation of toxic aggregates of misfolded proteins has been proposed as a therapeutic approach for PMDs. Due to their unique properties, nanomaterials have been extensively investigated for their ability to inhibit protein aggregation and have shown great potential in the diagnosis and treatment of PMDs. However, the precise mechanisms by which nanomaterials interact with amyloidogenic proteins and the factors influencing these interactions remain poorly understood. Consequently, developing a rational design strategy for nanomaterials that target specific proteins in PMDs has been challenging. In this review, we elucidate the effects of nanomaterials on protein aggregation and describe the mechanisms through which nanomaterials interfere with protein aggregation. The major factors impacting protein-nanomaterial interaction such as size, charge, concentration, surface modification and morphology that can be rationally addressed to achieve the desired effects of nanomaterials on protein aggregation are summarized. The prospects and challenges to the clinical application of nanomaterials for the treatment of PMDs are also discussed.

Sunset Yellow Dye Induces Amorphous Aggregation in β-Lactoglobulin at Acidic pH: A Multi-Techniques Approach

Protein aggregation is of two types: (i) amorphous and (ii) amyloid fibril. Several extrinsic factors (temperature, pH, and small ligands) stimulate protein aggregation in vitro. In this study, we have examined the role of sunset yellow (SY) on the β-lactoglobulin (BLG) aggregation at pH 2.0. We have used spectroscopic (turbidity, Rayleigh light scattering (RLS), far-UV CD) and microscopic (transmission electron microscopy [TEM]) techniques to describe the effects of SY on BLG aggregation. Our results showed that BLG aggregation is dependent on SY concentrations. Very low concentrations (0.0–0.07 mM) of SY were unable to induce aggregation, while SY in the concentrations range of 0.1–5.0 mM induces aggregation in BLG. The kinetics of SY-stimulated aggregation is very fast and monomeric form of BLG directly converted into polymeric aggregates. The kinetics results also showed SY-induced BLG aggregation disappeared in the presence of NaCl. The far-UV CD and TEM results indicated the amorphous nature of SY-induced BLG aggregates. We believe that our results clearly suggest that SY dye effectively stimulates BLG aggregation.

Change in conformational, digestive and immunological characteristics of bovine allergen β-lactoglobulin induced by metal ions in combination with heating

Aggregation of bovine β-lactoglobulin is affected easily by external factors. In this study, effects of metal ions combining with temperature on aggregation of β-lactoglobulin were explored. The conformational characteristics of aggregates were detected by environment scanning electron microscope, CD spectrum and free sulfhydryl group, respectively. Digestive and immunological characteristics were assessed by simulated digestion in vitro and ELISA respectively. The results showed that the morphology of β-lactoglobulin aggregates became more amorphous in Cu²⁺ and Mg²⁺ treated samples and more constricted in Zu²⁺-induced protein. Among them, Cu²⁺ altered the secondary structure of β-lactoglobulin aggregates and free sulfhydryl content most as well as that in gastric digestion. However, all ion-treated groups had similar digestive stability in intestinal digestion. Specially, Ca²⁺ and Mg²⁺ made the antigenicity and potential allergenicity of β-lactoglobulin aggregates decrease, which helps us understand the role of metal ions in immunological characteristics.

Modulation of amyloid fibrillation of bovine β-lactoglobulin by selective methionine oxidation

Deposition of oxidation-modified proteins during normal aging and oxidative stress are directly associated with systemic amyloidoses. Methionine (Met) is believed to be one of the most readily oxidisable amino acid residues of protein. Bovine beta-lactoglobulin (β-lg), a model globular whey protein, has been presented as a subsequent paradigm for studies on protein aggregation and amyloid formation. Herein, we investigated the effect of t-butyl hydroperoxide (tBHP)-induced oxidation on structure, compactness and fibrillation propensity of β-lg at physiological pH. Notably, whey protein modification, specifically Met residues, plays an important role in the dairy industry during milk processing and lowering nutritional value and ultimately affecting their technological properties. Several bio-physical studies revealed enhanced structural flexibility and aggregation propensity of oxidised β-lg in a temperature dependent manner. A molecular docking study is used to predict possible interactions with tBHP and infers selective oxidation of methionine residues at 7, 24 and 107 positions. From our studies, it can be corroborated that specific orientations of Met residues directs the formation of a partially unfolded state susceptible to fibrillation with possible different cytotoxic effects. Our studies have greater implications in deciphering the underlying mechanism of different whey proteins encountering oxidative stress. Our findings are also important to elucidate the understanding of oxidation induced amyloid fibrillation of protein which may constitute a new route to pave the way for a modulatory role of oxidatively stressed proteins in neurological disorders.

