Comparing the inhibitory abilities of epigallocatechin-3-gallate and gallocatechin gallate against tyrosinase and their combined effects with kojic acid

Potential application of nanoliposomes loaded with complex tannins from the seed shell of Euryale ferox in the anti-browning of fresh-cut asparagus lettuce

Surface browning of plant-derived fresh-cut products is mainly caused by conversion of the phenolic compounds into o-quinones under tyrosinase catalysis. In this study, the rarely reported complex tannins from Euryale ferox seed shell (ECTs) constituted by the units of 35.60% condensed tannins and 64.40% hydrolysable tannins were shown to suppress the activity of tyrosinase efficiently, supporting the exploitation of ECTs into novel anti-browning agents. However, the utilization of ECTs in food preservation is often restricted because of their chemical instability to external environment. Further fabrication of nanoliposomes loaded with ECTs (ECTs-NLs) herein was carried out to improve the stability of ECTs. DLS, TEM, FTIR, DSC and XRD confirmed that ECTs were encapsulated into nanoliposomes successfully, and ECTs-NLs appeared as vesicle-like spherical morphology with favorable encapsulation efficiency, uniform particle size distribution and negative zeta-potential. The resulting ECTs-NLs were relatively stable in the dark at 4 °C. Nanoliposomal encapsulation significantly enhanced ECTs stability, thus protecting inhibitory effect of ECTs against tyrosinase. Furthermore, anti-browning evaluation proved that ECTs-NLs had distinct advantages over free ECTs in alleviating surface browning of fresh-cut asparagus lettuces. These results suggested that nanoliposomes were effective in stabilizing ECTs and ECTs-NLs could be potentially applied to the fresh-cut food industry.

Relationships between degree of polymerization and activities: A study on condensed tannins from the bark of Ficus altissima

Condensed tannins were isolated from the bark of Ficus altissima and fractionated into four subcomponents on a Sephadex LH-20 column with 60 %, 80 %, 100 % methanol, and 70 % acetone, separately. Their structures were characterized by MALDI-TOF MS coupled with HPLC-ESI-MS and confirmed to be polymers of B-type procyanidin glucosides, procyanidins, and prodelphinidin glucosides. The degree of polymerization (DP) of these polymers was as high as 21, and the mDPs of the four subcomponents were calculated as 2.4, 6.6, 10.5 and 13.4, respectively. They competitively or noncompetitively suppressed the activities of tyrosinase and α-glucosidase through hydrogen bonding and hydrophobic interaction. And they also showed a powerful antioxidative activity. Correlation analyses verified that the anti-tyrosinase capacity exhibited a significant positive correlation (R2monophenolase = 0.9167 and R2diphenolase = 0.9302) with mDP within the methanol-water system, and the anti-α-glucosidase activity also showed a significant positive correlation with the mDP (R2 = 0.9187). In contrast, the antioxidant capability showed a significant negative correlation with the mDP (R2DPPH = 0.9258, R2ABTS = 0.9372). This study confirmed that condensed tannins from the bark of F. altissima were desirable anti-tyrosinase, anti-α-glucosidase, and antioxidant agents, and elucidated the relationships of their mDP (molecular weight) and activities, which provided a scientific basis for the comprehensive utilization of these polymers in the food, cosmetics, medicine and other fields.

Valuation of the significant hypoglycemic activity of black currant anthocyanin extract by both starch structure transformation and glycosidase activity inhibition

The objective of this study was to investigate the impact of anthocyanin-rich black currant extract (BCE) on the structural properties of starch and the inhibition of glycosidases, gathering data and research evidence to support the use of low glycemic index (GI) foods. The BCE induced a change in the starch crystal structure from A-type to V-type, resulting in a drop in digestibility from 81.41 % to 65.57 %. Furthermore, the inhibitory effects of BCE on glycosidases activity (α-glucosidase: IC50 = 0.13 ± 0.05 mg/mL and α-amylase: IC50 = 2.67 ± 0.16 mg/mL) by inducing a change in spatial conformation were confirmed through in vitro analysis. The presence of a 5'-OH group facilitated the interaction between anthocyanins and receptors of amylose, α-amylase, and α-glucosidase. The glycosyl moiety enhanced the affinity for amylose yet lowered the inhibitory effect on α-amylase. The in vivo analysis demonstrated that BCE resulted in a reduction of 3.96 mM·h in blood glucose levels (Area Under Curve). The significant hypoglycemic activity, particularly the decrease in postprandial blood glucose levels, highlights the potential of utilizing BCE in functional foods for preventing diabetes.

Identification of tyrosinase inhibitors in defatted seeds of evening primrose (Oenothera biennis L.) by affinity-labeled molecular networking

The defatted seeds of evening primrose (DE), a by-product of evening primrose oil extraction, are currently underutilized. This study aimed to valorize DE by examining its effects on melanogenesis and tyrosinase activity in zebrafish embryos and in vitro, and an innovative affinity-labeled molecular networking workflow was proposed for the rapid identification of tyrosinase inhibitors in DE. Our results indicated DE significantly reduced melanin content (53.3 % at 100 μg/mL) and tyrosinse activity (80.05 % for monophenolase and 70.40 % for diphenolase at 100 μg/mL). Furthermore, through the affinity-labeled molecular networking approach, 20 compounds were identified as potential tyrosinase inhibitors within DE, predominantly flavonoids and tannins characterized by catechin and galloyl substructures. Seven of these compounds were isolated and their inhibitory effects on tyrosinase were validated using functional assays. This study not only underscores the potential of DE as a rich source of natural tyrosinase inhibitors but also establishes the effectiveness of affinity-labeled molecular networking in pinpointing bioactive compounds in complex biological matrices.

