The galloyl moiety enhances the inhibitory activity of catechins and theaflavins against α-glucosidase by increasing the polyphenol-enzyme binding interactions

The combined effects of bisphenol S and hexavalent chromium on alpha-glucosidase: Intermolecular interaction, structural and functional changes

The co-contamination of heavy metal ions and organic pollutants has posed a threat to human health. Herein, this study investigated the intermolecular interactions of bisphenol S (BPS) and hexavalent chromium (Cr(VI)) under both individual and coexisting conditions, with alpha-glucosidase (AG), a key enzyme in carbohydrate metabolism, and the corresponding effects on the structure and function of AG. Multiple spectroscopic and molecular docking methods were employed to conduct the investigation in vitro and in silico. The results indicated that both BPS and Cr(VI) quenched the fluorescence of AG via a combined static and dynamic quenching processes. At 310 K, the binding constants of AG with BPS in the AG-BPS and (AG-Cr(VI))-BPS systems were 1.84 × 104 and 2.03 × 104 L mol-1, and the binding constants of AG with Cr(VI) in the AG-Cr(VI) and (AG-BPS)-Cr(VI) systems were 6.14 × 103 and 4.35 × 103 L mol-1. Cr(VI) could significantly affect the binding site of BPS in AG, while BPS had a minimal impact on the binding site of Cr(VI) in AG. BPS and Cr(VI) caused varied structural alterations of AG, and the impact of their coexistence on the structure of AG was related to the order in which they were added. Both BPS and Cr(VI) had a concentration-related effect on AG activity. This study provides valuable insights into the molecular mechanisms underlying the combined toxic effects of BPS and Cr(VI) on AG, highlighting the potential health risks associated with their environmental co-exposure.

Polyphenol oxidase mediates (-)-epigallocatechin gallate to inhibit endogenous cathepsin activity in Apostichopus japonicus

Apostichopus japonicus (A. japonicus) has rich nutritional value and is an important economic crop. Due to its rich endogenous enzyme system, fresh A. japonicus is prone to autolysis during market circulation and storage, resulting in economic losses. In order to alleviate this phenomenon, we investigated the effect of polyphenol oxidase (PPO) mediated (-)-epigallocatechin gallate (EGCG) on the activity and structure of endogenous cathepsin series protein (CEP) from A. japonicus. Research on cathepsin activity showed that PPO mediated EGCG could significantly reduce enzyme activity, resulting in a decrease in enzymatic reaction rate. SDS-PAGE and scanning electron microscopy results showed that PPO mediates EGCG could induce CEP aggregation to form protein aggregates. Various spectral results indicated that EGCG caused changes in the structure of CEP. Meanwhile, the conjugates formed by PPO mediated EGCG had lower thermal stability. In conclusion, PPO mediated EGCG was an effective method to inhibit the endogenous enzyme activity.

Inhibition mechanism of different structural polyphenols against α-amylase studied by solid-state NMR and molecular docking

Polyphenol has the considerable effects for inhibition of digestive enzymes, however, inhibition mechanism of molecular size-dependent polyphenols on enzyme activity is still lacking. Herein, inhibition effect and binding interactions of three different structural polyphenols (catechol, quercetin and hesperidin) on α-amylase were studied. Inhibition assays proved that polyphenols significantly inhibited α-amylase and their effects were increased with their molecular sizes. Hesperidin showed the highest inhibition ability of α-amylase, which was determined as IC50 = 0.43 mg/mL. Fluorescence and FT-IR spectroscopy proved that inter-molecular interactions between polyphenols and α-amylase occurred through non-covalent bonds. Besides, the secondary structure of α-amylase was obviously changed after binding with polyphenols. Inter-molecular interactions were investigated using solid-state NMR and molecular docking. Findings proved that hydrogen bonds and π-π stacking interactions were the mainly inter-molecular interactions. We hope this contribution could provide a theoretical basis for developing some digestive enzyme inhibitors from natural polyphenols.

Effective α-glycosidase inhibitors based on polyphenolic benzothiazole heterocycles

α-Glycosidase inhibition is one of the main approaches to treat Diabetes mellitus. Polyphenolic moieties are known to be responsible for yielding exhibit potent α-glycosidase inhibitory effects. In addition, compounds containing benzothiazole and Schiff base functionalities were previously reported to show α-glycosidase inhibition. In this paper, the synthesis of seven new phloroglucinol-containing benzothiazole Schiff base derivatives through the reaction of 6-substituted-2-aminobenzothiazole compounds with 2,4,6-trihydroxybenzaldehyde using acetic acid as a catalyst was reported. The synthesized compounds were characterized using spectroscopic methods such as FT-IR, 1H NMR, 13C NMR, and elemental analysis. The synthesized compounds were evaluated for their inhibitory effects on α-glycosidase, compounds 3f and 3g were found to show significant inhibitory properties when compared to the positive control. The IC50 values of 3f and 3g were calculated as 24.05 ± 2.28 and 18.51 ± 1.19 µM, respectively. Kinetic studies revealed that compounds 3f and 3g exhibited uncompetitive mode of inhibition against α-glycosidase. Molecular modeling predicted druglikeness for the title compounds and underpinned the importance of phloroglucinol hydroxyls for interacting with the key residues of α-glycosidase.

