Polyoxometalates: Study of inhibitory kinetics and mechanism against α-glucosidase

New Perspectives on the Risks of Hydroxylated Polychlorinated Biphenyl (OH-PCB) Exposure: Intestinal Flora α-Glucosidase Inhibition

Polychlorinated biphenyls (PCBs) are a group of colorless and odorless environmental pollutants with a wide range of toxic effects. Some PCBs, especially less chlorinated ones, will rapidly undergo phase I metabolism after entering the body, and hydroxylated polychlorinated biphenyls (OH-PCBs) are the main metabolites of PCBs. Intestinal flora α-glucosidase is a common carbohydrate-active enzyme which is ubiquitous in human intestinal flora. It can convert complex dietary polysaccharides into monosaccharides, assisting the body in degrading complex carbohydrates and providing energy for the survival and growth of bacterial flora. The present study aims to investigate the inhibition of the activity of intestinal flora α-glucosidase by OH-PCBs. 4-Nitrophenyl-α-D-glucopyranoside (PNPG) was used as a probe substrate for α-glucosidase, and in vitro incubation experiments were conducted to study the inhibition of 26 representative OH-PCBs on α-glucosidase. Preliminary screening of in vitro incubation was performed with 100 μM of OH-PCBs. The results showed that 26 OH-PCBs generally exhibited strong inhibition of α-glucosidase. The concentration-dependent inhibition and half inhibition concentrations (IC50s) of OH-PCBs on α-glucosidase were determined. 4'-OH-PCB 86 and 4'-OH-PCB 106 were chosen as representative OH-PCBs, and the inhibition kinetic parameters (Kis) of inhibitors for α-glucosidase were determined. The inhibition kinetic parameters (Kis) of 4'-OH-PCB 86 and 4'-OH-PCB 106 for α-glucosidase are 1.007 μM and 0.538 μM, respectively. The silico docking method was used to further analyze the interaction mechanism between OH-PCBs and α-glucosidase. All these results will help us to understand the risks of OH-PCB exposure from a new perspective.

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.

Adaptive Responses of a Peroxidase-like Polyoxometalate-Based Tri-Assembly to Bacterial Microenvironment (BME) Significantly Improved the Anti-Bacterial Effects

The present study presents the tertiary assembly of a POM, peptide, and biogenic amine, which is a concept to construct new hybrid bio-inorganic materials for antibacterial applications and will help to promote the development of antivirus agents in the future. To achieve this, a Eu-containing polyoxometalate (EuW₁₀) was first co-assembled with a biogenic amine of spermine (Spm), which improved both the luminescence and antibacterial effect of EuW₁₀. Further introduction of a basic peptide from HPV E6, GL-22, induced more extensive enhancements, both of them being attributed to the cooperation and synergistic effects between the constituents, particularly the adaptive responses of assembly to the bacterial microenvironment (BME). Further intrinsic mechanism investigations revealed in detail that the encapsulation of EuW₁₀ in Spm and further GL-22 enhanced the uptake abilities of EuW₁₀ in bacteria, which further improved the ROS generation in BME via the abundant H₂O₂ involved there and significantly promoted the antibacterial effects.

Inhibition of SERCA and PMCA Ca2+-ATPase activities by polyoxotungstates

Plasma membrane calcium ATPases (PMCA) and sarco(endo) reticulum calcium ATPases (SERCA) are key proteins in the maintenance of calcium homeostasis. Herein, we compare for the first time the inhibition of SERCA and PMCA calcium pumps by several polyoxotungstates (POTs), namely by Wells-Dawson phosphotungstate anions [P₂W₁₈O₆₂]^6- (intact, {P₂W₁₈}), [P₂W₁₇O₆₁]^10- (monolacunary, {P₂W₁₇}), [P₂W₁₅O₅₆]^12- (trilacunary, {P₂W₁₅}), [H₂P₂W₁₂O₄₈]^12- (hexalacunary, {P₂W₁₂}), [H₃P₂W₁₅V₃O₆₂]^6- (trivanadium-substituted, {P₂W₁₅V₃}) and by Preyssler-type anion [NaP₅W₃₀O₁₁₀]^14- ({P₅W₃₀}). The speciation in the solutions of tested POTs was investigated by ³¹P and ⁵¹V NMR spectroscopy. The tested POTs inhibited SERCA Ca²⁺-ATPase activity, whereby the Preyssler POT showed the strongest effect, with an IC₅₀ value of 0.37 μM. For {P₂W₁₇} and {P₂W₁₅V₃} higher IC₅₀ values were determined: 0.72 and 0.95 μM, respectively. The studied POTs showed to be more potent inhibitors of PMCA Ca²⁺-ATPase activity, with lower IC₅₀ values for {P₂W₁₇}, {P₅W₃₀} and {P₂W₁₅V₃}.

