An Optimized Two-Photon Fluorescent Probe for Biological Sensing and Imaging of Catechol-O-Methyltransferase

Rational engineering of isoform-specific hSULT1E1 fluorogenic substrates for functional analysis and inhibitor screening

Human estrogen sulfotransferase (hSULT1E1), an important conjugative enzyme, plays crucial roles in both estrogen homeostasis and xenobiotic metabolism. Herein, a rational substrate engineering strategy was adopted to construct highly specific fluorogenic substrates for hSULT1E1. In the 1st round of structure-based virtual screening, 4-hydroxyl-1,8-naphthalimide (4-HN) was identified as a suitable scaffold for constructing hSULT1E1 substrates. Subsequently, structural modifications on the north part of 4-HN generated a panel of derivatives as substrate candidates, in which HN-299 was identified as a highly selective fluorogenic substrate for hSULT1E1. In the 3rd round of structural optimization, a "molecular growth" strategy on the south part of HN-299 was used to develop a highly selective and reactive substrate (HN-375). Under physiological conditions, HN-375 could be readily sulfated by hSULT1E1 to generate a single fluorescent product, which emitted bright green signals at around 510 nm and was fully identified as HN-375 4-O-sulfate (HNS). Further investigations indicated that HN-375 exhibited excellent isoform-specificity, rapid-response, ultrahigh sensitivity, and high signal-to-noise ratio, and as such was subsequently used for sensing SULT1E1 activity in hepatocellular carcinoma specimens and live organs. With HN-375 in hand, a practical fluorescence-based assay was established for high-throughput screening and characterization of hSULT1E1 inhibitors, as such two potent hSULT1E1 inhibitors were identified from in-house compound libraries. Collectively, this study showcases a groundbreaking strategy for engineering highly specific and sensitive fluorogenic substrates for target conjugative enzyme(s), while HN-375 emerges as a practical tool for sensing SULT1E1 activity in a biological context and for the high-throughput screening of inhibitors.

Multisubstrate-based system: a kinetic mechanism study of catechol-O-methyltransferase

Catechol-O-methyltransferase (COMT, EC 2.1.1.6) can transfer the methyl group from S-adenosyl-l-methionine (SAM) to one of the hydroxyl groups of a catechol substrate in the presence of Mg2+. However, there is no consensus view of the kinetic mechanism of COMT involving multiple substrates. Further progress requires the development of methods for determining enzyme kinetic behavior and the binding mode of ligands to the protein. Here, we establish a multisubstrate kinetic system covering the fluorescence and mass spectrometry techniques to quantify the products in a COMT-catalyzed reaction. The catechol substrate, 3-BTD, can be methylated by COMT to form a single product, 3-BTMD, with a sensitive fluorescence response and the conversion of SAM to S-adenosyl-l-homocysteine (SAH) was monitored by LC-MS/MS. The kinetic assays suggested that the reaction occurred via an ordered sequential mechanism, in which SAM first bound to COMT, followed by the addition of Mg2+ and 3-BTD. The chemical step involved a quaternary complex of COMT-SAM-Mg2+-3-BTD, followed by the ordered dissociation of 3-BTMD, Mg2+, and SAH. In cooperation with molecular dynamics simulation, the binding of COMT to Mg2+ induced a shape change in the catechol-binding site, which accommodated 3-BTD binding and facilitated catalysis. These findings provide new insights into the kinetic mechanism of COMT, contributing to the development of previously undescribed functional COMT ligands.

An activity-based sensing fluorogenic probe for monitoring O-methyltransferase in plants

Activity-based sensing probes are powerful tools for monitoring enzymatic activities in complex biological samples such as cellular and live animals; however, their application in plants remains challenging. Herein, fourteen activity-based fluorescent probes were assayed against Arabidopsis O-methyltransferases (AtOMTs). One probe, 3-BTD, displayed a high selectivity, reactivity, and fluorescence response toward AtOMTs especially the isoform AtCCoAOMT. We further characterized the features of this probe and explored whether it could be used to detect OMT activities in living plant cells. Our results show that 3-BTD can be used to visualize OMT activity in Arabidopsis, and no fluorescent signal was observed in the comt/ccoaomt double mutant, indicating that it has good specificity. Interestingly, in contrast to the observation that AtCCoAOMT-YFP accumulated in both cytoplasm and nucleus, OMT enzymatic activity tracked by 3-BTD probe was found only in the cytoplasm. This underscores the importance of activity-based sensing in studying protein function. Moreover, 3-BTD can be successfully applied in OMT visualization of different plants. This study indicates that 3-BTD can serve as a potential probe for in situ monitoring the real activity of OMT in multiple plants and provides a strategy for visualizing the activity of other enzymes in plants.