Characterization of a novel recombinant D-mannose isomerase from Bifidobacterium bifidum and its catalytic mechanism

Due to the increasing demand for health-conscious and environmentally friendly products, D-mannose has gained significant attention as a natural, low-calorie sweetener. The use of D-mannose isomerases (D-MIases) for D-mannose production has emerged as a prominent area of research, offering superior advantages compared with conventional methods such as plant extraction and chemical synthesis. In this study, a gene encoding D-MIase was cloned from Bifidobacterium and expressed in E. coli BL21 (DE3). The heterologously expressed enzyme, Bifi-mannose, formed a trimer with a molecular weight of 146.3 kDa and a melting temperature (Tm) of 63.39 ± 1.3 °C. Bifi-mannose exhibited optimal catalytic activity at pH 7.5 and 55 °C, and retained more than 80% of its activity after a 3-hour incubation at 55 °C, demonstrating excellent thermal stability. The Km, Vmax, and kcat/Km values of Bifi-mannose for D-fructose isomerization were determined as 538.7 ± 62.5 mM, 11.7 ± 0.9 μmol·mg1·s1, and 1.02 ± 0.3 mM1·s1, respectively. Notably, under optimized conditions, catalytic yields of 29.4, 87.1, and 148.5 mg·mL1 were achieved when using 100, 300, and 500 mg·mL1 of D-fructose as substrates, resulting in a high conversion rate (29%). Furthermore, kinetic parameters and molecular docking studies revealed that His387 residue primarily participates in the opening of the pyranose ring, while His253 acts as a basic catalyst in the isomerization process.

Inhibition mechanism of fisetin on acetylcholinesterase and its synergistic effect with galantamine

The search for acetylcholinesterase (AChE) inhibitors produced by natural sources is of great significance for the prevention and therapy of Alzheimer's disease and has been widely concerned. In this study, fisetin, a flavonoid compound of plant origin, displayed a mixed inhibition mode on AChE (IC50 = 8.88 ± 0.14 μM). Fluorescence spectra analysis revealed that fisetin statically quenched AChE fluorescence, and the ground state complex was formed by hydrogen bonds and hydrophobic interactions. Circular dichroism assays showed that fisetin induced AChE structure loosened with a decrease in α-helix structure (from 20.6 % to 19.5 %). Computer simulation exhibited that fisetin bound to both the peripheral anionic site (PAS) and the catalytic active site (CAS) and increased the stability of the AChE. Interestingly, the combination of fisetin and galantamine enhanced the binding affinity between AChE and galantamine and induced AChE structure further loosened, while the inhibition mode was still the mixed type. The heatmap analysis indicated that galantamine (0.2 μM) combined with fisetin (2.25 μM) had a significant synergy on AChE inhibition, probably because fisetin binding at the PAS-AChE induced conformation changes of the gorge and CAS, which enhanced galantamine binding affinity with CAS, and a further loose structure of AChE was induced by the mixture, so finally the interaction between the substrate and AChE was strongly affected. This work may offer a theoretical reference for the functional research of fisetin as a potential AChE inhibitor and an enhanced supplement for galantamine.

Eupatilin inhibits xanthine oxidase in vitro and attenuates hyperuricemia and renal injury in vivo

Uric acid (UA) is the final metabolite of purines in the liver that can cause hyperuricemia at high levels. The kidneys are the main excretory organs for UA. The excessive accumulation of UA in the kidneys causes the development of hyperuricemia that often leads to renal injury. Eupatilin (Eup) is a flavonoid natural product that possesses various pharmacological properties such as antioxidant, anti-cancer, and anti-inflammatory. We were interested in exploring the potential role of Eup in lowering UA and nephroprotective. We initially investigated the effects of Eup on xanthin oxidase (XOD) activity in vitro, followed by investigating its ability to lower UA levels, anti-inflammatory effects, nephroprotective effects, and the underlying mechanisms using hyperuricemia rats sustained at high UA level. The results showed that Eup had an inhibitory effect on XOD activity in vitro and significantly reduced serum UA, creatinine, BUN, IL-1β and IL-6 levels in hyperuricemic rats, ameliorating inflammation, renal oxidative stress and pathological injury. Furthermore, Eup inhibited ADA and XOD enzyme activities in the liver and serum and modulated GLUT9, URAT1 and ABCG2 protein expression in the kidneys and ileum. Our findings provide a scientific basis for suggesting Eup as an option for a potential treatment for hyperuricemia.

Exploring the effect of a series of flavonoids on tyrosinase using integrated enzyme kinetics, multispectroscopic, and molecular modelling analyses

The control of food browning can be achieved by inhibiting tyrosinase (TY) activity, but current studies on the interaction of flavonoids as potent inhibitors with TY are inadequate. Herein, the effect of a library of flavonoids on TY was investigated using enzyme kinetics, multispectroscopic methods, and molecular modelling. Some flavonoids including 4, 8, 10, 17, 18, 28, 30, 33, and 34 exhibited potent TY inhibitory activity, with compound 10 demonstrating reversible inhibition in a mixed-competitive manner. Ultraviolet-visible spectral changes confirmed the formation of flavonoid-TY complexes. Fluorescence quenching analysis suggested effective intrinsic fluorescence quenching by flavonoids through static quenching with the ground-state complex formation. Synchronous fluorescence spectra showed the microenvironment change around the fluorophores induced by flavonoids. ANS-binding fluorescence assay indicated TY's surface hydrophobicity change by flavonoids and highlighted the change in secondary structure conformation, which was further confirmed by Fourier-transform infrared spectra. Molecular modelling results helped visualize the preferred binding conformation at the active site of TY, and demonstrated the important role of hydrophobic interaction and hydrogen bonding in stabilizing the flavonoid-TY complexes. These findings prove that diverse flavonoid structures distinctly impact their binding behavior on TY and contribute to understanding flavonoids' potential as TY inhibitors in controlling food browning.

