Concentration and activation biresponsive strategy in one analysis system with simultaneous use of G4 structure-specific signal probe and enzyme-catalyzed reaction

BACKGROUND

Enzymes with critical effects on life systems are regulated by expression and activation to modulate life processes. However, further insights into enzyme functions and mechanisms in various physiological processes are limited to concentration or activation analysis only. Currently, enzyme analysis has received notable attention, particularly simultaneous analysis of their concentration and activation in one system. Herein, N-methyl mesoporphyrin IX (NMM), a specific dye with notable structural selectivity for parallel G-quadruplex nucleic acid enzyme (G4h DNAzyme), is employed for the analysis of its concentration. In addition, the peroxidase activity of G4h DNAzyme is characterized based on G4h DNAzyme-catalyzed decomposition of H2O2 to continuously consume luminol. Accordingly, an increased fluorescence (FL) response of NMM and a decreased FL response of luminol could be simultaneously employed to analyze the concentration and activation of G4h DNAzyme.

RESULT

Herein, a novel concentration and activation biresponsive strategy is proposed using a G4h DNAzyme-based model that simultaneously employs a G4h structure-specific signal probe for enzyme concentration analysis and G4h DNAzyme-catalyzed reactions for enzyme activation analysis. Under optimal conditions, the biresponsive strategy can be effectively used for the simultaneous analysis of G4h DNAzyme concentration and activation, with detection limits of 718.7 pM and 233.4 nM respectively, delivering acceptable performances both in cell and in vitro.

SIGNIFICANCE

This strategy can not only be applied to concentration and activation analyses of G4h DNAzyme but can also be easily extended to other enzymes by simultaneously combining concentration analysis via target-induced direct reaction and activation analysis via target-induced catalytic reaction, offering deeper insights into various enzymes and enabling their effective implementation in bioanalysis and biochemistry.

Fluorometric Assay of Tyrosinase and Atrazine Based on the Use of Carbon Dots and the Inhibition of Tyrosinase Activity

Sensitive and convenient strategy of tyrosinase (TYR) and its inhibitor atrazine is in pressing demand for essential research as well as pragmatic application. In this work, an exquisite label-free fluorometric assay with high sensitivity, convenience and efficiency was described for detecting TYR and the herbicide atrazine on the basis of fluorescent nitrogen-doped carbon dots (CDs). The CDs were prepared via one-pot hydrothermal reaction starting from citric acid and diethylenetriamine. TYR catalyzed the oxidation of dopamine to dopaquinone derivative which could quench the fluorescence of CDs through a fluorescence resonance energy transfer (FRET) process. Thus, a sensitive and selective quantitative evaluation of TYR can be constructed on the basis of the relationship between the fluorescence of CDs and TYR activity. Atrazine, a typical inhibitor of TYR, inhibited the catalytic activity of TYR, leading to the reduced dopaquinone and the fluorescence was retained. The strategy covered a broad linear range of 0.1-150 U/mL and 4.0-80.0 nM for TYR and atrazine respectively with a low detection limit of 0.02 U/mL and 2.4 nM/mL. It is also demonstrated that the assay can be applied to detect TYR and atrazine in spiked complex real samples, which provides infinite potential in application of disease monitoring along with environmental analysis.

Tyrosinase-activated Nanocomposites for Double-Modals Imaging Guided Photodynamic and Photothermal Synergistic Therapy

Tyrosinase (TYR) is an important biomarker of melanoma. The exploration of fluorescent pr-obes-based composites is beneficial to build an integrative platform for the diagnosis and treatment of melanoma. Herein, a multifunctional nanocomposite IOBOH@BSA activated by TYR is developed for selective imaging and ablation of melanoma. The chemical structure of IOBOH enables the fluorescence (FL) imaging activated by TYR, photoacoustic (PA) imaging, and photodynamic-photothermal activity by regulating the balance between radiative decay and non-radiative decay. IOBOH combined with bovine serum albumin (IOBOH@BSA) presents the response to TYR and realizes FL imaging with mitochondria-targeting in melanoma. Moreover, IOBOH@BSA shows excellent photothermal ability and is applied for PA imaging. After IOBOH@BSA is activated by TYR, the singlet oxygen generation increases obviously. IOBOH@BSA can realize TYR-activated imaging and photodynamic-photothermal therapy of melanoma. The development of TYR-activated multifunctional nanocomposites promotes the precise imaging and improves the therapeutic effect of melanoma.