Exploring Nanocarriers for Boosting Entacapone Bioavailability: A Journey through System Characterization and Assessment of Toxicity and Pharmacological and 2D Permeability Paybacks

Catechol-O-methyltransferase inhibitors (iCOMT), such as entacapone, have been successfully employed to treat tremor-related symptoms of Parkinson's disease. However, iCOMT has been associated with a short half-life and poor oral bioavailability. Nanobased drug delivery systems have often been used to overcome this type of setbacks. Therefore, entacapone was encapsulated in PEGylated poly(lactic-co-glycolic acid) (PLGA)-based nanoparticles (NPs) via a nanoprecipitation process, as well as in PEGylated nanostructured lipid carriers (NLCs) using a solvent emulsification/evaporation method. Both nanoformulations presented sub-200 nm populations, with zeta-potential (ZP) values close to -30 mV, and showed stability at different pHs, while maintaining their physicochemical properties mostly intact, presenting only a change in their superficial charge (ZP values), indicating their interaction. Both nanoformulations presented interaction with mucins, which anticipates good permeation and bioavailability for oral and topical administration. No cytotoxic effects were observed for lyophilized PLGA NPs encapsulating entacapone, in which 2-hydroxypropyl-ß-cyclodextrin (HPβCD) was used as a cryoprotectant at 3% concentration (HP-PLGA@Ent), in human hepatocellular carcinoma (HepG2), human neuroblastoma (SH-SY5Y), or human epithelial colorectal adenocarcinoma (Caco-2) cell lines. Conversely, NLCs encapsulating entacapone (W-NLCs@Ent) presented cytotoxic effects on the HepG2 cell line, likely due to intracellular lipid accumulation or storage. Both nanoformulations maintained a COMT inhibition effect in HepG2 cells, using 3-BTD as the COMT probe. An increase of entacapone permeability in both monolayer and coculture models (Caco-2 and Caco-2/HT29-MTX, respectively) was observed for the developed nanoformulations. Overall, this work shows that encapsulated entacapone in different nanocarriers could be a stimulating alternative to solve entacapone setbacks, since they improve its physicochemical properties and permeability while still maintaining the COMT inhibitory activity.

The molecular effects underlying the pharmacological activities of daphnetin

As an increasingly well-known derivative of coumarin, daphnetin (7,8-dithydroxycoumarin) has demonstrated various pharmacological activities, including anti-inflammation, anti-cancer, anti-autoimmune diseases, antibacterial, organ protection, and neuroprotection properties. Various studies have been conducted to explore the action mechanisms and synthetic methods of daphnetin, given its therapeutic potential in clinical. Despite these initial insights, the precise mechanisms underlying the pharmacological activities of daphnetin remain largely unknown. In order to address this knowledge gap, we explore the molecular effects from the perspectives of signaling pathways, NOD-like receptor protein 3 (NLRP3) inflammasome and inflammatory factors; and try to find out how these mechanisms can be utilized to inform new combined therapeutic strategies.

Bioactive components and mechanisms of Pu-erh tea in improving levodopa metabolism in rats through COMT inhibition

Catechol-O-methyltransferase (COMT) plays a central role in the metabolic inactivation of endogenous neurotransmitters and xenobiotic drugs and hormones having catecholic structures. Its inhibitors are used in clinical practice to treat Parkinson's disease. In this study, a fluorescence-based visualization inhibitor screening method was developed to assess the inhibition activity on COMT both in vitro and in living cells. Following the screening of 94 natural products, Pu-erh tea extract exhibited the most potent inhibitory effect on COMT with an IC50 value of 0.34 μg mL-1. In vivo experiments revealed that Pu-erh tea extract substantially hindered COMT-mediated levodopa metabolism in rats, resulting in a significant increase in levodopa levels and a notable decrease in 3-O-methyldopa in plasma. Subsequently, the chemical components of Pu-erh tea were analyzed using UHPLC-Q-Exactive Orbitrap HRMS, identifying 24 major components. Among them, epigallocatechin gallate, gallocatechin gallate, epicatechin gallate, and catechin gallate exhibited potent inhibition of COMT activity with IC50 values from 93.7 nM to 125.8 nM and were the main bioactive constituents in Pu-erh tea responsible for its COMT inhibition effect. Inhibition kinetics analyses and docking simulations revealed that these compounds competitively inhibit COMT-mediated O-methylation at the catechol site. Overall, this study not only explained how Pu-erh tea catechins inhibit COMT, suggesting Pu-erh tea as a potential dietary intervention for Parkinson's disease, but also introduced a new strategy for discovering COMT inhibitors more effectively.

Rescuing a Troubled Tolcapone with PEGylated PLGA Nanoparticles: Design, Characterization, and Hepatotoxicity Evaluation

