Tyrosinase Inhibition and Kinetic Details of Puerol A Having But-2-Enolide Structure from Amorpha fruticosa

Development of chemometric-assisted supercritical fluid extraction of effective and natural tyrosinase inhibitor from Syzygium aqueum leaves

Tyrosinase is a key enzyme in enzymatic browning, causing quality losses in food through the oxidation process. Thus, the discovery of an effective and natural tyrosinase inhibitor via green technology is of great interest to the global food market due to food security and climate change issues. In this study, Syzygium aqueum (S. aqueum) leaves, which are known to be rich in phenolic compounds (PC), were chosen as a natural source of tyrosinase inhibitor, and the effect of the sustainable, supercritical fluid extraction (SFE) process was evaluated. Response surface methodology-assisted supercritical fluid extraction (RSM-assisted SFE) was utilized to optimize the PCs extracted from S. aqueum. The highest amount of PC was obtained at the optimum conditions (55 °C, 3350 psi, and 70 min). The IC50 (661.815 μg/mL) of the optimized extract was evaluated, and its antioxidant activity (96.8 %) was determined. Gas chromatography-mass spectrometry (GC-MS) results reveal that 2',6'-dihydroxy-4'-methoxychalcone (2,6-D4MC) (82.65 %) was the major PC in S. aqueum. Chemometric analysis indicated that 2,6-D4MC has similar chemical properties to the tyrosinase inhibitor control (kaempferol). The toxicity and physiochemical properties of the novel 2,6-D4MC from S. aqueum revealed that the 2,6-D4MC is safer than kaempferol as predicted via absorption, distribution, metabolism, and excretion (ADME) evaluation. Enzyme kinetic analysis shows that the type of inhibition of the optimized extract is non-competitive inhibition with Km = 1.55 mM and Vmax = 0.017 μM/s. High-performance liquid chromatography (HPLC) analysis shows the effectiveness of S. aqueum as a tyrosinase inhibitor. The mechanistic insight of the tyrosinase inhibition using 2,6-D4MC was successfully calculated using density functional theory (DFT) and molecular docking approaches. The findings could have a significant impact on food security development by devising a sustainable and effective tyrosinase inhibitor from waste by-products that is aligned with the United Nation's SDG 2, zero hunger.

6'-O-Caffeoylarbutin from Quezui Tea: A Highly Effective and Safe Tyrosinase Inhibitor

Tyrosinase is vital in fruit and vegetable browning and melanin synthesis, crucial for food preservation and pharmaceuticals. We investigated 6'-O-caffeoylarbutin's inhibition, safety, and preservation on tyrosinase. Using HPLC, we analyzed its effect on mushroom tyrosinase and confirmed reversible competitive inhibition. UV_vis and fluorescence spectroscopy revealed a stable complex formation with specific binding, causing enzyme conformational changes. Molecular docking and simulations highlighted strong binding, enabled by hydrogen bonds and hydrophobic interactions. Cellular tests showed growth reduction of A375 cells with mild HaCaT cell toxicity, indicating favorable safety. Animal experiments demonstrated slight toxicity within safe doses. Preservation trials on apple juice showcased 6'-O-caffeoylarbutin's potential in reducing browning. In essence, this study reveals intricate mechanisms and applications of 6'-O-caffeoylarbutin as an effective tyrosinase inhibitor, emphasizing its importance in food preservation and pharmaceuticals. Our research enhances understanding in this field, laying a solid foundation for future exploration.

New Class of Tyrosinase Inhibitors, Rotenoids, from Amorpha fruticosa

A series of rotenoids including a new one from the seeds of Amorpha fruticosa were found to have significant potential as tyrosinase inhibitors. All of the isolated rotenoids (1-6) displayed inhibitory activity against tyrosinase, both as a monophenolase for the oxidation of l-tyrosine and as a diphenolase for the oxidation of l-DOPA. The three most active compounds (1, 5, and 6) showed significant monophenolase inhibition with IC50 values of 2.1, 1.7, and 1.2 μM, respectively. They also inhibited diphenolase function with IC50 values in the range of 9.5-21.5 μM. The inhibition kinetics established all compounds to be competitive inhibitors of both oxidation processes. All rotenoids formed the Emet·I complex effectively around their IC50 values with long lag times. Tyrosinase inhibition of the new rotenoid 6 was additionally demonstrated using high-performance liquid chromatography (HPLC) analysis with N-acetyl-l-tyrosine. Molecular docking disclosed that the sugar moiety of 5 interacted with the bottom of the catalytic gorge.