Inhibition of amyloid fibrillation of human γD-crystallin by gold nanoparticles: Studies at molecular level

The capability of citrate-stabilized gold nanoparticles (AuNps) has been explored for the inhibition of amyloid fibrillation of human γD-crystallin (HGD), a major protein of eye lens. Citrate-capped AuNps were synthesized, characterized and used further for amyloid inhibition. The results from intrinsic and extrinsic (in the presence of Thioflavin T and ANS) fluorescence based assays and CD spectroscopy clearly suggest that AuNps at nanomolar concentrations can act as an effective inhibitor against fibrillation of HGD. Fluorescence microscopic and transmission electron microscopic images also supported this observation. Considering the inhibitory role of AuNps against HGD fibrillation, interactions between HGD and AuNps were studied to decipher the mechanism of amyloid inhibition. The binding and quenching constants were calculated as ~10⁹ M^-1 using the data of tryptophan fluorescence quenching of HGD by AuNps. Ground state complexation between the protein and nanoparticles was predicted. AuNps were not found to cause any major conformational changes in the native protein. Entropy-driven complexation process between the protein and nanoparticles indicates the interactions of AuNps with hydrophobic residues of HGD. Therefore, in the presence of AuNps, the exposure of the hydrophobic patches of HGD during its partial unfolding became restricted, which results inhibition in HGD fibrillation.

Biopolymer-coated gold nanoparticles inhibit human insulin amyloid fibrillation

Deposits of protein misfolding and/or aggregates are a pathological hallmark of amyloid-related diseases. For instance, insulin amyloid fibril deposits have been observed in patients with insulin-dependent diabetes mellitus after insulin administration. Here, we report on the use of AuNPs functionalized with linear- (i.e. dextrin and chitosan) and branched- (i.e. dextran-40 and dextran-10) biopolymers as potential agents to inhibit insulin fibril formation. Our dynamic light scattering analyses showed a size decrease of the amyloid fibrils in the presence of functionalized AuNPs. Circular dichroism spectroscopy as well as enzyme-linked immunosorbent assay data demonstrated that the secondary structural transition from α-helix to β-sheet (which is characteristic for insulin amyloid fibril formation) was significantly suppressed by all biopolymer-coated AuNPs, and in particular, by those functionalized with linear biopolymers. Both transmission electron microscopy and atomic force microscopy analyses showed that the long thick amyloid fibrils formed by insulin alone become shorter, thinner or cluster when incubated with biopolymer-coated AuNPs. Dextrin- and chitosan-coated AuNPs were found to be the best inhibitors of the fibril formation. Based on these results, we propose a mechanism for the inhibition of insulin amyloid fibrils: biopolymer-coated AuNPsstrongly interact with the insulin monomers and inhibit the oligomer formation as well as elongation of the protofibrils.Moreover, cytotoxicity experiments showed that AuNP-insulin amyloid fibrils are less toxic compared to insulin amyloid fibrils alone. Our results suggest that both dextrin- and chitosan-AuNPs could be used as therapeutic agents for the treatment of amyloid-related disorders.

Inhibition of Amyloid Fibrillation by Small Molecules and Nanomaterials: Strategic Development of Pharmaceuticals Against Amyloidosis