Tolcapone is an orally active catechol-O-methyltransferase (COMT) inhibitor used as adjuvant therapy in Parkinson's disease. However, it has a highly hepatotoxic profile, as recognized by the U.S. Food and Drug Administration. As a possible solution, nanoscience brought us several tools in the development of new functional nanomaterials with tunable physicochemical properties, which can be part of a solution to solve several drawbacks, including drug's short half-life and toxicity. This work aims to use PEGylated poly(lactic-co-glycolic acid) (PLGA) nanoparticles as a stable carrier with lower hydrodynamic size and polydispersity to encapsulate tolcapone in order to overcome its therapeutic drawbacks. Using the nanoprecipitation method, tolcapone-loaded nanoparticles with a DLC% of 5.7% were obtained (EE% of 47.0%) and subjected to a lyophilization optimization process to obtain a final shelf-stable formulation. Six different cryoprotectants in concentrations up to 10% (w/v) were tested. A formulation of PLGA nanoparticles with 3% hydroxypropyl-β-cyclodextrin (HPβCD) as a cryoprotectant (PLGA-HP@Tolc), presenting sub-200 nm sizes and low polydispersity (PdI < 0.200) was selected. Cytotoxicity assays, namely, MTT and SRB, were used to study the metabolic activity and cell density of tolcapone and PLGA-HP@Tolc-treated cells. In both assays, a hepatocarcinoma cell line (HepG2) growing in glucose or glucose-free media (galactose-supplemented medium) was used. The results demonstrated that the treatment with the PLGA-HP@Tolc formulation led to a decrease in cytotoxicity in comparison to free tolcapone-treated cells in both media tested. Moreover, the elected formulation also counteracted ATP-depletion and excessive ROS production induced by tolcapone. The results suggest that HPβCD might have a dual function in the formulation: cryoprotectant and anticytotoxic agent, protecting cells from tolcapone-induced damage. Using an in vitro COMT inhibition assay, the PLGA-HP@Tolc formulation demonstrated to inhibit COMT as efficiently as free tolcapone. Overall, the results suggest that tolcapone-loaded PLGA NPs could be an interesting alternative to free tolcapone, demonstrating the same in vitro efficacy in inhibiting COMT but with a safer cytotoxic profile.

More than double the fun with two-photon excitation microscopy

For generations researchers have been observing the dynamic processes of life through the lens of a microscope. This has offered tremendous insights into biological phenomena that span multiple orders of time- and length-scales ranging from the pure magic of molecular reorganization at the membrane of immune cells, to cell migration and differentiation during development or wound healing. Standard fluorescence microscopy techniques offer glimpses at such processes in vitro, however, when applied in intact systems, they are challenged by reduced signal strengths and signal-to-noise ratios that result from deeper imaging. As a remedy, two-photon excitation (TPE) microscopy takes a special place, because it allows us to investigate processes in vivo, in their natural environment, even in a living animal. Here, we review the fundamental principles underlying TPE aimed at basic and advanced microscopy users interested in adopting TPE for intravital imaging. We focus on applications in neurobiology, present current trends towards faster, wider and deeper imaging, discuss the combination with photon counting technologies for metabolic imaging and spectroscopy, as well as highlight outstanding issues and drawbacks in development and application of these methodologies.

A rationally engineered specific near-infrared fluorogenic substrate of human pancreatic lipase for functional imaging and inhibitor screening

Obesity, now widespread all over the world, is frequently associated with several chronic diseases. Human pancreatic lipase (hPL) is a crucial digestive enzyme responsible for the digestion of dietary lipids in humans, and the inhibition of hPL is effective in reducing triglyceride intake and thus preventing and treating obesity. In this work, a practical sequential screening strategy was developed to construct a highly selective near-infrared fluorogenic substrate 7-STCFC for hPL. Under physiological conditions, 7-STCFC can be rapidly hydrolyzed by hPL to form 7-HTCFC, which triggers 254-fold NIR signal enhancement at 670 nm. 7-STCFC was successfully applied for the sensing and imaging of endogenous PL in living systems (including living cells, tissues and organs) with low cytotoxicity and high imaging resolution. Moreover, a high-throughput screening platform was established using 7-STCFC, and the inhibitory effects of 94 kinds of herbs toward hPL were evaluated. Among them, Pu-erh tea stood out with outstanding hPL inhibitory effects, and the inhibitory ingredients and involved inhibitory mechanism were further revealed, which strongly facilitates the discovery of novel anti-obesity agents targeting hPL. Collectively, these findings suggested that our strategy was practical to develop an isoform-specific fluorogenic substrate for a target enzyme, and 7-STCFC was a powerful tool for monitoring PL activity in complex biological systems with value for exploring physiological functions and rapid screening of inhibitors.

Functional Imaging and Inhibitor Screening of Human Pancreatic Lipase by a Resorufin-Based Fluorescent Probe

Human pancreatic lipase (hPL) is a crucial digestive enzyme responsible for the digestion of dietary lipids in humans, and inhibition of hPL is effective in reducing triglyceride intake, thereby preventing and treating obesity. In this study, a series of fatty acids with different carbon chain lengths were constructed to the fluorophore resorufin based on the substrate preference of hPL. Among them, RLE was found to have the best combination of stability, specificity, sensitivity and reactivity towards hPL. Under physiological conditions, RLE can be rapidly hydrolyzed by hPL and released to resorufin, which triggered approximately 100-fold fluorescence enhancement at 590 nm. RLE was successfully applied for sensing and imaging of endogenous PL in living systems with low cytotoxicity and high imaging resolution. Moreover, a visual high-throughput screening platform was established using RLE, and the inhibitory effects of hundreds of drugs and natural products toward hPL were evaluated. Collectively, this study reports a novel and highly specific enzyme-activatable fluorogenic substrate for hPL that could serve as a powerful tool for monitoring hPL activity in complex biological systems and showcases the potential to explore physiological functions and rapid screening of inhibitors.

