Inhibitory mechanisms of glabridin on tyrosinase

Pleiotropic Potential of Evernia prunastri Extracts and Their Main Compounds Evernic Acid and Atranorin: In Vitro and In Silico Studies

Evernia prunastri is a lichen widely distributed in the Northern Hemisphere. Its biological properties still need to be discovered. Therefore, our paper focuses on studies of E. prunastri extracts, including its main metabolites evernic acid (EA) or atranorin (ATR). Phytochemical profiles using chromatographic analysis were confirmed. The antioxidant activity was evaluated using in vitro chemical tests and in vitro enzymatic cells-free tests, namely superoxide dismutase (SOD), glutathione peroxidase (GPx), glutathione reductase (GR), and catalase (CAT). The anti-inflammatory potential using cyclooxygenase-2 (COX-2) and hyaluronidase were determined. The neuroprotective potential using acetylcholinesterase, (AChE), butyrylcholinesterase (BChE), and tyrosinase (Tyr) was estimated. The hypoglycemic activity was also confirmed (α-glucosidase). Principal component analysis was performed to determine the relationship between the biological activity of extracts. The inhibitory effect of EA and ATR on COX-2 AChE, BChE, Tyr, and α-glucosidase was evaluated using molecular docking techniques and confirmed for EA and ATR (besides α-glucosidase). The penetration of EA and ATR from extracts through the blood-brain barrier was confirmed using the parallel artificial membrane permeability assay blood-brain barrier test. In conclusion, depending on chemical surroundings and the concentration, the E. prunastri extracts, EA or ATR, showed attractive pleiotropic properties, which should be further investigated.

Natural tyrosinase enzyme inhibitors: A path from melanin to melanoma and its reported pharmacological activities

The skin containing melanin pigment acts as a protective barrier and counteracts the UVR and other environmental stressors to maintain or restore disrupted cutaneous homeostasis. The production of melanin pigment is dependent on tyrosine levels. L-tyrosine and L-dihydroxyphenylalanine (L-DOPA) can serve both as a substrates and intermediates of melanin synthetic pathway and as inducers and positive regulators of melanogenesis. The biosynthesis of melanin is stimulated upon exposure to UVR, which can also stimulate local production of hormonal factors, which can stimulate melanoma development by altering the chemical properties of eu- and pheomelanin. The process of melanogenesis can be altered by several pathways. One involves activation of POMC, with the production of POMC peptides including MSH and ACTH, which increase intracellular cAMP levels, which activates the MITF, and helps to stimulate tyrosinase (TYR) expression and activity. Defects in OCA1 to 4 affects melanogenic activity via posttranslational modifications resulting in proteasomal degradation and reducing pigmentation. Further, altering, the MITF factor, helps to regulate the expression of MRGE in melanoma, and helps to increase the TYR glycosylation in ER. CRH stimulates POMC peptides that regulate melanogenesis and also by itself can stimulate melanogenesis. The POMC, P53, ACTH, MSH, MC1R, MITF, and 6-BH4 are found to be important regulators for pigmentation. Melanogenesis can affect melanoma behaviour and inhibit immune responses. Therefore, we reviewed natural products that would alter melanin production. Our special focus was on targeting melanin synthesis and TYR enzyme activity to inhibit melanogenesis as an adjuvant therapy of melanotic melanoma. Furthermore, this review also outlines the current updated pharmacological studies targeting the TYR enzyme from natural sources and its consequential effects on melanin production.

A ratiometric fluorescent probe for simultaneous detection of L-ascorbic acid and alkaline phosphatase activity based on red carbon dots/polydopamine nanocomposite

Herein, efficient red carbon dots (R-CDs) were synthesized by one-step hydrothermal treatment of N-(4-amino phenyl) acetamide and (2,3-difluoro phenyl) boronic acid. The optimal emission peak of R-CDs was at 602 nm (under 520 nm excitation) and the absolute fluorescence quantum yield of R-CDs was 12.9%. Polydopamine, which was formed by the self-polymerization and cyclization of dopamine in alkaline condition, emitted characteristic fluorescence with peak position of 517 nm (under 420 nm excitation) and affected the fluorescence intensity of R-CDs through inner filter effect. L-Ascorbic acid (AA), which was the hydrolysis product of L-ascorbic acid-2-phosphate trisodium salt under the catalytic reaction of alkaline phosphatase (ALP), effectively prevented the polymerization of dopamine. Combined with the ALP-mediated AA production and the AA-mediated polydopamine generation, the ratiometric fluorescence signal of polydopamine with R-CDs was correlated closely with the concentration of both AA and ALP. Under optimal conditions, the detection limits of AA and ALP were 0.28 μM during linear range of 0.5-30 μM and 0.044 U/L with linear range of 0.05-8 U/L, respectively. This ratiometric fluorescence detection platform can efficiently shield the background interference of sophisticated samples by introducing a self-calibration as reference signal in a multi-excitation mode, which can detect AA and ALP in human serum samples with satisfactory results. Such R-CDs/polydopamine nanocomposite provides a steadfast quantitative information and makes R-CDs be excellent candidate for biosensors via combining target recognition strategy.

Novel kojic acid-1,2,4-triazine hybrids as anti-tyrosinase agents: Synthesis, biological evaluation, mode of action, and anti-browning studies

A class of new kojic acid hybrids (7a-7o) bearing a 1,2,4-triazine moiety were prepared, and their inhibitory activities and mechanism on tyrosinase were investigated. All derivatives showed good to excellent anti-tyrosinase activity with IC₅₀ values ranging from 0.34 ± 0.06 μM to 8.44 ± 0.73 μM. In kinetic study, compound 7m was a mixed-type inhibitor with K_i and K_is of 0.73 and 1.27 μM, respectively. The interaction mechanism toward tyrosinase of compound 7m was further elaborated in combination with molecular docking and various spectral techniques. The results showed that compound 7m could change the secondary structure of tyrosinase to reduce its catalytic activity. Anti-browning assays demonstrated that 7m inhibited the browning of bananas effectively during storage. What's more, 7m was found to have low cytotoxicity in vitro. In conclusion, compound 7m has the potential to be applied as an anti-browning agent.

