Different inhibition mechanisms of gentisic acid and cyaniding-3-O-glucoside on polyphenoloxidase

Inhibitory effect of fermented avocado seed extract (Persea americana Mill. cv. Hass) on polyphenol oxidase and its application as anti-browning agent in avocado, apple, and banana pulps

Antioxidant components of avocado seed can be used to prevent browning caused by polyphenol oxidase (PPO). This research evaluated the inhibitory effect of avocado seed extract obtained through solid state fermentation on PPO and assessed the anti-browning performance of this extract on avocado, banana, and apple pulp in comparison to ascorbic acid and L-Cysteine. The Km and Vmax kinetic parameters of avocado PPO, using catechol as substrate, were 0.1627 g/L and 0.6577 Abs/min, respectively. The fermented extract completely inhibited the enzyme at 60 % v/v, likewise, the type of competitive inhibition of the fermented extract was determined by the Lineweaver-Burk method. Additionally, fermented extract effectively delayed browning in avocado, and banana at 0.5 % v/v and 1.0 % v/v in apple, the effect is linked to the presence of bioactive compounds as potential inhibitors capable of retarding the darkening. Mathematical modeling of enzyme inhibition was performed and it was found self-inhibition is present in most cases. In conclusion, fermented avocado seed extract is a potential product to protect other foodstuffs from oxidation in the food industry.

Insight into the mechanism of high hydrostatic pressure effect on inhibitory efficiency of three natural inhibitors on polyphenol oxidase

The development of natural inhibitors of polyphenol oxidase (PPO) is crucial in the prevention of enzymatic browning in fresh foods. However, few studies have focused on the effect of subsequent sterilization on their inhibition efficiency. This study investigated the influence and mechanism of high hydrostatic pressure (HHP) on the inhibition of PPO by epigallocatechin gallate (EGCG), cyanidin-3-O-glucoside (C3G), and ferulic acid. Results showed that under the conditions of 550 MPa/30 min, the activity of EGCG-PPO decreased to 55.92%, C3G-PPO decreased to 81.80%, whereas the activity of FA-PPO remained stable. Spectroscopic experiments displayed that HHP intensified the secondary structure transformation and fluorescence quenching of PPO. Molecular dynamics simulations revealed that at 550 MPa, the surface interaction between PPO with EGCG or C3G increased, potentially leading to a reduction in their activity. In contrast, FA-PPO demonstrated conformational stability. This study can provide a reference for the future industrial application of natural inhibitors.

Comparison of interactions between alpha-lactalbumin and three protopanaxadiol ginsenosides: Impacts on the structure and antitumor properties

In this research, interactions between α-lactalbumin (ALA) and three protopanaxadiol ginsenosides [20(S)-Rg3, 20(S)-Rh2, and 20(S)-PPD] were compared to explore the effects of similar ligand on structure and cytotoxicity of ALA. Multi-spectroscopy revealed the binding between ALA and ginsenoside changed the conformation of ALA, which related to different structures and solubility of ligands. Scanning electron microscope illustrated that all ALA-ginsenoside complexes exhibited denser structures via hydrophobic interactions. Additionally, the cytotoxic experiments confirmed that the cytotoxicity of ginsenoside was enhanced after binding with ALA. Molecular docking showed all three ginsenosides were bound to the sulcus depression region of ALA via hydrogen bonding and hydrophobic interaction. Furthermore, molecular dynamics simulation elucidated the precise binding sites and pertinent system properties. Among all three composite systems, 20(S)-Rh2 had optimal binding affinity. These findings enhanced understanding of the synergistic utilization of ALA and ginsenosides as functional ingredients in food, medicine, and cosmetics.