Catechol-O-methyltransferase Inhibitors from Calendula officinalis Leaf

Calendula officinalis is commonly known as marigold and its flowers are used in herbal medicines, cosmetics, perfumes, dyes, pharmaceutical preparations, and food products. However, the utility of its leaves has not been studied in depth. The purpose of the present study was to identify the major compounds in C. officinalis leaves and to determine the inhibitory properties of the isolated compounds toward human catechol-O-methyltransferase (COMT), a key neurotransmitter involved in Parkinson's disease and depression. We isolated and identified ten compounds, including two phenylpropanoids and seven flavonoids, from C. officinalis leaf extracts, of which four flavonoids were identified from C. officinalis leaves for the first time. Eight compounds exhibited COMT inhibitory activities with IC₅₀ values of less than 100 μM. Our results indicate that compounds in C. officinalis leaves are potentially effective for preventing Parkinson's disease and depression. Thus, C. officinalis leaves may hold promise as dietary supplements.

Hydroxylated Coumarin-Based Thiosemicarbazones as Dual Antityrosinase and Antioxidant Agents

The design of novel antityrosinase agents appears extremely important in medical and industrial sectors because an irregular production of melanin is related to the insurgence of several skin-related disorders (e.g., melanoma) and the browning process of fruits and vegetables. Because melanogenesis also involves a nonenzymatic oxidative process, developing dual antioxidant and antityrosinase agents is advantageous. In this work, we evaluated the antioxidant and tyrosinase inhibition ability of two new bishydroxylated and two new monohydroxylated derivatives of (1E)-2-(1-(2-oxo-2H-chromen-3-yl)ethylidene)hydrazine-1-carbothioamide (T1) using different experimental and computational approaches. The study was also carried out on another monohydroxylated derivative of T1 for comparison. Interestingly, these molecules have more potent tyrosinase-inhibitory properties than the reference compound, kojic acid. Moreover, the antioxidant activity appears to be influenced according to the number and substitution pattern of the hydroxyl groups. The safety of the compounds without (T1), with one (T3), and with two (T6) hydroxyl groups, has also been assessed by studying their cytotoxicity on melanocytes. These results indicate that (1E)-2-(1-(2-oxo-2H-chromen-3-yl)ethylidene)hydrazine-1-carbothioamide and its hydroxylated derivatives are promising molecules for further drug development studies.

Daphnetin, a Coumarin in Genus Stellera Chamaejasme Linn: Chemistry, Bioactivity and Therapeutic Potential

Coumarins is a huge family of phenolic compounds containing a common structure of 2 H -1-benzopyran-2-one. Nowadays, more than 1,300 natural-based coumarins have been identified in a variety of plants, bacteria and fungi, many of them exhibited promising biomedical performance. Daphnetin (7,8-dihydroxycoumarin) is a typical coumarin associated with a couple of bioactivities such as anti-cancer, antibacterial, anti-inflammatory and anti-arthritis. In the treatment of diseases, it has been verified that daphnetin has outstanding therapeutic effects on diabetes, arthritis, transplant rejection, cancer and even on central nervous system diseases. Herein, we summarized the chemical synthetic methodologies, bioactivities, therapeutic potentials and structure-activity relationships of daphnetin and its derivatives. Hopefully, this review would be beneficial for the discovery of new coumarin-based biomedicine in the near future.

Thermostability and catechol-O-methyltransferase inhibitory activity of acylated anthocyanins from purple yam

The thermostability of purple yam was investigated to be used as natural colorants. In addition, the inhibitory properties of purple yam and its isolated anthocyanins toward human catechol-O-methyltransferase (COMT), a key neurotransmitter involved in Parkinson's disease and depression, were also investigated. The thermostability of purple yam was higher than that of the reference samples (purple sweet potato and purple potato). Quantitative HPLC analysis revealed that alatanin A (2) contributed to the thermostability of purple yam. Methanol extracts of purple yam exhibited the highest COMT inhibitory activity of the tested samples. Alatanin D (1) showed the highest inhibitory activity of the anthocyanins in purple yam (IC50 19 µM). This study revealed the thermostability and COMT inhibitory activity of purple yam and may lead to its use not only as a thermostable natural source of colorants, but also for the prevention and treatment of Parkinson's disease and depression.

Optical substrates for drug-metabolizing enzymes: Recent advances and future perspectives

Drug-metabolizing enzymes (DMEs), a diverse group of enzymes responsible for the metabolic elimination of drugs and other xenobiotics, have been recognized as the critical determinants to drug safety and efficacy. Deciphering and understanding the key roles of individual DMEs in drug metabolism and toxicity, as well as characterizing the interactions of central DMEs with xenobiotics require reliable, practical and highly specific tools for sensing the activities of these enzymes in biological systems. In the last few decades, the scientists have developed a variety of optical substrates for sensing human DMEs, parts of them have been successfully used for studying target enzyme(s) in tissue preparations and living systems. Herein, molecular design principals and recent advances in the development and applications of optical substrates for human DMEs have been reviewed systematically. Furthermore, the challenges and future perspectives in this field are also highlighted. The presented information offers a group of practical approaches and imaging tools for sensing DMEs activities in complex biological systems, which strongly facilitates high-throughput screening the modulators of target DMEs and studies on drug/herb‒drug interactions, as well as promotes the fundamental researches for exploring the relevance of DMEs to human diseases and drug treatment outcomes.

