Influence of the Galloyl Moiety in Tea Catechins on Binding Affinity for Human Serum Albumin

Green tea catechin-grafted silk fibroin hydrogels with reactive oxygen species scavenging activity for wound healing applications

BACKGROUND

Overproduction of reactive oxygen species (ROS) is known to delay wound healing by causing oxidative tissue damage and inflammation. The green tea catechin, (-)-Epigallocatechin-3-O-gallate (EGCG), has drawn a great deal of interest due to its strong ROS scavenging and anti-inflammatory activities. In this study, we developed EGCG-grafted silk fibroin hydrogels as a potential wound dressing material.

METHODS

The introduction of EGCG to water-soluble silk fibroin (SF-WS) was accomplished by the nucleophilic addition reaction between lysine residues in silk proteins and EGCG quinone at mild basic pH. The resulting SF-EGCG conjugate was co-crosslinked with tyramine-substituted SF (SF-T) via horseradish peroxidase (HRP)/H2O2 mediated enzymatic reaction to form SF-T/SF-EGCG hydrogels with series of composition ratios.

RESULTS

Interestingly, SF-T70/SF-EGCG30 hydrogels exhibited rapid in situ gelation (< 30 s), similar storage modulus to human skin (≈ 1000 Pa) and superior wound healing performance over SF-T hydrogels and a commercial DuoDERM® gel dressings in a rat model of full thickness skin defect.

CONCLUSION

This study will provide useful insights into a rational design of ROS scavenging biomaterials for wound healing applications.

Insights into the Binding of Dietary Phenolic Compounds to Human Serum Albumin and Food-Drug Interactions

The distribution of drugs and dietary phenolic compounds in the systemic circulation de-pends on, among other factors, unspecific/specific reversible binding to plasma proteins such as human serum albumin (HSA). Phenolic substances, present in plant-derived feeds, foods, beverages, herbal medicines, and dietary supplements, are of great interest due to their biological activity. Recently, considerable research has been directed at the formation of phenol-HSA complexes, focusing above all on structure-affinity relationships. The nucleophilicity and planarity of molecules can be altered by the number and position of hydroxyl groups on the aromatic ring and by hydrogenation. Binding affinities towards HSA may also differ between phenolic compounds in their native form and conjugates derived from phase II reactions. On the other hand, food-drug interactions may increase the concentration of free drugs in the blood, affecting their transport and/or disposition and in some cases provoking adverse or toxic effects. This is caused mainly by a decrease in drug binding affinities for HSA in the presence of flavonoids. Accordingly, to avoid the side effects arising from changes in plasma protein binding, the intake of flavonoid-rich food and beverages should be taken into consideration when treating certain pathologies.

Consumption of a green tea extract-curcumin drink decreases blood urea nitrogen and redox iron in β-thalassemia patients

The most important cause of death in β-thalassemia major patients is organ dysfunction due to iron deposits. Non-transferrin bound iron (NTBI), labile plasma iron (LPI) and labile iron pool are redox-active forms of iron found in thalassemia. Iron chelation therapy is adopted to counteract the resulting iron overload. Extracts of green tea (GTE) and curcumin exhibit iron-chelating and antioxidant activities in iron-loaded cells and β-thalassemic mice. We have used our GTE-CUR drink to investigate the potential amelioration of iron overload and oxidative stress in transfusion-dependent β-thalassemia (TDT) patients. The patients were enrolled for a control group without and with GTE-CUR treatments (17.3 and 35.5 mg EGCG equivalent). Along with regular chelation therapy, they were daily administered the drink for 60 d. Blood samples were collected at the beginning of the study and after 30 d and 60 d for biochemical and hematological tests. Interestingly, we found a decrease of blood urea nitrogen levels (P < 0.05), along with a tendency for a decrease of NTBI and LPI, and a delay in increasing lipid-peroxidation product levels in the GTE-CUR groups. The findings suggest that GTE-CUR could increase kidney function and diminish redox-active iron in iron overloaded β-thalassemia patients.