Antidiabetic Potential of Tea and Its Active Compounds: From Molecular Mechanism to Clinical Evidence

Diabetes mellitus (DM) is a chronic endocrine disorder that poses a long-term risk to human health accompanied by serious complications. Common antidiabetic drugs are usually accompanied by side effects such as hepatotoxicity and nephrotoxicity. There is an urgent need for natural dietary alternatives for diabetic treatment. Tea (Camellia sinensis) consumption has been widely investigated to lower the risk of diabetes and its complications through restoring glucose metabolism homeostasis, safeguarding pancreatic β-cells, ameliorating insulin resistance, ameliorating oxidative stresses, inhibiting inflammatory response, and regulating intestinal microbiota. It is indispensable to develop effective strategies to improve the absorption of tea active compounds and exert combinational effects with other natural compounds to broaden its hypoglycemic potential. The advances in clinical trials and population-based investigations are also discussed. This review primarily delves into the antidiabetic potential and underlying mechanisms of tea active compounds, providing a theoretical basis for the practical application of tea and its active compounds against diabetes.

Clinical applications and mechanism insights of natural flavonoids against type 2 diabetes mellitus

Diabetes is a complex disease that affects a large percentage of the world's population, and it is associated with several risk factors. Self-management poses a significant challenge, but natural sources have shown great potential in providing effective glucose reducing solutions. Flavonoids, a class of bioactive substances found in different natural sources including medicinal plants, have emerged as promising candidates in this regard. Indeed, several flavonoids, including apigenin, arbutin, catechins, and cyanidin, have demonstrated remarkable anti-diabetic properties. The clinical effectiveness of these flavonoids is linked to their potential to decrease blood glucose concentration and increase insulin concentration. Thus, the regulation of certain metabolic pathways such as glycolysis and neoglycogenesis has also been demonstrated. In vitro and in vivo investigations revealed different mechanisms of action related to flavonoid compounds at subcellular, cellular, and molecular levels. The main actions reside in the activation of glycolytic signaling pathways and the inhibition of signaling that promotes glucose synthesis and storage. In this review, we highlight the clinical efficiency of natural flavonoids as well as the molecular mechanisms underlying this effectiveness.

Dietary phenolic compounds as promising therapeutic agents for diabetes and its complications: A comprehensive review

In the middle of an ever-changing landscape of diabetes care, precision medicine, and lifestyle therapies are becoming increasingly important. Dietary polyphenols are like hidden allies found in our everyday meals. These biomolecules, found commonly in fruits, vegetables, and various plant-based sources, hold revolutionary potential within their molecular structure in the way we approach diabetes and its intimidating consequences. There are currently numerous types of diabetes medications, but they are not appropriate for all patients due to limitations in dosages, side effects, drug resistance, a lack of efficacy, and ethnicity. Currently, there has been increased interest in practicing herbal remedies to manage diabetes and its related complications. This article aims to summarize the potential of dietary polyphenols as a foundation in the treatment of diabetes and its associated consequences. We found that most polyphenols inhibit enzymes linked to diabetes. This review outlines the potential benefits of selected molecules, including kaempferol, catechins, rosmarinic acid, apigenin, chlorogenic acid, and caffeic acid, in managing diabetes mellitus as these compounds have exhibited promising results in in vitro, in vivo, in silico, and some preclinical trials study. This encompassing exploration reveals the multifaceted impact of polyphenols not only in mitigating diabetes but also in addressing associated conditions like inflammation, obesity, and even cancer. Their mechanisms involve antioxidant functions, immune modulation, and proinflammatory enzyme regulation. Furthermore, these molecules exhibit anti-tumor activities, influence cellular pathways, and activate AMPK pathways, offering a less toxic, cost-effective, and sustainable approach to addressing diabetes and its complications.

