Novel hydroxypyridinone derivatives containing an oxime ether moiety: Synthesis, inhibition on mushroom tyrosinase and application in anti-browning of fresh-cut apples

Inhibition of polyphenol oxidase for preventing browning in edible mushrooms: A review

Edible mushrooms are rich in nutrients and bioactive compounds, but their browning affects their quality and commercial value. This article reviews various methods to inhibit polyphenol oxidase (PPO)-induced browning in mushrooms. Physical methods such as heat treatment, low temperatures, irradiation, and ultrasound effectively reduce PPO activity but may affect mushroom texture and flavor. Chemical inhibitors, including synthetic chemicals and natural plant extracts, provide effective PPO inhibition but require careful monitoring of their content. Biological methods, including gene editing and microbial fermentation, show promise in targeting PPO genes and enhancing antioxidant production. Combining these methods offers a comprehensive strategy for preserving mushroom quality, extending shelf life, and maintaining nutritional value. PRACTICAL APPLICATION: These approaches can be applied in the food industry to improve post-harvest mushroom preservation, enhance product quality, and reduce waste, benefiting both producers and consumers. Further research and innovation are needed to optimize the practical application of these methods in large-scale processing and storage conditions.

Design, synthesis, in silico ADME, DFT, molecular dynamics simulation, anti-tyrosinase, and antioxidant activity of some of the 3-hydroxypyridin-4-one hybrids in combination with acylhydrazone derivatives

Tyrosinase is the rate-limiting enzyme in synthesizing melanin. Melanin is responsible for changing the color of fruits and vegetables and protecting against skin photo-carcinogenesis. Herein, some of the hybrids of 3-hydroxypyridine-4-one and acylhydrazones were designed and synthesized to study the anti-tyrosinase and antioxidant activities. The diphenolase activity of mushroom tyrosinase using L-DOPA assayed the inhibitory effects, and the antioxidant activity was assessed using DPPH free radical. The synthesized derivatives were confirmed using 1H-NMR, 13C-NMR, IR, and Mass spectroscopy. Among analogs, compound 5h bearing furan ring with IC50=8.94 μM was more potent than kojic acid (IC50=16.68 μM). The pharmacokinetic profile of the compounds showed that the tested compounds had suitable oral bioavailability and drug-likeness properties. The molecular docking studies showed that compound 5h was located in the tyrosinase-binding site. Also, the molecular dynamics simulation was performed on compound 5h, proving the obtained molecular docking results. At the B3LYP/6-31 + G** level of theory, the reactivity descriptors for 5 g and 5h were investigated using DFT calculations. Also, IR frequency was calculated to verify DFT results with experimental data. The electrostatic potential energy of the surface and the HOMO and LUMO molecular orbitals were also studied. It agrees with experimental results that the 5h is a soft molecule and ready for chemical reaction with other interacting molecules.Communicated by Ramaswamy H. Sarma.

Design, Synthesis, and Anti-Melanogenic Activity of 2-Mercaptomethylbenzo[d]imidazole Derivatives Serving as Tyrosinase Inhibitors: An In Silico, In Vitro, and In Vivo Exploration

2-Mercaptomethylbenzo[d]imidazole (2-MMBI) derivatives were designed and synthesized as tyrosinase (TYR) chelators using 2-mercaptomethylimidazole scaffolds. Seven of the ten 2-MMBI derivatives exhibited stronger inhibition of mushroom TYR activity than kojic acid. Their ability to chelate copper ions was demonstrated through experiments using the copper chelator pyrocatechol violet and assays measuring TYR activity in the presence or absence of exogenous CuSO4. The inhibition mechanisms of derivatives 1, 3, 8, and 9, which showed excellent TYR inhibitory activity, were elucidated through kinetic studies and supported by the docking simulation results. Derivatives 3, 7, 8, and 10 significantly inhibited cellular TYR activity and melanin production in B16F10 cells in a dose-dependent manner, with stronger potency than kojic acid. Furthermore, in situ, derivatives 7 and 10 showed stronger inhibitory effects on B16F10 cell TYR activity than kojic acid. Six derivatives, including 8, showed highly potent depigmentation in zebrafish larvae, outpacing kojic acid even at 200-670 times lower concentrations. Additionally, all derivatives could scavenge for reactive oxygen species without causing cytotoxicity in epidermal cells. These results suggested that 2-MMBI derivatives are promising anti-melanogenic agents.

Synthesis, biological evaluation, molecular docking, MD simulation and DFT analysis of new 3-hydroxypyridine-4-one derivatives as anti-tyrosinase and antioxidant agents

In the present study, ten new substituted 3-hydroxypyridine-4-one derivatives were synthesized in a four-step method, and their chemical structures were confirmed using various spectroscopic techniques. Subsequently, the inhibitory activities of these derivatives against tyrosinase enzyme and their antioxidant activities were evaluated. Amongest the synthesized compounds, 6b bearing a 4-OH-3-OCH3 substitution was found to be a promising tyrosinase inhibitor with an IC50 value of 25.82 μM, which is comparable to the activity of kojic acid as control drug. Kinetic study indicated that compound 6b is a competitive inhibitor of tyrosinase enzyme, which was confirmed by molecular docking results. The molecular docking study and MD simulation showed that compound 6b was properly placed within the tyrosinase binding pocket and interacted with key residues, which is consistent with its biological activity. The DFT analysis demonstrated that compound 6b is kinetically more stable than the other compounds. In addition, compounds 6a and 6b exhibited the best antioxidant activities. The findings indicate that compound 6b could be a promising lead for further studies.

Considerations about the inhibition of monophenolase and diphenolase activities of tyrosinase. Characterization of the inhibitor concentration which generates 50 % of inhibition, type and inhibition constants. A review

Tyrosinase is a copper oxidase enzyme which catalyzes the first two steps in the melanogenesis pathway, L-tyrosine to L-dopa conversion and, then, to o-dopaquinone and dopachrome. Hypopigmentation and, above all, hyperpigmentation issues can be originated depending on their activity. This enzyme also promotes the browning of fruits and vegetables. Therefore, control of their activity by regulators is research topic of great relevance. In this work, we consider the use of inhibitors of monophenolase and diphenolase activities of the enzyme in order to accomplish such control. An experimental design and data analysis which allow the accurate calculation of the degree of inhibition of monophenolase activity (iM) and diphenolase activity (iD) are proposed. The IC50 values (amount of inhibitor that causes 50 % inhibition at a fixed substrate concentration) can be calculated for the two activities and from the values of IC50M (monophenolase) and IC50D(diphenolase). Additionally, the strength and type of inhibition can be deduced from these values. The data analysis from these IC50D values allows to obtain the values of [Formula: see text] or [Formula: see text] , or and [Formula: see text] from the values of IC50M. In all cases, the values of the different must satisfy their relationship with IC50M and IC50D.