Photoinduced electron transfer (PeT) based fluorescent probes for cellular imaging and disease therapy

Typical PeT-based fluorescent probes are multi-component systems where a fluorophore is connected to a recognition/activating group by an unconjugated linker. PeT-based fluorescent probes are powerful tools for cell imaging and disease diagnosis due to their low fluorescence background and significant fluorescence enhancement towards the target. This review provides research progress towards PeT-based fluorescent probes that target cell polarity, pH and biological species (reactive oxygen species, biothiols, biomacromolecules, etc.) over the last five years. In particular, we emphasise the molecular design strategies, mechanisms, and application of these probes. As such, this review aims to provide guidance and to enable researchers to develop new and improved PeT-based fluorescent probes, as well as promoting the use of PeT-based systems for sensing, imaging, and disease therapy.

Persistent and Efficient Multimodal Imaging for Tyrosinase Based on Two-Photon Excited Fluorescent and Room-Temperature Phosphorescent Probes

Tyrosinase is crucial to regulate the metabolism of phenol derivatives, playing an important role in the biosynthesis of melanin pigments, whereas an abnormal level of tyrosinase would lead to severe diseases. It is rather necessary to develop a sensitive and selective imaging tool to assess the level of tyrosinase in vivo. We thoroughly researched the luminous mechanism of the existing TPTYR probe and provided design strategies to improve its two-photon excited fluorescence properties. The designed probes benza2-TPTYR and product benza2-TPTYR-coumarin have large two-photon absorption cross sections at the NIR spectral region (41 GM/706 nm, 71 GM/852 nm), while benza2-TPTYR-coumarin possesses easily distinguishable spectrum in the visible region and a high fluorescence efficiency (Φ_F = 0.27). What is more, novel two-photon excited multimodal imaging based on the pure organic small molecule benza1-TPTYR-coumarin (61 GM/936 nm) is proposed first, simultaneously possessing strong instantaneous fluorescent (563.79 nm) and persistent room-temperature phosphorescent emissions (767.68 nm, 0.54 ms).

Development of a Selective Assay of Tyrosine and Its Producing and Metabolizing Enzymes Utilizing Pulse-UV Irradiation-Induced Chemiluminescence

A new pulse UV irradiation-induced chemiluminescence (CL) determination method was developed for l-tyrosine using the luminol derivative L-012. The proposed method depends on the formation of reactive oxygen species (ROS) upon pulse UV irradiation of l-tyrosine; then, these ROS react with L-012 producing strong CL. The proposed method showed excellent sensitivity and ultraselectivity toward l-tyrosine. The mechanism of the developed CL method was studied using ROS scavengers, HPLC, and mass spectrometry. The method was linear for l-tyrosine in the range of 0.03-50 μM. Minor changes in the l-tyrosine structure, including hydroxylation, dehydroxylation, phosphorylation, or decarboxylation, were found to lead to a strong decrease in CL. Using the excellent selectivity of the proposed method for l-tyrosine, we have developed a CL assay for measuring alkaline phosphatase activity in the range of 0.02-15 U/L with the limit of detection (LOD) of 4 mU/L using the nonchemiluminescent O-phospho-l-tyrosine as a substrate. Furthermore, the CL reaction was applied for tyrosinase activity assay as this enzyme can convert l-tyrosine to the nonchemiluminescent l-dopa. The decrease in CL is correlated with the tyrosinase activity in the range of 0.025-0.75 U/mL with an LOD of 1.5 mU/mL. Moreover, the tyrosinase activity assay was successfully applied for the determination of IC₅₀ of the tyrosinase inhibitors kojic acid and benzoic acid. Therefore, our novel pulse UV irradiation CL method for the determination of l-tyrosine was not only suitable for the determination of this vital amino acid but also extended to the successful determination of its producing and metabolizing enzymes and their inhibitors.