Clinical Development and Evaluation of a Multi-Component Dissolving Microneedle Patch for Skin Pigmentation Disorders

Excessive melanin deposition in the skin leads to various skin pigmentation diseases, such as chloasma and age spots. The deposition is induced by several factors, including tyrosinase activities and ultraviolet-induced oxidative stress. Herein, we propose a multi-component, multi-pathway drug combination, with glabridin, 3-O-ethyl-L-ascorbic acid, and tranexamic acid employed as, respectively, a tyrosinase inhibitor, an antioxidant, and a melanin transmission inhibitor. Considering the poor skin permeability associated with topical application, dissolving microneedles (MNs) prepared with hyaluronic acid/poly(vinyl alcohol)/poly(vinylpyrrolidone) were developed to load the drug combination. The drug-loaded microneedles (DMNs) presented outstanding skin insertion, dissolution, and drug delivery properties. In vitro experiments confirmed that DMNs loaded with active ingredients had significant antioxidant and inhibitory effects on tyrosinase activity. Furthermore, the production of melanin both in melanoma cells (B16-F10) and in zebrafish was directly reduced after using DMNs. Clinical studies demonstrated the DMNs' safety and showed that they have the ability to effectively reduce chloasma and age spots. This study indicated that a complex DMN based on a multifunctional combination is a valuable depigmentation product worthy of clinical application.

Xanthine oxidase inhibitory kinetics and mechanism of ellagic acid: In vitro, in silico and in vivo studies

Ellagic acid (EA), which is widely distributed in many foods, has been found to possess inhibitory activity against xanthine oxidase (XO). However, there is ongoing debate about the difference in XO inhibitory activity between EA and allopurinol. Additionally, the inhibitory kinetics and mechanism of EA on XO are still unclear. Herein, the authors systematically studied the inhibitory effects of EA on XO. The authors' findings showed that EA is a reversible inhibitor with mixed-type inhibition, and its inhibitory activity is weaker than allopurinol. Fluorescence quenching experiments suggested that the generation of EA-XO complex was exothermic and spontaneous. In silico analysis further confirmed that EA entered the XO catalytic centre. Furthermore, the authors verified the anti-hyperuricemia effect of EA in vivo. This study elucidates the inhibition kinetics and mechanism of EA on XO, and lays a theoretical foundation for the further development of drugs and functional foods containing EA for the treatment of hyperuricemia.

Naturally-Occurring Tyrosinase Inhibitors Classified by Enzyme Kinetics and Copper Chelation

Currently, there are three major assaying methods used to validate in vitro whitening activity from natural products: methods using mushroom tyrosinase, human tyrosinase, and dopachrome tautomerase (or tyrosinase-related protein-2, TRP-2). Whitening agent development consists of two ways, melanin synthesis inhibition in melanocytes and downregulation of melanocyte stimulation. For melanin levels, the melanocyte cell line has been used to examine melanin synthesis with the expression levels of TRP-1 and TRP-2. The proliferation of epidermal surfaced cells and melanocytes is stimulated by cellular signaling receptors, factors, or mediators including endothelin-1, α-melanocyte-stimulating hormone, nitric oxide, histamine, paired box 3, microphthalmia-associated transcription factor, pyrimidine dimer, ceramide, stem cell factors, melanocortin-1 receptor, and cAMP. In addition, the promoter region of melanin synthetic genes including tyrosinase is upregulated by melanocyte-specific transcription factors. Thus, the inhibition of growth and melanin synthesis in gene expression levels represents a whitening research method that serves as an alternative to tyrosinase inhibition. Many researchers have recently presented the bioactivity-guided fractionation, discovery, purification, and identification of whitening agents. Melanogenesis inhibition can be obtained using three different methods: tyrosinase inhibition, copper chelation, and melanin-related protein downregulation. There are currently four different types of inhibitors characterized based on their enzyme inhibition mechanisms: competitive, uncompetitive, competitive/uncompetitive mixed-type, and noncompetitive inhibitors. Reversible inhibitor types act as suicide substrates, where traditional inhibitors are classified as inactivators and reversible inhibitors based on the molecule-recognizing properties of the enzyme. In a minor role, transcription factors can also be downregulated by inhibitors. Currently, the active site copper iron-binding inhibitors such as kojic acid and chalcone exhibit tyrosinase inhibitory activity. Because the tyrosinase catalysis site structure is important for the mechanism determination of tyrosinase inhibitors, understanding the enzyme recognition and inhibitory mechanism of inhibitors is essential for the new development of tyrosinase inhibitors. The present review intends to classify current natural products identified by means of enzyme kinetics and copper chelation to exhibit tyrosinase enzyme inhibition.

Recent discovery of tyrosinase inhibitors in traditional Chinese medicines and screening methods

ETHNOPHARMACOLOGICAL RELEVANCE

Tyrosinase, the key rate-limiting enzyme for melanogenesis, is one of the main targets for skin senescence and some pigmented skin diseases, such as albinism and melanoma. Tyrosinase inhibitors are capable of reducing melanin generation and deposition in the skin through blocking the reaction chain of formation. Thus, it has been used for anti-melanoma and showed the potential to be developed into novel skin whitening and spot removing products. With the trend of back-to-nature, natural tyrosinase inhibitors are receiving more and more attention. Traditional Chinese medicines (TCMs) as the promising source of novel chemotypes and pharmacophores, are huge treasures for the discovery of natural tyrosinase inhibitors characterized with green, safe, and highly efficient.

AIM OF THIS REVIEW

This review aims to provide a systematic overview of natural tyrosinase inhibitors and a detailed summary of the types of TCMs from which they originate. In addition, this paper also highlights the screening methods developed for exploring tyrosinase inhibitors in recent years, compares the advantages and disadvantages of various methods under the guidance of different screening principles, and predicts their applications in the future.

MATERIALS AND METHODS

Relevant literature have been obtained using the keywords "tyrosinase inhibitors", "traditional Chinese medicines", "whitening", and "screening" in scientific databases, such as "PubMed", "SciFinder", "Web of Science", "Elsevier", "China Knowledge Resource Integrated databases". Information was also collected from Chinese pharmacopoeia, Chinese herbal classics books, "Google Scholar", "Baidu Scholar", and other literature sources, etc. RESULTS: An overview about the tyrosinase inhibitors derived from TCMs since 2002 has been compiled via the above-mentioned sources. Up to now, 186 components, mainly belonging to flavonoids, lignans, terpenoids, Diels-Alder adducts, simple phenylpropanoids and stilbenes, from 61 kinds of TCMs have been reported to possess tyrosinase inhibitory activity, among which flavonoids are mainly focused on. Furthermore, on the basis of bioactive detection strategies, the screening methods for tyrosinase inhibitors have been classified into bioaffinity-based, intrinsic enzymatic-based, and computer-aided drug design (CADD). Precisely because screening approaches are essential for rapid identification of tyrosinase inhibitors from TCMs, the principles, advantages and disadvantages, and specific applications of each method are presented along with a comparison of applicability.

CONCLUSIONS

The summary of TCMs-derived inhibitors gives a clue on the discovery of candidates with the property to whiten the skin. Meanwhile, the outlook of developed screening methods provides technical references for the efficient exploration of safer and more effective tyrosinase inhibitors from TCMs.