Protein Binding Characteristics of the Principal Green Tea Catechins: A QCM Study Comparing Crude Extract to Pure EGCG

Label-free detection methods such as the quartz crystal microbalance (QCM) are well suited to the analysis of molecular interactions in complex mixtures such as crude botanical extracts. In the present study, the binding characteristics of epigallocatechin gallate (EGCG) and crude green tea extract solutions to bovine serum albumin (BSA) have been investigated. The adsorbed mass levels onto BSA-functionalized surfaces were measured at various solution concentrations. Langmuir and Freundlich isotherms were used to model the adsorption data. The Langmuir isotherm better described the adsorption behavior with correlations of 0.68 and 0.70 for the EGCG and the crude extract solutions, respectively. The better fit of the Langmuir model indicates that adsorption occurs homogeneously and that aggregation is negligible. The mass saturation is estimated to be 58% higher for the crude green tea solution as compared to the pure EGCG solution (7.9 ng/cm² for green tea and 5 ng/cm² for EGCG). The increased adsorption for the crude extract indicates that the additional tea chemical constituents are binding to alternate sites on the protein molecule and that competitive binding is a nondominant effect. However, a reduced adsorption rate for the crude extract was also observed, indicating some presence of competitive mechanisms. The results demonstrate the utility of the QCM for the analysis of protein binding in crude mixtures as well as pure compounds.

In Vitro and In Silico Studies of the Molecular Interactions of Epigallocatechin-3-O-gallate (EGCG) with Proteins That Explain the Health Benefits of Green Tea

Green tea has been shown to have beneficial effects on many diseases such as cancer, obesity, inflammatory diseases, and neurodegenerative disorders. The major green tea component, epigallocatechin-3-O-gallate (EGCG), has been demonstrated to contribute to these effects through its anti-oxidative and pro-oxidative properties. Furthermore, several lines of evidence have indicated that the binding affinity of EGCG to specific proteins may explain its mechanism of action. This review article aims to reveal how EGCG-protein interactions can explain the mechanism by which green tea/EGCG can exhibit health beneficial effects. We conducted a literature search, using mainly the PubMed database. The results showed that several methods such as dot assays, affinity gel chromatography, surface plasmon resonance, computational docking analyses, and X-ray crystallography have been used for this purpose. These studies have provided evidence to show how EGCG can fit or occupy the position in or near functional sites and induce a conformational change, including a quaternary conformational change in some cases. Active site blocking, steric hindrance by binding of EGCG near an active site or induced conformational change appeared to cause inhibition of enzymatic activity and other biological activities of proteins, which are related to EGCG’s biological oligomer and formation of their toxic aggregates, leading to the prevention of neurodegenerative diseases and amyloidosis. In conclusion, these studies have provided useful information on the action of green tea/catechins and would lead to future studies that will provide further evidence for rational EGCG therapy and use EGCG as a lead compound for drug design.

Binding of Catechins to Staphylococcal Enterotoxin A

Staphylococcal enterotoxin A (SEA) is a toxin protein, and is the most common cause of staphylococcal food poisoning. Polyphenols, such as catechins, are known to interact with proteins. In this study, we investigated the binding of catechins to SEA using SPR (Biacore), Fourier transform infrared spectroscopy (FT-IR), isothermal titration calorimetry (ITC), and protein-ligand docking. We found that (−)-epigallocatechin gallate (EGCG) could strongly bind to SEA. According to thermodynamic parameters, a negative ΔG indicated that the interaction between EGCG and SEA was spontaneous, and the electrostatic force accompanied by hydrophobic binding forces may play a major role in the binding. Data from Western blot analysis and docking simulation suggest that the hydroxyl group at position 3 of the galloyl group in the catechin structure was responsible for binding affinity with the Y91 of the A-6 region of SEA active sites. Our results provide further understanding of the binding interactions between catechins and SEA, and the inhibition of toxin activities by catechins.

