Calorimetric and spectroscopic studies of the interactions between insulin and (−)-epigallocatechin-3-gallate

Enhanced Cytotoxicity of Cadmium by a Sulfated Polysaccharide from Abalone

Consumption of seafood is a common route of cadmium ion (Cd²⁺) exposure to consumers. The seafood matrices may alter the toxicity profile of Cd²⁺ due to the interaction between Cd²⁺ and biomacromolecules in seafood. In this study, enhanced cytotoxicity of Cd²⁺ was found in the presence of an abalone gonad sulfated polysaccharide (AGSP) and the mechanism was investigated at a metabolic level. The formation of the AGSP-Cd²⁺ complex was demonstrated by isothermal titration calorimetry. The level of reactive oxygen species (ROS) increased and mitochondrial membrane potential reduced upon exposure to the AGSP-Cd²⁺ complex as compared with those of Cd²⁺ exposure. The decreased cell viability after incubation with the AGSP-Cd²⁺ complex also suggested enhanced Cd²⁺ toxicity induced by AGSP. The metabolomics and lipidomics analysis revealed that, compared with the Cd²⁺ group, the AGSP-Cd²⁺ downregulated the phospholipid metabolism and resulted in more serious damage in the cellular membrane. The lipid metabolism disorder, in turn, amplified the generation of ROS, leading to a decrease in cell viability. These results provided new evidence of the enhanced Cd²⁺ toxicity upon interaction with seafood polysaccharides, and much attention should be paid to the effect of food ingredients on heavy metal ion toxicity.

Poly(amino acid) Multilayers Modified Dendritic Mesoporous Silica Nanoparticles Achieve Effective Enzyme Stability for Ultrasensitive Immunoassay

Enzyme-linked immunosorbent assay (ELISA) is one of the most common techniques in biomedical detection; however, the poor sensitivity in early diagnosis for some diseases seriously limits its application. In this work, we developed an ultrasensitive ELISA system that is based on horseradish peroxidase (HRP)-loaded dendritic mesoporous silica nanoparticles (DMSN) modified with poly(amino acid) multilayers (defined as DSHP). A large amount of HRP adsorption was achieved in center-radial mesoporous channels of DMSN because of the high specific surface area and large pore size, leading to significant signal amplification. Additionally, DSHP could not only effectively maintain HRP activity for at least 10 days but also provide preferable protection for HRP activity even at high temperatures or a wide pH range. Moreover, the DSHP system exhibited admirable signal amplification performance with a limit of detection of 0.667 fM and a wide detectable range from 6.67 × 10^-4 to 6.67 × 10⁵ pM, whose sensitivity was 10⁴ times higher than that of the conventional ELISA. We believe that the DSHP will offer a new strategy for signal amplification of the ELISA system in clinical diagnosis.

Hydrophobic Modification of Carboxyl-Terminated Polyamidoamine Dendrimer Surface Creates a Potent Inhibitor of Amyloid-β Fibrillation

Amyloid β-peptide (Aβ) fibrillogenesis is a major hallmark of Alzheimer's disease (AD); inhibition of Aβ fibrillation is thus considered as a promising strategy for AD prevention and treatment. Our group has previously proposed the hydrophobic binding-electrostatic repulsion (HyBER) hypothesis, which provides guidance for the design of new amyloid inhibitors. Inspired by the HyBER hypothesis, we have herein proposed to synthesize hydrophobic-modified generation 5 carboxyl-terminated polyamidoamine dendrimer, denoted as PAMP, to create a potent inhibitor with a negatively charged hydrophobic surface. Results indicate that the PAMP with a proper degree of phenyl substitution (30-42%) alters the conformation of Aβ₄₂ through both hydrophobic binding and electrostatic repulsive forces on its surface. With these well-balanced interactions, the inhibitor can even completely inhibit the formation of β-sheet structure of the peptide, accompanied by changes at the level of the fibrillary architecture. Moreover, the results also indicate that changes of Aβ₄₂ aggregation pathway influenced by the PAMP occur at the very early stage, so the PAMP can significantly avoid the formation of toxic intermediates of Aβ₄₂ aggregation.

Binary and Tertiary Complex Based on Short-Chain Glucan and Proanthocyanidins for Oral Insulin Delivery

The present study was performed to investigate binary and tertiary nanocomposites between short-chain glucan (SCG) and proanthocyanidins (PAC) for the oral delivery of insulin. There was a large decrease in fluorescence intensity of insulin in the presence of SCG or the combination of SCG with PAC. Fourier transform infrared spectroscopy revealed that the binary and tertiary nanocomposites were synthesized due to the hydrogen bonding and hydrophobic interactions. The insulin entrapped in the nanocomposites was in an amorphous state confirmed by X-ray diffraction. The cell culture demonstrated that both the nanocomposites showed no detectable cytotoxicity with relative cell viability all above 85%. The pharmacological bioavailability after oral administration of insulin-SCG-PAC at a dose of 100 IU/kg was found to be 6.98 ± 1.20% in diabetic rats without any sharp fluctuations in 8 h.