Anti-Melanogenic Potential of Natural and Synthetic Substances: Application in Zebrafish Model

Melanogenesis is a biosynthetic pathway for the formation of the pigment melanin in human skin. A key enzyme in the process of pigmentation through melanin is tyrosinase, which catalyzes the first and only limiting step in melanogenesis. Since the discovery of its methanogenic properties, tyrosinase has been the focus of research related to the anti-melanogenesis. In addition to developing more effective and commercially safe inhibitors, more studies are required to better understand the mechanisms involved in the skin depigmentation process. However, in vivo assays are necessary to develop and validate new drugs or molecules for this purpose, and to accomplish this, zebrafish has been identified as a model organism for in vivo application. In addition, such model would allow tracking and studying the depigmenting activity of many bioactive compounds, important to genetics, medicinal chemistry and even the cosmetic industry. Studies have shown the similarity between human and zebrafish genomes, encouraging their use as a model to understand the mechanism of action of a tested compound. Interestingly, zebrafish skin shares many similarities with human skin, suggesting that this model organism is suitable for studying melanogenesis inhibitors. Accordingly, several bioactive compounds reported herein for this model are compared in terms of their molecular structure and possible mode of action in zebrafish embryos. In particular, this article described the main metabolites of Trichoderma fungi, in addition to substances from natural and synthetic sources.

In Vitro, In Vivo, and In Silico Analyses of Molecular Anti-Pigmentation Mechanisms of Selected Thai Rejuvenating Remedy and Bioactive Metabolites

Thai rejuvenating remedies are mixed herbal formulas promoting longevity. Due to the complexity, the biological activities of these remedies are minimal. Therefore, in this study, the authors evaluated the anti-pigmentation effect at the molecular level of the selected Thai rejuvenating remedy to fulfill the knowledge gap. First, the authors found that the selected remedy showed promising activity against the tyrosinase enzyme with an IC₅₀ value of 9.41 µg/mL. In the comparison, kojic acid (positive control) exhibited an IC₅₀ value of 3.92 µg/mL against the same enzyme. Later, the authors identified glabridin as a bioactive molecule against tyrosinase with an IC₅₀ value of 0.08 µg/mL. However, ethyl p-methoxycinnamate was the most abundant metabolite found in the remedy. The authors also found that the selected remedy and glabridin reduced the melanin content in the cell-based assay (B16F1) but not in the zebrafish larvae experiment. Finally, the authors conducted a computational investigation through molecular docking proposing a theoretical molecular interplay between glabridin, ethyl p-methoxycinnamate, and target proteins (tyrosinase and melanocortin-1 receptor, MC1R). Hence, in this study, the authors reported the molecular anti-pigmentation mechanism of the selected Thai rejuvenating remedy for the first time by combining the results from in silico, in vitro, and in vivo experiments.

Review on the Diverse Biological Effects of Glabridin

Glabridin is a prenylated isoflavan from the roots of Glycyrrhiza glabra Linne and has posed great impact on the areas of drug development and medicine, due to various biological properties such as anti-inflammation, anti-oxidation, anti-tumor, anti-microorganism, bone protection, cardiovascular protection, neuroprotection, hepatoprotection, anti-obesity, and anti-diabetes. Many signaling pathways, including NF-κB, MAPK, Wnt/β-catenin, ERα/SRC-1, PI3K/AKT, and AMPK, have been implicated in the regulatory activities of glabridin. Interestingly, glabridin has been considered as an inhibitor of tyrosinase, P-glycoprotein (P-gp), and CYP2E1 and an activator of peroxisome proliferator-activated receptor γ (PPARγ), although their molecular regulating mechanisms still need further investigation. However, poor water solubility and low bioavailability have greatly limited the clinical applications of glabridin. Hopefully, several effective strategies, such as nanoemulsions, microneedles, and smartPearls formulation, have been developed for improvement.

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.

Persistent and Efficient Multimodal Imaging for Tyrosinase Based on Two-Photon Excited Fluorescent and Room-Temperature Phosphorescent Probes

Tyrosinase is crucial to regulate the metabolism of phenol derivatives, playing an important role in the biosynthesis of melanin pigments, whereas an abnormal level of tyrosinase would lead to severe diseases. It is rather necessary to develop a sensitive and selective imaging tool to assess the level of tyrosinase in vivo. We thoroughly researched the luminous mechanism of the existing TPTYR probe and provided design strategies to improve its two-photon excited fluorescence properties. The designed probes benza2-TPTYR and product benza2-TPTYR-coumarin have large two-photon absorption cross sections at the NIR spectral region (41 GM/706 nm, 71 GM/852 nm), while benza2-TPTYR-coumarin possesses easily distinguishable spectrum in the visible region and a high fluorescence efficiency (Φ_F = 0.27). What is more, novel two-photon excited multimodal imaging based on the pure organic small molecule benza1-TPTYR-coumarin (61 GM/936 nm) is proposed first, simultaneously possessing strong instantaneous fluorescent (563.79 nm) and persistent room-temperature phosphorescent emissions (767.68 nm, 0.54 ms).

Regulation of Tyrosinase Enzyme Activity by Glutathione Peroxidase Mimics

Hydrogen peroxide plays a crucial role in the melanogenesis process by regulating the activity of the key melanin-forming enzyme tyrosinase, responsible for the browning of fruits, vegetables, and seafood. Therefore, a molecule with dual activities, both efficient tyrosinase inhibition and strong hydrogen peroxide degrading ability, may act as a promising antibrowning agent. Herein, we report highly efficient selone-based mushroom tyrosinase inhibitors 2 and 3 with remarkable glutathione peroxidase (GPx) enzyme-like activity. The presence of benzimidazole moiety enhances the tyrosinase inhibition efficiency of selone 2 (IC₅₀ = 0.4 μM) by almost 600 times higher than imidazole-based selone 1 (IC₅₀ = 238 μM). Interestingly, the addition of another aromatic ring to the benzimidazole moiety has led to the development of an efficient lipid-soluble tyrosinase inhibitor 3 (IC₅₀ = 2.4 μM). The selenium center and the -NH group of 2 and 3 are extremely crucial to exhibit high GPx-like activity and tyrosinase inhibition potency. The hydrophobic moiety of the inhibitors (2 and 3) further assists them in tightly binding at the active site of the enzyme and facilitates the C═Se group to strongly coordinate with the copper ions. Inhibitor 2 exhibited excellent antibrowning and polyphenol oxidase inhibition properties in banana and apple juice extracts.