Antioxidant, Tyrosinase, α-Glucosidase, and Elastase Enzyme Inhibition Activities of Optimized Unripe Ajwa Date Pulp (Phoenix dactylifera) Extracts by Response Surface Methodology

The Ajwa date (Phoenix dactylifera L., Arecaceae family) is a popular edible fruit consumed all over the world. The profiling of the polyphenolic compounds of optimized unripe Ajwa date pulp (URADP) extracts is scarce. The aim of this study was to extract polyphenols from URADP as effectively as possible by using response surface methodology (RSM). A central composite design (CCD) was used to optimize the extraction conditions with respect to ethanol concentration, extraction time, and temperature and to achieve the maximum amount of polyphenolic compounds. High-resolution mass spectrometry was used to identify the URADP's polyphenolic compounds. The DPPH-, ABTS-radical scavenging, α-glucosidase, elastase and tyrosinase enzyme inhibition of optimized extracts of URADP was also evaluated. According to RSM, the highest amounts of TPC (24.25 ± 1.02 mgGAE/g) and TFC (23.98 ± 0.65 mgCAE/g) were obtained at 52% ethanol, 81 min time, and 63 °C. Seventy (70) secondary metabolites, including phenolic, flavonoids, fatty acids, and sugar, were discovered using high-resolution mass spectrometry. In addition, twelve (12) new phytoconstituents were identified for the first time in this plant. Optimized URADP extract showed inhibition of DPPH-radical (IC₅₀ = 87.56 mg/mL), ABTS-radical (IC₅₀ = 172.36 mg/mL), α-glucosidase (IC₅₀ = 221.59 mg/mL), elastase (IC₅₀ = 372.25 mg/mL) and tyrosinase (IC₅₀ = 59.53 mg/mL) enzymes. The results revealed a significant amount of phytoconstituents, making it an excellent contender for the pharmaceutical and food industries.

Discovery of Novel Tyrosinase Inhibitors From Marine Cyanobacteria

Tyrosinase, an important oxidase involved in the primary immune response in humans, can sometimes become problematic as it can catalyze undesirable oxidation reactions. Therefore, for decades there has been a strong pharmaceutical interest in the discovery of novel inhibitors of this enzyme. Recent studies have also indicated that tyrosinase inhibitors can potentially be used in the treatment of melanoma cancer. Over the years, many new tyrosinase inhibitors have been discovered from various natural sources; however, marine natural products (MNPs) have contributed only a small number of promising candidates. Therefore, in this study we focused on the discovery of new MNP tyrosinase inhibitors of marine cyanobacterial and algal origins. A colorimetric tyrosinase inhibitory assay was used to screen over 4,500 marine extracts against mushroom tyrosinase (A. bisporus). Our results revealed that scytonemin monomer (ScyM), a pure compound from our compound library and also the monomeric last-step precursor in the biosynthesis of the well-known cyanobacterial sunscreen pigment “scytonemin,” consistently showed the highest tyrosinase inhibitory score. Determination of the half maximal inhibitory concentration (IC₅₀) further indicated that ScyM is more potent than the commonly used commercial inhibitor standard “kojic acid” (KA; IC₅₀ of ScyM: 4.90 μM vs. IC₅₀ of KA: 11.31 μM). After a scaled-up chemical synthesis of ScyM as well as its O-methyl analog (ScyM-OMe), we conducted a series of follow-up studies on their structures, inhibitory properties, and mode of inhibition. Our results supported ScyM as the second case ever of a novel tyrosinase inhibitory compound based on a marine cyanobacterial natural product. The excellent in vitro performance of ScyM makes it a promising candidate for applications such as a skin-whitening agent or an adjuvant therapy for melanoma cancer treatment.

Inhibition of Bacterial Neuraminidase and Biofilm Formation by Ugonins Isolated From Helminthostachys Zeylanica (L.) Hook

Bacterial neuraminidase (BNA) plays a pivotal role in the pathogenesis of several microbial diseases including biofilm formation. The aim of this study is to reveal the neuraminidase inhibitory potential of metabolites from Helminthostachys zeylanica (L.) Hook. which have diverse biological activities including PTP1B and α-glucosidase. The six ugonins (1–6) from the target plant showed significant neuraminidase inhibition. The inhibitory potencies were observed at a nanomolar level of 35–50 nM, which means they are 100 times more active than their corresponding mother compounds (eriodyctiol and luteolin). A detailed kinetic study revealed that all ugonins were reversible noncompetitive inhibitors. An in-depth investigation of the most potent compound 1 showed its time-dependent inhibition with the isomerization model having k₅ = 0.0103 min⁻¹, k₆ = 0.0486 min⁻¹, and K_i^app = 0.062 μM. The binding affinities (K_sv) were agreed closely with our prediction based on the inhibitory potencies. Particularly, ugonin J (1) blocked the biofilm formation of E. coli dose-dependently up to 150 µM without the inhibition of bacteria. The major compounds (1–6) in the extract were characterized by UPLC-ESI-Q-TOF/MS.

