Tyrosinase inhibitory mechanism and the anti-browning properties of piceid and its ester

Hydroxyl-Amide Alkaloids from Pepper Roots: Potential Sources of Natural Antioxidants and Tyrosinase Inhibitors

Pepper (Piper nigrum L.) is a widely used spice plant known for its fruits and roots, which serve as flavor enhancers in culinary applications and hold significant economic value. Despite the popularity of pepper fruits, their roots remain relatively understudied, with limited research conducted on their bioactive components. This study focused on discovering and separating the primary bioactive amide alkaloids found in pepper roots. The process involved using the antioxidant activity of crude fractions and the Global Natural Products Social Molecular Networking analysis platform. The process led to the discovery of 23 previously unknown hydroxyl-amide alkaloids. Notably, compounds 11, 12, and 14 showed excellent antioxidant activity, while compound 11 exhibited significant inhibitory effects on mushroom tyrosinase. Theoretical exploration of enzyme-ligand interactions was conducted through molecular docking and molecular dynamics simulation. The findings of this study highlight the potential of hydroxyl-amide alkaloids as antioxidant products and natural food preservatives in the pharmaceutical and food cosmetic industries.

Cyrene™ as a tyrosinase inhibitor and anti-browning agent

The tyrosinase pathway takes part in the enzymatic process of food browning and is primarily responsible for food spoilage - manifesting itself from a decrease in its nutritional value to a deterioration of taste, which consequently leads to a gradual loss of shelf life. Finding safe and bio-based tyrosinase inhibitors and anti-browning agents may be of great importance in agriculture and food industries. Herein, we showed that Cyrene™ exhibits tyrosinase inhibitory activity (IC50: 268.2 µM), the 1.44 times higher than ascorbic acid (IC50: 386.5 μM). Binding mode studies demonstrated that the carbonyl oxygen of Cyrene™ coordinates with both copper ions. Surprisingly, both hydroxyl groups of Cyrene gem-diol perform a monodentate binding mode with both copper ions, at similar distances. This fact suggests that both compounds could have a similar binding mode and, as consequence, similar biological activities in tyrosinase inhibition assays and anti-browning activities.

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.

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.

Investigation on the inhibition mechanism and binding behavior of paeonol to tyrosinase and its anti-browning property by multi-spectroscopic and molecular docking methods

Paeonol, as one effective tyrosinase inhibitor, had been used as food preservative and clinical medication for skin disorders. In this study, the inhibition mechanism and binding behavior of paeonol to tyrosinase and its anti-browning property were investigated using multi-spectroscopic and molecular docking methods. Activity assay and kinetic results confirmed paeonol as a reversible mixed-type tyrosinase inhibitor. Results of the mechanistic studies were clarified using fluorescence quenching, synchronous fluorescence, CD spectra and 3D fluorescence, and showed that the binding of paeonol to tyrosinase might change the chromophore microenvironment and conformation of tyrosinase to inhibit enzyme catalytic activity. Molecular docking results revealed the detailed binding between paeonol and tyrosinase. Moreover, paeonol could prevent the browning of fresh-cut apples, as well as inhibiting PPO and POD activities and increasing APX activity. All above findings established a reliable basis for the inhibitory mechanism of paeonol against tyrosinase and therefore contributed to its application in anti-browning.

Fractionation and evaluation of light-colored lignin extracted from bamboo shoot shells using hydrated deep eutectic solvents

In this study, light-colored lignin was extracted from bamboo shoot shells (BSS) using a hydrated deep eutectic solvent (DES) pretreatment. The hydrated DES used in pretreatment consist of formic acid, benzyl triethylammonium chloride (BTEAC) and water. The pretreatment using a hydrated DES containing 30% water (H30) demonstrate efficient delignification (82.9%). Additionally, the hydrated DES protected the β-O-4 linkage from excessive cleavage and recondensation as well as keep the light-colored of lignin. Moreover, the hydrated DES extracted lignin exhibits superior antioxidant performance and tyrosinase inhibitory capacity compared to the control. Notably, incorporating 5% lignin of H30-extracted lignin into a commercial suncream led to a remarkable enhancement of the SPF value, elevating from 14.8 to 32.6. In summary, the proposed hydrated DES pretreatment method offers significant benefits for extracting light-colored lignin, thereby promoting the multifunctional application of lignin in cosmetics.