Whole-transcriptome sequencing reveals a melanin-related ceRNA regulatory network in the breast muscle of Xichuan black-bone chicken

The economic losses incurred due to reduced muscle pigmentation highlight the crucial role of melanin-based coloration in the meat of black-bone chickens. Melanogenesis in the breast muscle of black-bone chickens is currently poorly understood in terms of molecular mechanisms. This study employed whole-transcriptome sequencing to analyze black and white breast muscle samples from black-bone chickens, leading to the identification of 367 differentially expressed (DE) mRNAs, 48 DElncRNAs, 104 DEcircRNAs, and 112 DEmiRNAs involved in melanin deposition. Based on these findings, a competitive endogenous RNA (ceRNA) network was developed to better understand the complex mechanisms of melanin deposition. Furthermore, our analysis revealed key DEmRNAs (TYR, DCT, EDNRB, MLPH and OCA2) regulated by DEmiRNAs (gga-miR-140-5p, gga-miR-1682, gga-miR-3529, gga-miR-499-3p, novel-m0012-3p, gga-miR-200b-5p, gga-miR-203a, gga-miR-6651-5p, gga-miR-7455-3p, gga-miR-31-5p, miR-140-x, miR-455-x, novel-m0065-3p, gga-miR-29b-1-5p, miR-455-y, novel-m0085-3p, and gga-miR-196-1-3p). These DEmiRNAs competitively interacted with DElncRNAs including MSTRG.2609.2, MSTRG.4185.1, LOC112530666, LOC112533366, LOC771030, LOC107054724, LOC121107411, LOC100859072, LOC101750037, LOC121108550, LOC121109224, LOC121110876, and LOC101749016, as well as DEcircRNAs, such as novel_circ_000158, novel_circ_000623, novel_001518, and novel_circ_003596. The findings from this study provide insight into the mechanisms that regulate lncRNA, circRNA, miRNA, and mRNA expression in chicken melanin deposition.

Synthesis, anti-browning effect and mechanism research of kojic acid-coumarin derivatives as anti-tyrosinase inhibitors

Thirteen kojic acid-coumarin derivatives were synthesized using the principle of molecular hybridization, and their structures were characterized by 1H NMR, 13C NMR, and HRMS. In vitro enzyme inhibition experiments showed that all newly synthesized derivatives have excellent inhibition of tyrosinase (TYR) activity. As a mixed inhibitor, compound 6f has the strongest activity, with an IC50 value of 0.88 ± 0.10 µM. Multispectral experiments have confirmed that the mode of action of compound 6f on TYR was static quenching. In addition, compound 6f formed a new complex with TYR, which increased the hydrophobicity of the enzyme microenvironment, reduced the content of the α-helix in the enzyme, and changed the secondary structure. The experimental results showed that compound 6f effectively inhibited the browning of lotus root slices and had low cytotoxicity. Therefore, compound 6f is believed to have great development potential as a TYR inhibitor in the food industry.

Design and synthesis of novel dihydropyridine- and benzylideneimine-based tyrosinase inhibitors

Tyrosinase (TYR) inhibitors are very significant as they inhibit enzyme tyrosinase activity, and its inhibition is vital for skin care, anticancer medication, and antibrowning of fruits and vegetables. This work presents a novel and economical route for the preparation of new synthetic tyrosinase inhibitors using amlodipine (4). The novel conjugates 6 (a-o) were designed, synthesized, and characterized by spectroscopic analyses, including Fourier transform infrared and low- and high-resolution mass spectroscopy. The purified compound 4 was refluxed with various aldehydes and ketones 5 (a-o) for 5-8 h in methanol at 60°C-90°C. This research modified the drug in a step-by-step manner to develop therapeutic properties as a tyrosinase inhibitor. The structures of synthesized ligands 6 (a-o) were established based on spectral and analytical data. The synthesized compounds 6 (a-o) were screened against tyrosinase enzyme. Kojic acid was taken as standard. All the prepared compounds 6 (a-o) have good inhibition potential against the enzyme tyrosinase. Compounds 6o, 6b, 6f, and 6k depicted excellent antityrosinase activity. Compound 6k, with an IC50 value of 5.34 ± 0.58 µM, is as potent as the standard kojic acid (IC50 6.04 ± 0.11 µM), standing out among all synthesized compounds 6 (a-o). The in silico studies of the conjugates 6 (a-o) were evaluated via PatchDock. Compound 6k showed a binding affinity score of 8,999 and an atomic contact energy (ACE) value of -219.66 kcal/mol. The structure-activity relationship illustrated that the presence of dihydropyridine nuclei and some activating groups at the ortho and para positions of the benzylideneimine moiety is the main factor for good tyrosinase activity. The compound 6k could be used as a lead compound for drug modification as a tyrosinase inhibitor for skin care, anticancer medication, and antibrowning for fruits and vegetables.

Methyl 4-pyridyl ketone thiosemicarbazone (4-PT) as an effective and safe inhibitor of mushroom tyrosinase and antibrowning agent

Enzymatic browning is of concern as it can affect food safety and quality. In this study, an effective and safe tyrosinase inhibitor and anti-browning agent, methyl 4-pyridyl ketone thiosemicarbazone (4-PT), was synthesised and characterised using Fourier-transform infrared (FTIR) spectroscopy, CHNS elemental analysis, and proton (1H) and carbon-13 (13C) nuclear magnetic resonance (NMR) spectroscopy. The vibrational frequencies of 4-PT were studied theoretically using vibrational energy distribution analysis (VEDA). Density functional theory (DFT) was applied to elucidate its chemical properties, including the Mulliken atomic charges, molecular electrostatic potential (MEP), quantum theory of atoms in molecules (QTAIM) and reduced density gradient non-covalent interactions (RDG-NCIs). Moreover, 4-PT was compared with kojic acid in terms of its effectiveness as a tyrosinase inhibitor and anti-browning agent. The toxicity and physicochemical properties of 4-PT were predicted via ADME evaluation, which proved that 4-PT is safer than kojic acid. Experimentally, 4-PT (IC50 = 5.82 μM, browning index (10 days) = 0.292 ± 0.002) was proven to be an effective tyrosinase inhibitor and anti-browning agent compared to kojic acid (IC50 = 128.17 μM, browning index (10 days) = 0.332 ± 0.002). Furthermore, kinetic analyses indicated that the type of tyrosinase inhibition is a mixed inhibition, with Km and Vmax values of 0.85 mM and 2.78 E-09 μM/s, respectively. Finally, the mechanism of 4-PT for tyrosinase inhibition was proven by 1D, second derivative and 2D IR spectroscopy, molecular docking and molecular dynamic simulation approaches.