Synthesis and bioactivities evaluation of oleanolic acid oxime ester derivatives as α-glucosidase and α-amylase inhibitors

Different oleanolic acid (OA) oxime ester derivatives (3a-3t) were designed and synthesised to develop inhibitors against α-glucosidase and α-amylase. All the synthesised OA derivatives were evaluated against α-glucosidase and α-amylase in vitro. Among them, compound 3a showed the highest α-glucosidase inhibition with an IC₅₀ of 0.35 µM, which was ∼1900 times stronger than that of acarbose, meanwhile compound 3f exhibited the highest α-amylase inhibitory with an IC₅₀ of 3.80 µM that was ∼26 times higher than that of acarbose. The inhibition kinetic studies showed that the inhibitory mechanism of compounds 3a and 3f were reversible and mixed types towards α-glucosidase and α-amylase, respectively. Molecular docking studies analysed the interaction between compound and two enzymes, respectively. Furthermore, cytotoxicity evaluation assay demonstrated a high level of safety profile of compounds 3a and 3f against 3T3-L1 and HepG2 cells.

Highlights

Oleanolic acid oxime ester derivatives (3a–3t) were synthesised and screened against α-glucosidase and α-amylase.

Compound 3a showed the highest α-glucosidase inhibitory with IC50 of 0.35 µM.

Compound 3f presented the highest α-amylase inhibitory with IC50 of 3.80 µM.

Kinetic studies and in silico studies analysed the binding between compounds and α-glucosidase or α-amylase.

Abrus precatorius Leaf Extract Reverses Alloxan/Nicotinamide-Induced Diabetes Mellitus in Rats through Hormonal (Insulin, GLP-1, and Glucagon) and Enzymatic (α-Amylase/α-Glucosidase) Modulation

BACKGROUND

Abrus precatorius is used in folk medicine across Afro-Asian regions of the world. Earlier, glucose lowering and pancreato-protective effects of Abrus precatorius leaf extract (APLE) was confirmed experimentally in STZ/nicotinamide-induced diabetic rats; however, the underlying mechanism of antidiabetic effect and pancreato-protection remained unknown.

OBJECTIVE

This study elucidated antidiabetic mechanisms and pancreato-protective effects of APLE in diabetic rats.

MATERIALS AND METHODS

APLE was prepared by ethanol/Soxhlet extraction method. Total phenols and flavonoids were quantified calorimetrically after initial phytochemical screening. Diabetes mellitus (DM) was established in adult Sprague-Dawley rats (weighing 120-180 g) of both sexes by daily sequential injection of nicotinamide (48 mg/kg; ip) and Alloxan (120 mg/kg; ip) over a period of 7 days. Except control rats which had fasting blood glucose (FBG) of 4.60 mmol/L, rats having stable FBG (16-21 mmol/L) 7 days post-nicotinamide/Alloxan injection were considered diabetic and were randomly reassigned to one of the following groups (model, APLE (100, 200, and 400 mg/kg, respectively; po) and metformin (300 mg/kg; po)) and treated daily for 18 days. Bodyweight and FBG were measured every 72 hours for 18 days. On day 18, rats were sacrificed under deep anesthesia; organs (kidney, liver, pancreas, and spleen) were isolated and weighed. Blood was collected for estimation of serum insulin, glucagon, and GLP-1 using a rat-specific ELISA kit. The pancreas was processed, sectioned, and H&E-stained for histological examination. Effect of APLE on enzymatic activity of alpha (α)-amylase and α-glucosidase was assessed. Antioxidant and free radical scavenging properties of APLE were assessed using standard methods.

RESULTS

APLE dose-dependently decreased the initial FBG by 68.67%, 31.07%, and 4.39% compared to model (4.34%) and metformin (43.63%). APLE (100 mg/kg) treatment restored weight loss relative to model. APLE increased serum insulin and GLP-1 but decreased serum glucagon relative to model. APLE increased both the number and median crosssectional area (×106 μm2) of pancreatic islets compared to that of model. APLE produced concentration-dependent inhibition of α-amylase and α-glucosidase relative to acarbose. APLE concentration dependently scavenged DPPH and nitric oxide (NO) radicals and demonstrated increased ferric reducing antioxidant capacity (FRAC) relative to standards.

CONCLUSION

Antidiabetic effect of APLE is mediated through modulation of insulin and GLP-1 inversely with glucagon, noncompetitive inhibition of α-amylase and α-glucosidase, free radical scavenging, and recovery of damaged/necro-apoptosized pancreatic β-cells.

Oleanolic acid indole derivatives as novel α-glucosidase inhibitors: Synthesis, biological evaluation, and mechanistic analysis

Research efforts have been directed to the development of oleanolic acid (OA) based α-glucosidase inhibitors and various OA derivatives showed improved anti-α-glucosidase activity. However, the inhibitory effects of indole infused OA derivatives on α-glucosidase is unknown. Herein, we synthesized a series of indole-OA (2a-2o) and -OA methyl ester (3a-3 l) derivatives with various electron withdrawing groups inducted to indole benzene ring and evaluated their anti-α-glucosidase activity. Indole OA derivatives (2a-2o) exhibited superior α-glucosidase inhibitory effects as compared to OA methyl ester derivatives (3a-3l) and OA (with IC₅₀ values of 4.02 μM-5.30 μM v.s. over 10 μM and 5.52 µM, respectively). In addition, mechanistic studies using biochemical (kinetic assay), biophysical (circular dichroism), and computational (docking) methods revealed that OA-indole derivatives (2a and 2f) are mixed type of α-glucosidase inhibitors and their inhibitory effects were attributed to their capacity of forming the ligand-enzyme complex with α-glucosidase enzyme. Findings from this study support that OA indole derivatives are promising α-glucosidase inhibitors as a potential management of diabetes mellitus.