Self-Assembled pH-Responsive Metal-Organic Frameworks for Enhancing the Encapsulation and Anti-Oxidation and Melanogenesis Inhibition Activities of Glabridin

Metal organic frameworks (MOFs) are formed by self-assembly of metal ions and organic ligands. A special type of MOF called ZIF-8, which is formed by self-assembly of zinc ions and 2-methylimidazole, shows excellent stability in aqueous solutions and disintegrates under acidic conditions. These properties make ZIF-8 a suitable carrier material for pH-stimulated drug delivery systems. Glabridin is an isoflavane compound that is widely present in the roots of licorice. Because of its outstanding skin whitening properties, glabridin is widely used as a whitener in the cosmetics industry. In this study, ZIF-8 was employed to encapsulate glabridin. Glabridin-loaded ZIF-8 was successfully prepared with a drug encapsulation efficiency of 98.67%. The prepared sample showed a fusiform or cruciate flower-like structure, and its size was about 3 μm. ZIF-8 enabled pH-controlled release of glabridin. Moreover, ZIF-8 encapsulation significantly enhanced the intracellular anti-oxidant activity and melanogenesis inhibitory activity of glabridin. This study provides a new approach that shows great potential to improve the biological application of glabridin.

In silico ADMET, molecular docking and molecular simulation-based study of glabridin’s natural and semisynthetic derivatives as potential tyrosinase inhibitors

Hyper-pigmentation conditions may develop due to erroneous melanogenesis cascade which leads to excess melanin production. Recently, inhibition of tyrosinase is the main focus of investigation as it majorly contributes to melanin production. This inhibition property can be exploited in medicine, agriculture, and in cosmetics. Present study aims to find a natural and safe alternative molecule as tyrosinase inhibitor. In this study, human tyrosinase enzyme was modelled due to unavailability of its crystal structure to look into the degree of efficacy of glabridin and its 15 derivatives as tyrosinase inhibitor. Docking was performed by Autodock Vina at the catalytic core enzyme. Glabridin effects on melanoma cell lines was also elucidated by analysing cytotoxicity and effect on melanin production. Computational ADME analysis was done by SwissADME. Molecular dynamic simulation was also performed to further evaluate the interaction profile of these molecules and kojic acid (positive inhibitor) with respect to apo protein. Notably, four derivatives 5′-formylglabridin, glabridin dimer, 5′-prenyl glabridin and R-glabridin exhibited better binding affinity than glabridin. Glabridin effectively inhibited melanin production in a dose dependent manner. Among these, 5′-formylglabridin displayed highest binding affinity with docking score − 9.2 kcal/mol. Molecular properties and bioactivity analysis by Molinspiration web server and by SwissADME also presented these molecules as potential drug candidates. The study explores the understanding for the development of suitable tyrosinase inhibitor/s for the prevention of hyperpigmentation. However, a detailed in vivo study is required for glabridin derivatives to suggest these molecules as anti-melanogenic compound.

Glabrol impurity exacerbates glabridin toxicity in zebrafish embryos by increasing myofibril disorganization

ETHNOPHARMACOLOGICAL RELEVANCE

Glabridin, extracted from Glycyrrhiza glabra L., is widely used for the treatment of hyperpigmentation because of its anti-inflammatory and antioxidant activities and its ability to inhibit melanin synthesis. This led to the strict regulation of its quality and safety. However, traditional quality control methods used for plant extracts cannot reflect the product quality owing to multiple unknown impurities, which necessitates the further analysis of impurities.

AIM OF THE STUDY

The study identified the toxic impurities of glabridin and their toxicological mechanism.

MATERIALS AND METHODS

In total, 10 glabridin samples from different sources were quantified using high-performance liquid chromatography. Sample toxicities were evaluated using zebrafish and cell models. To identify impurities, samples with different toxicity were analyzed by ultra-high-performance liquid chromatography coupled with quadrupole-Orbitrap mass spectrometry. The toxicity of related impurities was verified in the zebrafish model. Phalloidin stain was used to evaluate subtle changes in myofibril alignment.

RESULTS

Although glabridin content in the samples was similar, there were significant differences in toxicity. The results were verified using four different mammalian cell lines. Higher contents of glabrone and glabrol were identified in the sample with the highest toxicity. In the zebrafish model, the addition of glabrol reduced the LC₅₀ of glabridin to 9.224, 6.229, and 5.370 μM at 48, 72, and 96 h post-fertilization, respectively, whereas glabrone did not have any toxic effect. Phalloidin staining indicated that a glabrol impurity exacerbates the myotoxicity of glabridin in zebrafish embryos.

CONCLUSION

Glabrol, but not glabrone, was identified as a key impurity that increased glabridin toxicity. This finding indicates that controlling glabrol content is necessary during glabridin product production.

EV-T synergizes with AZD5582 to overcome TRAIL resistance through concomitant suppression of cFLIP, MCL-1, and IAPs in hepatocarcinoma

Hepatocellular carcinoma (HCC) is an aggressive malignancy, and its effective treatment has been hampered by drug resistance. Extracellular vesicle (EV) delivery of TNF-related apoptosis-inducing ligand (TRAIL) (EV-T) was demonstrated to be superior to recombinant TRAIL (rTRAIL) for cancer treatment previously. And AZD5582, a potent antagonist of inhibitors of apoptosis proteins (IAPs) can potentiate apoptosis-based cancer therapies. However, the combination of EV-T and AZD5582 has never been examined for their possible apoptosis inducing synergism in cancers. In this study, we proposed and tested the combination of EV-T and AZD5582 as a potential novel therapy for effective treatment of HCC. Two HCC lines Huh7 and HepG2 that are both resistant to rTRAIL were examined. The results confirmed that AZD5582 and EV-T are synergistic for apoptosis induction in some cancer lines including Huh7 and HepG2 while sparing normal cells. More importantly, this study revealed that TRAIL sensitization by AZD5582 is mediated through the concomitant suppression of anti-apoptotic factors including cFLIP, MCL-1, and IAPs (XIAP, Survivin and cIAP-1). Particularly the downregulation of cFLIP and IAP's appeared to be essential and necessary for the synergism between AZD5582 and TRAIL. In vivo, we first time demonstrated that the combined therapy with low doses of AZD5582 and EV-Ts triggered drastically enhanced apoptosis leading to the complete eradication of Huh7 tumor development without any apparent adverse side effects examined. We thus have unraveled the important molecular mechanism underlying TRAIL sensitization by AZD5582, rationalizing the next development of a combination therapy with AZD5582 and EV-T for HCC treatment. KEY MESSAGES: It confirmed the TRAIL sensitization by AZD5582, a potent antagonist of IAPs in hepatocarcinoma. It revealed that the sensitization is via the concomitant suppression of antiapoptotic factors including cFLIP, MCL-1, and IAPs. The downregulation of cFLIP and IAPs like Survivin appeared to be essential and necessary for the synergism between AZD5582 and nanosomal TRAIL. In vivo the combined therapy with AZD5582 and nanosomal TRAIL led to complete eradication of hepatocarcinoma tumors. This study has rationalized the next development of a combination therapy with AZD5582 and nanosomal TRAIL for cancer treatment.