Encapsulation Alleviates the Auto-browning of Epigallocatechin-3-gallate in Aqueous Solutions through Regulating Molecular Self-Aggregation Behavior

Catechins are widely recognized for superb antioxidant capability, but their application as food antioxidants is hindered by susceptibility to auto-browning under high-moisture conditions. Here, we proposed a strategy of ordered encapsulation with cyclodextrin-based metal-organic frameworks (CD-MOFs) to alleviate the auto-browning phenomenon of catechins while preserving their antioxidant capability and demonstrated the feasibility of this strategy via selecting epigallocatechin-3-gallate (EGCG) as a model. Even in aqueous solutions, EGCG@CD-MOFs still possessed delayed browning, in contrast with pristine EGCG, characterized by suppressed efficiencies on the generation of oxidative dimers (theasinensin A) and semiquinone radicals. Mechanism insights revealed that ordered encapsulation brought dual regulations on the self-aggregation behavior of EGCG: EGCG@CD-MOFs exhibited a gradual structural collapse from the framework toward irregular aggregates as O-K bonds broke progressively, which restricted molecular mobility of EGCG, and EGCG molecular conformations became constrained by the structure of EGCG@CD-MOFs as well as rich intermolecular forces, even after structural collapse.

Potent Antioxidant and Anti-Tyrosinase Activity of Butein and Homobutein Probed by Molecular Kinetic and Mechanistic Studies

Butein (BU) and homobutein (HB) are bioactive polyhydroxylated chalcones widespread in dietary plants, whose antioxidant properties require mechanistic definition. They were investigated by inhibited autoxidation kinetic studies of methyl linoleate in Triton™ X-100 micelles at pH 7.4, 37 °C. Butein had kinh = (3.0 ± 0.9) × 104 M-1s-1 showing a chain-breaking mechanism with higher antioxidant activity than reference α-tocopherol (kinh = (2.2 ± 0.6) × 104 M-1s-1), particularly concerning the stoichiometry or peroxyl radical trapping n = 3.7 ± 1.1 vs. 2.0 for tocopherol. Homobutein had kinh = (2.8 ± 0.9) × 103 M-1s-1, pairing the relative BDEOH measured by radical equilibration EPR as 78.4 ± 0.2 kcal/mol for BU and estimated as 82.6 kcal/mol for HB. The inhibition of mushroom tyrosinase (mTYR) by HB and BU was also investigated. BU gives a reversible uncompetitive inhibition of monophenolase reaction with KI' = 9.95 ± 2.69 µM and mixed-type diphenolase inhibition with KI = 3.30 ± 0.75 µM and KI' = 18.75 ± 5.15 µM, while HB was nearly competitive toward both mono- and diphenolase with respective KI of 2.76 ± 0.70 µM and 2.50 ± 1.56 µM. IC50 values (monophenolase/diphenolase at 1 mM substrate) were 10.88 ± 2.19 µM/15.20 ± 1.25 µM, 14.78 ± 1.05 µM/12.36 ± 2.00 µM, and 33.14 ± 5.03 µM/18.27 ± 3.42 µM, respectively, for BU, HB, and reference kojic acid. Molecular docking studies confirmed the mechanism. Results indicate very potent antioxidant activity for BU and potent anti-tyrosinase activity for both chalcones, which is discussed in relation to bioactivity toward protection from skin disorders and food oxidative spoilage.

Design, synthesis, spectroscopic characterizations, in vitro pancreatic lipase as well as tyrosinase inhibition evaluations and in silico analysis of novel aryl sulfonate-naphthalene hybrids

One of the primary purposes of this study is to synthesize new aryl sulfonate-naphthalene hybrid structures possessing divergent electron-withdrawing and electron-releasing functional groups. Following the improved reaction conditions, we successfully gathered ten distinct sulfonate derivatives (3a-j) with good yields. The synthesized naphthalene-based sulfonate derivatives were then characterized using appropriate analytical methods (FT-IR, 1H-NMR, 13C-NMR, HRMS, and elemental analysis). Additionally, in vitro and in silico enzyme inhibitory properties of the prepared aryl sulfonate-naphthalene hybrid structures were evaluated against pancreatic lipase and tyrosinase enzymes. Corresponding in vitro enzyme activity investigations revealed that the produced compounds inhibit pancreatic lipase and tyrosinase enzymes significantly. According to the lowest IC50 values, 3h (95.3 ± 4.0 µM) demonstrated the most effective inhibition against pancreatic lipase, whereas 3a (40.8 ± 3.3 µM) was found as the most effective inhibition against the tyrosinase. According to in silico studies, 3a exhibited the highest affinity value (-9.9 kcal/mol) against pancreatic lipase, whereas 3f demonstrated the best affinity value (-8.7 kcal/mol) against tyrosinase.Furthermore, we investigated various structural and physicochemical properties of the target molecules, namely frontier orbital' (HOMO, LUMO, and bandgap) energies (including their corresponding contour plots), global reactivity descriptors (ionization energy and electron affinity), and electronegativity values gathered from ground-state (GS) density functional theory (DFT) calculations. These investigations demonstrated that the observed electrostatic interactions effectively contributed to the studied molecules' experimentally demonstrated enzyme inhibition potential. Also, ADMET studies were evaluated to enlighten the molecular interactions of the compounds with the enzymes.Communicated by Ramaswamy H. Sarma.

Exploring inhibitory effect and mechanism of hesperetin-Cu (II) complex against protein glycation

Inhibition of advanced glycation end products (AGEs) formed in protein glycosylation is crucial for minimizing diabetic complications. Herein, the anti-glycation potential of hesperetin-Cu (II) complex was investigated. Hesperetin-Cu (II) complex strongly inhibited three stages glycosylation products in bovine serum albumin (BSA)-fructose model, especially for the inhibition of AGEs (88.45%), which was stronger than hesperetin (51.76%) and aminoguanidine (22.89%). Meanwhile, hesperetin-Cu (II) complex decreased the levels of BSA carbonylation and oxidation products. 182.50 µg/mL of hesperetin-Cu (II) complex inhibited 66.71% β-crosslinking structures of BSA, and scavenged 59.80% superoxide anions and 79.76% hydroxyl radicals. Moreover, after incubating with methylglyoxal for 24 h, hesperetin-Cu (II) complex removed 85.70% methylglyoxal. The mechanisms of protein antiglycation by hesperetin-Cu (II) complex may be through protecting structure, trapping methylglyoxal, scavenging free radicals and interacting with BSA. This study may contribute to the development of hesperetin-Cu (II) complex as a functional food additive against protein glycation.