Natural Products and Diabetes: (-)-Epicatechin and Mechanisms Involved in the Regulation of Insulin Sensitivity

Type 2 diabetes (T2D) is a disease that occurs when cells do not respond normally to insulin, a condition called insulin resistance, which leads to high blood glucose levels. Although it can be treated pharmacologically, dietary habits beyond carbohydrate restriction can be highly relevant in the management of T2D. Emerging evidence supports the possibility that natural products (NPs) could contribute to managing blood glucose or counteract the undesirable effects of hyperglycemia and insulin resistance. This chapter summarizes the relevant preclinical evidence involving the flavonoid (-)-epicatechin (EC) in the optimization of glucose homeostasis, reducing insulin resistance and/or diabetes-associated disorders. Major effects of EC are observed on (i) intestinal functions, including digestive enzymes, glucose transporters, microbiota, and intestinal permeability, and (ii) redox homeostasis, including oxidative stress and inflammation. There is still a need for further clinical studies to confirm the in vitro and rodent data, allowing recommendations for EC, particularly in prediabetic and T2D patients. The collection of similar data and the lack of clinical evidence for EC is also applicable to other NPs.

Inhibition effect of epicatechin gallate on acid phosphatases from rainbow trout (Oncorhynchus mykiss) liver by multispectral and molecular docking

Inhibition of acid phosphatase, which significantly contributes to inosine 5'-monophosphate (IMP) degradation, is crucial for preventing flavor deterioration of aquatic products during storage. In this study, the inhibitory effect of epicatechin gallate (ECG) on the activity of acid phosphatase isozymes (ACPI and ACPII) was analyzed using inhibition kinetics, fluorescence spectroscopy, isothermal titration calorimetry, and molecular simulation. ACPI and ACPII with molecular weights of 59.5 and 37.3 kDa, respectively, were purified from rainbow trout liver. ECG reversibly inhibited ACPI and ACPII activities via mixed-type inhibition, with half maximal inhibitory concentration (IC50) of 0.24 ± 0.01 mmol/L and 0.27 ± 0.03 mmol/L, respectively. Fluorescence spectra indicated that ECG statically quenched the intrinsic fluorescence of ACPI and ACPII. ECG could spontaneously bind to ACPI and ACPII through hydrogen bonding and van der Waals forces and exhibited a higher affinity for ACPI than for ACPII. In addition, molecular dynamic simulation revealed that ECG-ACPI and ECG-ACPII complexes were relatively stable during the entire simulation process. Our findings provide a theoretical basis for the use of ECG as an inhibitor of ACP to improve the flavor of aquatic products.

Epigallocatechin gallate-immobilized antimicrobial resin with rechargeable fluorinated synergistic composite for enhanced caries control

OBJECTIVES

Given the global prevalence of dental caries, impacting 2.5 billion individuals, the development of sophisticated prevention filling materials is crucial. Streptococcus mutans, the principal caries-causing strain, produces acids that demineralize teeth and initiate dental caries. To address this issue, we aimed to develop a synergistic resin-based composite for enhancing caries control.

METHODS

The synergistic resin composite incorporates fluorinated kaolinite and silanized Al2O3 nanoparticle fillers into an epigallocatechin gallate (EGCG) immobilized urethane-modified epoxy acrylate (U-EA) resin matrix, referred to the as-prepared resin composite. The EGCG-modified TPGDA/U-EA network was synthesized by preparing methacrylate-functionalized isocyanate (HI), reacting it with EGCG to form HI-EGCG, and then incorporating HI-EGCG into the TPGDA/U-EA matrix. The lamellar space within the kaolinite layer was expanded through the intercalation of acrylamide into kaolinite, enhancing its capability to adsorb and release fluoride ions (F-). The layered structure of acrylamide/ kaolinite in the U-EA resin composite acts as a F- reservoir.

RESULTS

The physico-mechanical properties of the as-prepared resin composites are comparable to those of commercial products, exhibiting lower polymerization shrinkage, substantial F- release and recharge and favorable diametral tensile strength. The immobilized EGCG in the composite exhibits potent antimicrobial properties, effectively reducing the biofilm biomass. Furthermore, the synergistic effect of EGCG and fluorinated kaolinite efficiently counteracts acid-induced hydroxyapatite dissolution, thereby suppressing demineralization and promoting enamel remineralization.

SIGNIFICANCE

Our innovative EGCG and fluoride synergistic composite provides enhanced antimicrobial properties, durable anti-demineralization, and tooth remineralization effects, positioning it as a promising solution for effective caries control and long-term dental maintenance.