Preparation, characterisation and comparison of glabridin-loaded hydrogel-forming microneedles by chemical and physical cross-linking

Poly(vinyl alcohol) (PVA) and carbomer were used as the hydrogel system to fabricate glabridin-loaded hydrogel-forming microneedles (HFMNs) by chemical cross-linking (CCMNs) and physical cross-linking (PCMNs). The properties and drug permeation effect of glabridin-loaded HFMNs with different methods were compared. They both owned excellent shape, mechanical and insertion properties. PCMNs showed a collapsed shape during swelling due to the low cross-linking rate and high porosity, probably resulting in resealing of skin pores during transdermal delivery. However, CCMNs could rapidly swell within 2 h with slightly bending. The infrared spectra indicated that CCMNs and PCMNs might form the hydrogel network by generating hydrogen and covalent bonds, respectively. The in vitro release studies showed that cumulative permeation amount within 24 h (1654 μg/cm²) of CCMNs, significantly higher than that (372 μg/cm²) achieved by PCMNs and that (118 μg/cm²) achieved by glabridin-loaded gel. The skin barrier recovery test suggests the desirable security of both microneedles (MNs), notwithstanding the presence of mild erythema in the mouse skin applied CCMNs. These results indicated that CCMNs were more desirable for glabridin delivery using PVA and carbomer as a skeleton of the hydrogel network.

Pharmacological Effects and Underlying Mechanisms of Licorice-Derived Flavonoids

Glycyrrhizae Radix et Rhizoma is the most frequently prescribed natural medicine in China and has been used for more than 2,000 years. The flavonoids of licorice have garnered considerable attention in recent decades due to their structural diversity and myriad pharmacological effects, especially as novel therapeutic agents against inflammation and cancer. Although many articles have been published to summarize different pharmacological activities of licorice in recent years, the systematic summary for flavonoid components is not comprehensive. Therefore, in this review, we summarized the pharmacological and mechanistic data from recent researches on licorice flavonoids and their bioactive components.

Licorice (Glycyrrhiza glabra, G. uralensis, and G. inflata) and Their Constituents as Active Cosmeceutical Ingredients

The interest in plant extracts and natural compounds in cosmetic formulations is growing. Natural products may significantly improve cosmetics performance since they have both cosmetic and therapeutic-like properties, known as cosmeceutical effects. Glycyrrhiza genus, belonging to the Leguminosae family, comprises more than 30 species, widely distributed worldwide. The rhizomes and roots are the most important medicinal parts currently used in pharmaceutical industries and in the production of functional foods and food supplements. In the last few years, the interest in their potential activities in cosmetic formulations has greatly increased. Glycyrrhiza spp. extracts are widely implemented in cosmetic products for their good whitening effect. The biological effects of Glycyrrhiza extracts are especially ascribable to the occurrence of specialized metabolites belonging to the flavonoid class. This review focuses on the botany and the chemistry of the main investigated Glycyrrhiza spp. (G. glabra, G. uralensis, and G. inflata) along with their cosmeceutical activities categorized as skin anti-aging, photoprotective, hair care, and anti-acne. It has been highlighted how, along with Glycyrrhiza extracts, three main flavonoids namely licochalcone A, glabridin, and dehydroglyasperin C are the most investigated compounds. It is noteworthy that other molecules from licorice show potential cosmeceutical effects. These data suggest further investigations to clarify their potential value for cosmetic industries.

Glycyrrhiza glabra (Licorice): A Comprehensive Review on Its Phytochemistry, Biological Activities, Clinical Evidence and Toxicology

There are more than 30 species of Glycyrrhiza genus extensively spread worldwide. It was the most prescribed herb in Ancient Egyptian, Roman, Greek, East China, and the West from the Former Han era. There are various beneficial effects of licorice root extracts, such as treating throat infections, tuberculosis, respiratory, liver diseases, antibacterial, anti-inflammatory, and immunodeficiency. On the other hand, traditional medicines are getting the attraction to treat many diseases. Therefore, it is vital to screen the medicinal plants to find the potential of new compounds to treat chronic diseases such as respiratory, cardiovascular, anticancer, hepatoprotective, etc. This work comprehensively reviews ethnopharmacological uses, phytochemistry, biological activities, clinical evidence, and the toxicology of licorice, which will serve as a resource for future clinical and fundamental studies. An attempt has been made to establish the pharmacological effect of licorice in different diseases. In addition, the focus of this review article is on the molecular mechanism of licorice extracts and their four flavonoids (isoliquiritigenin, liquiritigenin, lichalocone, and glabridin) pharmacologic activities. Licorice could be a natural alternative for current therapy to exterminate new emerging disorders with mild side effects. This review will provide systematic insights into this ancient drug for further development and clinical use.

Insights on the Inhibitory Power of Flavonoids on Tyrosinase Activity: A Survey from 2016 to 2021

Tyrosinase is a multifunctional copper-containing oxidase enzyme that initiates melanin synthesis in humans. Excessive accumulation of melanin pigments or the overexpression of tyrosinase may result in skin-related disorders such as aging spots, wrinkles, melasma, freckles, lentigo, ephelides, nevus, browning and melanoma. Nature expresses itself through the plants as a source of phytochemicals with diverse biological properties. Among these bioactive compounds, flavonoids represent a huge natural class with different categories such as flavones, flavonols, isoflavones, flavan-3-ols, flavanones and chalcones that display antioxidant and tyrosinase inhibitor activities with a diversity of mechanistic approaches. In this review, we explore the role of novel or known flavonoids isolated from different plant species and their participation as tyrosinase inhibitors reported in the last five years from 2016 to 2021. We also discuss the mechanistic approaches through the different studies carried out on these compounds, including in vitro, in vivo and in silico computational research. Information was obtained from Google Scholar, PubMed, and Science Direct. We hope that the updated comprehensive data presented in this review will help researchers to develop new safe, efficacious, and effective drug or skin care products for the prevention of and/or protection against skin-aging disorders.

Theoretical Exploring of a Molecular Mechanism for Melanin Inhibitory Activity of Calycosin in Zebrafish

Tyrosinase is an oxidase that is the rate-limiting enzyme for controlling the production of melanin in the human body. Overproduction of melanin can lead to a variety of skin disorders. Calycosin is an isoflavone from Astragali Radix, which is a traditional Chinese medicine that exhibits several pharmacological activities including skin whitening. In our study, the inhibitory effect of calycosin on melanin production is confirmed in a zebrafish in vivo model by comparing with hydroquinone, kojic acid, and arbutin, known as tyrosinase inhibitors. Moreover, the inhibitory kinetics of calycosin on tyrosinase and their binding mechanisms are determined using molecular docking techniques, molecular dynamic simulations, and free energy analysis. The results indicate that calycosin has an obvious inhibitory effect on zebrafish pigmentation at the concentration of 7.5 μM, 15 μM, and 30 μM. The IC₅₀ of calycosin is 30.35 μM, which is lower than hydroquinone (37.35 μM), kojic acid (6.51 × 10³ μM), and arbutin (3.67 × 10⁴ μM). Furthermore, all the results of molecular docking, molecular dynamics simulations, and free energy analysis suggest that calycosin can directly bind to the active site of tyrosinase with very good binding affinity. The study indicates that the combination of computer molecular modeling and zebrafish in vivo assay would be feasible in confirming the result of the in vitro test and illustrating the target-binding information.