New 3-Hydroxypyridine-4-one Analogues: Their Synthesis, Antimicrobial Evaluation, Molecular Docking, and In Silico ADME Prediction

INTRODUCTION

Drug resistance to existing antimicrobial drugs has become a serious threat to human health, which highlights the need to develop new antimicrobial agents.

METHODS

In this study, a new set of 3-hydroxypyridine-4-one derivatives (6a-j) was synthesized, and the antimicrobial effects of these derivatives were evaluated against a variety of microorganisms using the microdilution method. The antimicrobial evaluation indicated that compound 6c, with an electron-donating group -OCH3 at the meta position of the phenyl ring, was the most active compound against S. aureus and E. coli species with an MIC value of 32 μg/mL. Compound 6c was more potent than ampicillin as a reference drug.

RESULTS

The in vitro antifungal results showed that the studied derivatives had moderate effects (MIC = 128-512 μg/mL) against C. albicans and A. niger species. The molecular modeling studies revealed the possible mechanism and suitable interactions of these derivatives with the target protein.

CONCLUSION

The obtained biological results offer valuable insights into the design of more effective antimicrobial agents.

Catechol-mimicking transition-state analogues as non-oxidizable inhibitors of tyrosinases

Tyrosinases are copper-containing metalloenzymes involved in several processes in both mammals, insects, bacteria, fungi and plants. Their phenol oxidation properties are especially responsible for human melanogenesis, potentially leading to abnormal pigmentation, and for postharvest vegetable tissue browning. Thus, targeting tyrosinases attracts interest for applications both in dermocosmetic and agrofood fields. However, a large part of the literature about tyrosinase inhibitors is dedicated to the report of copper-interacting phenolic compounds, that are more likely alternative substrates leading to undesirable toxic quinones production. To circumvent this issue, the use of catechol-mimicking copper-chelating groups that are analogues of the tyrosinase oxidation transition state appears as a valuable strategy. Relying on several non-oxidizable pyridinone, pyrone or tropolone moieties, innovative inhibitors were developed, especially within the past decade, and the best reported analogues reached IC50 values in the nanomolar range. Herein, we review the design, the activity against several tyrosinases, and the proposed binding modes of reported catechol-mimicking, non-oxidizable molecules, in light of recent structural data.

Synthesis of 3-hydroxypyridin-4-one derivatives bearing benzyl hydrazide substitutions towards anti-tyrosinase and free radical scavenging activities

Tyrosinase is a vital enzyme in the biosynthesis of melanin, which has a significant role in skin protection. Due to the importance of the tyrosinase enzyme in the cosmetics and health industries, studies to design new tyrosinase inhibitors have been expanded. In this study, the design and synthesis of 3-dihydroxypyridine-4-one derivatives containing benzo hydrazide groups with different substitutions were carried out, and their antioxidant and anti-tyrosinase activities were also evaluated. The proposed compounds showed tyrosinase inhibitory effects (IC50) in the 25.29 to 64.13 μM range. Among all compounds, 6i showed potent anti-tyrosinase activity with an IC50 = 25.29 μM. Also, the antioxidant activity of derivatives by using DPPH radical scavenging indicates an EC50 value between 0.039 and 0.389 mM. Molecular docking studies were performed to reveal the position and interactions of 6i as the most potent inhibitor within the tyrosinase active site. The results showed that 6i binds well to the proposed binding site and forms a stable complex with the target protein. Furthermore, the physicochemical profiles of the tested compounds indicated drug-like and bioavailability properties. The kinetic assay revealed that 6i acts as a competitive inhibitor. Also, for the estimation of the reactivity of the best compound (6i), the density functional theory (DFT) was performed at the B3LYP/6-31+G**.

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.

Heterocyclic Compounds as Synthetic Tyrosinase Inhibitors: Recent Advances

Tyrosinase is a copper-containing enzyme which is widely distributed in nature (e.g., bacteria, mammals, fungi) and involved in two consecutive steps of melanin biosynthesis. In humans, an excessive production of melanin can determine hyperpigmentation disorders as well as neurodegenerative processes in Parkinson's disease. The development of molecules able to inhibit the high activity of the enzyme remain a current topic in medicinal chemistry, because the inhibitors reported so far present several side effects. Heterocycle-bearing molecules are largely diffuse in this sense. Due to their importance as biologically active compounds, we decided to report a comprehensive review of synthetic tyrosinase inhibitors possessing heterocyclic moieties reported within the last five years. For the reader's convenience, we classified them as inhibitors of mushroom tyrosinase (Agaricus bisporus) and human tyrosinase.

Mechanistic Understanding of Tyrosinase Inhibition by Polymeric Proanthocyanidins from Acacia confusa Stem Bark and Their Effect on the Browning Resistance of Fresh-Cut Asparagus Lettuce

Tyrosinase inhibitors are capable of preventing unfavorable enzymatic browning of fruits and vegetables. In this study, the capacity of Acacia confusa stem bark proanthocyanidins (ASBPs) to inhibit tyrosinase activity was evaluated. ASBPs were shown to be a high-potential inhibitor of tyrosinase with IC50 values of 92.49 ± 4.70 and 61.74 ± 8.93 μg/mL when using L-tyrosine and L-DOPA as the substrate, respectively. The structural elucidation performed with UV-vis, FT-IR spectroscopy, ESI-MS and thiolysis coupled to HPLC-ESI-MS suggested that ASBPs had structural heterogeneity in monomer units and interflavan linkages and consisted mainly of procyanidins dominant with B-type linkages. To gain insights into the inhibitory mechanisms of ASBPs against tyrosinase, different spectroscopic and molecular docking methods were further conducted. Results validated that ASBPs possessed the ability to chelate copper ions and could prevent the oxidation process of substrates by tyrosinase. The hydrogen bond formed with Lys-376 residue played a key role in the binding force of ASBPs with tyrosinase that induced a certain alteration in the microenvironment and secondary structure of tyrosinase, resulting in the enzymatic activity being ultimately restricted. It was also observed that ASBPs treatment effectively inhibited the activities of PPO and POD to retard the surface browning of fresh-cut asparagus lettuce and thus extended their shelf-life. The results provided preliminary evidence supporting the exploitation of ASBPs into potential antibrowning agents for the fresh-cut food industry.