Amyloid fibrils are a special class of self-assembled protein molecules, which exhibit various toxic effects in cells. Different physiological disorders such as Alzheimer's, Parkinson's, Huntington's diseases, etc. happen due to amyloid formation and lack of proper cellular mechanism for the removal of fibrils. Therefore, inhibition of amyloid fibrillation will find immense applications to combat the diseases associated with amyloidosis. The development of therapeutics against amyloidosis is definitely challenging and numerous strategies have been followed to find out anti-amyloidogenic molecules. Inhibition of amyloid aggregation of proteins can be achieved either by stabilizing the native conformation or by decreasing the chances of assembly formation by the unfolded/misfolded structures. Various small molecules such as naturally occurring polyphenols, flavonoids, small organic molecules, surfactants, dyes, chaperones, etc. have demonstrated their capability to interrupt the amyloid fibrillation of proteins. In addition to that, in last few years, different nanomaterials were evolved as effective therapeutic inhibitors against amyloidosis. Aromatic and hydrophobic interactions between the partially unfolded protein molecules and the inhibitors had been pointed as a general mechanism for inhibition. In this review article, we are presenting an overview on the inhibition of amyloidosis by using different small molecules (both natural and synthetic origin) as well as nanomaterials for development of pharmaceutical strategies against amyloid diseases.

A quercetin-based flavanoid (rutin) reverses amyloid fibrillation in β-lactoglobulin at pH 2.0 and 358 K

β-lactoglobulin (BLG) is a well characterized milk protein and a model for folding and aggregation studies. Rutin is a quercetin based-flavanoid and a famous dietary supplement. It is a potential protector from coronary heart disease, cancers, and inflammatory bowel disease. In this study, amyloid fibrillation is reported in BLG at pH 2.0 and temperature 358 K. It is inhibited to some extent by rutin with a rate of 99.3 h^-1 M^-1. Amyloid fibrillation started taking place after 10 h of incubation and completed near 40 h at a rate of 16.6 × 10^-3 h^-1, with a plateau during 40-108 h. Disruption of tertiary structure of BLG and increased solvent accessibility of hydrophobic core seem to trigger intermolecular assembly. Increase in 7% β-sheet structure at the cost of 10% α-helical structures and the electron micrograph of BLG fibrils at 108 h further support the formation of amyloid. Although it could not block amyloidosis completely, and even the time required to reach plateau remains the same, a decrease of growth rate from 16.6 × 10^-3 to 13.5 × 10^-3 h^-1 was observed in the presence of 30.0 μM rutin. Rutin seems to block solvent accessibility of the hydrophobic core of BLG. A decrease in the fibril population was observed in electron micrographs, with the increase in rutin concentration. All evidences indicate reversal of fibrillation in BLG in the presence of rutin.

Silver nanoparticle modulates the aggregation of beta-lactoglobulin and induces to form rod-like aggregates

Silver nanoparticles (SNPs) have been increasingly used in medicines and biomaterials as a drug carriers and diagnostic or therapeutic material due to their smaller size, large surface area and cell penetration ability. Here we report the preparation of SNPs of diameter 10 ± 3 nm by using silver nitrate and sodium borohydride and the interaction of synthesized SNPs with our model protein β-lactoglobulin (β-lg) in 10 mM phosphate buffer at pH 7.5 after thermal exposure at 75 °C. Heat exposed β-lg forms amyloidal fibrillar aggregates whereas this protein aggregates adopt rod-like shape instead of fibrillar structure in presence of SNP under the same conditions. Size of the synthesized SNPs is confirmed by UV-Visible spectroscopy, SEM and TEM. Interactions and subsequent formation of molecular assembly of heat stressed β-lg with SNP were investigated using Th-T assay and ANS binding assay, DLS, RLS, CD, FT-IR, SEM, TEM. Docking study parallely also support the experimental findings.

Inhibition of amyloid fibril formation of β-lactoglobulin by natural and synthetic curcuminoids

The aggregation of proteins has been associated with several aspects of daily life, including food processing, blood coagulation and many neurodegenerative infections.

The effect of green synthesis silver nanoparticles (AgNPs) from Pulicaria undulata on the amyloid formation in α-lactalbumin and the chaperon action of α-casein