Advanced interactional characterization of the inhibitory effect of anthocyanin extract from Hibiscus sabdariffa L. on apple polyphenol oxidase

In this study, a comprehensive approach to advance the inhibitory effect of Hibiscus sabdariffa extract on apple polyphenol oxidase (PPO) was performed. PPO was extracted, purified, and characterized for optimal activity, whereas response surface methodology generated a quadratic polynomial model to fit the experimental results for hibiscus extraction. The optimum conditions allowed to predict a maximum recovery of anthocyanins (256.11 mg delphinidin-3-O-glucoside/g), with a validated value of 272.87 mg delphinidin-3-O-glucoside/g dry weight (DW). The chromatographic methods highlighted the presence of gallic acid (36,812.90 µg/g DW extract), myricetin (141,933.84 µg/g DW extract), caffeic acid (101,394.07 µg/g DW extract), sinapic acid (1157.46 µg/g DW extract), kaempferol (2136.76 µg/g DW extract), and delphinidin 3-O-β-d-glucoside (226,367.08 µg/g DW extract). The inactivation of PPO followed a first-order kinetic model. A temperature-mediated flexible fit between PPO and anthocyanins was suggested, whereas the molecular docking tests indicated that PPO is a good receptor for cafestol, gallic acid, and catechin, involving hydrophobic and hydrogen bond interactions. PRACTICAL APPLICATION: It is well known that enzymatic browning is one of the most important challenges in the industrial minimal processing of selected fruit and vegetable products. Novel inhibitors for polyphenol oxidase are proposed in this study by using an anthocyanin-enriched extract from Hibiscus sabdariffa L. Based on our results, combining the chemical effect of phytochemicals from hibiscus extract with different functional groups with minimal heating could be an interesting approach for the development of a new strategy to inhibit apple polyphenol oxidase.

Dried grape pomace with lactic acid bacteria as a potential source for probiotic and antidiabetic value-added powders

Two drying methods (convective (CD) and infrared (IR)) on grape pomace with probiotics were analysed, based on kinetic models and survival rate. The moisture ratio decreases linearly with drying time. The IR drying time reduced up to 14.3% at 50 °C. The Page model allowed to calculate the drying constant (0.188-0.404 s-1), whereas the effective moisture diffusivity ranged from 6.64 × 10-9 to 9.38 × 10-9 m2/s for CD and from 8.83 × 10-9 to 11.16 × 10-9 m2/s for IR, respectively. Chromatographic analysis highlighted the presence of 28 anthocyanins, with cyanidin-3-O-monoglucoside as a main bioactive in both powder. The probiotic survivale rate reached 7.0 log CFU/g dry weight after 14 days of storage at 4 °C. The extracts affected conformation of α-amylase, with binding constants lower for IR extract (15.94 ± 1.61 × 10-2 Mol/L) when compared with CD (25.09 ± 2.14 × 10-2 Mol/L). The IC50 values were significant higher for the IR (6.92 ± 0.09 μMol C3G/mL) when compared with CD extract (10.70 ± 0.12 μMol C3G/mL).

Polyphenol oxidase inhibited by 4-hydroxycinnamic acid and naringenin: Multi-spectroscopic analyses and molecular docking simulation at different pH

It is still unclear how pH affects the inhibitory effects of phenolic acids and flavonoids on polyphenol oxidase (PPO). In this study, 4-hydroxycinnamic acid and naringenin were selected to investigate their interactions with PPO from pH 6.8 to 5.0. Results showed that acidic pH could enhance the inhibitory effect of inhibitors and a greater enhancement effect was observed in 4-hydroxycinnamic acid. Fluorescence emission spectra indicated that 4-hydroxycinnamic acid and naringenin interacted with PPO and quenched its intrinsic fluorescence, which was also enhanced by acidic pH. Circular dichroism suggested that 4-hydroxycinnamic acid and naringenin could reversibly bind to PPO molecules and transform α-helix into β-sheet. Molecular docking results revealed that 4-hydroxycinnamic acid and naringenin interacted with PPO through hydrogen bond and hydrophobic interaction, and more interactions were observed near the carboxyl group. These results indicated that acidic pH could significantly enhance the inhibitory effect of phenolic acid on PPO.