Antioxidant and Anti-Melanogenic Activities of Heat-Treated Licorice (Wongam, Glycyrrhiza glabra × G. uralensis) Extract

Melanin is a brown or black pigment that protects skin from ultraviolet radiation and reactive oxygen species (ROS). However, overproduction of melanin is associated with lentigines, melasma, freckles and skin cancer. Licorice has shown antioxidant, anti-tumor, anti-platelet, anti-inflammatory and immunomodulatory activities and is used as a natural treatment for skin whitening. We aimed to confirm the potential of Wongam, a new cultivar of licorice developed by the Rural Development Administration (RDA), as a whitening agent in cosmetics. In addition, we verified the effect of heat treatment on the bioactivity of licorice by comparing antioxidant and anti-melanogenic activities of licorice extract before and after heating (130 °C). The heat-treated licorice extract (WH-130) showed higher radical-scavenging activities in the ABTS⁺ (2,2′-azino-bis-(3-ethylbenzothiazolin-6-sulfonic acid) diammonium salt) and DPPH (2,2-diphenyl-1-picrylhydrazyl) assays. In addition, WH-130 inhibited melanogenesis more effectively due to downregulation of tyrosinase in B16F10 melanoma cells than non-heated licorice extract. Moreover, heat treatment increased total phenolic content. In particular, isoliquiritigenin, an antioxidant and anti-melanogenic compound of licorice, was produced by heat treatment. In conclusion, WH-130, with increased levels of bioactive phenolics such as isoliquiritigenin, has potential for development into a novel skin whitening material with applications in cosmetics.

Investigation on conformational variation and activity of trypsin affected by black phosphorus quantum dots via multi-spectroscopy and molecular modeling

Binding interaction between black phosphorus quantum dots (BPQDs) and trypsin was researched deeply to illustrate the variations on conformation and activity of trypsin affected by BPQDs via multi-spectroscopy and molecular modeling. Experimental results implied that inherent fluorescence of trypsin was quenched by BPQDs via static fluorescence quenching mode. BPQDs bound with trypsin to construct ground-state complex under the binding forces of van der Waal interaction and hydrophobic interaction, resulting in the conformational change of trypsin to be more hydrophilic and incompact. The result of molecular modeling indicated that BPQDs interacted with trypsin at its allosteric site and inhibited the activity of trypsin via non-competitive manner. Finally, BPQDs efficiently inhibited the digestion activity of trypsin on human serum albumin, human cervical carcinoma HeLa cells, and human lung adenocarcinoma A549 cells. This work not only explores the in-depth understanding on the influence of BPQDs on proteinases but also paves the way for further application of BPQDs on human beings for diseases treatments.

Structure-activity relationships of antityrosinase and antioxidant activities of cinnamic acid and its derivatives

The related structure-activity relationships of cinnamic acid and its derivates have not been studied in details yet. Herein, antityrosinase and antioxidant activities of 18 compounds were evaluated. The results demonstrated that the substituents on the phenyl ring of cinnamic acid led to the enhancement of the inhibition on monophenolase and the weakening of the inhibition on diphenolase. Among these tested compounds, 9 was first discovered as a tyrosinase inhibitor in a reversible competitive manner with IC50 value of 68.6 ± 4.2 µm. Docking results demonstrated 9 located into the catalytic center of tyrosinase. Antioxidant assay indicated that only 1 hydroxyl group on the phenyl ring was not enough to possess the radical scavenging activity, and the number of hydroxyl groups may be more important. This study will be helpful in the development of new cinnamic acid derivates as tyrosinase inhibitors and antioxidants with higher efficacy.

Evaluation of antityrosinase activity and mechanism, antioxidation, and UV filter properties of theaflavin

Tyrosinase is a key metalloenzyme for the biosynthesis of melanin that plays a critical role in the prevention of skin damage caused by ultraviolet (UV) radiation. However, the overproduction of melanin may cause a variety of skin diseases. Due to the toxicity and inefficiency of existing tyrosinase inhibitors, it is urgent to identify safe and potent alternatives from natural sources. Theaflavin, a single-component extracted from black tea, has been found to possess a variety of pharmacological activities. Herein, the inhibition kinetics of theaflavin on tyrosinase and inhibitory mechanism were determined using spectroscopy, molecular docking, and zebrafish model. The results showed that theaflavin inhibited the diphenolase activity of tyrosinase in a reversible mixed type manner with IC₅₀ of 229.75 μmol/L and hindered the synthesis of melanin in zebrafish. This may be due to the formation of eight hydrogen bonds and hydrophobic effects between theaflavin and tyrosinase according to the results of molecular docking. To study the possible effects on the prevention of free radical-mediated skin cancer and photoaging caused by UV radiation, the antioxidation and UV filter properties of theaflavin were further verified. This study demonstrates that theaflavin is a potential multifunctional compound that can be used in cosmetic and medicinal products.

Inhibitory activity and mechanism of trilobatin on tyrosinase: kinetics, interaction mechanism and molecular docking

Tyrosinase is the rate-limiting enzyme controlling the production of melanin, and tyrosinase inhibitors can regulate the overproduction of melanin by inhibiting tyrosinase activity, which is an effective method to treat pigmentation disorders. In this study, kinetic analysis, multispectroscopic methods and molecular simulation were used to investigate the inhibitory activity and mechanism of trilobatin on tyrosinase. The kinetic analysis showed that trilobatin had significant inhibitory activity on tyrosinase in a reversible and mixed-type manner with IC₅₀ values of (2.24 ± 0.35) × 10^-5 mol L^-1. The intrinsic fluorescence of tyrosinase was quenched by trilobatin through a static quenching mechanism. Different spectroscopic measurements demonstrated that trilobatin could change the microenvironments and conformation of tyrosinase and molecular docking determined the binding site of quercetin on tyrosinase.