Preparation, antioxidant and tyrosinase inhibitory activities of chitosan oligosaccharide-hydroxypyridinone conjugates

Two novel chitosan oligosaccharide (COS)-hydroxypyridone (HPO) conjugates were prepared by reacting chitosan oligosaccharide with 2-chloromethyl-5-hydroxypyridone (HPO), which was synthesized by a series of reactions starting from kojic acid. The degree of substitution of COS-HPO2 reached 1.2, with a yield of 74.9%. The structure of the two conjugates (COS-HPO1 and COS-HPO2) was identified by NMR and FT-IR analysis. The two conjugates showed significantly higher free radical (DPPH•, ABTS^+• and •OH) scavenging activity and reducing power than those of COS and HPO (p < 0.05). Both COS-HPO1 and COS-HPO2 possessed significantly stronger tyrosinase inhibitory activity than those of COS, with IC₅₀ values of 0.67 and 0.28 mg/mL for monophenolase, 0.73 and 0.30 mg/mL for diphenolase, respectively. In addition, the conjugates were found to be non-toxic to RAW264.7 macrophages and MRC-5 human lung cells. This work proposes a facile method to enhance the oxidative and tyrosinase inhibitory properties of COS.

Antioxidant, Bacteriostatic and Preservative Effects of Extractable Condensed Tannins Isolated from Longan Pericarps and Seeds

In the process of longan production and processing, a large amount of remnants is produced, such as dried longan pericarps and seeds, which have been reported to be rich in polyphenols but not effectively utilized. In this paper, the total phenolic contents in the remnants of longan pericarps and seeds were found to be 39.58 ± 3.54 and 69.53 ± 1.99 mg/g (DW), respectively, accounting for 60-80% of those in fresh samples. The contents of extractable condensed tannins (ECTs) in the remnants of longan pericarps and seeds were 19.25 ± 6.71 mg/g (DW) and 44.59 ± 2.05 mg/g (DW), respectively, accounting for 60-70% of the fresh samples. These data indicate that the polyphenols in the remnants of the sampled longan pericarps and seeds were effectively retained. The antioxidant capacity of ECTs from the longan pericarps and seeds was more than 60% of the fresh samples measured with the 1, 1-diphenyl-2-trinitrophenylhydrazine and ferric reducing ability of plasma methods. Further exploration showed that ECTs from the longan pericarps and seeds had significant inhibitory effects on Pseudomonas aeruginosa, Escherichia coli, Salmonella and Staphylococcus aureus. The minimum inhibitory concentration (MIC) of the longan pericarp ECTs on all four studied bacteria was 3 mg/mL. The MIC of longan seed ECTs on Salmonella was 3 mg/mL, and that of the other three bacteria was 1.5 mg/mL. In view of the good antioxidant and antibacterial activities of longan pericarps and seeds, we applied them to the preservation of fresh-cut lotus roots. When the concentration of ECTs in the longan pericarps and seeds was 2 mg/mL and 1 mg/mL, respectively, the two kinds of ECTs showed an obvious preservative effect. After the ECT treatment of the lotus roots, their browning degree was reduced, their color was better maintained, their respiration was inhibited and their nutrient loss was reduced. Bacterial reproduction was inhibited, and cell senescence was slowed. Accordingly, the shelf life of ECT-treated fruits and vegetables can be effectively extended. Overall, we can suggest that ECTs from the remnants of dried longan pericarps and seeds could be used as natural preservatives for fresh-cut fruits and vegetables.

Tyrosinase Inhibitors Naturally Present in Plants and Synthetic Modifications of These Natural Products as Anti-Melanogenic Agents: A Review

Tyrosinase is a key enzyme target to design new chemical ligands against melanogenesis. In the current review, different chemical derivatives are explored which have been used as anti-melanogenic compounds. These are different chemical compounds naturally present in plants and semi-synthetic and synthetic compounds inspired by these natural products, such as kojic acid produced by several species of fungi; arbutin-a glycosylated hydroquinone extracted from the bearberry plant; vanillin-a phenolic aldehyde extracted from the vanilla bean, etc. After enzyme inhibition screening, various chemical compounds showed different therapeutic effects as tyrosinase inhibitors with different values of the inhibition constant and IC₅₀. We show how appropriately designed scaffolds inspired by the structures of natural compounds are used to develop novel synthetic inhibitors. We review the results of numerous studies, which could lead to the development of effective anti-tyrosinase agents with increased efficiency and safety in the near future, with many applications in the food, pharmaceutical and cosmetics industries.

Copper chelating peptides derived from tilapia (Oreochromis niloticus) skin as tyrosinase inhibitor: Biological evaluation, in silico investigation and in vivo effects

Binuclear copper ions at the active site determine the catalysis of tyrosinase (TYR)¹ whose activity can be inhibited by copper's chelation with other compounds. In this study, tilapia (Oreochromis niloticus) skin was used to generate TYR-inhibitory peptides after being treated by different enzymes and 4 h-Alcaline protease hydrolysate exhibited the highest TYR inhibition and copper chelation. Immobilized metal affinity chromatography was used for purifying copper chelating peptides, among which PFRMY (IC₅₀: 0.43 ± 0.08 mg/mL) and RGFTGM (IC₅₀: 1.61 ± 0.04 mg/mL) exhibited the highest TYR-inhibitory capacity and the lowest docking energy. Both two peptides inhibited TYR in a mixed manner and interacted with key residues binding to copper ions within TYR mainly by hydrogen bonds and hydrophobic forces, while PFRMY had a more compact and stable conjugation with TYR. Zebrafish assay revealed that PFRMY reduced not only melanin synthesis but in vivo TYR activity.