Protocatechuic acid and gallic acid improve the emulsion and thermal stability of whey protein by covalent binding

This study aimed to explore the impacts of gallic acid (GA)/protocatechuic acid (PA) on the structural and functional characteristics of whey proteins (WP) through covalent binding. To this purpose, the covalent complexes of WP-PA and WP-GA at different concentration gradients were prepared by the alkaline method. SDS-PAGE indicated that PA/GA was cross-linked by covalent bonds. The decreased contents of free amino and sulfhydryl groups suggested that WP formed covalent bonds with PA/GA by amino and sulfhydryl groups, and the structure of WP became slightly looser after covalent modification by PA/GA. When the concentration of GA was added up to 10 mM, the structure of WP was slightly loosened with a reduction of α-helix content by 2.3% and an increase in random coil content by 3.0%. The emulsion stability index of WP increased by 14.9 min after interaction with GA. Moreover, the binding of WP and 2-10 mM PA/GA increased the denaturation temperature by 1.95 to 19.87 °C, indicating the improved thermal stability of the PA/GA-WP covalent complex. Additionally, the antioxidant capacity of WP was increased with increasing GA/PA concentration. This work may offer worthful information for enhancing the functional properties of WP and the application of the PA/GA-WP covalent complexes in food emulsifiers.

Treatments of heating and ultrasound improve the inhibition of gallocatechin gallate on tyrosinase

BACKGROUND

Gallocatechin gallate (GCG), a catechin of tea polyphenols, possesses inhibitory ability against tyrosinase, but few studies have reported how common processing methods affect it. In this research, the influence of heating and ultrasound treatments on the inhibition of GCG against tyrosinase was explored by ultraviolet-visible absorption, fluorescence spectroscopy, high-performance liquid chromatography and liquid chromatography-tandem mass spectrometry.

RESULTS

Both heating and ultrasound treatments of GCG alone improved GCG's inhibitory ability against tyrosinase compared with the untreated, and a combination of heating and ultrasound treatment (100 °C, 20 min + 630 W, 20 min) further decreased the relative tyrosinase activity to 26.8%. The treated GCG exhibited a stronger fluorescence quenching effect on tyrosinase, but did not have any influence on the static quenching mechanism. Compared to the untreated GCG, the binding constants of treated GCG by heating, ultrasound and their combination with tyrosinase significantly increased, but the number of binding sites was still approximately one and the main driving force of the treated GCG was still hydrophobic interaction. After treatments of heating, ultrasound and their combination, the composition of GCG solutions was changed.

CONCLUSION

The enhanced inhibition of treated GCG on tyrosinase may be due to partial conversion of GCG into epigallocatechin-3-gallate (EGCG) and gallic acid (GA), which may cooperate with GCG to better inhibit the enzyme activity. This study has provided some valuable information for the application of catechins against tyrosinase in food processing and cosmetic industry. © 2022 Society of Chemical Industry.

Synthesis and biological evaluation of new kojic acid-1,3,4-oxadiazole hybrids as tyrosinase inhibitors and their application in the anti-browning of fresh-cut mushrooms

In the food industry, inhibition of tyrosinase activity is considered as one of the main means to prevent browning. Therefore, fourteen kojic acid-1,3,4-oxadiazole hybrids (5a-5n) were prepared and tested for their tyrosinase inhibitory effects. Among them, 5f (IC₅₀ = 5.32 ± 0.58 μM) has the best anti-tyrosinase activity and was 9 times higher than that of kojic acid (IC₅₀ = 49.77 ± 1.19 μM). Additionally, the inhibitory mechanism was studied by copper-chelating assay, ultraviolet spectrophotometry, fluorescence quenching, molecular docking, etc. The results had shown that 5f could not only bind to the copper ion in the active region of tyrosinase but also change the secondary structure of tyrosinase. Combined with the outstanding anti-browning effect and low cytotoxicity of 5f, it is concluded that these title derivatives could be used as the leading molecules in the development of new anti-browning agents.

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.

Amelioration of ovalbumin gel properties by EGCG via protein aggregation, hydrogen, and van der Waals force

The mechanism of epigallocatechin gallate (EGCG)-modified ovalbumin gel (EMOG) was investigated. Results indicated that, with the increase of EGCG concentration from 0% to 0.05%, the opacity, hardness, and cohesiveness of EMOG increased significantly from 0.058 to 0.133, 321.0 g to 377.6 g, and 0.879 to 0.951, respectively, while the soluble protein, surface hydrophobicity, and free sulfhydryl decreased significantly by 41.74%, 28.26%, and 39.36%, respectively. Moreover, EGCG promoted the formation of dense and stable microstructures of EMOG, changed the expansion rate, and improved the stability of EMOG. Moreover, the results of silico simulation showed that EGCG would insert into ovalbumin and interact with the amino acids through van der Waals force and hydrogen bonds, leading to a compact and stable protein structure. In this paper, the mechanism of modification of ovalbumin by EGCG was elucidated at the macro and micro levels, providing insights into the action of polyphenols and proteins.