The formation and deposition of protein fibrillar aggregates in the tissues is associated with several neurodegenerative diseases such as Alzheimer's and Parkinson's disease. Molecular chaperones are a family of proteins that are believed to have the ability to inhibit protein aggregation. The present study examines the effect of different concentrations of green synthesis silver nanoparticles (AgNPs) from Pulicaria undulata L. on the aggregation of α-lactalbumin (α-LA) and the chaperone action of α_s-casein. The effects of the AgNPs were determined by measuring light scattering absorption, fluorescence (ThT assay, intrinsic fluorescence assay and ANS binding assay), TEM, CD spectroscopy and SDS-PAGE. The results showed that AgNPs have the ability to prevent the aggregation of α-LA in a concentration-dependent manner. In fact, by increasing the concentration of AgNPs within a specified range, the adsorption and interaction between AgNPs and protein have increased and protein conformational changes and self-association decreased, thus amyloid aggregation is prevented. Our results also showed that α-casein effectively prevented the aggregation of the α-lactalbumin which increased in the presence of the AgNPs. Standard experimental results, however, proved that nanoparticles had no effect on the structure and hence the chaperone ability of α-casein. Our findings showed that AgNPs can prevent protein aggregation and have no effect on the chaperone ability of α_s-casein. In the main, results of this study show that biosynthesized AgNPs mediated by Pulicaria undulata L. has the capability in inhibiting amyloid fibril formation and thus could be consider as a therapeutic agent in the treatment of amyloidosis disorders.

Protein Adsorption to Charged Gold Nanospheres as a Function of Protein Deformability

The corona that forms as protein adsorbs to gold nanospheres (AuNSs) is directly influenced by the surface chemistry of the AuNS. Tools to predict adsorption outcomes are needed for intelligent design of nanomaterials for biological applications. We hypothesized that the denaturation behavior of a protein might be a useful predictor of adsorption behavior to AuNSs, and used this idea to study protein adsorption to anionic citrate-capped AuNSs and to cationic poly(allylamine hydrochloride) (PAH) wrapped AuNSs. Three proteins (α-amylase (A-Amy), β-lactoglobulin (BLG), and bovine serum albumin (BSA)), representing three different classes of acid denaturation behavior, were selected with BLG being the least deformable and BSA being the most deformable. Protein adsorption to AuNSs was monitored via UV-vis spectrophotometry and dynamic light scattering. Changes to the protein structure upon AuNS interaction were monitored via circular dichroism spectroscopy. Binding constants were determined using the Langmuir adsorption isotherm, resulting in BSA > BLG ≫ A-Amy affinities for citrate-capped gold nanospheres. PAH-coated AuNSs displayed little affinity for these proteins at similar concentrations as citrate-coated AuNSs and became agglomerated at high protein concentrations. The enzymatic activity of A-Amy/citrate AuNS conjugates was measured via colorimetric assay, and found to be 11% of free A-Amy, suggesting that binding restricts access to the active site. Across both citrate AuNSs and PAH AuNSs, the changes in secondary structure were greatest for BSA > A-Amy > BLG, which does follow the trends predicted by acid denaturation characteristics.

Al cation induces aggregation of serum proteins

Al cation is known to induce protein fibrillation and causes several neurodegenerative disorders. We report the spectroscopic, thermodynamic analysis and AFM imaging for the Al cation binding process with human serum albumin (HSA), bovine serum albumin (BSA) and milk beta-lactoglobulin (b-LG) in aqueous solution at physiological pH. Hydrophobicity played a major role in Al-protein interactions with more hydrophobic b-LG forming stronger Al-protein complexes. Thermodynamic parameters ΔS, ΔH and ΔG showed Al-protein bindings occur via hydrophobic and H-bonding contacts for b-LG, while van der Waals and H-bonding interactions prevail in HSA and BSA adducts. AFM clearly indicated that aluminum cations are able to force BSA and b-LG into larger or more robust aggregates than HSA, with HSA 4±0.2 (SE, n=801) proteins per aggregate, for BSA 17±2 (SE, n=148), and for b-LG 12±3 (SE, n=151). Thioflavin T test showed no major protein fibrillation in the presence of Al cation. Al complexation induced major alterations of protein conformations with the order of perturbations b-LG>BSA>HSA.

Aggregation of trypsin and trypsin inhibitor by Al cation

Al cation may trigger protein structural changes such as aggregation and fibrillation, causing neurodegenerative diseases. We report the effect of Al cation on the solution structures of trypsin (try) and trypsin inhibitor (tryi), using thermodynamic analysis, UV-Visible, Fourier transform infrared (FTIR) spectroscopic methods and atomic force microscopy (AFM). Thermodynamic parameters showed Al-protein bindings occur via H-bonding and van der Waals contacts for trypsin and trypsin inhibitor. AFM showed that Al cations are able to force trypsin into larger or more robust aggregates than trypsin inhibitor, with trypsin 5±1 SE (n=52) proteins per aggregate and for trypsin inhibitor 8.3±0.7 SE (n=118). Thioflavin T test showed no major protein fibrillation in the presence of Al cation. Al complexation induced more alterations of trypsin inhibitor conformation than trypsin.