The substitution sites of hydroxyl and galloyl groups determine the inhibitory activity of human pancreatic α-amylase in twelve tea polyphenol monomers

Tea polyphenols have been reported as potential α-amylase inhibitors. However, the quantitative structure-activity relationship (QSAR) between tea polyphenols and human pancreas α-amylase (HPA) is not well understood. Herein, the inhibitory effect of twelve tea polyphenol monomers on HPA was investigated in terms of inhibitory activity, as well as QSAR analysis and interaction mechanism. The results revealed that the HPA inhibitory activity of theaflavins (TFs), especially theaflavin-3'-gallate (TF-3'-G, IC50: 0.313 mg/mL), was much stronger than that of catechins (IC50: 18.387-458.932 mg/mL). The QSAR analysis demonstrated that the determinant for the inhibitory activity of HPA was not the number of hydroxyl and galloyl groups in tea polyphenol monomers, while the substitution sites of these groups potentially might play a more important role in modulating the inhibitory activity. The inhibition kinetics and molecular docking revealed that TF-3'-G as a mixed-type inhibitor had the lowest inhibition constant and bound to the active sites of HPA with the lowest binding energy (-7.74 kcal/mol). These findings could provide valuable insights into the structures-activity relationships between tea polyphenols and the HPA inhibitors.

Lowering glycemic levels via gastrointestinal tract factors: the roles of dietary fiber, polyphenols, and their combination

Dietary fiber (DF) and polyphenols (DP) are typical blood sugar-lowering components, and both play distinct yet interconnected roles in exerting their blood sugar-lowering effects. We comprehensively summarized the single and combined effects of DF and DP on blood glucose homeostasis through regulating the relevant factors in the upper gastrointestinal tract (UGT) and lower gastrointestinal tract (LGT). In the UGT, DF slowed down glucose metabolism by enhancing digesta viscosity and hindering enzyme-substrate interaction. DP primarily targeted enzymes and substrates. When combined, DP enhanced the adsorption capacity of DF for glucose. DF weakened DP's inhibitory effect on enzymes. Both DF and DP disrupted glucose intestinal uptake via physical or genomic modulation, but the co-consumption of DF and DP demonstrated a lower inhibitory effect on glucose uptake than DP alone. In the LGT, DF and DP showed synergistic or antagonistic effects on gut microbiota. Remarkably, whole foods exhibited potent prebiotic effects due to their compound-rich matrix, potentially enhancing glucose homeostasis and expanding dietary options for glucose regulation research.

Thermally Induced Covalent Cross-Linking of Proanthocyanidins and Pectin in Processed Fruit-Based Foods

The covalent interactions between proanthocyanidins (PAs) and pectin during thermal processing was investigated. An acid-butanol assay clearly showed that PAs were covalently bound to pectin. Computational studies indicated that a nucleophilic substitution reaction occurred between the carbocation generated by the PAs and carboxyl or hydroxyl groups on the pectin, leading to the formation of PAs-pectin adducts. Thermal processing and PAs significantly affected the physicochemical, functional, and biological properties of pectin. Thermal processing reduced the molecular weight and increased the gelling properties of pectin, whereas PAs increased both the molecular weight and the gelling properties. Finally, we found that the covalent attachment of PAs to pectin greatly enhanced its antioxidant, prebiotic, and α-glucosidase inhibitory activity. Overall, our results suggest that the thermal processing of fruits has the potential to induce a covalent interaction between PAs and pectin, which would impact the physicochemical characteristics and functional properties of pectin.

Soluble dietary fibres decrease α-glucosidase inhibition of epigallocatechin gallate through affecting polyphenol-enzyme binding interactions

The effects of soluble dietary fibres (SDFs) on α-glucosidase inhibition of EGCG were studied. Three arabinoxylans and polygalacturonic acid (PGA) significantly decreased inhibitory activity of EGCG against α-glucosidase, while two β-glucans hardly affected the inhibition. Although arabinoxylans and PGA weakened the competitive inhibition character of EGCG, they maintained the fluorescence quenching effect of EGCG. Then, arabinoxylans and PGA significantly decreased the particle size and turbidity of EGCG-enzyme complex. These results suggest that there formed SDFs-EGCG-enzyme ternary complexes. The stronger decreasing-effects of arabinoxylans and PGA on α-glucosidase inhibition of EGCG than β-glucans resulted from the stronger non-covalent interactions of arabinoxylans and PGA with EGCG. This is considered to arise from the short-branches of arabinoxylans that provided more opportunity for capturing EGCG, and from the strong polarity of PGA carboxyl that promoted hydrogen bondings with EGCG. Conclusively, SDFs should be considered as an impact factor when evaluating α-glucosidase inhibition of dietary polyphenols.

Exploration of the inhibitory mechanisms of trans-polydatin/resveratrol on α-glucosidase by multi-spectroscopic analysis, in silico docking and molecular dynamics simulation

Plant-derived phenolics as natural α-glucosidase (α-GLU) inhibitors have attached great attention in the treatment of type-II diabetes mellitus currently. In this study, trans-polydatin and its aglycone resveratrol were found to show a notable inhibitory activity on α-GLU in a mixed-type manner with IC₅₀ values of 18.07 and 16.73 μg/mL, respectively, which were further stronger than anti-diabetic drug acrabose (IC₅₀ = 179.86 μg/mL). Multi-spectroscopic analysis results indicated that polydatin/resveratrol bound to α-GLU with one affinity binding site which was mainly driven by hydrogen bonds and van der Waals forces, and this binding process resulted in conformational alteration of α-GLU. In silico docking study showed that polydatin/resveratrol can well interact with the surrounding amino acid residues in the active cavity of α-GLU. Molecular dynamics simulation further clarified the structure and characterization of α-GLU-polydatin/resveratrol complexes. This study might supply a theoretical basis for the designing of novel functional foods with polydatin/resveratrol.