Tyrosinase inhibitory mechanism and anti-browning properties of novel kojic acid derivatives bearing aromatic aldehyde moiety

Kojic acid-aromatic aldehydes 6a-6m were synthesized and screened for their anti-tyrosinase activities. These compounds showed potently anti-tyrosinase activity with IC₅₀ values in the range of 5.32 ± 0.23 to 77.89 ± 3.36 μM compared with kojic acid (IC₅₀ = 48.05 ± 3.28 μM). Thereinto, compound 6j with 3-fluorine and 4-aldehyde substitutions showed the most potent anti-tyrosinase activity (IC₅₀ = 5.32 ± 0.23 μM). Enzyme kinetic study revealed that 6j is a noncompetitive tyrosinase inhibitor (Ki = 2.73 μM). The action mechanism of 6j was evaluated by fluorescence spectrum quenching, molecular docking, ¹H NMR titration, etc. The anti-browning assay showed that 6j could delay the enzymatic browning of fresh-cut apples. Besides, the cell viability assay proved that 6j had a good safety profile as an anti-browning agent. Hence, these results identify a new class of anti-tyrosinase and anti-browning agents for further investigation in the food industry.

Enzymatic browning in apple products and its inhibition treatments: A comprehensive review

Apple (Malus domestica) is widely consumed by consumers from various regions. It contains a high number of phenolic compounds (majorly hydroxybenzoic acids, hydroxycinnamic acids, flavanols, flavonols, dihydrochalcones, and anthocyanins) and antioxidant activity, which are beneficial for human health. The trends on healthy and fresh food have driven the food industry to produce minimally processed apple, such as fresh-cut, puree, juice, and so on without degrading the quality of products. Enzymatic browning is one of the problems found in minimally processed apple as it causes the undesirable dark color as well as the degradation of phenolics and antioxidant activity, which then reduces the health benefits of apple. Proper inhibition is needed to maintain the quality of minimally processed apple with minimal changes in sensory properties. This review summarizes the inhibition of enzymatic browning of apple products based on recent studies using the conventional and nonconventional processing, as well as using synthetic and natural antibrowning agents. Nonconventional processing and the use of natural antibrowning agents can be used as promising treatments to prevent enzymatic browning in minimally processed apple products. Combination of 2-3 treatments can improve the effective inhibition of enzymatic browning. Further studies, such on as other potential natural antibrowning agents and their mechanisms of action, should be conducted.

Hydroxypyrone derivatives in drug discovery: from chelation therapy to rational design of metalloenzyme inhibitors

The versatile structural motif of hydroxypyrone is found in natural products and can be easily converted into hydroxypyridone and hydroxythiopyridone analogues. The favourable toxicity profile and ease of functionalization to access a vast library of compounds make them an ideal structural scaffold for drug design and discovery. This versatile scaffold possesses excellent metal chelating properties that can be exploited for chelation therapy in clinics. Deferiprone [1,2-dimethyl-3-hydroxy-4(1H)-one] was the first orally active chelator to treat iron overload in thalassemia major. Metal complexes of hydroxy-(thio)pyr(id)ones have been investigated as magnetic resonance imaging contrast agents, and anticancer and antidiabetic agents. In recent years, this compound class has demonstrated potential in discovering and developing metalloenzyme inhibitors. This review article summarizes recent literature on hydroxy-(thio)pyr(id)ones as inhibitors for metalloenzymes such as histone deacetylases, tyrosinase and metallo-β-lactamase. Different approaches to the design of hydroxy-(thio)pyr(id)ones and their biological properties against selected metalloenzymes are discussed.

Design and synthesis of novel stilbene-hydroxypyridinone hybrids as tyrosinase inhibitors and their application in the anti-browning of freshly-cut apples

In order to develop the tyrosinase inhibitors with potential application in food industry, a series of stilbene-hydroxypyridinone hybrids were prepared. Among these compounds, 1h was found to possess the most potent tyrosinase inhibitory effect on both monophenolase and diphenolase activities, with IC₅₀ values of 2.72 μM and 15.86 μM, respectively. The inhibitory effect of 1h on monophenolase activity was 4.6 times that of kojic acid. An inhibition kinetic assay indicated that 1h was a mixed-type and reversible inhibitor. The copper-binding and reducing ability assays, molecular docking study, intrinsic and ANS-binding fluorescence assays indicated that copper coordination and reduction is likely to be the causative mechanism for 1h-induced inhibition on tyrosinase. The results of color measurement and browning index determination indicated that treatment with 1h retarded effectively the browning of freshly-cut apples during their storage. Meanwhile, PPO and POD activities in apple slices were found to be effectively inhibited.

Antioxidant and anti-tyrosinase activity of a novel stilbene analogue as an anti-browning agent

BACKGROUND

Tyrosinase inhibitors find potential application in food, cosmetic and medicinal products, but most of the identified tyrosinase inhibitors are not suitable for practical use because of safety regulations or other problems. For the purpose of development of novel tyrosinase inhibitors that meet the requirement for practical application, a novel stilbene analogue (SA) was designed.

RESULTS

SA was found to possess a potent inhibitory effect against both mono- and diphenolase activities of mushroom tyrosinase, with IC₅₀ values of 1.56 and 7.15 μmol L^-1 , respectively. Compared with a natural tyrosinase inhibitor - kojic acid - the anti-tyrosinase effect of SA was significantly improved. Analysis of inhibition kinetics indicated that SA was a reversible and competitive-noncompetitive mixed-type inhibitor. SA was also found to possess more potent antioxidant activities (DPPH, superoxide anion radical and hydroxyl radical scavenging ability) than those of kojic acid. Cell viability studies revealed that SA was non-toxic to two cell lines. Furthermore, an anti-browning test demonstrated that SA effectively delayed the blackening of shrimp.

CONCLUSION

SA has potential as an anti-browning agent in foods. © 2021 Society of Chemical Industry.

Azole inhibitors of mushroom and human tyrosinases: Current advances and prospects of drug development for melanogenic dermatological disorders