Chemical Profiles of Korean Bee Pollens and Their Catechol-O-methyltransferase Inhibitory Activities

Bee pollen is an apicultural product collected by honeybees from flower stamens and is consumed to help maintain a healthy diet. In this study, the chemical profiles of 11 Korean bee pollens were investigated using molecular networking analysis. This analysis elucidated the presence of two major clusters, hydroxycinnamoyl acid amides (HCAAs, molecular network 1 (MN₁)) and flavonoid glycosides (MN₂), in the bee pollen samples. The inhibitory properties of the bee pollens and the isolated HCAAs toward human catechol-O-methyltransferase (COMT), a key neurotransmitter involved in Parkinson's disease and depression, were determined. N¹,N⁵,N¹⁰-(E)-tricaffeoylspermidine ((E,E,E)-1) exhibited the highest activity of the four compounds isolated, with an IC₅₀ value 16 μM, and inhibited COMT competitively. Quantitative analysis of HCAAs showed that the amounts of N¹,N¹⁰-dicaffeoyl-N⁵-p-coumaroylspermidine (2) and N¹⁰-caffeoyl-N¹,N⁵-di-p-coumaroylspermidine (3) contributed to the observed differences in the COMT inhibitory activities of Korean bee pollens. This study may lead to the prevention and treatment of Parkinson's disease and depression using bee pollens.

Inhibition of catechol-O-methyltransferase by natural pentacyclic triterpenes: structure-activity relationships and kinetic mechanism

Inhibitors of COMT are clinically used for the treatment of Parkinson's disease. Here, we report the first natural pentacyclic triterpenoid-type COMT inhibitors and their structure-activity relationships and inhibition mechanism. The most potent compounds were found to be oleanic acid, betulinic acid and celastrol with IC₅₀ values of 3.89-5.07 μM, that acted as mixed (uncompetitive plus non-competitive) inhibitors of COMT, representing a new skeleton of COMT inhibitor. Molecular docking suggested that they can specifically recognise and bind with the unique hydrophobic residues surrounding the catechol pocket. Furthermore, oleanic acid and betulinic acid proved to be less disruptive of mitochondrial membrane potential (MMP) compared to tolcapone, thus reducing the risk of liver toxicity. These findings could be used to produce an ideal lead compound and to guide synthetic efforts in generating related derivatives for further preclinical testing.

Characterization of Dopamine Receptor Associated Drugs on the Proliferation and Apoptosis of Prostate Cancer Cell Lines

BACKGROUND

Dopamine Receptor (DR) gene family play an essential role in the regulation of Interleukin- 6 (IL-6) production. Our prior analysis of human prostate biopsy samples demonstrated the increased expression of IL-6 and a downregulating trend for dopamine receptor gene family.

OBJECTIVE

The objective was to investigate the expression of dopamine receptors, their catabolizing enzyme and IL-6 in prostate cancer cell lines and assess pharmacological effect of dopamine receptor modulators as a novel class of drugs repurposed for the treatment of prostate cancer.

METHODS

The therapeutic effect of dopamine, DR agonists, and DR antagonist were examined using LNCaP and PC3 cell lines. Cell viability and proliferation were assessed by MTT assay and proliferating cell nuclear antigen expression analysis, respectively. Furthermore, bax/bcl2 ratio, immunofluorescence assay and flow cytometric assay were performed for apoptosis analysis. RT- qPCR analysis was used to characterize the relative expression of dopamine-related genes, catabolic enzyme Catechol-o-Methyl-Transferase (COMT) and IL-6 before and after treatment to assess the therapeutic effects of drugs.

RESULTS

LNCaP cells express DRD1, DRD2, DRD5 and COMT genes and PC3 cells only express IL-6 gene. In-vitro, dopamine receptor agonists reduced cell viability of LNCaP and PC3 cells. In contrast, dopamine and dopamine receptor antagonist significantly increased tumor growth in PC3 cells.

CONCLUSION

Our results offer novel suggestion for a pathogenic role of dopamine receptor signaling in prostate cancer adenocarcinoma and indicates that modulators of DR- IL-6 pathway, including FDA-approved drug bromocriptine, might be utilized as novel drug repurposing strategy.

Rapid Two-Photon Fluorescence Imaging of Monoamine Oxidase B for Diagnosis of Early-Stage Liver Fibrosis in Mice

Liver fibrosis could induce cirrhosis and liver cancer, causing serious damages to liver function and even death. Early diagnosis of fibrosis is extremely requisite for optimizing treatment schedule to improve cure rate. In early-stage fibrosis, overexpressed monoamine oxidase B (MAO-B) can serve as a biomarker, which greatly contributes to the diagnosis of early liver fibrosis. However, there is still a lack of desired strategy to precisely monitor MAO-B in situ. In this work, we established a two-photon fluorescence imaging method for in vivo detection of MAO-B activity counting on a simply prepared probe, BiPhAA. The BiPhAA could be activated by MAO-B within 10 min and fluoresced brightly. To our knowledge, this BiPhAA-based imaging platform for MAO-B is more rapid than other current detection methods. Furthermore, BiPhAA allowed the dynamic observation of endogenous MAO-B level changes in hepatic stellate cells (LX-2). Through two-photon fluorescence imaging, we observed six times higher fluorescence brightness in the liver tissue of fibrosis mice than that of normal mice, thus successfully distinguishing mice with liver fibrosis from normal mice. Our work offers a simple, fast, and highly sensitive approach for imaging MAO-B in situ and paves a way to the diagnosis of early liver fibrosis with accuracy.