Gallate Moiety of Catechin Is Essential for Inhibiting CCL2 Chemokine-Mediated Monocyte Recruitment

Leukocyte recruitment witnesses an orchestrated complex formation between the chemokines and their molecular partners. CCL2 chemokine that regulates monocyte trafficking is a worthwhile system from the pharmaceutical perspective. In the current study, four major catechins (EC/EGC/ECG/EGCG) were assessed for their inhibitory potential against CCL2-regulated monocyte/macrophage recruitment. Interestingly, catechins with the gallate moiety (ECG/EGCG) could only attenuate the CCL2-induced macrophage migration. These molecules specifically bound to CCL2 on a pocket comprising the N-terminal, β₀-sheets, and β₃-sheets, and the binding affinity of ECGC (K_d = 22 ± 4 μM) is ∼4 times higher than that of the ECG complex (K_d = 85 ± 6 μM). MD simulation analysis evidenced that the molecular specificity/stability of CCL2-catechin complexes is regulated by multiple factors, including stereospecificity, number of hydroxyl groups on the annular ring-B, the positioning of the carbonyl group, and the methylation of the galloyl ring. Further, a significant overlap on the binding surface of CCL2 for EGCG/ECG and receptor interactions as evidenced from NMR data provided the rationale for the observed inhibition of macrophage migration in response to EGCG/ECG binding. In summary, these galloylated epicatechins can be considered as potent protein-protein interaction (PPI) inhibitors that regulate CCL2-directed leukocyte recruitment for resolving inflammatory/immunomodulatory disorders.

Inconsistency between polyphenol-enzyme binding interactions and enzyme inhibition: Galloyl moiety decreases amyloglucosidase inhibition of catechins

Inhibiting carbohydrate-hydrolyzing enzymes has been considered as an effective approach for controlling starch digestion and postprandial blood glucose level. α-Amylase and amyloglucosidase (AMG) are commonly applied in analysis of starch digestion behaviour. Catechins have been shown with the inhibiting effects on α-amylase. However, the inhibitory activity of catechins against AMG needs to be further studied. Therefore, AMG inhibition of 8 catechins and the mechanisms were studied in this work through substrate depletion, inhibition kinetics, molecular docking, fluorescence quenching, differential scanning calorimetry, and isothermal titration calorimetry. The inhibitory activity of catechins with galloyl moiety (CGMs) was found to be lower than the corresponding catechins without the moiety (Cs). All catechins were anti-competitive inhibitors, indicating that they tended to bind with AMG-starch complex in the digestion system, rather than with AMG directly. Interestingly, CGMs had higher quenching effects on AMG fluorescence than Cs, due to the additional π-stacking between aromatic rings of GM and AMG fluorophores. Also, CGMs had a higher binding affinity to AMG, due to the tendency of GM to AMG active site, although the affinity was much weaker than that of starch to AMG. Besides, catechins did not affect AMG thermostability. Therefore, there was an inconsistency between catechins-AMG binding interactions and the enzyme inhibition because the predominant sites for catechins binding were the non-active sites on AMG-starch complex, rather than the enzyme active ones. Conclusively, inhibition mode should also be considered when evaluating the inhibitory activity of a polyphenol based on the polyphenol-enzyme binding affinity.

Water Caltrop (Trapa quadrispinosa Roxb.) Husk Improves Oxidative Stress and Postprandial Blood Glucose in Diabetes: Phenolic Profiles, Antioxidant Activities and α-Glycosidase Inhibition of Different Fractions with In Vitro and In Silico Analyses

The aim of this study was to investigate the phenolic profiles, antioxidant activities and α-glycosidase inhibitory activities of three different phenolic fractions from water caltrop (Trapa quadrispinosa Roxb.) husk and to further explore the predominant compounds and their mechanisms on α-glycosidase inhibition by virtual screening and molecular dynamics. A total of 29 substances were identified and quantified in this study. Tannins were the main constituents of water caltrop husk extract. All of the free phenolic (FP), esterified phenolic (EP) and insoluble-bound phenolic (BP) fractions exhibited good antioxidant activities, and the BP had the highest radical scavenging ability with IC50 values of 0.82 ± 0.12 μg/mL (ABTS) and 1.15 ± 0.02 μg/mL (DPPH), respectively (p < 0.05). However, compared with the EP and BP, the FP showed the strongest inhibition towards the α-glycosidase and the IC50 value of FP was 1.43 ± 0.12 μg/mL. The 1,2,6-trigalloylglucose and α-glycosidase complex had better root mean square deviations (RMSD) stability via molecular dynamics simulation study. Results obtained from this study may provide a good potential natural resource for the improvement of oxidative stress injury and blood glucose control in diabetes mellitus, which could expand the use of water caltrop husk and improve its economic value.