Azoles are a famous and promising class of drugs for treatment of a range of ailments especially fungal infections. A wide variety of azole derivatives are also known to exhibit tyrosinase inhibition, some of which possess promising activity with potential for treatment of dermatological disorders such as post-inflammatory hyperpigmentation, nevus, flecks, melasma, and melanoma. Recently, thiazolyl-resorcinol derivatives have demonstrated potent human tyrosinase inhibition with a safe and effective therapeutic profile for treatment of skin hyperpigmentation in humans, which are currently under clinical trials. If approved these derivatives would be the first azole drugs to be used for treatment of skin hyperpigmentation. Although the scientific literature has been witnessing general reviews on tyrosinase inhibitors to date, there is none that specifically and comprehensively discusses azole inhibitors of tyrosinase. Appreciating such potential of azoles, this focused review highlights a wide range of their derivatives with promising mushroom and human tyrosinase inhibitory activities and clinical potential for treatment of melanogenic dermatological disorders. Presently, these disorders have been treated with kojic acid, hydroquinone and other drugs, the design and development of which are based on their ability to inhibit mushroom tyrosinase. The active sites of mushroom and human tyrosinases carry structural differences which affect substrate or inhibitor binding. For this reason, kojic acid and other drugs pose efficacy and safety issues since they were originally developed using mushroom tyrosinase and have been clinically used on human tyrosinase. Design and development of tyrosinase inhibitors should be based on human tyrosinase, however, there are challenges in obtaining the human enzyme and understanding its structure and function. The review discusses these challenges that encompass structural and functional differences between mushroom and human tyrosinases and the manner in which they are inhibited. The review also gauges promising azole derivatives with potential for development of drugs against skin hyperpigmentation by analyzing and comparing their tyrosinase inhibitory activities against mushroom and human tyrosinases, computational data, and clinical profile where available. It aims to lay groundwork for development of new azole drugs for treatment of skin hyperpigmentation, melanoma, and related dermatological disorders.

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.

Chemical inhibition of polyphenol oxidase and cut surface browning of fresh-cut apples

Fresh-cut apples, which offer consumers health benefits and convenience, have become popular in recent years. One of the main challenges for processing fresh-cut apples is rapid development of cut surface browning, immediately after fruits are cut. Browning, a physiological response that impacts organoleptic properties and deters consumer purchase of fresh-cut fresh produce, is mainly a result of enzymatic reaction of phenolic compounds with oxygen catalyzed by polyphenol oxidase (PPO), a decapper enzyme. Many antibrowning agents have been developed and evaluated to inhibit PPO activities by using reducing agents (antioxidants), chelating agents, acidulants, etc. The present manuscript reviews the diverse characteristics of PPO (such as optimum pH and temperature, and molecular weight) in apples reported in the literature and the enzyme's latency, multiplicity and copper states in the active site. It also summarizes the latest development in the investigation and formulations of antibrowning compounds, and discusses future research needs. This review should stimulate further research to discover more effective, low cost, and natural antibrowning compounds to meet the demand of consumers as well as the food industry for clean label and long shelf-life of fresh-cut apples.

Hydroxypyridinone-Based Metal Chelators towards Ecotoxicity: Remediation and Biological Mechanisms

Hydroxypyridinones (HPs) are recognized as excellent chemical tools for engineering a diversity of metal chelating agents, with high affinity for hard metal ions, exhibiting a broad range of activities and applications, namely in medical, biological and environmental contexts. They are easily made and functionalizable towards the tuning of their pharmacokinetic properties or the improving of their metal complex thermodynamic stabilities. In this review, an analysis of the recently published works on hydroxypyridinone-based ligands, that have been mostly addressed for environmental applications, namely for remediation of hard metal ion ecotoxicity in living beings and other biological matrices is carried out. In particular, herein the most recent developments in the design of new chelating systems, from bidentate mono-HP to polydentate multi-HP derivatives, with a structural diversity of soluble or solid-supported backbones are outlined. Along with the ligand design, an analysis of the relationship between their structures and activities is presented and discussed, namely associated with the metal affinity and the thermodynamic stability of the corresponding metal complexes.

Kojic Acid Showed Consistent Inhibitory Activity on Tyrosinase from Mushroom and in Cultured B16F10 Cells Compared with Arbutins

Kojic acid, β-arbutin, α-arbutin, and deoxyarbutin have been reported as tyrosinase inhibitors in many articles, but some contradictions exist in their differing results. In order to provide some explanations for these contradictions and to find the most suitable compound as a positive control for screening potential tyrosinase inhibitors, the activity and inhibition type of the aforementioned compounds on monophenolase and diphenolase of mushroom tyrosinase (MTYR) were studied. Their effects on B16F10 cells melanin content, tyrosinase (BTYR) activity, and cell viability were also exposed. Results indicated that α-arbutin competitively inhibited monophenolase activity, whereas they uncompetitively activated diphenolase activity of MTYR. β-arbutin noncompetitively and competitively inhibited monophenolase activity at high molarity (4000 µM) and moderate molarity (250–1000 µM) respectively, whereas it activated the diphenolase activity of MTYR. Deoxyarbutin competitively inhibited diphenolase activity, but could not inhibit monophenolase activity and only extended the lag time. Kojic acid competitively inhibited monophenolase activity and competitive–noncompetitive mixed-type inhibited diphenolase activity of MTYR. In a cellular experiment, deoxyarbutin effectively inhibited BTYR activity and reduced melanin content, but it also potently decreased cell viability. α-arbutin and β-arbutin dose-dependently inhibited BTYR activity, reduced melanin content, and increased cell viability. Kojic acid did not affect cell viability at 43.8–700 µM, but inhibited BTYR activity and reduced melanin content in a dose-dependent manner. Therefore, kojic acid was considered as the most suitable positive control among these four compounds, because it could inhibit both monophenolase and diphenolase activity of MTYR and reduce intercellular melanin content by inhibiting BTYR activity without cytotoxicity. Some explanations for the contradictions in the reported articles were provided.

Anti-tyrosinase, antioxidant and antibacterial activities of gallic acid-benzylidenehydrazine hybrids and their application in preservation of fresh-cut apples and shrimps

A series of gallic acid-benzylidenehydrazine hybrids were synthesized and evaluated for their tyrosinase inhibitory activity. Thereinto, compounds 5d and 5f potently inhibited tyrosinase with IC₅₀ of 15.3 and 3.3 μM, compared to kojic acid (44.4 μM). The inhibition mechanism suggested that 5d and 5f not only chelated with Cu²⁺, but also reduced Cu²⁺ to Cu¹⁺ in the tyrosinase active site. Additionally, 5d and 5f exhibited strong DPPH scavenging and antibacterial activities against Vibrio parahaemolyticu and Staphylococcus aureus, which can be attributed to the function of gallic acid and hydrazone moiety. These compounds also exhibited capacity to preserve fresh-cut apples and shrimps. Finally, 5d and 5f exhibited low cytotoxic activity in a human cell line (HEK293). Therefore, these compounds possess anti-tyrosinase, antioxidant, and antibacterial activities, and can be used in the development of novel food preservatives.