Isolation, identification, and inhibitory enzyme activity of phenolic substances present in Spirulina

Spirulina species are edible with high nutritional as well as potential therapeutic values. In this work, we show that phenolic extracts from Spirulina (p-Coumaric acid) possessed inhibitory potential on α-glucosidase (IC₅₀  = 1.67 ± 0.02 mM) and tyrosinase (IC₅₀  = 52.71 ± 3.01 mM). Moreover, p-Coumaric acid inhibited α-glucosidase and tyrosinase in a reversible mixed-type manner. Interestingly, molecular docking demonstrated that p-Coumaric acid penetrated in depth of the active-site of tyrosinase and α-glucosidase by the noncovalent force or interaction. Among them, making polar interactions with Cu²⁺ ions and the amino acid residue capable of forming cation-π significantly contribute to the strong binding of p-Coumaric acid on tyrosinase. p-Coumaric acid was isolated and identified from Spirulina for the first time, which can be used as a lead compound for the design of functional food additives and skin-lightening active ingredient in cosmetics, and pharmaceuticals against type 2 diabetes. PRACTICAL APPLICATIONS: A natural, food-derived compound possessing the potential for the development of an anti-hyperglycaemic and skin-lightening supplement is very promising in cosmetics, functional food, and pharmaceuticals against type 2 diabetes. Herein, the present study is the first to present high levels of p-Coumaric acid from Spirulina, which simultaneously possessed inhibition potential on α-glucosidase and tyrosinase. Importantly, we gained initial information about the polypeptide-inhibitor interactions and underlying mechanisms for Spirulina's therapeutic effects, which will provide the bases for developing new drugs for preventing or treating type 2 diabetes and enzyme inhibitors. Moreover, this work also demonstrates the potential of the extraction of high-value chemicals from Spirulina waste.

Biological evaluation of Keggin-type polyoxometalates on tyrosinase: Kinetics and molecular modeling

Abnormal overexpression of tyrosinase activity can lead to the production of hyperpigmentation in human skin and enzymatic browning in fruits and vegetables. Herein, the inhibition and mechanism of the H₃ PMo₁₂ O₄₀ and two transition metal-substituted Keggin-type polyoxometalates (Na₇ PMo₁₁ CoO₄₀ and Na₇ PMo₁₁ ZnO₄₀ ) on tyrosinase were studied by kinetics and molecular modeling. Kinetic studies indicated that all compounds had more potent inhibitory activities than standard arbutin, and H₃ PMo₁₂ O₄₀ (IC₅₀  = 0.443 ± 0.006 mm) is ~15-fold stronger inhibition than arbutin. Additionally, all compounds inhibited tyrosinase in a reversible competitive manner. Intriguingly, molecular modeling elucidated that three compounds competitively bind to tyrosinase mainly through more interactions with Cu²⁺ ions and the amino acid residue capable of forming cation-π and hydrogen bonding, forming a reversible non-covalent complex. Molecular simulation study correlated well with the biological activity of three compounds in vitro. This work provided new insights into design and synthesis of polyoxometalates as tyrosinase inhibitors in the field of medicine, cosmetic, and food.

The Aquaporin-3-Inhibiting Potential of Polyoxotungstates

Polyoxometalates (POMs) are of increasing interest due to their proven anticancer activities. Aquaporins (AQPs) were found to be overexpressed in tumors bringing particular attention to their inhibitors as anticancer drugs. Herein, we report for the first time the ability of polyoxotungstates (POTs), such as of Wells-Dawson P2W18, P2W12, and P2W15, and Preyssler P5W30 structures, to affect aquaporin-3 (AQP3) activity and impair melanoma cell migration. The tested POTs were revealed to inhibit AQP3 function with different effects, with P2W18, P2W12, and P5W30 being the most potent (50% inhibitory concentration (IC50) = 0.8, 2.8, and 3.2 µM), and P2W15 being the weakest (IC50 > 100 µM). The selectivity of P2W18 toward AQP3 was confirmed in yeast cells transformed with human aquaglyceroporins. The effect of P2W12 and P2W18 on melanoma cells that highly express AQP3 revealed an impairment of cell migration between 55% and 65% after 24 h, indicating that the anticancer properties of these compounds may in part be due to the blockage of AQP3-mediated permeability. Altogether, our data revealed that P2W18 strongly affects AQP3 activity and cancer cell growth, unveiling its potential as an anticancer drug against tumors where AQP3 is highly expressed.

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.