Comparing the effect of benzoic acid and cinnamic acid hydroxyl derivatives on polyphenol oxidase: activity, action mechanism, and molecular docking

BACKGROUND

Polyphenol oxidase (PPO) is considered to have a key role in the food industry because it initiates enzymatic browning in the processing and storage of fruit and vegetables. Increasing numbers of benzoic and cinnamic acid derivatives have been found to be efficient inhibitors of polyphenol oxidase, but a comparison study on activity and action mechanism is lacking. In this study, 18 benzoic acid and cinnamic acid hydroxy derivatives were selected and investigated.

RESULTS

Three substrates, four activators and 11 inhibitors were identified from benzoic and cinnamic acid derivatives. 2,4-Dihydroxycinnamic acid and benzoic acid showed the strongest inhibitory effect on PPO, with IC₅₀ of 0.092 and1.425 mmol L^-1 , respectively. Benzoic acid reversibly inhibited PPO in a competitive manner, while 2,4-dihydroxycinnamic acid showed a mixed-type inhibition. Both of them showed that static-type fluorescence quenching and electrostatic interaction were the main driving force in the bonding process. Compared with benzoic acid, 2,4-dihydroxycinnamic acid more easily formed hydrogen bonds in the active site of PPO, making the interaction more stable.

CONCLUSION

Comparative analysis showed that the inhibition effect of cinnamic acid hydroxyl derivatives on PPO was stronger than that of benzoic acid derivatives. Benzoic acid and 2,4-dihydroxycinnamic acid were the strongest inhibitors. PPO inhibitors identified from benzoic and cinnamic acid derivatives are expected to be promising inhibitors for controlling fruit and vegetable browning. © 2021 Society of Chemical Industry.

The mechanism of interaction between lotus rhizome polyphenol oxidase and ascorbic acid: Inhibitory activity, thermodynamics, and conformation analysis

In this study, the interaction between lotus rhizome polyphenol oxidase (PPO) and ascorbic acid (AA) was discussed from the aspects of inhibitory activity, thermodynamics, and conformation. Results showed that PPO was purified from lotus rhizome by DEAE-52 anion exchange chromatography and Sephadex G-100 gel filtration chromatography, with its optimum substrate being determined as pyrogallic acid. Spectrophotometric and polarographic assays demonstrated that AA exhibited strong inhibitory activity against PPO. Thermodynamics, fluorescence, and circular dichroism spectral analysis showed that hydrophobic interactions caused the formation of AA-PPO complex, leading to the remarkable fluorescence quenching and conformational change of PPO. Atomic force microscopic analysis revealed that binding to AA induced significant changes in the surface morphology and molecular aggregation of PPO molecules. In this study, the interaction mechanism between PPO and AA was proposed for the first time, which provided a theoretical basis for AA to inhibit lotus rhizome browning. PRACTICAL APPLICATIONS: Lotus rhizome, an aquatic vegetable, is prone to enzymatic browning in processing operations, which leads to a decrease in market value and economic loss. At present, ascorbic acid (AA) is widely used in industries as an excellent antioxidant because of its good antibrowning effect and relatively low cost. However, the interaction between the enzymatic browning-related polyphenol oxidase (PPO) from lotus rhizome and ascorbic acid has not been clearly studied. Understanding the mechanism of inhibiting PPO will help to prevent vegetable browning, especially fresh-cut products. The inhibitory effect of AA on PPO in lotus rhizome favors simultaneous use with other types of PPO inhibitors because of their likely synergistic effects.

Interaction mechanism of pelargonidin against tyrosinase by multi-spectroscopy and molecular docking

The interaction mechanism of pelargonidin (PG) with tyrosinase was investigated by multi-spectroscopy and molecular docking. As a result, PG had strong inhibitory activity on tyrosinase with the IC₅₀ value of 41.94×10^-6 mol·L^-1 . The inhibition type of PG against tyrosinase was determined as a mixed mode. Meanwhile, the fluorescence of tyrosinase was quenched statically by PG, and accompanied by non-radiative energy transfer. The three-dimensional (3-D) fluorescence, ultraviolet-visible spectroscopy (UV-Vis) and circular dichroism spectroscopies (CD) indicated that PG decreased the hydrophobicity of the micro-environment around tryptophan (Trp) and tyrosine (Tyr), which resulted in the conformational change of tyrosinase. In addition, fluorescence and molecular docking analysis indicated that PG bound to tyrosinase via hydrogen bonds (H-bonds) and van der Waals force (vdW force). We herein recommended that PG might be a potential candidate drug for the treatment of melanin-related diseases.