Hesperetin-Cu(II) complex as potential α-amylase and α-glucosidase inhibitor: Inhibition mechanism and molecular docking

Inhibition of α-amylase and α-glucosidase activity is an effective way for controlling postprandial blood glucose-related diabetes. The study found that hesperetin-Cu(II) complex (Hsp-Cu(II)) exhibited a stronger inhibitory ability on α-amylase and α-glucosidase compared to hesperetin (Hsp), with smaller IC₅₀ values of Hsp-Cu(II) (60.3 ± 0.9 µM for α-amylase; 1.25 ± 0.03 µM for α-glucosidase) than Hsp (115.6 ± 1.1 µM for α-amylase; 55.2 ± 0.1 µM for α-glucosidase). Interestingly, Hsp-Cu(II) and acarbose exerted a synergistic effect on inhibition of α-glucosidase. The binding affinities of Hsp-Cu(II) to α-amylase and α-glucosidase were strong with the Ka values (binding constant) in the magnitude order of 10⁵, which was 9 times larger than Hsp. After interacting, Hsp-Cu(II) reduced α-helix contents of α-amylase and α-glucosidase, resulting in a looser conformation of these two enzymes. Molecular simulations manifested that Hsp-Cu(II) bound to the active center of enzymes driven by hydrogen bonds and interacted with the key catalytic amino acids (α-amylase: Gln63, Asp300 and His305; α-glucosidase: Tyr158, Asp215, Glu277 and Glu411), altering the conformation of enzymes, blocking the entrance of substrates, ultimately reducing the activities of α-glucosidase and α-amylase. This study has demonstrated that Hsp-Cu(II) may be a promising candidate of functional nutritional additive and medicine for the prevention of diabetes.

Inhibition of albendazole and 2-(2-aminophenyl)-1H-benzimidazole against tyrosinase: mechanism, structure-activity relationship, and anti-browning effect

BACKGROUND

Tyrosinase is the key enzyme involved in enzymatic browning of plant-derived foods. Inhibition of tyrosinase activity contributes to the control of food browning. Due to safety regulations or other issues, most identified tyrosinase inhibitors are not suitable for practical use. Therefore, it is necessary to search for novel tyrosinase inhibitors. In this study, the anti-tyrosinase activity and mechanism of albendazole and 2-(2-aminophenyl)-1H-benzimidazole (2-2-A-1HB) were investigated through ultraviolet-visible absorption spectroscopy, fluorescence spectra, molecular docking, and molecular dynamic (MD) simulation. The anti-browning effect of albendazole on fresh-cut apples was then elucidated.

RESULTS

Albendazole and 2-2-A-1HB were both efficient tyrosinase inhibitors with IC₅₀ of 51 ± 1.5 and 128 ± 1.3 μmol L^-1 , respectively. Albendazole suppressed tyrosinase non-competitively and formed tyrosinase-albendazole complex statically. Hydrogen bond and hydrophobic interaction were major driving forces in stabilizing the tyrosinase-albendazole complex. While 2-2-A-1HB inhibited the enzyme competitively and quenched its intrinsic fluorescence through a static mechanism, it generated strong binding affinity with tyrosinase through hydrophobic interaction. MD simulations further validated that albendazole/2-2-A-1HB could form stable complexes with tyrosinase and loosened its basic framework structure, leading to a change in secondary structure and conformation. In addition, albendazole could delay the browning of fresh-cut apples by inhibiting the activity of polyphenol oxidase, peroxidase and phenylalanine ammonia-lyase, and reducing the oxidation of phenolic compounds.

CONCLUSION

This research might provide a deep view of tyrosinase inhibition by benzimidazole derivatives and a theoretical basis for developing albendazole as a potential fresh-keeping agent. © 2023 Society of Chemical Industry.

Fungal production of kojic acid and its industrial applications

Kojic acid has gained its importance after it was known worldwide that the substance functions primarily as skin-lightening agent. Kojic acid plays a vital role in skin care products, as it enhances the ability to prevent exposure to UV radiation. It inhibits the tyrosinase formation which suppresses hyperpigmentation in human skin. Besides cosmetics, kojic acid is also greatly used in food, agriculture, and pharmaceuticals industries. Conversely, according to Global Industry Analysts, the Middle East, Asia, and in Africa especially, the demand of whitening cream is very high, and probably the market will reach to $31.2 billion by 2024 from $17.9 billion of 2017. The important kojic acid-producing strains were mainly belongs to the genus Aspergillus and Penicillium. Due to its commercial potential, it continues to attract the attention for its green synthesis, and the studies are still widely conducted to improve kojic acid production. Thus, the present review is focused on the current production processes, gene regulation, and limitation of its commercial production, probable reasons, and possible solutions. For the first time, detailed information on the metabolic pathway and the genes involved in kojic acid production, along with illustrations of genes, are highlighted in the present review. Demand and market applications of kojic acid and its regulatory approvals for its safer use are also discussed. KEY POINTS: • Kojic acid is an organic acid that is primarily produced by Aspergillus species. • It is mainly used in the field of health care and cosmetic industries. • Kojic acid and its derivatives seem to be safe molecules for human use.

New Insights into the Inhibition of Hesperetin on Polyphenol Oxidase: Inhibitory Kinetics, Binding Characteristics, Conformational Change and Computational Simulation

The inhibitory activity of hesperetin on polyphenol oxidase (PPO) and their interaction characteristics were investigated using multiple spectroscopic methods and computational simulation. Hesperetin, a mixed inhibitor, reversibly inhibited PPO activity, and its half-maximum inhibitory concentration (IC₅₀) values on monophenolase and diphenolase were 80.8 ± 1.4 μM and 776.0 ± 15.5 μM, respectively. Multivariate curve resolution-alternate least squares (MCR-ALS) analysis suggested PPO interacted with hesperetin and formed PPO-hesperetin complex. Hesperetin statically quenched PPO's endogenous fluorescence, and hydrophobic interactions mainly drove their binding. Hesperetin affected the polarity of the microenvironment around the Trp residues in PPO, but had no effect on that around Tyr residues. Circular dichroism (CD) results showed that hesperetin increased α-helix content and decreased β-fold and random coil contents, thus tightening PPO's structure. Molecular docking showed that hesperetin entered the hydrophobic cavity of PPO, bound near the dinuclear copper active center, interacted with Val283, Phe264, His85, Asn260, Val248, and His263 via hydrophobic interactions, formed hydrogen bonds with Met280, His89, and His259 residues and also interacted with Phe292, His61, Phe90, Glu256, His244, Asn260, Phe264, and Gly281 via van der Waals forces. The molecular dynamics simulation results also demonstrated that the addition of hesperetin reduced the stability and hydrophobicity of PPO and increased PPO's structural denseness. Thus, the inhibition of hesperetin on PPO may be because hesperetin bound near the active center of PPO, interacted with the surrounding residues, occupied the binding site for substrate, and induced the changes in PPO's secondary structure, thus inhibiting the catalytic activity of PPO. This study may provide novel views for the inhibition of hesperetin on PPO and theoretical guidance for developing flavonoids as new and efficient PPO inhibitors.