Influence of gold nanoparticle surface chemistry and diameter upon Alzheimer's disease amyloid-β protein aggregation

Background

Deposits of aggregated amyloid-β protein (Aβ) are a pathological hallmark of Alzheimer’s disease (AD). Thus, one therapeutic strategy is to eliminate these deposits by halting Aβ aggregation. While a variety of possible aggregation inhibitors have been explored, only nanoparticles (NPs) exhibit promise at low substoichiometric ratios. With tunable size, shape, and surface properties, NPs present an ideal platform for rationally designed Aβ aggregation inhibitors. In this study, we characterized the inhibitory capabilities of gold nanospheres exhibiting different surface coatings and diameters.

Results

Both NP diameter and surface chemistry were found to modulate the extent of aggregation, while NP electric charge influenced aggregate morphology. Notably, 8 nm and 18 nm poly(acrylic acid)-coated NPs abrogated Aβ aggregation at a substoichiometric ratio of 1:2,000,000. Theoretical calculations suggest that this low stoichiometry could arise from altered solution conditions near the NP surface. Specifically, local solution pH and charge density are congruent with conditions that influence aggregation.

Conclusions

These findings demonstrate the potential of surface-coated gold nanospheres to serve as tunable therapeutic agents for the inhibition of Aβ aggregation. Insights gained into the physiochemical properties of effective NP inhibitors will inform future rational design of effective NP-based therapeutics for AD.

Electronic supplementary material

The online version of this article (doi:10.1186/s13036-017-0047-6) contains supplementary material, which is available to authorized users.

Loading of halloysite nanotubes with BSA, α-Lac and β-Lg: a Fourier transform infrared spectroscopic and thermogravimetric study

Halloysite nanotubes (HNTs) are considered as ideal materials for biotechnological and medical applications. An important feature of halloysite is that it has a different surface chemistry on the inner and outer sides of the tubes. This property means that negatively-charged molecules can be selectively loaded inside the halloysite nanoscale its lumen. Loaded HNTs can be used for the controlled or sustained release of proteins, drugs, bioactive molecules and other agents. We studied the interaction between HNTs and bovine serum albumin, α lactalbumin and β -lactoglobulin loaded into HTNs using Fourier transform infrared spectroscopy and thermogravimetry. These techniques enabled us to study the protein conformation and thermal stability, respectively, and to estimate the amount of protein loaded into the HNTs. TEM images confirmed the loading of proteins into HTNs.

Impact of stirring speed on β-lactoglobulin fibril formation

β-Lactoglobulin (β-lg) can produce fibrils that have multi-functional properties. Impacts of different stirring speeds on characteristics of β-lg fibrils as a stable form in β-lg fibril solutions were investigated. Fibril concentration, fibril morphology, turbidity, particle size distribution, zeta potential, and rheological behavior of solutions were studied. Stirring enhanced fibril formation and stability of a fibril solution, in comparison with unstirred solutions. Increasing the stirring speed produced more turbidity and a greater distribution of particle sizes, higher viscosity values, but no differences in zeta potential values of β-lg fibril solutions. However, a high stirring speed is not feasible due to reduction of the fibril yield and changes in fibril morphology.

Multispectroscopic analysis and molecular modeling to investigate the binding of beta lactoglobulin with curcumin derivatives

Bovine beta lactoglobulin (β-lg), the major whey protein, has a great affinity for a wide range of organic compounds like fatty acids, retinoletc.

Curcumin inhibits the Al(iii) and Zn(ii) induced amyloid fibrillation of β-lactoglobulin in vitro

Accumulation of ordered protein aggregates (or amyloids) is responsible for several neurodegenerative diseases.

New insight into the alcohol induced conformational change and aggregation of the alkaline unfolded state of bovine β-lactoglobulin

Here we report for first time the self-assembly of β-lg from its alkaline unfolded state.