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.

Combinatorial metabolic engineering enables high yield production of α-arbutin from sucrose by biocatalysis

α-Arbutin has been widely used as a skin-whitening ingredient. Previously, we successfully produced α-arbutin via whole-cell biocatalysis and found that the conversion rate of sucrose to α-arbutin was low (~45%). To overcome this issue, herein, we knocked out the genes of enzymes related to the sucrose hydrolysis, including sacB, sacC, levB, and sacA. The sucrose consumption was reduced by 17.4% in 24 h, and the sucrose conversion rate was increased to 51.5%. Furthermore, we developed an inducible protein degradation system with Lon protease isolated from Mesoplasma florum (MfLon) and proteolytic tag to control the PfkA activity, so that more fructose-6-phosphate (F6P) can be converted into glucose-1-phosphate (Glc1P) for α-arbutin synthesis, which can reduce the addition of sucrose and increase the sucrose conversion efficiency. Finally, the pathway of F6P to Glc1P was enhanced by integrating another copy of glucose 6-phosphate isomerase (Pgi) and phosphoglucomutase (PgcA); a high α-arbutin titer (~120 g/L) was obtained. The sucrose conversion rate was increased to 60.4% (mol/mol). In this study, the substrate utilization rate was boosted due to the attenuation of its hydrolysis and the assistance of the intracellular enzymes that converted the side product back into the substrate for α-arbutin synthesis. This strategy provides a new idea for the whole-cell biocatalytic synthesis of other products using sucrose as substrate, especially valuable glycosides.Key points The genes of sucrose metabolic pathway were knocked out to reduce the sucrose consumption. The by-product fructose was reused to synthesize α-arbutin. The optimized whole-cell system improved sucrose conversion by 15.3%.

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

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

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.

Maillard reaction between oligopeptides and reducing sugar at body temperature: The putative anti-glycation agents

Type 2 Diabetes mellitus (T2DM) is one of the most common chronic multifactorial diseases, which is associated with the increased concentration of glucose in the blood. Therefore, the utilization of blood lowering agents is clearly a promising approach which can lead to a suppression of the evaluated blood glucose, and thus curing T2DM and other complication. In this study, we evaluated the glucose lowering effect of a varieties of amino acids (alanine and histidine), dipeptides (carnosine and α-alanine-L-histidine), and tripeptide (glutathione) by reacting with glucose, fructose, and sucrose under 37°C and pH 7.4 to mimic their reaction in physiological condition. By measuring the reduction of reactants and the formation of Maillard reaction products over the course of 21 days’ storage, we found that the glucose lowering effect of carnosine was better than the counterparts. The histidine residue in carnosine may contribute to its glucose lowing effect while β-amino acid β-alanine residue could facilitate the glucose lowering effect of carnosine by maintaining its chemical stability during the storage. These results may open up new avenues for the applications of bioactive peptide carnosine as a natural blood sugar lowering agent to control T2DM.

Characterization, Stability, and Antibrowning Effects of Oxyresveratrol Cyclodextrin Complexes Combined Use of Hydroxypropyl Methylcellulose

Oxyresveratrol (Oxy) has attracted much attention by employing it as an antibrowning agent in fruits and vegetables. In this study, the formation of cyclodextrin (CD) inclusion exhibited a certain protective effect on Oxy oxidative degradation, while hydroxypropyl-β-cyclodextrin (HP–β-CD) inclusion complex showed stronger stabilizing effects than those of β-cyclodextrin (β-CD). The combined use of CD and hydroxypropyl methylcellulose (HPMC) greatly improved the stability of Oxy–CD inclusion complexes, with approximately 70% of the trans-Oxy retained after 30 days of storage under light conditions at 25 °C. The results of the interaction between CD and Oxy determined by phase solubility studies and fluorescence spectroscopic analysis showed that the binding strength of CD and Oxy increased in the presence of HPMC. Moreover, Oxy combined with ascorbic acid and HPMC showed an excellent antibrowning effect on fresh-cut apple slices during the 48 h test period, indicating that adding HPMC as the third component will not influence the antibrowning activity of Oxy.