The critical role of epigallocatechin gallate in regulating mitochondrial metabolism

Epigallocatechin gallate (EGCG), one of polyphenols isolated from green tea, exhibits biology-benefiting effects with minimum severe adverse. EGCG is known to be a mitochondrion-targeting medicinal agent, regulating mitochondrial metabolism, including mitochondrial biogenesis, mitochondrial bioenergetics, and mitochondria-mediated cell cycle and apoptosis. EGCG might exhibit either antioxidative activity to prevent against oxidative stress or pro-oxidative activity to counteract cancer cells, which depends on the cellular stress situations, cell types and the concentration of EGCG. Recent research has gained positive and promising data. This review will discuss the interaction between EGCG and mitochondrion.

The critical role of epigallocatechin gallate in regulating mitochondrial metabolism

Epigallocatechin gallate (EGCG), one of polyphenols isolated from green tea, exhibits biology-benefiting effects with minimum severe adverse. EGCG is known to be a mitochondrion-targeting medicinal agent, regulating mitochondrial metabolism, including mitochondrial biogenesis, mitochondrial bioenergetics, and mitochondria-mediated cell cycle and apoptosis. EGCG might exhibit either antioxidative activity to prevent against oxidative stress or pro-oxidative activity to counteract cancer cells, which depends on the cellular stress situations, cell types and the concentration of EGCG. Recent research has gained positive and promising data. This review will discuss the interaction between EGCG and mitochondrion.

Multi-site binding of epigallocatechin gallate to human serum albumin measured by NMR and isothermal titration calorimetry

The affinity of epigallocatechin gallate (EGCG) for human serum albumin (HSA) was measured in physiological conditions using NMR and isothermal titration calorimetry (ITC). NMR estimated the K_a (self-dissociation constant) of EGCG as 50 mM. NMR showed two binding events: strong (n₁=1.8 ± 0.2; K_d1 =19 ± 12 μM) and weak (n₂∼20; K_d2 =40 ± 20 mM). ITC also showed two binding events: strong (n₁=2.5 ± 0.03; K_d1 =21.6 ± 4.0 μM) and weak (n₂=9 ± 1; K_d2 =22 ± 4 mM). The two techniques are consistent, with an unexpectedly high number of bound EGCG. The strong binding is consistent with binding in the two Sudlow pockets. These results imply that almost all EGCG is transported in the blood bound to albumin and explains the wide tissue distribution and chemical stability of EGCG in vivo.

Tannins from Hamamelis virginiana bark extract: characterization and improvement of the antiviral efficacy against influenza A virus and human papillomavirus

Antiviral activity has been demonstrated for different tannin-rich plant extracts. Since tannins of different classes and molecular weights are often found together in plant extracts and may differ in their antiviral activity, we have compared the effect against influenza A virus (IAV) of Hamamelis virginiana L. bark extract, fractions enriched in tannins of different molecular weights and individual tannins of defined structures, including pseudotannins. We demonstrate antiviral activity of the bark extract against different IAV strains, including the recently emerged H7N9, and show for the first time that a tannin-rich extract inhibits human papillomavirus (HPV) type 16 infection. As the best performing antiviral candidate, we identified a highly potent fraction against both IAV and HPV, enriched in high molecular weight condensed tannins by ultrafiltration, a simple, reproducible and easily upscalable method. This ultrafiltration concentrate and the bark extract inhibited early and, to a minor extent, later steps in the IAV life cycle and tannin-dependently inhibited HPV attachment. We observed interesting mechanistic differences between tannin structures: High molecular weight tannin containing extracts and tannic acid (1702 g/mol) inhibited both IAV receptor binding and neuraminidase activity. In contrast, low molecular weight compounds (<500 g/mol) such as gallic acid, epigallocatechin gallate or hamamelitannin inhibited neuraminidase but not hemagglutination. Average molecular weight of the compounds seemed to positively correlate with receptor binding (but not neuraminidase) inhibition. In general, neuraminidase inhibition seemed to contribute little to the antiviral activity. Importantly, antiviral use of the ultrafiltration fraction enriched in high molecular weight condensed tannins and, to a lesser extent, the unfractionated bark extract was preferable over individual isolated compounds. These results are of interest for developing and improving plant-based antivirals.