Facile synthesis and characterization of beta lactoglobulin–copper nanocomposites having antibacterial applications

The synthesis of Cu⁰ nanoparticles and Cu–protein nanocomposites is a great challenge.

Insight into the co-solvent induced conformational changes and aggregation of bovine β-lactoglobulin

Many proteins form ordered irreversible aggregates called amyloid fibrils which are responsible for several neurodegenerative diseases. β-lactoglobulin (β-lg), an important globular milk protein, self-assembles to form amyloid-like fibrils on heating at low pH. The present study investigated the effects of two commonly used organic solvents acetonitrile (MeCN) and antimicrobial preservative benzyl alcohol (BA) on the conformation and self-assembly of β-lg at ambient condition. Both MeCN and BA induced a concentration-dependent conformational change showing exposure of hydrophobic patches, loss of tertiary structure and higher α-helical structure at moderate concentrations. In the presence of 50-80% (v/v) MeCN and 1.5-3% (v/v) BA further structural transitions from α-helical to non-native β-sheet structure were observed with a molten globule-like intermediate at 70% MeCN. These non-native β-sheet structures have high tendency to form aggregates. The formation of β-lg self-assembly was confirmed by Thioflavin T studies, Congo red assay, Rayleigh scattering and dynamic light scattering analysis. Transmission electron microscopy studies showed amyloid fibril formation in both MeCN and BA. Our results showed that BA enhances the unfolding and self-assembly of β-lg at much lower concentration than MeCN. Thus solvent composition forces the protein to achieve the non-native structures which are responsible for protein aggregation.

Curcumin Protects β-Lactoglobulin Fibril Formation and Fibril-Induced Neurotoxicity in PC12 Cells

In this study the β-lactoglobulin fibrillation, in the presence or absence of lead ions, aflatoxin M1 and curcumin, was evaluated using ThT fluorescence, Circular dichroism spectroscopy and atomic force microscopy. To investigate the toxicity of the different form of β-Lg fibrils, in the presence or absence of above toxins and curcumin, we monitored changes in the level of reactive oxygen species and morphology of the differentiated neuron-like PC12 cells. The cell viability, cell body area, average neurite length, neurite width, number of primary neurites, percent of bipolar cells and node/primary neurite ratios were used to assess the growth and complexity of PC12 cells exposed to different form of β-Lg fibrils. Incubation of β-Lg with curcumin resulted in a significant decrease in ROS levels even in the presence of lead ions and aflatoxin M1. The β-Lg fibrils formed in the presence of lead ions and aflatoxin M1 attenuated the growth and complexity of PC12 cells compared with other form of β-Lg fibrils. However, the adverse effects of these toxins and protein fibrils were negated in the presence of curcumin. Furthermore, the antioxidant and inhibitory effects of curcumin protected PC12 cells against fibril neurotoxicity and enhanced their survival. Thus, curcumin may provide a protective effect toward β-Lg, and perhaps other protein, fibrils mediated neurotoxicity.

Encapsulation of testosterone and its aliphatic and aromatic dimers by milk beta-lactoglobulin

The encapsulation of testosterone and it aliphatic dimer (alip) and aromatic dimer (arom) with milk β-lactoglobulin (β-LG) was studied in aqueous solution at pH 7.4. Multiple spectroscopic methods, transmission electron microscopy (TEM) and molecular modeling were used to characterize testosterone-β-LG binding and protein aggregation process. Spectroscopic analysis showed that steroids bind β-LG via hydrophobic and H-bonding interactions with overall binding constants K test-β-LG = 5.6 (± 0.6) × 10(4)M(-1), K test-dimeralip-β-LG = 4.8 (± 0.5) × 10(3)M(-1) and K test-dimer-arom-β-LG = 2.9 (± 0.4) × 10(4)M(-1). The binding affinity was testosterone > testosterone dimer-aromatic > testosterone dimer-aliphatic. Transmission electron microscopy showed major changes in protein morphology as testosterone-protein complexation occurred with increase in the diameter of the protein aggregate indicating encapsulation of steroids by β-LG. Modeling showed the presence of H-bonding stabilized testosterone-β-LG complexes with the free binding energy of -9.82 Kcal/mol indicating that the interaction process is spontaneous at room temperature.