Discovery and characterization of flavonoids in vine tea as catechol-O-methyltransferase inhibitors

Vine tea has been used as a traditionally functional herbal tea in China for centuries, which exhibits paramount potential for chronic metabolic diseases. Herein, the inhibitory potential of vine tea toward human catechol-O-methyltransferase (hCOMT) was investigated. A practical bioactivity-guided fractionation combined with chemical profiling strategy was developed to identify the naturally occurring hCOMT inhibitors. Five flavonoids in vine tea displayed moderate to strong inhibition on hCOMT with IC₅₀ values ranging from 0.96 μM to 42.47 μM, in which myricetin was the critically potent constituent against hCOMT. Inhibition kinetics assays and molecular docking simulations showed that myricetin could bind to the active site of COMT and inhibited COMT-catalyzed 3-BTD methylation in a mixed manner. Collectively, our findings not only suggested that the strong hCOMT inhibition of vine tea has guiding significance in the drug exposure of catechol drugs, but also identified a promising lead compound for developing more efficacious hCOMT inhibitors.

Discovery and characterization of naturally occurring potent inhibitors of catechol-O-methyltransferase from herbal medicines

Human catechol-O-methyltransferase (hCOMT) is considered a therapeutic target due to its crucial roles in the metabolic inactivation of endogenous neurotransmitters and xenobiotic drugs. There are nevertheless few safe and effective COMT inhibitors and there lacks a diversity in structure. To discover novel safe and effective hCOMT inhibitors from herbal products, in this study, 53 herbal products were collected and their inhibitory effects against hCOMT were investigated. Among them, Scutellariae radix (SR) displayed the most potent inhibitory effect on hCOMT with an IC50 value of 0.75 μg mL-1. To further determine specific chemicals as COMT inhibitors, an affinity ultrafiltration coupled with liquid chromatography-mass spectrometry method was developed and successfully applied to identify COMT inhibitors from SR extract. The results demonstrated that scutellarein 2, baicalein 9 and oroxylin A 12 were potent COMT inhibitors, showing a high binding index (>3) and very low IC50 values (32.9 ± 3.43 nM, 37.3 ± 4.32 nM and 18.3 ± 2.96 nM). The results of inhibition kinetics assays and docking simulations showed that compounds 2, 9 and 12 were potent competitive inhibitors against COMT-mediated 3-BTD methylation, and they could stably bind to the active site of COMT. These findings suggested that affinity ultrafiltration allows a rapid identification of natural COMT inhibitors from a complex plant extract matrix. Furthermore, scutellarein 2, baicalein 9 and oroxylin A 12 are potent inhibitors of hCOMT in SR, which could be used as promising lead compounds to develop more efficacious non-nitrocatechol COMT inhibitors for biomedical applications.

Analytical methodologies for sensing catechol-O-methyltransferase activity and their applications

Mammalian catechol-O-methyltransferases (COMT) are an important class of conjugative enzymes, which play a key role in the metabolism and inactivation of catechol neurotransmitters, catechol estrogens and a wide range of endobiotics and xenobiotics that bear the catechol group. Currently, COMT inhibitors are used in combination with levodopa for the treatment of Parkinson's disease in clinical practice. The crucial role of COMT in human health has raised great interest in the development of more practical assays for highly selective and sensitive detection of COMT activity in real samples, as well as for rapid screening and characterization of COMT inhibitors as drug candidates. This review summarizes recent advances in analytical methodologies for sensing COMT activity and their applications. Several lists of biochemical assays for measuring COMT activity, including the probe substrates, along with their analytical conditions and kinetic parameters, are presented. Finally, the challenges and future perspectives in the field, such as visualization of COMT activity in vivo and in situ, are highlighted. Collectively, this review article overviews the practical assays for measuring COMT activities in complex biological samples, which will strongly facilitate the investigations on the relevance of COMT to human diseases and promote the discovery of COMT inhibitors via high-throughput screening.

Accurate and sensitive detection of Catechol-O-methyltransferase activity by liquid chromatography with fluorescence detection

Catechol-O-methyltransferase (COMT) is a major drug metabolizing enzyme in humans. COMT expression is directedly associated with various mental diseases and cancers due to its essential role in catalyzing metabolic inactivation of endogenous catecholamines and catechol estrogens. However, a practical method to precisely measure COMT activities in biological samples is lacking. In the current study, we established a liquid chromatography-fluorescence detection (LC-FD) method based on fluorometric detection of the methylated product of 3-BTD, a fluorescent probe for COMT, to sensitively quantify COMT activities in human erythrocytes and cell homogenates. Assay validation of the established LC-FD based method was conducted for selectivity and sensitivity, range of linearity, precision and accuracy, recovery, biological matrices effect and stability. The limit of quantification for 3-BTMD (the methylated product of 3-BTD by COMT) of this method was 0.0083 nM, which is nearly 10-fold lower than that for previously published methods. The method was precise with intra- and inter-day relative standard deviation (RSD) lower than 5%. In addition, this method showed an excellent anti-interference ability with no effects of the endogenous substances on the fluorometric detection of 3-BTMD. The practical use of this method was established by its successful application for the measurement of COMT activities in individual human erythrocytes (n = 13), and in cell homogenates generated from four different human cell lines. Our results suggest that this method will be of great value in accurately determining the native activity of COMT in biological samples, which is beneficial for a complete understand of the role of COMT both in physiological and pathological conditions.