Inhibitory mechanism of two homoisoflavonoids from Ophiopogon japonicus on tyrosinase activity: insight from spectroscopic analysis and molecular docking

The inhibition mechanism of two homoisoflavonoids from Ophiopogon japonicus including methylophiopogonanone A (MO-A) and methylophiopogonanone B (MO-B) on tyrosinase (Tyr) was studied by multiple spectroscopic techniques and molecular docking. The results showed that the two homoisoflavonoids both inhibited Tyr activity via a reversible mixed-inhibition, with a half inhibitory concentration (IC50) of (10.87 ± 0.25) × 10-5 and (18.76 ± 0.14) × 10-5 mol L-1, respectively. The fluorescence quenching and secondary structure change of Tyr caused by MO-A and B are mainly driven by hydrophobic interaction and hydrogen bonding. Molecular docking analysis indicated that phenylmalandioxin in MO-A and methoxy in MO-B could coordinate with a Cu ion in the active center of Tyr, and interacted with amino acid Glu322 to form hydrogen bonding, occupying the catalytic center to block the entry of the substrate and consequently inhibit Tyr activity. This study may provide new perspectives on the inhibition mechanism of MO-A and MO-B on Tyr and serve a scientific basis for screening effective Tyr inhibitors.

Novel kojic acid derivatives with anti-inflammatory effects from Aspergillus versicolor

Two novel kojic acid derivatives, kojicones A and B (1 and 2), along with the precursors kojic acid (3) and (2R,4R)-4-hydroxy-5-methoxy-2,4-dimethyl-2- [(2R)-2-methylbutyryloxy]-5-cyclohexen-1,3-dione (4), were isolated from a fungal strain Aspergillus versicolor. Their structures and absolute configurations were accurately confirmed by HRESIMS data, NMR analysis, and electronic circular dichroism (ECD) calculations. Kojicones A and B were the first examples of kojic acid adducts with cyclohexen-1,3-dione possessing unprecedented tricycle skeletons. Compounds 1-3 were found to have inhibition on the NO production of murine RAW 264.7 cells. They can also reduce the mRNA expression of four cytokines (IL-6, IL-1β, TNF-α, and iNOS) and promote the expression of IL-4 at 20 μM. Moreover, kojic acid (3) could treat the DSS (dextran sulfate sodium)-induced colitis on mice with the effectiveness similar to that of the positive control. The results suggested that kojic acid and its derivatives could be a promising anti-inflammatory source for the medicinal and cosmetic industry.

Recent advances in the design and discovery of synthetic tyrosinase inhibitors

Tyrosinase is a copper-containing metalloenzyme that is responsible for the rate-limiting catalytic step in the melanin biosynthesis and enzymatic browning. As a promising target, tyrosinase inhibitors can be used as skin whitening agents and food preservatives, thus having broad potential in the fields of food, cosmetics, agriculture and medicine. From 2015 to 2020, numerous synthetic inhibitors of tyrosinase have been developed to overcome the challenges of low efficacy and side effects. This review summarizes the enzyme structure and biological functions of tyrosinase and demonstrates the recent advances of synthetic tyrosinase inhibitors from the perspective of medicinal chemistry, providing a better understanding of the catalytic mechanisms and more effective tyrosinase inhibitors.

Efficient tyrosinase nano-inhibitor based on carbon dots behaving as a gathering of hydrophobic cores and key chemical group

Small organic molecules have been extensively applied to achieve enzymatic inhibition. Although numerous efforts have been made to deliver efficient inhibitors, small inhibitors applications are hindered by many drawbacks. Moreover, reporters comprising nanoparticle inhibitory activity against enzymes are very scarce in the literature. In this scenario, carbon nanodots (CDs) emerge as promising candidates for efficient enzyme inhibition due to their unique properties. Here, CDs specific molecular characteristics (core composition and chemical surface groups) have been investigated to produce a more potent enzyme inhibition. Mushroom tyrosinase (mTyr) has been adopted as an enzymatic prototype. The CDs revealed a high affinity to mTyr (Ka ≈ 10⁶ M^-1), mainly through hydrophobic forces and followed by slight mTyr structural alteration. CDs competitively inhibit mTyr, with low inhibition constant (K_I = 517.7 ± 17.0 nM), which is up 70 fold smaller then the commercial inhibitor (kojic acid) and the starch nanoparticles previously reported. The results expose that the CDs act as a hydrophobic agglomerate with carboxyl groups on its surface, mimicking characteristics found on small molecule inhibitors (but with superior performance). All these results highlight the CD excellent potential as an efficient low toxic Tyr inhibitor, opening the prospect of using these nanoparticles in the cosmetic and food industries.

Design, synthesis, molecular modeling, and biological evaluation of novel kojic acid derivatives containing bioactive heterocycle moiety as inhibitors of tyrosinase and antibrowning agents

Tyrosinase plays an important role in melanin biosynthesis and enzymatic browning of fresh-cut fruit and vegetables. To discover potent tyrosinase inhibitors and antibrowning agents, a series of novel kojic acid derivatives containing bioactive heterocycle moiety (4a-4l) were designed and synthesized. Thereinto, 4d displayed the most potent tyrosinase inhibitory activity with IC₅₀ of 3.23 ± 0.26 μM and behaved as a competitive inhibitor with a K_i of 1.96 μM, compared to kojic acid (IC₅₀ = 32.23 ± 2.01 μM). Besides, copper-chelating assay, fluorescence spectrum quenching experiment, ANS-binding fluorescence quenching analysis, and molecular modeling studies indicated that 4d may inhibit tyrosinase activity by chelating with copper ions in the active site of tyrosinase. Furthermore, 4d exhibited low cytotoxic activity and significant antibrowning effects.This study suggests that these compounds may serve as lead molecules for developing novel tyrosinase inhibitors and antibrowning agents.

An overview of hydroxypyranone and hydroxypyridinone as privileged scaffolds for novel drug discovery

Hydroxypyranone and hydroxypyridinone are important oxygen-containing or nitrogen-containing heterocyclic nucleus and attracted increasing attention in medicinal chemistry and drug discovery over the past decade. Previous literature reports revealed that hydroxypyranone and hydroxypyridinone derivatives exhibit a wide range of pharmacological activities such as antibacterial, antifungal, antiviral, anticancer, anti-inflammatory, antioxidant, anticonvulsant, and anti-diabetic activities. In this review, we systematically summarized the literature reported biological activities of hydroxypyranone and hydroxypyridinone derivatives. In particular, we focus on their biological activity, structure-activity relationship (SAR), mechanism of action, and interaction mechanisms with the target. The collected information is expected to provide rational guidance for the development of clinically useful agents from these pharmacophores.

