Mechanism of melanogenesis inhibition by Keggin-type polyoxometalates

Abnormal melanin overproduction can result in hyperpigmentation syndrome in human skin diseases and enzymatic browning of fruits and vegetables. Recently, our group found that Keggin-type polyoxometalates (POMs) can efficiently inhibit tyrosinase activity. However, it remains unclear whether Keggin-type POMs exhibit optimal effects in vivo. Additionally, the inhibitory effect and mechanism of action of POMs on cellular tyrosinase activity and melanogenesis have been rarely reported. Here we demonstrate that our screened and synthesised PMo11Zn and GaMo12 show superior inhibitory effects on melanin formation as well as inhibition of cellular tyrosinase activity compared to other Keggin-type POMs. Intriguingly, we reveal that Keggin-type POMs competitively bind to tyrosinase mainly through more interactions with Cu2+ ions and the amino acid residue is capable of forming van der Waals, cation-π and hydrogen bonds, resulting in a reversible non-covalent complex formation. Our findings provide valuable insights into the design, synthesis and screening of polyoxometalates as multifunctional metallodrugs and food preservatives against hyperpigmentation.

Speciation atlas of polyoxometalates in aqueous solutions

Speciation is the key parameter in solution chemistry that describes the composition, concentration, and oxidation state of each chemical form of an element present in a sample. The speciation study of complex polyatomic ions has remained challenging because of the large number of factors affecting stability and the limited number of direct methods. To address these challenges, we developed the speciation atlas of 10 polyoxometalates commonly used in catalytic and biological applications in aqueous solutions, where the speciation atlas provides both a species distribution database and a predictive model for other polyoxometalates to be used. Compiled for six different polyoxometalate archetypes with three types of addenda ions based on 1309 nuclear magnetic resonance spectra under 54 different conditions, the atlas has revealed a previously unknown behavior of polyoxometalates that may account for their potency as biological agents and catalysts. The atlas is intended to promote the interdisciplinary use of metal oxides in various scientific fields.

Wells-Dawson phosphotungstates as mushroom tyrosinase inhibitors: a speciation study

In order to elucidate the active polyoxotungstate (POT) species that inhibit fungal polyphenol oxidase (AbPPO4) in sodium citrate buffer at pH 6.8, four Wells-Dawson phosphotungstates [α/β-P^V₂W^VI₁₈O₆₂]^6- (intact form), [α₂-P^V₂W^VI₁₇O₆₁]^10- (monolacunary), [P^V₂W^VI₁₅O₅₆]^12- (trilacunary) and [H₂P^V₂W^VI₁₂O₄₈]^12- (hexalacunary) were investigated. The speciation of the POT solutions under the dopachrome assay (50 mM Na-citrate buffer, pH 6.8; L-3,4-dihydroxyphenylalanine as a substrate) conditions were determined by ¹⁸³W-NMR, ³¹P-NMR spectroscopy and mass spectrometry. The intact Wells-Dawson POT [α/β-P^V₂W^VI₁₈O₆₂]^6- shows partial (~ 69%) disintegration into the monolacunary [α₂-P^V₂W^VI₁₇O₆₁]^10- anion with moderate activity (K_i = 9.7 mM). The monolacunary [α₂-P^V₂W^VI₁₇O₆₁]^10- retains its structural integrity and exhibits the strongest inhibition of AbPPO4 (K_i = 6.5 mM). The trilacunary POT [P^V₂W^VI₁₅O₅₆]^12- rearranges to the more stable monolacunary [α₂-P^V₂W^VI₁₇O₆₁]^10- (~ 62%) accompanied by release of free phosphates and shows the weakest inhibition (K_i = 13.6 mM). The hexalacunary anion [H₂P^V₂W^VI₁₂O₄₈]^12- undergoes time-dependent hydrolysis resulting in a mixture of [H₂P^V₂W^VI₁₂O₄₈]^12-, [P^V₈W^VI₄₈O₁₈₄]^40-, [P^V₂W^VI₁₉O₆₉(H₂O)]^14- and [α₂-P^V₂W^VI₁₇O₆₁]^10- which together leads to comparable inhibitory activity (K_i = 7.5 mM) after 48 h. For the solutions of [α/β-P^V₂W^VI₁₈O₆₂]^6-, [α₂-P^V₂W^VI₁₇O₆₁]^10- and [P^V₂W^VI₁₅O₅₆]^12- the inhibitory activity is correlated to the degree of their rearrangement to [α₂-P^V₂W^VI₁₇O₆₁]^10-. The rearrangement of hexalacunary [H₂P^V₂W^VI₁₂O₄₈]^12- into at least four POTs with a negligible amount of monolacunary anion interferes with the correlation of activity to the degree of their rearrangement to [α₂-P^V₂W^VI₁₇O₆₁]^10-. The good inhibitory effect of the Wells-Dawson [α₂-P^V₂W^VI₁₇O₆₁]^10- anion is explained by the low charge density of its protonated forms H_x[α₂-P^V₂W^VI₁₇O₆₁]^(10-x)- (x = 3 or 4) at pH 6.8.