Simultaneous detection of green tea catechins and gallic acid in human serum after ingestion of green tea tablets using ion-pair high-performance liquid chromatography with electrochemical detection

We developed an analytical method for the simultaneous determination of tea catechins and gallic acid (GA) in human serum using ion-pair high-performance liquid chromatography (HPLC) with electrochemical detection. GA was measured to estimate the amount of gallate moiety produced by degradation of gallated catechins ((-)-epicatechin-3-gallate, ECG; (-)-epigallocatechin-3-gallate, EGCG). Ethyl gallate was adopted as an internal standard to correct for the extraction efficiency. To maximize extraction efficiency, a hydrophobic polytetrafluoroethylene (PTFE) filter was selected for pre-treatment prior to separation. HPLC separation was performed using a C18 reversed-phase column with a gradient mobile phase of phosphate buffer (pH 2.5) containing tetrahexylammonium hydrogensulfate as an ion-pair reagent. Using this method, (-)-epicatechin (EC), (-)-epigallocatechin (EGC), ECG, EGCG, ethyl gallate, and GA were detected as single peaks. The resolution values for target analytes were 4.0-13.0 and the mean values of the absolute recoveries of catechins and GA were 77.3-93.9%. The detection limits for catechins and GA in serum were 0.4-3.1ng/mL. The serum catechin levels of eight healthy volunteers after ingestion of a single dose of green tea tablets were measured using this method. The concentration of total catechins (free+conjugated forms) in serum peaked 60min after ingestion. From these results, this method is thought to enable the simultaneous quantification of GA, the hydrolysis product of gallated catechins, and target catechins, and to be sufficiently sensitive for pharmacokinetic studies of catechins following oral administration of green tea.

Does flavor impact function? Potential consequences of polyphenol-protein interactions in delivery and bioactivity of flavan-3-ols from foods

Astringency is a component of the overall flavor experienced when consuming polyphenol rich foods and beverages such as tea, wine, cocoa and select fruits. Following consumption, the astringent sensation results from the well documented ability of polyphenols to bind to salivary proline rich proteins (PRP) and facilitate their precipitation in the oral cavity. In a similar fashion, polyphenols are also known to non-specifically bind food and other biological proteins. While much is known regarding the polyphenol-protein interactions leading to astringency, significantly less information is available regarding the impact of these polyphenol-protein interactions with food or other biological proteins on relevant physiological outcomes. This paper focuses on the interactions between flavan-3-ols, one of the most abundant dietary polyphenol forms, with proteins in food, salivary PRP and other physiological proteins. The physiological implications of these interactions in food and through the gut will be discussed in relation to manipulation of flavan-3-ol bioavailability, metabolism and biological activities including inhibition of digestive enzymes in the gut.

Investigation on the interaction between ilaprazole and bovine serum albumin without or with different C-ring flavonoids from the viewpoint of food-drug interference

The interaction between ilaprazole and bovine serum albumin (BSA) has been investigated in the absence and presence of four popular flavonoids with different C-ring structures, quercetin, luteolin, taxifolin, and (+)-catechin, by means of fluorescence spectroscopy. The results indicated that ilaprazole had a strong ability to quench the intrinsic fluorescence of BSA, and site marker competitive experiments indicated that the binding of ilaprazole to BSA primarily took place in subdomain IIA. The quenching process of ilaprazole with BSA was easily affected by flavonoids,; however, they did not change the quchenching mechanism of ilaprazole with BSA, whereas all of the fluorescence quenching was initiated by a static quenching procedure combining with nonradiative energy transfer. The presence of flavonoids decreased the quenching constants of ilaprazole with BSA from 2.2 to 23.7% and decreased the binding constants from 73.7 to 98.3%, which depended on the different flavonoids' structures. The decreased binding constants and unchangeable spatial distance of ilaprazole with BSA by the introduction of quercetin, luteolin, and taxifolin may result from the competition of flavonoids and ilaprazole binding to BSA, whereas in the presence of (+)-catichin, decreased binding constants and increased spatial distance possibly resulted from the formation of a ternary ilaprazole-BSA-(+)-catechin complex. All of these results may have relevant consequences in rationalizing the interferences of common food to gastric ulcer treatments.