Functionality study of chalcone-hydroxypyridinone hybrids as tyrosinase inhibitors and influence on anti-tyrosinase activity

In an attempt to synthesise new tyrosinase inhibitors, we designed and synthesised a series of chalcone-hydroxypyridinone hybrids as potential tyrosinase inhibitors adopting strategic modifications of kojic acid. All the newly synthesised compounds were characterised by NMR and mass spectrometry. Initial screening of the target compounds demonstrated that compounds 1a, 1d, and 1n had relatively strong inhibitory activities against tyrosinase monophenolase, with IC₅₀ values of 3.07 ± 0.85, 2.25 ± 0.8 and 2.75 ± 1.19 μM, respectively. The inhibitory activity against monophenolase was 6- to 8-fold higher than that of kojic acid. Compounds 1a, 1d, and 1n also showed inhibition of diphenolase, with IC₅₀ values of 17.05 ± 0.07, 11.70 ± 0.03 and 19.3 ± 0.28 μM, respectively. The inhibition kinetics of diphenolase indicates that compounds 1a and 1d induce reversible inhibition on tyrosinase. Finally, we found that copper coordination should be one of the important inhibitory mechanism of these compounds in tyrosinase.

Separation, identification, and molecular docking of tyrosinase inhibitory peptides from the hydrolysates of defatted walnut (Juglans regia L.) meal

Defatted walnut meal protein was hydrolyzed using alcalase to yield tyrosinase inhibitory peptides. After separation by ultrafiltration and Sephadex G-25, the fraction with the highest tyrosinase inhibitory activity was identified using liquid chromatography-tandem mass spectrometry and 606 peptides were obtained. Then, molecular docking was used to screen for tyrosinase inhibitory peptides and to clarify the theoretical interaction mechanism between the peptides and tyrosinase. A peptide with the sequence Phe-Pro-Tyr (FPY, MW: 425.2 Da) was identified and the synthesized peptide inhibited tyrosine monophenolase and diphenolase with IC₅₀ values of 1.11 ± 0.05 and 3.22 ± 0.09 mM, respectively. The inhibition of tyrosinase by FPY was competitive and reversible. Good stability of FPY toward digestion was observed in an in vitro gastrointestinal digestion simulation experiment. These results indicated that FPY can be used as a potential tyrosinase inhibitor in the food, medicine, and cosmetics industries.

Structure Analysis and Study of Biological Activities of Condensed Tannins from Bruguiera gymnorhiza (L.) Lam and Their Effect on Fresh-Cut Lotus Roots

Bruguiera gymnorhiza (L.) Lam is a mangrove plant that spread in many parts of the world. Though mangrove plant polyphenols have been reported to exhibit many biological activities, little is known about mangrove plant tannins. To explore the application value of tannins from B. gymnorhiza, analyses on the structure and biological activity of condensed tannins (CTs) from Bruguiera gymnorhiza (L.) Lam were carried out. The results from ¹³C nuclear magnetic resonance (¹³C-NMR) and reversed-phase, high-performance liquid chromatography (RP-HPLC) showed that the CTs were dominated by procyanidins, with a small quantity of prodelphinidins and propelargonidins; and that the monomeric constituents of B. gymnorhiza tannins were catechin/epicatechin, gallocatechin/epigallocatechin and afzelechin/epiafzelechin. The CTs were reversible and mixed competitive inhibitors of tyrosinase and the 50% inhibiting concentration (IC₅₀) was estimated to be 123.90 ± 0.140 μg/mL. The antioxidant activities of CTs from B. gymnorhiza leaves were evaluated, the IC₅₀ for 2,2-diphenyl-1-picrylhydrazyl (DPPH) and 2,2'-azino-bis (3-ethylbenzo-thiazoline-6-sulfonic acid diammonium salt) (ABTS) scavenging activities were 88.81 ± 0.135 and 105.03 ± 0.130 μg/mL, respectively, and the ferric ion reducing antioxidant power (FRAP) value was 1052.27 ± 4.17 mgAAE/g. In addition, the results from fresh-keeping assays on fresh-cut lotus root reveal that CTs from B. gymnorhiza had excellent effects on inhibiting the activities of polyphenol oxidase (PPO) and peroxidase (POD), protecting fresh-cut lotus root from the oxidation of total phenolics and malondialdehyde (MDA) content and slowing the increase in total phenol content (TPC) at 4 °C during the whole storage period. Therefore, CTs showed good effects against the browning of fresh-cut lotus root. Together, these results suggested that B. gymnorhiza CTs are promising antibrowning agents for fresh-cut fruits.

Thiosemicarbazide binds with the dicopper center in the competitive inhibition of mushroom tyrosinase enzyme: Synthesis and molecular modeling of theophylline analogues

Theophylline is long known for its anti-ageing and anti-oxidative properties. Moreover, Tyrosinase is a crucial enzyme that regulates the melanin synthetic pathway, which is involved in various physiological metabolic processes including aging. The current paper describes the synthesis of various heterocyclic systems coupled with theophylline moiety along with their tyrosinase inhibition activity in view to identify the potent nucleus. Around 19 compounds were synthesized and screened for enzyme inhibition. Based on the current study, it is suggested that compound 18 having thiosemicarbazide has strong enzyme inhibition potential. The enzyme kinetics and docking studies provide important insights into how the compound interacts with the mushroom tyrosinase active site. The work will provide clue to developing new, potent tyrosinase inhibitors for drug development.

A systematic review of synthetic tyrosinase inhibitors and their structure-activity relationship

Tyrosinase is a copper-containing oxidation enzyme, which is responsible for the production of melanin. This enzyme is widely distributed in microorganisms, animals and plants, and plays an essential role in undesirable browning of fruits and vegetables, antibiotic resistance, skin pigment formation, sclerotization of cuticle, neurodegeneration, etc. Hence, it has been recognized as a therapeutic target for the development of antibrowning agents, antibacterial agents, skin-whitening agents, insecticides, and other therapeutic agents. With great potential application in food, agricultural, cosmetic and pharmaceutical industries, a large number of synthetic tyrosinase inhibitors have been widely reported in recent years. In this review, we systematically summarized the advances of synthetic tyrosinase inhibitors in the literatures, including their inhibitory activity, cytotoxicity, structure-activity relationship (SAR), inhibition kinetics, and interaction mechanisms with the enzyme. The collected information is expected to provide a rational guidance and effective strategy to develop novel, potent and safe tyrosinase inhibitors for better practical applications in the future.