Design and applications of a novel fluorescent probe for detecting glutathione in biological samples

This study aimed to develop a novel and practical fluorescent method for GSH detection in complex biological samples. To this end, a series of coumarin-based fluorescent probes was designed and synthesized using various aliphatic halogens as the sensing group. By using a new evaluation method of GSH/Cys/Hcy coexisting conditions, the probe with chloropropionate (CBF3) showed a high selectivity, excellent sensitivity, good stability for GSH detection. The reaction mechanism is proposed as nucleophilic substitution/cyclization and intramolecular charge transfer (ICT), which was confirmed by LC-MS and NMR analysis, as well as density functional theory calculations. In addition, CBF3 was demonstrated to be competent not only for the quantitative detection of GSH in real serum samples, but also for sensing GSH changes in different oxidative stress models in living cells and nematodes. This study showed a practical strategy for constructing GSH-specific fluorescent probes, and provided a sensitive tool for real-time sensing of GSH in real biological samples. The findings would greatly facilitate further investigations on GSH-associated clinical diagnosis and biomedical studies.

Design, Synthesis, and Cholinesterase Inhibition Assay of Coumarin-3-carboxamide-N-morpholine Hybrids as New Anti-Alzheimer Agents

A new series of coumarin-3-carboxamide-N-morpholine hybrids 5a-5l was designed and synthesized as cholinesterases inhibitors. The synthetic approach for title compounds was started from the reaction between 2-hydroxybenzaldehyde derivatives and Meldrum's acid to afford corresponding coumarin-3-carboxylic acids. Then, amidation of the latter compounds with 2-morpholinoethylamine or N-(3-aminopropyl)morpholine led to the formation of the compounds 5a-5l. The in vitro inhibition screen against acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) revealed that most of the synthesized compounds had potent AChE inhibitory while their BuChE inhibitions are moderate to weak. Among them, propylmorpholine derivative 5g (N-[3-(morpholin-4-yl)propyl]-2-oxo-2H-chromene-3-carboxamide) bearing an unsubstituted coumarin moiety and ethylmorpholine derivative 5d (6-bromo-N-[2-(morpholin-4-yl)ethyl]-2-oxo-2H-chromene-3-carboxamide) bearing a 6-bromocoumarin moiety showed the most activity against AChE and BuChE, respectively. The inhibitory activity of compound 5g against AChE was 1.78 times more than that of rivastigmine and anti-BuChE activity of compound 5d is approximately same as rivastigmine. Kinetic and docking studies confirmed the dual binding site ability of compound 5g to inhibit AChE.

Recent progress and challenges in screening and characterization of UGT1A1 inhibitors

Uridine-diphosphate glucuronosyltransferase 1A1 (UGT1A1) is an important conjugative enzyme in mammals that is responsible for the conjugation and detoxification of both endogenous and xenobiotic compounds. Strong inhibition of UGT1A1 may trigger adverse drug/herb-drug interactions, or result in metabolic disorders of endobiotic metabolism. Therefore, both the US Food and Drug Administration (FDA) and the European Medicines Agency (EMA) have recommended assaying the inhibitory potential of drugs under development on the human UGT1A1 prior to approval. This review focuses on the significance, progress and challenges in discovery and characterization of UGT1A1 inhibitors. Recent advances in the development of UGT1A1 probes and their application for screening UGT1A1 inhibitors are summarized and discussed in this review for the first time. Furthermore, a long list of UGT1A1 inhibitors, including information on their inhibition potency, inhibition mode, and affinity, has been prepared and analyzed. Challenges and future directions in this field are highlighted in the final section. The information and knowledge that are presented in this review provide guidance for rational use of drugs/herbs in order to avoid the occurrence of adverse effects via UGT1A1 inhibition, as well as presenting methods for rapid screening and characterization of UGT1A1 inhibitors and for facilitating investigations on UGT1A1-ligand interactions.

Synthesis and Structure-Activity Relationship of Daphnetin Derivatives as Potent Antioxidant Agents

In this study, daphnetin 1 was chosen as the lead compound, and C-3 or C-4-substituted daphnetins were designed and synthesized to explore the potential relationship between the antioxidant activities and the chemical structures of daphnetin derivatives. The antioxidant activities of the generated compounds were evaluated utilizing the free radical scavenging effect on 2,2'-diphenyl-1-picrylhydrazyl, 2,2'-azinobis-(3-ethylbenzthiazoline-6-sulfonate) cation, and the ferric reducing power assays, and were then compared with those of the standard antioxidant Trolox. The results showed that the catechol group was the key pharmacophore for the antioxidant activity of the daphnetins. The introduction of an electron-withdrawing hydrophilic group at the C-4 position of daphnetin enhanced the antioxidative capacity, but this trend was not observed for C-3 substitution. In addition, introduction of a a hydrophobic phenyl group exerted negative effects on the antioxidant activity in both the C-3 and C-4 substitutions. Among all of the derivatives tested, the most powerful antioxidant was 4-carboxymethyl daphnetin (compound 9), for which the strongest antioxidant activity was observed in all of the assays. In addition, compound 9 also displayed strong pharmaceutical properties in the form of metabolic stability. To summarize, compound 9 holds great potential to be developed as an antioxidant agent with excellent antioxidant activity and proper pharmacokinetic behavior.