Absorption, metabolism, bioactivity, and biotransformation of epigallocatechin gallate

Epigallocatechin gallate (EGCG), a typical flavone-3-ol polyphenol containing eight free hydroxyl groups, is associated with a variety of bioactivities, such as antioxidant, anti-inflammatory, anti-cancer, and antibacterial activities. However, the poor bioavailability of EGCG restricts its use. In this review, we discuss the processes involved in the absorption and metabolism of EGCG, with a focus on its metabolic interactions with the gut microbiota. Next, we summarize the bioactivities of some key metabolites, describe the biotransformation of EGCG by different microorganisms, and discuss its catabolism by specific bacteria. A deeper understanding of the absorption, metabolism, and biotransformation of EGCG may enable its disease-preventive and therapeutic properties to be better utilized. This review provides a theoretical basis for further development and utilization of EGCG and its metabolites for improving the gut microbiota and physiological health.

Structural insight into polyphenol oxidation during black tea fermentation

Polyphenol oxidation during fermentation plays a critical role in the formation of flavor and function of black tea. However, how the specific structures of tea polyphenols affect their oxidation kinetics during black tea fermentation is still unknown. Here, we found that the oxidations of tea polyphenols, including 7 catechins, 4 phenolic acids and 11 flavonoid glycosides followed pseudo-first-order kinetics during fermentation. Molecular structure and oxygen concentration collaboratively regulated the oxidation rate of different polyphenols. Pyrogallol structure was more easily to be oxidized than catechol and monophenol structure in B-ring, the gallic group in C-ring could inhibit oxidation of catechins, while the role of sugar moiety of flavonoid glycosides was differentiating. In addition, oxygen was found to be the key factor limiting the oxidation rate of polyphenols in regular black tea fermentation, and the oxidation rate constants of tea polyphenols were linearly and positively correlated with oxygen concentration.

Inhibition characteristics and mechanism of tyrosinase using five citrus flavonoids: A spectroscopic and molecular dynamics simulation study

This work presents a comparative analysis of the tyrosinase inhibitory effects of five citrus flavonoids, namely hesperetin, hesperidin, neohesperidin, naringenin and naringin. Visbile, fluorescence, and fourier transform infrared (FITR) spectroscopies, and molecular dynamic methods were employed to compare the anti-tyrosinase mechanisms of each flavonoid. Hesperetin, neohesperidin, naringenin and naringin exhibited potent inhibitory activities with IC50 values of 0.74 ± 0.05, 2.19 ± 0.03, 7.50 ± 9.82 and 24.94 ± 8.43 μmol/ml, respectively, all of which were higher than that of kojic acid (0.04 ± 0.02 μmol/ml). The enzymatic kinetics results suggested that hesperetin and naringenin were reversible inhibitors on tyrosinase in the mixed-type manner. H-bond and hydrophobic interactions were found to drive the binding of tyrosinase with hesperetin or naringenin, which subsequently changed the FTIR spectroscopy results by decreasing the α-helix ratio and increasing the β-turn, β-sheet and random coil ratio in tyrosinase. Molecular dynamics simulation not only verified some of the experimental results, but also suggested that the binding of hesperetin and naringenin to tyrosinase was spontaneous. The findings of this study indicate that citrus flavonoids are a promising dietary resource for tyrosinase inhibition. PRACTICAL APPLICATIONS: Hesperetin, hesperidin, neohesperidin, naringenin and naringin were typical citrus flavonoids that have anti-obesity, anti-oxidation, anti-inflammation and anti-diabetes activities. Current study suggested that hesperetin and naringenin were effective reversible inhibitors on tyrosinase in the mixed-type manner. Hesperetin and naringenin might serve as nutritional and chemical agents for regulating the tyrosinase activity to control melanin level in vivo.

Action mechanisms of two key xanthine oxidase inhibitors in tea polyphenols and their combined effect with allopurinol

BACKGROUND

Tea polyphenols have been reported to have the effect of lowering uric acid. However, there are few studies on the inhibitory effects and molecular mechanisms of specific catechins on the urate-metabolizing enzyme xanthine oxidase (XO). In this research, multiple spectroscopic methods and computer simulations were used to determine the inhibitory ability and mechanisms of epigallocatechin gallate (EGCG) and gallocatechin gallate (GCG) on XO.

RESULTS

Herein, EGCG and GCG reversibly inhibited XO activity in a mixed manner, with IC₅₀ values of 40.50 ± 0.32 and 33.60 ± 0.53 μmol L^-1 , and also decreased the superoxide anion radical (O₂ ^- ) of the catalytic system by reducing the XO molecule and inhibiting the formation of uric acid. The combination of EGCG or GCG with allopurinol showed synergistic inhibition on XO. The binding of EGCG or GCG to XO with moderate affinity formed a stable complex by hydrogen bonds and van der Waals forces. The presence of EGCG and GCG made the structure of XO more stable and compact. The two inhibitors bound to the vicinity of flavin adenine dinucleotide (FAD) in XO, hindering the entry of substrate; thus the activity of XO was suppressed.

CONCLUSION

Both EGCG and GCG are excellent natural XO inhibitors, and inhibited the activity of XO by occupying the channel of the substrate to enter the active center and interfering with the dual substrate reaction catalyzed by XO. These findings provide a scientific basis for the application of catechins in dietary supplements and medicines with lowering uric acid effects. © 2022 Society of Chemical Industry.