Inhibition of Three Diabetes-Related Enzymes by Procyanidins from Lotus (Nelumbo nucifera Gaertn.) Seedpods

The inhibitory effects of procyanidins from lotus (Nelumbo nucifera Gaertn.) seedpods on the activities of α-amylase, α-glucosidase and protein tyrosine phosphatase 1B (PTP1B), were studied and compared with those of (+)-catechin, (-)-epicatechin, epigallocatechin gallate (EGCG), procyanidin dimer B2 and trimer C1. The results showed that Lotus procyanidin extract (LPE) significantly inhibited α-amylase, α-glucosidase and PTP1B with IC₅₀ values of 5.5, 1.0, and 0.33 μg/mL, respectively. The inhibition increased with the degree of polymerization and the existence of galloyl or gallocatechin units. Kinetic analysis showed that LPE inhibited α-glucosidase activity in a mixed competitive and noncompetitive mode. Fluorescence quenching revealed that α-glucosidase interacted with LPE or EGCG in an apparent static mode, or the model of "sphere of action". The apparent static (K) and bimolecular (k_q) constants were 4375 M^-1 and 4.375 × 10¹¹ M^-1 s^-1, respectively, for LPE and 1195 M^-1 and 1.195 × 10¹¹ M^-1 s^-1, respectively, for EGCG. Molecular docking analysis provided further information on the interactions of (+)-catechin, (-)-epicatechin, EGCG, B2 and C1 with α-glucosidase. It is hypothesized that LPE may bind to multiple sites of the enzyme through hydrogen bonding and hydrophobic interactions, leading to conformational changes in the enzyme and thus inhibiting its activity. These findings first elucidate the inhibitory effect of LPE on diabetes-related enzymes and highlight the usefulness of LPE as a dietary supplement for the prophylaxis of diabetes.

Zein enhanced the digestive stability of five citrus flavonoids via different binding interaction

BACKGROUND

Zein is commonly used to construct food flavonoid delivery systems. This study investigated the effect and mechanism of zein on the digestive stability of five citrus flavonoids, namely hesperetin (HET), hesperidin (HED), neohesperidin (NHD), naringenin (NEN), and naringin (NIN).

RESULTS

Zein enhanced the digestive stability of the five citrus flavonoids, especially that of HET and NEN, during digestion in the stomach and small intestine. Fluorescence spectroscopy results suggested that citrus flavonoids spontaneously quenched the endogenous fluorescence of zein in static quenching mode. The binding of HET, HED and NHD to zein was driven respectively by electrostatic, hydrophobic and electrostatic interaction. However, Van der Waals' force and hydrogen (H)-bond interaction represented the primary driving force for binding NEN, and NIN to zein to form complexes. The binding of the five citrus flavonoids to zein also caused a diverse bathochromic shift in ultraviolet absorbance. Analysis using Fourier-transform infrared and Raman spectroscopy revealed that the binding behavior of the five citrus flavonoids had different effects on changes in the secondary structures, disulfide bonds, and tyrosine exposure of zein. The results were also partially verified by molecular dynamic simulation.

CONCLUSIONS

Zein enhanced the digestive stability of the five citrus flavonoids via different binding interactions that was due to the difference in molecular structure of citrus flavonoids. © 2022 Society of Chemical Industry.

Receptor-ligand affinity-based screening and isolation of water-soluble 5-lipoxygenase inhibitors from Phellinus igniarius

We developed an efficient combination method for extraction, biological activity screening, and preparation of 5-lipoxygenase inhibitors from Phellinus igniarius. 5-Lipoxygenase inhibitors were rapidly screened using ultrafiltration-liquid chromatography based on the receptor-ligand affinity. Parameters such as extraction time, extraction times, and temperature as well as liquid-solid ratio were optimized using response surface methodology to maximize the total yield of the three target compounds. Next, bioactive ingredients were isolated using high-speed countercurrent chromatography and semi-preparative liquid chromatography. Three active ingredients, phellibaumin E, protocatechuic aldehyde, and osmundacetone, were obtained via ultrafiltration-liquid chromatography. Subsequently, the potential anti-dementia effects of the obtained bioactive compounds were verified using molecular docking assays. The above-mentioned target compounds, with purities of 98.82%, 98.89%, and 99.51%, respectively, were separated using a two-phase solvent system consisting of n-hexane-ethyl acetate-ethanol-water (2.5:2:0.75:3, v/v/v/v) coupled with semi-preparative liquid chromatography.