The interaction of graphene oxide-silver nanoparticles with trypsin: Insights from adsorption behaviors, conformational structure and enzymatic activity investigations

In this work, we synthesized graphene oxide-silver nanoparticles (GO-AgNPs) hybrids by one-pot method. Since there are relatively few reports on whether GO-AgNPs bind and change the structure and function of trypsin, A variety of methods were employed to systematically characterize the molecular interaction between GO-AgNPs and trypsin. Results exhibited that GO-AgNPs bound with trypsin to form a ground state complex. GO-AgNPs had higher adsorption capacity for trypsin compared with single GO. Langmuir-Blodgett assembly method was used to confirm that AgNPs did not interfere with the adsorption of trypsin by GO. The secondary structure and the microenvironment of amino acid residues of trypsin were altered after interacting with GO-AgNPs. In addition, GO-AgNPs can enhance the activity of trypsin and promote the hydrolysis of bovine serum protein (BSA) by trypsin. These findings provide important support for the application of GO-based nanocomposites in the efficient immobilization of enzymes.

Isolindleyin exerts anti-melanogenic effects in human epidermal melanocytes via direct binding to tyrosinase

To overcome dermatological concerns causing abnormally excessive melanin synthesis, highly effective and safe skin depigmentation compounds have been identified in the cosmetic and pharmaceutical industries. Among several methods used to achieve skin depigmentation, inhibition of tyrosinase is one of the most effective, since tyrosinase is a crucial enzyme in melanogenesis. Herein, isolindleyin, a novel inhibitor of human tyrosinase, was introduced and evaluated for its anti-melanogenic effects in human epidermal melanocytes. The results revealed that isolindleyin was directly bound to tyrosinase and it suppressed melanin synthesis. The binding mode between isolindleyin and the active sites of human tyrosinase was investigated using computational molecular docking at the atomic level. Isolindleyin binding was found to be stabilized by hydrophobic interactions between His 367 and Val 377 and by hydrogen bonds between Ser 380 and Asn 364. The results of this study revealed the anti-melanogenic effects of isolindleyin that could contribute toward overcoming dermatological concerns that cause abnormally excessive melanin synthesis.

Synthesis, antioxidant and anti-tyrosinase activity of 1,2,4-triazole hydrazones as antibrowning agents

A series of 1,2,4-triazole hydrazones (1-16) were synthesized, and their inhibitory activities and mechanisms on tyrosinase were investigated by ultraviolet spectrophotometry, fluorescence quenching, molecular docking study, etc. Most of compounds possessed potent tyrosinase inhibitory activity. Thereinto, compound 9 presented the superior activity with IC₅₀ of 0.9 μM, which was markedly lower than the standard kojic acid (IC₅₀ = 64.1 μM). Compound 9 not only interacted with copper ions in the active center of the enzyme but also bound to the enzyme-substrate complex, indicating that it was a competitive-noncompetitive mixed inhibitor. Additionally, it also displayed potent DPPH scavenging activity. Antibrowning test showed that compound 9 effectively reduced the enzymatic browning of fresh-cut potatoes. Furthermore, compound 9 exhibited low cytotoxic activity against human normal cell line with IC₅₀ of 49.9 µM. Overall, the present study suggests that these compounds may serve as lead molecules for developing novel antibrowning agents in food industry.

Traditional Asian Herbs in Skin Whitening: The Current Development and Limitations

In Asia, the market for whitening cosmetics is expanding rapidly, more and more people prefer to use natural products. Driven by natural product demand and technical advances, herbal research is also developing fast. Lots of studies reported that Asian herbal reagents can reduce melanogenesis, these findings provide evidence for the whitening application of Asian herbs. However, the current development status and challenges of herbal research need attention too. By reviewing these studies, different problems in studying herbal formulas, extracts, and active ingredients were presented. One of the most influential troubles is that the components of herbs are too complex to obtain reliable results. Thus, an understanding of the overall quality of herbal research is necessary. Further, 90 most cited Asian herbal studies on whitening were collected, which were conducted between 2017 and 2020, then statistical analysis was carried out. This work provided a comprehensive understanding of Asian herbal research in skin whitening, including the overall status and quality, as well as the focuses and limitations of these studies. By proactively confronting and analyzing these issues, it is suggested that the focus of herbal medicine research needs to shift from quantity to quality, and the new stage of development should emphasize transformation from research findings to whitening products.

Evaluating the Performance of a Non-Bonded Cu2+ Model Including Jahn-Teller Effect into the Binding of Tyrosinase Inhibitors

Tyrosinase (TYR) is a metalloenzyme classified as a type-3 copper protein, which is involved in the synthesis of melanin through a catalytic process beginning with the conversion of the amino acid l-Tyrosine (l-Tyr) to l-3,4-dihydroxyphenylalanine (l-DOPA). It plays an important role in the mechanism of melanogenesis in various organisms including mammals, plants, and fungi. Herein, we used a combination of computational molecular modeling techniques including molecular dynamic (MD) simulations and the linear interaction energy (LIE) model to evaluate the binding free energy of a set of analogs of kojic acid (KA) in complex with TYR. For the MD simulations, we used a dummy model including the description of the Jahn–Teller effect for Cu²⁺ ions in the active site of this enzyme. Our results show that the LIE model predicts the TYR binding affinities of the inhibitor in close agreement to experimental results. Overall, we demonstrate that the classical model provides a suitable description of the main interactions between analogs of KA and Cu²⁺ ions in the active site of TYR.

Investigations of conformational structure and enzymatic activity of trypsin after its binding interaction with graphene oxide

Herein, interaction between graphene oxide (GO) and trypsin was systematically characterized for deep investigations of conformational structure and enzymatic activity of trypsin affected by GO. Results indicated that GO bound with trypsin to form ground state complex with molar ratio of 1 to 1. Intrinsic fluorescence of trypsin was statically quenched by GO through van der Waal interaction, hydrophobic interaction, hydrogen bond, and electrostatic interaction. Both tertiary structure and secondary structure of trypsin were changed obviously after its binding with trypsin, resulting in the structure transformation of trypsin from the β-sheet structure to the α-helix structure. Since GO bound with the allosteric site of trypsin to inhibit its enzymatic activity via non-competitive manner, GO efficiently protected human serum albumin and human cervical carcinoma HeLa cells from the digestion of trypsin. These results explored the exact binding mechanism of GO with protease, which provides more important information for possible biological risk of GO on human beings.