Exploring the Inhibition of Quercetin on Acetylcholinesterase by Multispectroscopic and In Silico Approaches and Evaluation of Its Neuroprotective Effects on PC12 Cells

This study investigated the inhibitory mechanism of quercetin in acetylcholinesterase (AChE) and its neuroprotective effects on β-amyloid_25-35-induced oxidative stress injury in PC12 cells. Quercetin inhibited AChE in a reversible mixed manner with an IC₅₀ of 4.59 ± 0.27 µM. The binding constant of quercetin with AChE at 25 °C was (5.52 ± 0.05) × 10⁴ L mol^-1. Hydrogen bonding and van der Waals forces were the main interactions in forming the stable quercetin-AChE complex. Computational docking revealed that quercetin was dominant at the peripheral aromatic site in AChE and induced enzymatic allosterism; meanwhile, it extended deep into the active center of AChE and destabilized the hydrogen bond network, which caused the constriction of the gorge entrance and prevented the substrate from entering the enzyme, thus resulting in the inhibition of AChE. Molecular dynamics (MD) simulation emphasized the stability of the quercetin-AChE complex and corroborated the previous findings. Interestingly, a combination of galantamine hydrobromide and quercetin exhibited the synergistic inhibition effect by binding to different active sites of AChE. In a β-amyloid_25-35-induced oxidative stress injury model in PC12 cells, quercetin exerted neuroprotective effects by increasing the glutathione level and reducing the malondialdehyde content and reactive oxygen species levels. These findings may provide novel insights into the development and application of quercetin in the dietary treatment of Alzheimer's disease.

Simultaneous loading of (-)-epigallocatechin gallate and ferulic acid in chitosan-based nanoparticles as effective antioxidant and potential skin-whitening agents

Chitosan (CS) based nanoparticles simultaneously loaded with (-)-epigallocatechin gallate (EGCG) and ferulic acid (FA) were fabricated via ionic gelation method modified by sodium tripolyphosphate and genipin (G-CS-EGCG-FA NPs). The particle size, morphology, entrapment efficiency, rheological properties, antioxidant and tyrosinase inhibitory activity of NPs were investigated. The G-CS-EGCG-FA NPs exhibited irregular ellipsoidal shape with average diameter of 412.3 nm and high DPPH and ABTS·⁺ scavenging ability. The entrapment efficiency of EGCG and FA in NPs was 46.0 ± 1.3 % and 46.8 ± 1.6 %, respectively. CS-based NPs show no toxic effects on NIH 3 T3 cells and B16-F10 melanoma cells with concentration <200 μg/mL and 25 μg/mL, respectively and the cell viability ranged from 100 % to 118 %. Meanwhile, the oxidative repaired capacity of G-CS-EGCG-FA NPs (200 μg/mL) in H₂O₂-induced cells was over 100 %, higher than that of the same dose of free EGCG or FA. Moreover, the tyrosinase inhibition activity of G-CS-EGCG-FA NPs (25 μg/mL) (84.6 %) was more potent than that of free EGCG (55.3 %), free FA (47.1 %) and kojic acid, indicating the good skin repairing and whitening ability of G-CS-EGCG-FA NPs. Given these results, this research provides new insights for designing novel particles loaded with dual bioactive agents that possess synergistic benefits.

The Hypopigmentation Mechanism of Tyrosinase Inhibitory Peptides Derived from Food Proteins: An Overview

Skin hyperpigmentation resulting from excessive tyrosinase expression has long been a problem for beauty lovers, which has not yet been completely solved. Although researchers are working on finding effective tyrosinase inhibitors, most of them are restricted, due to cell mutation and cytotoxicity. Therefore, functional foods are developing rapidly for their good biocompatibility. Food-derived peptides have been proven to display excellent anti-tyrosinase activity, and the mechanisms involved mainly include inhibition of oxidation, occupation of tyrosinase’s bioactive site and regulation of related gene expression. For anti-oxidation, peptides can interrupt the oxidative reactions catalyzed by tyrosinase or activate an enzyme system, including SOD, CAT, and GSH-Px to scavenge free radicals that stimulate tyrosinase. In addition, researchers predict that peptides probably occupy the site of the substrate by chelating with copper ions or combining with surrounding amino acid residues, ultimately inhibiting the catalytic activity of tyrosinase. More importantly, peptides reduce the tyrosinase expression content, primarily through the cAMP/PKA/CREB pathway, with PI3K/AKT/GSK3β, MEK/ERK/MITF and p38 MAPK/CREB/MITF as side pathways. The objective of this overview is to recap three main mechanisms for peptides to inhibit tyrosinase and the emerging bioinformatic technologies used in developing new inhibitors.

Antifungal Mechanism and Efficacy of Kojic Acid for the Control of Sclerotinia sclerotiorum in Soybean

Sclerotinia stem rot, which is caused by the fungal pathogen Sclerotinia sclerotiorum, is a soybean disease that results in enormous economic losses worldwide. The control of S. sclerotiorum is a difficult task due to the pathogen's wide host range and its persistent structures, called sclerotia. In addition, there is lack of soybean cultivars with medium to high levels of resistance to S. sclerotiorum. In this work, kojic acid (KA), a natural bioactive compound commonly used in cosmetic industry, was evaluated for the management of Sclerotinia stem rot. Interestingly, KA showed strong antifungal activity against S. sclerotiorum by inhibiting chitin and melanin syntheses and, subsequently, sclerotia formation. The antifungal activity of KA was not obviously affected by pH, but was reduced in the presence of metal ions. Treatment with KA reduced the content of virulence factor oxalic acid in S. sclerotiorum secretions. Preventive applications of 50 mM KA (7.1 mg/ml) completely inhibited S. sclerotiorum symptoms in soybean; whereas, in curative applications, the combination of KA with prochloraz and carbendazim improved the efficacy of these commercial fungicides. Taken together, the antifungal activity of KA against S. sclerotiorum was studied for the first time, revealing new insights on the potential application of KA for the control of Sclerotinia stem rot in soybean.