Enzymic catalyzing affinity to substrate affects inhibitor-enzyme binding interactions: Inhibition behaviors of EGCG against starch digestion by individual and co-existing α-amylase and amyloglucosidase

The inhibition of (-)-epigallocatechin-gallate (EGCG) against starch digestion by α-amylase (AA), amyloglucosidase (AMG) and co-existing enzymes (AA/AMG) were comparatively studied. EGCG inhibited AA only at slowly-digestible-starch (SDS) phase. This resulted from high catalytic efficiency of AA for rapidly-digestible-starch (RDS), counteracting the inhibition at this phase. EGCG inhibited AMG and AA/AMG during whole process. At RDS phase, the catalytic velocity of AMG was always higher than AA/AMG because of an antagonistic effect of two enzymes. However, at SDS phase with EGCG, the catalytic velocity of AA/AMG was higher than AMG. This is because binding of EGCG with both enzymes caused more unbound AMG that generated more glucose in co-existing AA/AMG than AMG. Although EGCG-AA binding affinity was higher than EGCG-AMG, competitive inhibition of EGCG against AA was weaker than AMG, indicating relatively higher binding/catalyzing affinity of AA to starch significantly weakened EGCG-AA binding due to competitive relationship between starch and EGCG.

Two Myricetin-Derived Flavonols from Morella rubra Leaves as Potent α-Glucosidase Inhibitors and Structure-Activity Relationship Study by Computational Chemistry

Diabetes mellitus (DM) is a chronic disease characterized by hyperglycemia, and oxidative stress is an important cause and therapeutic target of DM. Phytochemicals such as flavonols are important natural antioxidants that can be used for prevention and treatment of DM. In the present study, six flavonols were precisely prepared and structurally elucidated from Morella rubra leaves, which were screened based on antioxidant assays and α-glucosidase inhibitory activities of different plant tissues. Myricetin-3-O-(2″-O-galloyl)-α-L-rhamnoside (2) and myricetin-3-O-(4″-O-galloyl)-α-L-rhamnoside (3) showed excellent α-glucosidase inhibitory effects with IC50 values of 1.32 and 1.77 μM, respectively, which were hundredfold higher than those of positive control acarbose. Molecular docking simulation illustrated that the presence of galloyl group altered the binding orientation of flavonols, where it occupied the opening of the cavity pocket of α-glucosidase along with Pi-anion interaction with Glu304 and Pi-Pi stacked with His279. Pi-conjugations generated between galloyl moiety and key residues at the active site of α-glucosidase reinforced the flavonol-enzyme binding, which might explain the greatly increased activity of compounds 2 and 3. In addition, 26 flavonols were evaluated for systematic analysis of structure-activity relationship (SAR) between flavonols and α-glucosidase inhibitory activity. By using their pIC50 (-log IC50) values, three-dimensional quantitative SAR (3D-QSAR) models were developed via comparative molecular field analysis (CoMFA) and comparative similarity index analysis (CoMSIA), both of which were validated to possess high accuracy and predictive power as indicated by the reasonable cross-validated coefficient (q 2) and non-cross-validated coefficient (r 2) values. Through analyzing 3D contour maps of both CoMFA and CoMSIA models, QSAR results were in agreement with in vitro experimental data. Therefore, such results showed that the galloyl group in compounds 2 and 3 is crucial for interacting with key residues of α-glucosidase and the established 3D-QSAR models could provide valuable information for the prediction of flavonols with great antidiabetic potential.

Inhibitory activity and mechanism of guavinoside B from guava fruits against α-glucosidase: Insights by spectroscopy and molecular docking analyses

Guavinoside B (GUB) is the main active substance in guava fruit and shows promising biological activities. In this study, the inhibitory activity and mechanism of GUB on α-glucosidase were studied by using spectroscopic techniques, kinetic analysis, and molecular docking. Results indicated that GUB possessed significant inhibition ability on α-glucosidase, which was about 10 times that of acarbose. The GUB was a mixed-type inhibitor, which suppressed the activity of α-glucosidase through a reversible process. Fluorescence analysis revealed that GUB quenched the fluorescence of α-glucosidase statically, the formation of GUB-α-glucosidase complex was a spontaneous and exothermic process, van der Waals forces, hydrogen bonding, and hydrophobic interaction were the predominant driving forces, only one single-binding site on α-glucosidase was involved in the binding process. GUB inserted into the hydrophobic pocket of α-glucosidase with 11 hydrogen bonds and two π-π stacking formed. The presence of GUB changed the microenvironment near the fluorescent amino acids of α-glucosidase, and the structure of α-glucosidase was slightly changed, eventually leading to the decrease of α-glucosidase activity. PRACTICAL APPLICATIONS: Diabetes mellitus (DM) is a worldwide chronic metabolic disease threatening human health seriously. Guava fruit is a popular fruit, and its extracts were reported to show many biological activities. GUB is the main benzophenone glycoside in guava fruits. However, the inhibitory activity and mechanism of its specific active compound GUB are still unclear. Studies have shown that GUB could reversibly inhibit the activity of α-glucosidase, and its inhibitory ability was about 10 times that of acarbose. The kinetics and mechanism of inhibition were revealed. These will facilitate the further research and application of guava fruit and GUB in functional and healthy foods against hyperglycinaemia or even DM.