Recent advances and trends in extraction techniques to recover polyphenols compounds from apple by-products

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Apple by-products are a source of phenolic compounds associated with bioactivities.

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Apple processing industries generate by-products that could be better used.

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This work provides an up-to-date literature overview on extraction techniques.

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Gaps and future trends related to apple by-products are critically presented.

Apple is one of the most consumed fruits worldwide and has recognized nutritional properties. Besides being consumed fresh, it is the raw material for several food products, whose production chain generates a considerable amount of by-products that currently have an underestimated use. These by-products are a rich source of chemical compounds with several potential applications. Therefore, new ambitious platforms focused on reusing are needed, targeting a process chain that achieves well-defined products and mitigates waste generation. This review covers an essential part of the apple by-products reuse chain. The apple composition regarding phenolic compounds subclasses is addressed and related to biological activities. The extraction processes to recover apple biocompounds have been revised, and an up-to-date overview of the scientific literature on conventional and emerging extraction techniques adopted over the past decade is reported. Finally, gaps and future trends related to the management of apple by-products are critically presented.

Spectroscopic studies and molecular docking on the interaction of delphinidin-3-O-galactoside with tyrosinase

The inhibitory effects of delphinidin-3-O-galactoside (DG) on the activities of tyrosinase (EC 1.14.18.1) (TY) from the edible Agaricus bisporus mushroom were investigated by enzyme kinetics, multispectroscopic methods, and molecular docking. As a result, DG showed strong inhibition on TY with the IC₅₀ of 34.14 × 10^-6  mol L^-1 . The inhibition mode of DG against TY was mixed type with α values of 5.09. The binding constant K_a and related thermodynamic parameters at the three different temperatures showed that the fluorescence quenching of TY by DG was static quenching. Synchronous fluorescence, three-dimensional fluorescence, ultraviolet-visible spectroscopy, and circular dichroism spectroscopies confirmed that the conformation or microenvironment of the TY protein were changed after binding with DG. Molecular docking revealed that DG had strong binding affinity to TY through hydrogen bonding and van der Waals force, and the results were consistent with the fluorescence data. Our findings suggested that DG may be potential TY inhibitor.

The Hull of Ripe Pistachio Nuts (Pistacia vera L.) as a Source of New Promising Melanogenesis Inhibitors

In the present study an acidified methanol pistachio hull extract was investigated for antioxidant and inhibitory effects on melanin biosynthesis by in vitro and in vivo assays. The chromatographic analysis revealed that cyanidin-3-O-galactoside represents the main compound (98.37%). The pistachio hull extract efficiently inhibits the mono and diphenolase activity of mushroom tyrosinase (IC₅₀= 141.07 and 116.08 μg/mL, respectively) and it was able, thanks to its strong antioxidant and free-radical scavenging activities, to hinder the L-DOPA auto-oxidation in a concentration-dependent manner (125-500 μg/mL). Results of in vivo assay showed that the treatment with pistachio hull extract (10 μg/mL) reduced pigmentation in zebrafish embryos at early stages of development (60.01% of inhibition vs control). In conclusion, these findings suggest that the ripe pistachio hull may be considered as a promising source of antioxidant and skin whitening agents for the development of new products useful in preventing the pigmentation disorders in humans and/or to improve the food quality.