Genetically encoded dihydroxyphenylalanine coupled with tyrosinase for strain promoted labeling

Protein modifications through genetic code engineering have a remarkable impact on macromolecule engineering, protein translocation, protein-protein interaction, and cell biology. We used the newly developed molecular biology approach, genetic code engineering, for fine-tuning of proteins for biological availability. Here, we have introduced 3, 4-dihydroxy-l-phenylalanine in recombinant proteins by selective pressure incorporation method for protein-based cell labeling applications. The congener proteins treated with tyrosinase convert 3, 4-dihydroxy-l-phenylalanine to dopaquinone for strain-promoted click chemistry. Initially, the single-step Strain-Promoted Oxidation-Controlled Cyclooctyne-1,2-quinone Cycloaddition was studied using tyrosinase catalyzed congener protein and optimized the temporally controlled conjugation with (1R,8S,9s)-Bicyclo[6.1.0]non-4-yn-9-ylmethanol. Then, the feasibility of tyrosinase-treated congener annexin A5 with easily reactive quinone functional moiety was conjugated with fluorescent tag dibenzocyclooctyne-PEG4-TAMRA for labeling of apoptotic cells. Thus, the congener proteins-based products demonstrate selective cell labeling and apoptosis detection in EA.hy926 cells even after the protein modifications. Hence, genetic code engineering can be coupled with click chemistry to develop various protein-based fluorescent labels.

Iridal-Type Triterpenoids Displaying Human Neutrophil Elastase Inhibition and Anti-Inflammatory Effects from Belamcanda chinensis

The aim of this study is to explore anti-inflammatory phytochemicals from B. chinensis based on the inhibition of pro-inflammatory enzyme, human neutrophil elastase (HNE) and anti-inflammatory activities in lipopolysaccharide (LPS)-stimulated RAW264.7 macrophage. Three stereoisomers of iridal-type triterpenoids (1–3) were isolated from the roots of B. chinensis and their stereochemistries were completely identified by NOESY spectra. These compounds were confirmed as reversible noncompetitive inhibitors against HNE with IC₅₀ values of 6.8–27.0 µM. The binding affinity experiment proved that iridal-type triterpenoids had only a single binding site to the HNE enzyme. Among them, isoiridogermanal (1) and iridobelamal A (2) displayed significant anti-inflammatory effects by suppressing the expressions of pro-inflammatory cytokines, such as iNOS, IL-1β, and TNF-α through the NF-κB pathway in LPS-stimulated RAW264.7 cells. This is the first report that iridal-type triterpenoids are considered responsible phytochemicals for anti-inflammatory effects of B. chinensis.

SAR and QSAR research on tyrosinase inhibitors using machine learning methods

Tyrosinase is a key rate-limiting enzyme in the process of melanin synthesis, which is closely related to human pigmentation disorders. Tyrosinase inhibitors can down-regulate tyrosinase to effectively reduce melanin synthesis. In this work, we conducted structure-activity relationship (SAR) study on 1097 diverse mushroom tyrosinase inhibitors. We applied five kinds of machine learning methods to develop 15 classification models. Model 5B built by fully connected neural networks and ECFP4 fingerprints achieved the highest prediction accuracy of 91.36% and Matthews correlation coefficient (MCC) of 0.81 on the test set. The applicability domains (AD) of classification models were defined by d S T D - P R O method. Moreover, we clustered the 1097 inhibitors into eight subsets by K-Means to figure out inhibitors' structural features. In addition, 10 quantitative structure-activity relationship (QSAR) models were constructed by four machine learning methods based on 813 inhibitors. Model 6 J, the best QSAR model, was developed by fully connected neural networks with 50 RDKit descriptors. It resulted in a coefficient of determination (r ²) of 0.770 and a root mean squared error (RMSE) of 0.482 on the test set. The AD of Model 6 J was visualized by Williams plot. The models built in this study can be obtained from the authors.

Effectiveness of cyclohexyl functionality in ugonins from Helminthostachys zeylanica to PTP1B and α-glucosidase inhibitions

Ugonins are unique flavonoids with cyclohexyl motif from Helminthostachys zeylanica. Ugonins (1-6) from the target plant displayed significant inhibitions against both PTP1B (IC₅₀s = 0.6-7.3 μM) and α-glucosidase (IC₅₀s = 3.9-32.9 μM), which are crucial enzymes associated with diabetes. A cyclohexyl motif was proved to be the key functionality for PTP1B and α-glucosidase. For example, 1 was 26-fold effective to PTP1B and 15-fold to α-glucosidase than its mother compound, luteolin. This tendency was well elucidated with distinctive differences of binding affinities (K_SV) between ugonins and mother compounds to PTP1B enzyme. Inhibitory mechanisms to PTP1B and α-glucosidase were fully characterized to be competitive, non-competitive and mixed type I according to the position of cyclohexyl functionality. In particular, the ugonin J (1) has a cyclohexyl on the B ring was estimated as a reversible, competitive and a slow binding inhibitor with parameters: K_i^app = 0.1234 μM, k₃ = 0.5713 μM^-1 min^-1, and k₄ = 0.0705 min^-1. In-depth molecular docking experiments disclosed the specific binding sites and residues of competitive inhibitor (1) and non-competitive inhibitor (4) to PTP1B enzymes. As well, all six ugonins (1-6) also inhibited α-glucosidase effectively, in which cyclohexyl motif was also the key functionality of inhibitions.