Protective effects of gallocatechin gallate against ultraviolet B induced skin damages in hairless mice

Epigallocatechin gallate (EGCG) has the effect to protect skin from ultraviolet B (UVB) induced damages, but it is unstable under ambient conditions, being susceptible to become brown in color. Gallocatechin gallate (GCG), an epimer counterpart of EGCG, is more stable chemically than EGCG. The potential effects of GCG against UVB-induced skin damages has not been available. The objective of this study was to investigate the protective effects of GCG against UVB-induced skin photodamages. GCG was topically applied on the skin of hairless mice at three dosage levels (LL, 12.5 mg/mL; ML 25 mg/mL; HL, 50 mg/mL), with EGCG and a commercially available baby sunscreen lotion SPF50 PA⁺⁺⁺ as control. The mice were then irradiated by UVB (fluence rate 1.7 µmol/m² s) for 45 min. The treatments were carried out once a day for 6 consecutive days. Skin measurements and histological studies were performed at the end of experiment. The results show that GCG treatments at ML and HL levels inhibited the increase in levels of skin oil and pigmentation induced by UVB irradiation, and improved the skin elasticity and collagen fibers. GCG at ML and HL levels inhibited the formation of melanosomes and aberrations in mitochondria of UVB-irradiated skin in hairless mice. It is concluded that GCG protected skin from UVB-induced photodamages by improving skin elasticity and collagen fibers, and inhibiting aberrations in mitochondria and formation of melanosomes.

Inhibitory Mechanism of Baicalein on Acetylcholinesterase: Inhibitory Interaction, Conformational Change, and Computational Simulation

Alzheimer’s disease (AD) is the most prevalent chronic neurodegenerative disease in elderly individuals, causing dementia. Acetylcholinesterase (AChE) is regarded as one of the most popular drug targets for AD. Herbal secondary metabolites are frequently cited as a major source of AChE inhibitors. In the current study, baicalein, a typical bioactive flavonoid, was found to inhibit AChE competitively, with an associated IC₅₀ value of 6.42 ± 0.07 µM, through a monophasic kinetic process. The AChE fluorescence quenching by baicalein was a static process. The binding constant between baicalein and AChE was an order of magnitude of 10⁴ L mol⁻¹, and hydrogen bonding and hydrophobic interaction were the major forces for forming the baicalein−AChE complex. Circular dichroism analysis revealed that baicalein caused the AChE structure to shrink and increased its surface hydrophobicity by increasing the α-helix and β-turn contents and decreasing the β-sheet and random coil structure content. Molecular docking revealed that baicalein predominated at the active site of AChE, likely tightening the gorge entrance and preventing the substrate from entering and binding with the enzyme, resulting in AChE inhibition. The preceding findings were confirmed by molecular dynamics simulation. The current study provides an insight into the molecular-level mechanism of baicalein interaction with AChE, which may offer new ideas for the research and development of anti-AD functional foods and drugs.

Epicatechin Gallate as Xanthine Oxidase Inhibitor: Inhibitory Kinetics, Binding Characteristics, Synergistic Inhibition, and Action Mechanism

Epicatechin gallate (ECG) is one of the main components of catechins and has multiple bioactivities. In this work, the inhibitory ability and molecular mechanism of ECG on XO were investigated systematically. ECG was determined as a mixed xanthine oxidase (XO) inhibitor with an IC50 value of 19.33 ± 0.45 μM. The promotion of reduced XO and the inhibition of the formation of uric acid by ECG led to a decrease in O2- radical. The stable ECG-XO complex was formed by hydrogen bonds and van der Waals forces, with the binding constant of the magnitude of 104 L mol-1, and ECG influenced the stability of the polypeptide skeleton and resulted in a more compact conformation of XO. Computational simulations further characterized the binding characteristics and revealed that the inhibitory mechanism of ECG on XO was likely that ECG bound to the vicinity of flavin adenine dinucleotide (FAD) and altered the conformation of XO, hindering the entry of substrate and the diffusion of catalytic products. ECG and allopurinol bound to different active sites of XO and exerted a synergistic inhibitory effect through enhancing their binding stability with XO and changing the target amino acid residues of XO. These findings may provide a theoretical basis for the further application of ECG in the fields of food nutrition and functional foods.

Identification of Phenolic Compounds by LC-MS/MS and Evaluation of Bioactive Properties of Two Edible Halophytes: Limonium effusum and L. sinuatum

This work aimed to evaluate the phenolic content and in vitro antioxidant, antimicrobial and enzyme inhibitory activities of the methanol extracts and their fractions of two edible halophytic Limonium species, L. effusum (LE) and L. sinuatum (LS). The total phenolic content resulted about two-fold higher in the ethyl acetate fraction of LE (522.82 ± 5.67 mg GAE/g extract) than in that of LS (274.87 ± 1.87 mg GAE/g extract). LC-MS/MS analysis indicated that tannic acid was the most abundant phenolic acid in both species (71,439.56 ± 3643.3 µg/g extract in LE and 105,453.5 ± 5328.1 µg/g extract in LS), whereas hyperoside was the most abundant flavonoid (14,006.90 ± 686.1 µg/g extract in LE and 1708.51 ± 83.6 µg/g extract in LS). The antioxidant capacity was evaluated by DPPH and TAC assays, and the stronger antioxidant activity in ethyl acetate fractions was highlighted. Both species were more active against Gram-positive bacteria than Gram negatives and showed considerable growth inhibitions against tested fungi. Interestingly, selective acetylcholinesterase (AChE) activity was observed with LE and LS. Particularly, the water fraction of LS strongly inhibited AChE (IC₅₀ = 0.199 ± 0.009 µg/mL). The ethyl acetate fractions of LE and LS, as well as the n-hexane fraction of LE, exhibited significant antityrosinase activity (IC₅₀ = 245.56 ± 3.6, 295.18 ± 10.57 and 148.27 ± 3.33 µg/mL, respectively). The ethyl acetate fraction and methanol extract of LS also significantly inhibited pancreatic lipase (IC₅₀ = 83.76 ± 4.19 and 162.2 ± 7.29 µg/mL, respectively). Taken together, these findings warrant further investigations to assess the potential of LE and LS as a bioactive source that can be exploited in pharmaceutical, cosmetics and food industries.