Inhibitory mechanism of salicylic acid on polyphenol oxidase: A cooperation between acidification and binding effects

Salicylic acid is generally considered to combine with polyphenol oxidase (PPO) to inhibit activity and enzymatic browning, while its acidification effect on PPO activity was usually neglected. In this study, the inhibitory mechanism of salicylic acid on PPO was examined from acidification and binding effects by altering the buffer conditions. As the buffer concentration increased, contribution of acidification decreased while the binding effect became more predominant. Salicylic acid exhibited competitive inhibition on PPO, inducing the changes in secondary structure with a reduction in α-helix. Molecular docking results showed that salicylic acid interacted with residues HIS61, HIS85, HIS259, HIS263 and VAL283 through hydrogen bond and hydrophobic interaction. Furthermore, acidic pH enhanced the binding of salicylic acid to PPO with lower binding energy, additional hydrogen bond and electrostatic interactions. Therefore, both acidification and binding effects were important for salicylic acid on PPO inhibition and enzymatic browning control in fruit and vegetables.

Polyphenol oxidase dominates the conversions of flavonol glycosides in tea leaves

Flavonol glycosides are associated with astringency and bitterness of teas. To clarify the dominant enzymatic reaction of flavonol glycosides in tea leaves, the catalytic effects of polyphenol oxidase (PPO), peroxidase (POD) and β-glucosidase were studied, with the maintaining rates of total flavonol glycosides (TFG) being 73.0%, 99.8% and 94.3%. PPO was selected for further investigations, including the effects of pH value (3.5 ~ 6.5), temperature (25 °C ~ 55 °C) and dosage (39 ~ 72 U/mL PPO and 36 U/mL PPO, 3 ~ 36 U/mL POD). The oxidation of flavonol glycosides were intensified at pH 6.5, with 51.8% and 15.4% of TFG maintained after PPO and PPO + POD treatments, suggesting an enhancement from POD. The sensitivity ranking to PPO was: myricetin glycosides > quercetin glycosides > kaempferol glycosides. The inhibitor treatment testified the leading role of PPO in catalyzing flavonol glycosides in tea leaves. Sugar moiety enhanced the docking affinity of flavonol glycosides for PPO. PPO shows the potential of modifying flavonol glycoside composition.

Inhibitory effects of organic acids on polyphenol oxidase: From model systems to food systems

Organic acids are widely utilized in the food industry for inhibiting the activity of polyphenol oxidase (PPO) and enzymatic browning. This review discusses the mechanisms of inhibition of PPO and enzymatic browning by various organic acids based on studies in model systems, critically evaluates the relevance of such studies to real food systems and assesses the implication of the synergistic inhibitory effects of organic acids with other physicochemical processing techniques on product quality and safety. Organic acids inhibit the activity of PPO and enzymatic browning via different mechanisms and therefore the suitability of a particular organic acid depends on the structure and the catalytic properties of PPO and the physicochemical properties of the food matrix. Studies in model systems provide an invaluable insight into the inhibitory mechanisms of various organics acids. However, the difference in the effectiveness of PPO inhibitors between model systems and food systems and the lack of correlation between the degree of PPO inhibition based on in vitro assays and enzymatic browning imply that the effectiveness of organic acids can be accurately evaluated only via direct assessment of browning inhibition in a particular food system. Combination of organic acids with physical processing techniques is one of the most viable approaches for PPO inhibition since the observed synergistic effect helps to reduce the undesirable organoleptic quality changes from the use of excessive concentration of organic acids or intense physical processing.

Effect of gentisic acid on the structural-functional properties of liposomes incorporating β-sitosterol

Multifunctional liposomes incorporating β-sitosterol were developed for delivery of gentisic acid (GA). The interactions of both compounds with phospholipid bilayer were interpreted viaeffects of different β-sitosterol content (0, 20 and 50 mol %) and different gentisic acid to lipid ratio (n_GA/n_lip from 10^-5 to 1) on membrane fluidity and thermotropic properties. Multilamellar vesicles of phosphatidylcholines (with size range between 1350 and 1900 nm) effectively encapsulated GA (54%) when n_GA/n_lip was higher than 0.01. Suppression of lipid peroxidation was directly related to concentration of GA. The resistance to diffusion of gentisic acid from liposomes increased for ˜50% in samples incorporating 50 mol % β-sitosterol compared to sterol-free liposomes. Finally, simulated in vitro gastrointestinal conditions showed that the release was mainly affected by low pH of simulated gastric fluid and the presence of cholates in simulated intestinal fluid, rather than by enzymes activity.