Chemical Probes for Human UDP-Glucuronosyltransferases: A Comprehensive Review

UGTs play crucial roles in the metabolism and detoxification of both endogenous and xenobiotic compounds. The key roles of UGTs in human health have garnered great interest in the design and development of specific probes for human UGTs. However, in contrast to other human enzymes, the probe substrates for human UGTs are rarely reported, owing to the highly overlapping substrate specificities of UGTs and the lack of the integrated crystal structures of UGTs. Over the past decades, many efforts are made to develop specific probe substrates for UGTs and use them in both basic research and drug discovery. This review focuses on recent progress in the development of probe substrates for UGTs and their biomedical applications. A long list of chemical probes for UGTs, including non-fluorescent and fluorescent probes along with their structural information and kinetic parameters, are prepared and analyzed. Additionally, challenges and future directions in this field are highlighted in the final section. All information and knowledge presented in this review provide practical tools/methods for measuring UGT activities in complex biological samples, which will be very helpful for rapid screening and characterization of UGT modulators, and for exploring the relevance of UGT enzymes to human diseases.

Human carboxylesterases: a comprehensive review

Mammalian carboxylesterases (CEs) are key enzymes from the serine hydrolase superfamily. In the human body, two predominant carboxylesterases (CES1 and CES2) have been identified and extensively studied over the past decade. These two enzymes play crucial roles in the metabolism of a wide variety of endogenous esters, ester-containing drugs and environmental toxicants. The key roles of CES in both human health and xenobiotic metabolism arouse great interest in the discovery of potent CES modulators to regulate endobiotic metabolism or to improve the efficacy of ester drugs. This review covers the structural and catalytic features of CES, tissue distributions, biological functions, genetic polymorphisms, substrate specificities and inhibitor properties of CES1 and CES2, as well as the significance and recent progress on the discovery of CES modulators. The information presented here will help pharmacologists explore the relevance of CES to human diseases or to assign the contribution of certain CES in xenobiotic metabolism. It will also facilitate medicinal chemistry efforts to design prodrugs activated by a given CES isoform, or to develop potent and selective modulators of CES for potential biomedical applications.

Recent progresses in small-molecule enzymatic fluorescent probes for cancer imaging

An overview of recent advances in small-molecule enzymatic fluorescent probes for cancer imaging, including design strategies and cancer imaging applications.

Interspecies comparison in the COMT-mediated methylation of 3-BTD

Catechol-O-methyltransferase (COMT) is a druggable biological target and COMT modulators have been widely applied in the treatment of various central and peripheral nervous system disorders. The interspecies differences of COMT were carefully investigated using 3-BTD (a newly developed fluorescent probe of COMT) methylation as the probe reaction, and liver S9 from humans and seven experimental animals including monkeys, dogs, mice, rats, minipigs, guinea pigs and New Zealand rabbits as the enzyme source. Metabolite profiling demonstrated that all the tested liver S9 samples from the different animals could catalyse 3-BTD methylation but displayed significant differences in reaction rate. Also, the differential effects of tolcapone (a potent inhibitor against COMT) on 3-BTD methylation among various species were observed. The apparent kinetic parameters and the maximum intrinsic clearances (Cl_int) for 3-BTD methylation in liver S9 from the different animals were determined, and the order of the Cl_int values for the formation of 3-BTD was RLS9 > DLS9 ≈ PLS9 > MLS9 > CyLS9 > RaLS9 > GpLS9 > HLS9. These findings are helpful for further exploring COMT-associated biological processes in animal models, as well as for developing therapeutic molecules that target COMT.

The methylation behaviour of 3-BTD in liver S9 from eight different species was characterized with respect to the similarities and differences of their metabolic profiles, catalytic efficacy and inhibitory potency by a known chemical inhibitor.

A Practical and High-Affinity Fluorescent Probe for Uridine Diphosphate Glucuronosyltransferase 1A1: A Good Surrogate for Bilirubin

This study aimed to develop a practical and high-affinity fluorescent probe for uridine diphosphate glucuronosyltransferase 1A1 (UGT1A1), a key conjugative enzyme responsible for the elimination and detoxification of many potentially harmful compounds. Several substrates derived from N-butyl-4-phenyl-1,8-naphthalimide were designed and synthesized on the basis of the substrate preference of UGT1A1 and the principle of photoinduced electron transfer (PET). Following the preliminary screening, substrate 2 was found with a high specificity and high affinity toward UGT1A1, while such biotransformation brought remarkable changes in fluorescence emission. Both inhibition kinetic analyses and molecular docking simulations demonstrated that 2 could bind on UGT1A1 at the same ligand-binding site as bilirubin. Furthermore, this newly developed probe was successfully used for sensing UGT1A1 activities and the high-throughput screening of UGT1A1 modulators in complex biological samples. In conclusion, a practical and high-affinity fluorescent probe for UGT1A1 was designed and well-characterized, which could serve as a good surrogate for bilirubin to investigate UGT1A1-ligand interactions.