Grape Cane Extracts as Multifunctional Rejuvenating Cosmetic Ingredient: Evaluation of Sirtuin Activity, Tyrosinase Inhibition and Bioavailability Potential

Grape canes are waste biomass of viticulture containing bioactive polyphenols valuable in cosmetics. Whereas several studies reported the cosmetic activities of E-resveratrol, only few described the potential of E-ε-viniferin, the second major constituent of grape cane extracts (GCE), and none of them investigated GCE as a natural blend of polyphenols for cosmetic applications. In this study, we considered the potential of GCE from polyphenol-rich grape varieties as multifunctional cosmetic ingredients. HPLC analysis was performed to quantify major polyphenols in GCE i.e., catechin, epicatechin, E-resveratrol, E-piceatannol, ampelopsin A, E-ε-viniferin, hopeaphenol, isohopeaphenol, E-miyabenol C and E-vitisin B from selected cultivars. Skin whitening potential through tyrosinase inhibition assay and the activation capacity of cell longevity protein (SIRT1) of GCE were compared to pure E-resveratrol and E-ε-viniferin. Drug-likeness of GCE polyphenols were calculated, allowing the prediction of skin permeability and bioavailability. Finally, the present data enabled the consideration of GCE from polyphenol-rich varieties as multifunctional cosmetic ingredients in accordance with green chemistry practices.

Flavones as tyrosinase inhibitors: kinetic studies in vitro and in silico

INTRODUCTION

Tyrosinase is a multifunctional copper-containing oxidase enzyme that catalyses the first steps in the formation of melanin pigments. Identification of tyrosinase inhibitors is of value for applications in cosmetics, medicine and agriculture.

OBJECTIVE

To develop an analytical method that allows identification of drug-like natural products that can be further developed as tyrosinase inhibitors. Results of in vitro and in silico studies will be compared in order to gain a deeper insight into the mechanism of action of enzyme inhibition.

METHOD

Using an in vitro assay we tested tyrosinase inhibitor effects of five structurally related flavones, i.e. luteolin (1), eupafolin (2), genkwanin (3), nobiletin (4), and chrysosplenetin (5). The strongest inhibitors were further investigated in silico, using enzyme docking simulations.

RESULTS

All compounds tested showed modest tyrosinase inhibitory effect compared to the positive control, kojic acid. The polymethoxy flavones 4 and 5 exhibited the strongest tyrosinase inhibitory effect with the half maximal inhibitory concentration (IC₅₀ ) values of 131.92 ± 1.75 μM and 99.87 ± 2.38 μM respectively. According to kinetic analysis 2, 4 and 5 were competitive inhibitors, whereas 1 and 3 were non-competitive inhibitors of tyrosinase. Docking studies indicated that methoxy groups on 4 and 5 caused steric hindrance which prevented alternative binding modes in the tyrosinase; the methoxy groups on the B-ring of these flavones faced the catalytic site in the enzyme.

CONCLUSIONS

The docking simulations nicely complemented the in vitro kinetic studies, opening the way for the development of predictive models for use in drug design.

Tyrosinase-Targeting Gallacetophenone Inhibits Melanogenesis in Melanocytes and Human Skin-Equivalents

Demands for safe depigmentation compounds are constantly increasing in the pharmaceutical and cosmetic industry, since the numerous relevant compounds reported to date have shown undesirable side effects or low anti-melanogenic effects. In this study, we reported three novel inhibitors of tyrosinase, which is the key enzyme in melanogenesis, identified using docking-based high throughput virtual screening of an in-house natural compound library followed by mushroom tyrosinase inhibition assay. Of the three compounds, gallacetophenone showed high anti-melanogenic effect in both human epidermal melanocytes and a 3D human skin model, MelanoDerm. The inhibitory effect of gallacetophenone on tyrosinase was elucidated by computational molecular modeling at the atomic level. Binding of gallacetophenone to the active site of tyrosinase was found to be stabilized by hydrophobic interactions with His367, Ile368, and Val377; hydrogen bonding with Ser380 and a water molecule bridging the copper ions. Thus, our results strongly suggested gallacetophenone as an anti-melanogenic ingredient that inhibits tyrosinase.

Natural skin-whitening compounds for the treatment of melanogenesis (Review)

Melanogenesis is the process for the production of melanin, which is the primary cause of human skin pigmentation. Skin-whitening agents are commercially available for those who wish to have a lighter skin complexions. To date, although numerous natural compounds have been proposed to alleviate hyperpigmentation, insufficient attention has been focused on potential natural skin-whitening agents and their mechanism of action from the perspective of compound classification. In the present article, the synthetic process of melanogenesis and associated core signaling pathways are summarized. An overview of the list of natural skin-lightening agents, along with their compound classifications, is also presented, where their efficacy based on their respective mechanisms of action on melanogenesis is discussed.

Inhibition of Tyrosinase by Mercury Chloride: Spectroscopic and Docking Studies

Inorganic mercury compounds have been used in skin-lightening products since ancient times. Although a previous study demonstrated that mercury impeded the transfer of Cu²⁺ to the apotyrosinase, the effect of mercury on tyrosinase is still unclear. In the present study, the mechanism of mercury chloride (HgCl₂) induced inactivation of tyrosinase was investigated for the first time. The IC₅₀ values were 29.97 and 77.93 μmol/L for monophenolase and diphenolase, respectively. A kinetic analysis revealed that HgCl₂ inhibited tyrosinase activity in an irreversible non-competitive manner. The strong intrinsic fluorescence quenching suggested that the formation of the HgCl₂-tyrosinase complex induced conformational changes of the enzyme, and HgCl₂ had only one single binding site or a single class of binding site on tyrosinase. The molecular docking and further experiments demonstrated that HgCl₂ bound to the amino residuals (His) in the catalytic center of tyrosinase. To our knowledge, these findings presented in this paper were the first evidence of the direct interactions between HgCl₂ and tyrosinase, which provided a deep understanding of the inhibition mechanism of mercury on tyrosinase.

A label-free luminescent assay for tyrosinase activity monitoring and inhibitor screening with responsive lanthanide coordination polymer nanoparticles

In this work, a label-free, selective, and sensitive luminescent sensing platform was established for tyrosinase (TYR) activity monitoring and its inhibitor screening using one kind of lanthanide coordination polymer nanoparticles AMP-Tb/Ag⁺. By taking advantage of the specific binding and redox properties of Ag⁺ incorporated into the AMP-Tb network and dopamine (DA) as the product of the model substrate tyramine, the enzymatic reaction and the signal change of the sensing platform was effectively linked. The cooperative effect of a weakened energy transfer from AMP to Tb³⁺ by altering the electronic structure of Ag⁺ and an efficient photoinduced election transfer (PET) process caused by dopaquinone facilitated the luminescence quenching of Tb³⁺. Thus, this luminescent sensing platform could be employed for quantitative evaluation of TYR activity. There was a good linear range for TYR activity from 0.08 to 0.20 U mL^-1 with a low detection limit of 0.004 U mL^-1. Furthermore, this assay was successfully applied to accurate determination of TYR activity in human serum samples and efficient screening of TYR inhibitors. Considering unique spectral characteristics of lanthanides along with operation simplicity and superior analytical performance, this sensing platform is very promising in clinical diagnosis and drugs screening for TYR-associated diseases.