Plant-Based Nanoparticles Prepared from Proteins and Phospholipids Consisting of a Core-Multilayer-Shell Structure: Fabrication, Stability, and Foamability

Gliadin-phospholipid hybrid nanoparticles with a core-multilayered-shell internal structure were fabricated using a coassembly approach based on antisolvent coprecipitation. The mean particle diameters of the nanoparticles depended on their composition, increasing from 78 to 145 nm as the mass ratio of gliadin to phospholipid (G/P) increased from 7:3 to 3:7. The hybrid nanoparticles had better pH, salt, and thermal stabilities than simple gliadin nanoparticles. Hybrid nanoparticles with the highest phospholipid fraction (G/P 3:7) had the best pH, salt, and thermal stabilities, remaining stable from pH 3 to 8, from 0 to 300 mmol/L NaCl, and when boiled at pH 4 or 5 for 90 min. Hybrid nanoparticles also had better foam-formation and stability characteristics than gliadin nanoparticles, particularly at the highest phospholipid level. The structured nanoparticles developed in this study may therefore be useful for application in commercial products, for example, as delivery systems, emulsifiers, or foaming agents.

New Insights into the Inhibition Mechanism of Betulinic Acid on α-Glucosidase

Betulinic acid (BA), an important pentacyclic triterpene widely distributed in many foods, possesses high antidiabetic activity. In this study, BA was found to exhibit stronger inhibition of α-glucosidase than acarbose with an IC₅₀ value of (1.06 ± 0.02) × 10^-5 mol L^-1 in a mixed-type manner. BA bound with α-glucosidase to form a BA-α-glucosidase complex, resulting in a more compact structure of the enzyme. The obtained concentrations and spectra profiles of the components resolved by the multivariate-curve resolution-alternating least-squares confirmed the formation of the BA-α-glucosidase complex. Molecular docking showed that BA tightly bound to the active cavity of α-glucosidase, which might hinder the entrance of the substrate leading to a decline in enzyme activity. The chemical modification of α-glucosidase verified the results of the computer simulation that the order of importance of the four amino acid residues in the binding process was His > Tyr > Lys > Arg.

Understanding the inhibitory mechanism of tea polyphenols against tyrosinase using fluorescence spectroscopy, cyclic voltammetry, oximetry, and molecular simulations

Inhibiting the activity of tyrosinase is a very effective and safe way to prevent enzymatic browning in food and to resist pests in agriculture. Tea polyphenols (TPs), regarded as safe and non-toxic food additives, have been reported due to their potential inhibitory capability against tyrosinase, but their ambiguous inhibitory mechanisms have severely limited their application. In the present work, fluorescence spectroscopy, cyclic voltammetry (CV), oximetry and molecular simulation approaches were employed to shed light on the underlying inhibitory mechanisms of TPs with different structures including (+)-catechin, (-)-epicatechin gallate (ECG) and (-)-epigallocatechin gallate (EGCG) against tyrosinase. Fluorescence spectra show that the three TPs are capable of binding tyrosinase with a molar proportion of 1 : 1. The analysis of CV curves and oxygen utilization suggests that these three TPs can be oxidized by tyrosinase, revealing that these three TPs are suicide inhibitors of tyrosinase. Furthermore, ECG and catechin make tyrosinase irreversibly inactivated due to their catechol group (ring B) being catalyzed by tyrosinase through a cresolase-like pathway, while EGCG inhibits the activity of tyrosinase by competing with or delaying the oxidation of substrate. Molecular simulations further confirm that ring B of ECG and catechin makes a significant contribution to tyrosinase inhibitory activities, and has a direct interaction with the coupled binuclear copper ions in the optimal orientation required by the cresolase-like pathway.