Hematoxylin Inhibits Amyloid β-Protein Fibrillation and Alleviates Amyloid-Induced Cytotoxicity

Accumulation and aggregation of amyloid β-protein (Aβ) play an important role in the pathogenesis of Alzheimer's disease. There has been increased interest in finding new anti-amyloidogenic compounds to inhibit Aβ aggregation. Herein, thioflavin T fluorescent assay and transmission electron microscopy results showed that hematoxylin, a natural organic molecule extracted from Caesalpinia sappan, was a powerful inhibitor of Aβ42 fibrillogenesis. Circular dichroism studies revealed hematoxylin reduced the β-sheet content of Aβ42 and made it assemble into antiparallel arrangement, which induced Aβ42 to form off-pathway aggregates. As a result, hematoxylin greatly alleviated Aβ42-induced cytotoxicity. Molecular dynamics simulations revealed the detailed interactions between hematoxylin and Aβ42. Four binding sites of hematoxylin on Aβ42 hexamer were identified, including the N-terminal region, S8GY10 region, turn region, and C-terminal region. Notably, abundant hydroxyl groups made hematoxylin prefer to interact with Aβ42 via hydrogen bonds. This also contributed to the formation of π-π stacking and hydrophobic interactions. Taken together, the research proved that hematoxylin was a potential agent against Aβ fibrillogenesis and cytotoxicity.

Complexes of green tea polyphenol, epigalocatechin-3-gallate, and 2S albumins of peanut

2S albumins of peanuts are seed storage proteins, highly homologous in structure and described as major elicitors of anaphylactic reactions to peanut (allergens Ara h 2 and Ara h 6). Epigallocatechin-3-gallate (EGCG) is the most biologically potent polyphenol of green tea. Non-covalent interactions of EGCG with proteins contribute to its diverse biological activities. Here we used the methods of circular dichroism, fluorescence quenching titration, isothermal titration calorimetry and computational chemistry to elucidate interactions of EGCG and 2S albumins. Similarity in structure and overall fold of 2S albumins yielded similar putative binding sites and similar binding modes with EGCG. Binding affinity determined for Ara h 2 was in the range described for complexes of EGCG and other dietary proteins. Binding of EGCG to 2S albumins affects protein conformation, by causing an α-helix to β-structures transition in both proteins. 2S albumins of peanuts may be good carriers of physiologically active green tea catechin.

Molecular interactions between (-)-epigallocatechin gallate analogs and pancreatic lipase

The molecular interactions between pancreatic lipase (PL) and four tea polyphenols (EGCG analogs), like (−)-epigallocatechin gallate (EGCG), (−)-gallocatechin gallate (GCG), (−)-epicatechin gallate (ECG), and (−)-epigallocatechin (EC), were studied from PL activity, conformation, kinetics and thermodynamics. It was observed that EGCG analogs inhibited PL activity, and their inhibitory rates decreased by the order of EGCG>GCG>ECG>EC. PL activity at first decreased rapidly and then slowly with the increase of EGCG analogs concentrations. α-Helix content of PL secondary structure decreased dependent on EGCG analogs concentration by the order of EGCG>GCG>ECG>EC. EGCG, ECG, and EC could quench PL fluorescence both dynamically and statically, while GCG only quenched statically. EGCG analogs would induce PL self-assembly into complexes and the hydrodynamic radii of the complexes possessed a close relationship with the inhibitory rates. Kinetics analysis showed that EGCG analogs non-competitively inhibited PL activity and did not bind to PL catalytic site. DSC measurement revealed that EGCG analogs decreased the transition midpoint temperature of PL enzyme, suggesting that these compounds reduced PL enzyme thermostability. In vitro renaturation through urea solution indicated that interactions between PL and EGCG analogs were weak and non-covalent.

Investigation into the mechanism of (-)-epigallocatechin-3-gallate-induced precipitation of insulin

The molecular interactions between EGCG and insulin were investigated to probe the mechanism of EGCG-induced insulin precipitation. The results indicated that 1-5mM EGCG induced insulin into reversible globular precipitates of 185-365 nm. The formation of precipitates was facilitated at high salt concentration and pH values close to insulin's isoelectric point, indicating that hydrophobic interaction was the main driving force. The precipitation was positively related to insulin concentration, but for EGCG, there was a suitable concentration (2 mM at 2 mg/mL of insulin) at which the precipitate content reached maximum. Mass spectroscopy analysis indicated that EGCG formed clusters in the aqueous solution and the clusters correlate with the insulin precipitation. Based on extensive investigation, a physical model was proposed to explain the molecular interactions between EGCG and insulin. Namely, EGCG monomers and clusters first bound to insulin dimers via hydrophobic interaction, leading to the reduction of the thickness of the hydration layer and the partial denaturation of insulin. Then, EGCG clusters acted as bridges to induce the aggregation and precipitation of insulin.