Both Acidic pH Value and Binding Interactions of Tartaric Acid With α-Glucosidase Cause the Enzyme Inhibition: The Mechanism in α-Glucosidase Inhibition of Four Caffeic and Tartaric Acid Derivates

The inhibition mechanism of four caffeic and tartaric acid derivates, including caffeic acid (CA), tartaric acid (TA), caftaric acid (CFA) and chicoric acid (CHA) against α-glucosidase was characterized by substrate depletion, fluorescence quenching, isothermal titration calorimetry (ITC) and molecular docking. TA and CA were found with the highest and no inhibition effect respectively, and caffeoyl substitution at 2 and/or 3-OH of TA significantly decreased its inhibition. The enzyme inhibition effects of organic acids were not in an inhibitor concentration-dependent mode, and there was a rush increase in inhibition at a respective acidic pH value, especially for CFA and CHA, suggesting the important role of acidic pH in the enzyme inhibition for both compounds. Besides, CA, CFA and CHA were shown with strong quenching effects on α-glucosidase fluorescence because of π-conjugations between aromatic ring of caffeoyl moiety and that of enzyme fluorescent residues. However, no fluorescence quenching effect was observed for TA due to lack of aromatic ring. Additionally, a direct binding interaction behavior was observed for TA with α-glucosidase according to the fitted independent binding model in ITC, but not for CFA and CHA. Therefore, both acidic pH and binding interactions of TA with α-glucosidase resulted in the enzyme inhibition.

Essential moieties of myricetins, quercetins and catechins for binding and inhibitory activity against α-Glucosidase

α-Glucosidase inhibition of 11 flavonoids, including myricetins, quercetins and catechins were studied through initial reaction velocity, IC₅₀ value, inhibition kinetics, fluorescence quenching and molecular docking. It was found that C4 = O, C2 = C3, 3-OH and 5'-OH were essential moieties for α-glucosidase inhibition of myricetin that was shown with the highest inhibitory activity. The trans-conformational catechins was shown with stronger inhibition effects than the cis-conformational ones. Further, gallocatechin was an uncompetitive inhibitor, while myricetin, myricetrin, quercetin and catechin were competitive ones. 3-OH and 5'-OH promoted myricetin to bind with the enzyme active site through hydrogen bondings. The presence of C4 = O and C2 = C3 increased electron delocalization in ring A-C for myricetin and quercetin, and this enhanced stability of π-conjugations with aromatic residues of amino acids. However, 5'-OH decreased the quenching effects because it limited π-conjugations of ring B with key fluorescent residues. Notably, for same flavonoid sort, the constants that indicate binding affinity of flavonoids to α-glucosidase, including reciprocal of competitive inhibition constant, fluorescence quenching constant and binding energy followed same order as the inhibitory activity, indicating that α-glucosidase inhibition of the flavonoids resulted from binding interactions between them, and that the methods above can be combined reasonably to characterize flavonoid-enzyme binding interactions.

Unravelling the effects of procyanidin on gliadin digestion and immunogenicity

The effect of procyanidin dimer B3, a common food tannin, on the digestion of gliadin proteins was investigated by monitoring the changes in the immunogenic peptides produced during in vitro digestion and immunoreactivity. Interaction studies between procyanidin dimer B3, gluten proteins and/or digestive enzymes were performed by SDS-PAGE. The effect of procyanidin B3 on the enzymatic activity of trypsin, chymotrypsin and pancreatin was evaluated. The differences in the number and nature of immunogenic peptides released during digestion were identified by mass spectrometry. Briefly, the enzymatic activity of gastrointestinal enzymes was only slightly affected but a significant decrease in the immunological properties of the peptides produced during digestion was observed. Overall, although further studies are needed, the interaction between polyphenols and gluten proteins clearly influences gluten protein digestion and immunogenicity, thus suggesting that the consumption of dietary polyphenols can significantly affect the degree of celiac disease downstream immune reactions.