Electrocatalytic performance of tyrosinase detection in Penaeus vannamei based on a [(PSS/PPy)(P2Mo18/PPy)5] multilayer composite film modified electrode

Polyoxometalates (POMs) are widely used in the preparation of sensors that detect the content of substances because of their excellent electron transfer capabilities. In this paper, a [(PSS/PPy)(P₂Mo₁₈/PPy)₅] multilayer composite film modified electrode was prepared by the potentiostatic deposition method. The electrochemical performance of the modified electrode was studied by cyclic voltammetry under the conditions of different modified layers, different supporting electrolytes and different sweep rates. Different concentrations of tyrosinase were catalyzed by the modified electrode under a suitable supporting electrolyte, and the electrochemical sensing of tyrosinase by the modified electrode was studied. The research results show that the modified electrode has good stability and reproducibility for electrochemical sensing of tyrosinase, and the response current has a good linear relationship with the amount of tyrosinase added. Taking peak III as an example, the detection limit (S/N = 3) was 2.7649 U mL^-1. It can be known from the timing ampere curve that as the concentration of tyrosinase in the reaction system continues to increase, its response current increases stepwise, providing a linear curve in the range of 3.66 U mL^-1 to 26.87 U mL^-1, and the minimum detection limit (S/N = 3) reaches 0.0021 U mL^-1. The [(PSS/PPy)(P₂Mo₁₈/PPy)₅] multilayer composite membrane modified electrode was used to detect tyrosinase in Penaeus vannamei. The spiked recovery of the sample was 96.3-100.8%, indicating that the modified electrode has high accuracy and can be used for the detection of tyrosinase in actual samples.

Nineteen New Flavanol-Fatty Alcohol Hybrids with α-Glucosidase and PTP1B Dual Inhibition: One Unusual Type of Antidiabetic Constituent from Amomum tsao-ko

The dried fruits of Amomum tsao-ko were first revealed to have hypoglycemic effects on db/db mice at a concentration of 200 mg/kg. In order to clarify the antidiabetic constituents, 19 new flavanol-fatty alcohol hybrids, tsaokoflavanols A-S (1-19), were isolated and determined by extensive spectroscopic data and ECD calculations. Most of the compounds showed α-glucosidase and PTP1B dual inhibition, among which 1, 2, 6, 11, and 18 exhibited obvious activity against α-glucosidase with IC₅₀ values of 5.2-9.0 μM, 20-35 times stronger than that of acarbose (IC₅₀, 180.0 μM); meanwhile, 6, 10-12, and 19 were PTP1B/TCPTP-selective inhibitors with IC₅₀ values of 56.4-80.4 μM, 2-4 times stronger than that of suramin sodium (IC₅₀, 200.5 μM). Enzyme kinetics study indicated that compounds 1, 2, 6, and 11 were α-glucosidase and PTP1B mixed-type inhibitors with K_i values of 13.0, 11.7, 2.9, and 5.3 μM and 142.3, 88.9, 39.2, and 40.8 μM, respectively. Docking simulations proved the importance of hemiacetal hydroxy, the orientation of 3,4-dihydroxyphenyl, and the length of alkyl in binding with α-glucosidase and PTP1B.

Research progress on the inhibition of enzymes by polyoxometalates

Polyoxometalates (POMs) are a kind of inorganic cluster metal complex with various biological activities, such as anti-Alzheimer's disease, antibacterial, anti-cancer, anti-diabetes, anti-virus and so on.

Molecular docking of polyoxometalates as potential α-glucosidase inhibitors

α-Glucosidase is an important target enzyme for the treatment of type 2 diabetes in humans. In our previous studies, it was found that polyoxometalates exhibited an effective inhibitory effect on the activity of α-glucosidase, while polyoxometalates have the characteristics of structural diversity and unique properties. Herein, we investigated the inhibition of two different series of polyoxometalates on α-glucosidases by enzyme kinetics and molecular docking. The results demonstrated that all of the studied compounds had a significant inhibitory ability on α-glucosidase as compared with the positive control acarbose. H₈[P₂Mo₁₇Cr(OH₂)O₆₁] reversibly inhibited α-glucosidase in a competitive manner with IC₅₀ of 115.50 ± 1.64 μM and K_I value of 44.31 μM. All other compounds reversibly inhibited enzymatic activity in a mixed manner. H₆PMo₉V₃O₄₀ and H₈[P₂Mo₁₇Cu(OH₂)O₆₁] were the best inhibitors in the Keggin and Dawson series, respectively, with IC₅₀ of 9.63 ± 0.43 and 40.13 ± 0.61 μM, respectively. We conducted molecular docking study and found that the compound and α-glucosidase were mainly non-covalently interacting with hydrogen bonds and van der Waals forces. This result further confirmed the inhibition mechanism of enzyme kinetic experiments.