Ultrasensitive Fibroblast Activation Protein-α-Activated Fluorogenic Probe Enables Selective Imaging and Killing of Melanoma In Vivo

Melanoma is a malignant cancer with a high risk of metastasis and continued increase in death rates over the past decades, and its prognosis is highly related to the disease's stage, while early detection and treatment of melanoma are significant to the improvement of its therapy outcome. Different from the traditional methods for disease diagnosis, enzyme-activated fluorescent probes were developed rapidly due to their high sensitivity and temporal-spatial ratio and have been widely applied in tumor detection, surgical navigation, and cancer-related research. Fibroblast activation protein-α (FAPα), a serine-type cell surface protease that plays important roles in cell invasion and extracellular matrix degradation, is widely involved in tumor progression such as malignant melanoma, so developing a FAPα activity-based molecular tool would be of great potential for the early diagnosis and therapy of melanoma. However, few fluorescent probes targeting FAPα have been applied in melanoma-related studies, and thus, the construction of FAPα activity-based fluorescent probes for melanoma detection is in urgent need. By incorporating the selective recognition unit with a red-emission fluorophore, cresyl violet, we herein report an ultrasensitive (limit of detection = 5.3 ng/mL) fluorogenic probe for FAPα activity sensing, named CV-FAP; the acquired probe showed a significantly higher binding affinity (15.7-fold) and overall catalytic efficiency (2.6-fold) when compared with those of the best reported FAPα probes. The good performance of CV-FAP made it possible to discriminate malignant melanoma cells and tumor-bearing mice from normal cells and mice with high contrast. More importantly, CV-FAP showed significant antitumor activity toward melanoma in cultured cells and tumor-bearing nude mice (over 95% inhibited tumor growth) with good safety, which made it an ideal theranostic agent for melanoma.

Recent Advances in BODIPY Compounds: Synthetic Methods, Optical and Nonlinear Optical Properties, and Their Medical Applications

Organoboron compounds are attracting immense research interest due to their wide range of applications. Particularly, low-coordinate organoboron complexes are receiving more attention due to their improbable optical and nonlinear optical properties, which makes them better candidates for medical applications. In this review, we summarize the various synthetic methods including multicomponent reactions, microwave-assisted and traditional pathways of organoboron complexes, and their optical and nonlinear properties. This review also includes the usage of organoboron complexes in various fields including biomedical applications.

A novel fluorescent off–on probe based on 4-methylumbelliferone for highly sensitive determination of tyrosinase

A novel fluorescent probe for high-sensitivity determination of tyrosinase, with 4-methylumbelliferone as the fluorophore and 3-hydroxybenzyl as the recognition group.

Fluorescent probes for visualizing ROS-associated proteins in disease

Abnormal expression of proteins, including catalytic and expression dysfunction, is directly related to the development of various diseases in living organisms. Reactive oxygen species (ROS) could regulate protein expression by redox modification or cellular signal pathway and thus influence the development of disease. Determining the expression level and activity of these ROS-associated proteins is of considerable importance in early-stage disease diagnosis and the identification of new drug targets. Fluorescence imaging technology has emerged as a powerful tool for specific in situ imaging of target proteins by virtue of its non-invasiveness, high sensitivity and good spatiotemporal resolution. In this review, we summarize advances made in the past decade for the design of fluorescent probes that have contributed to tracking ROS-associated proteins in disease. We envision that this review will attract significant attention from a wide range of researchers in their utilization of fluorescent probes for in situ investigation of pathological processes synergistically regulated by both ROS and proteins.

Highly selective two-photon fluorescent off-on probes for imaging tyrosinase activity in living cells and tissues

A coumarin-based two-photon (TP) fluorescent off-on probe has been developed for detecting tyrosinase activity. High selectivity, sensitivity and biocompatibility enable the probes to successfully image tyrosinase activity in live cells and tissues using TP microscopy.

Spectrophotometric Assays for Sensing Tyrosinase Activity and Their Applications

Tyrosinase (TYR, E.C. 1.14.18.1), a critical enzyme participating in melanogenesis, catalyzes the first two steps in melanin biosynthesis including the ortho-hydroxylation of L-tyrosine and the oxidation of L-DOPA. Previous pharmacological investigations have revealed that an abnormal level of TYR is tightly associated with various dermatoses, including albinism, age spots, and malignant melanoma. TYR inhibitors can partially block the formation of pigment, which are always used for improving skin tone and treating dermatoses. The practical and reliable assays for monitoring TYR activity levels are very useful for both disease diagnosis and drug discovery. This review comprehensively summarizes structural and enzymatic characteristics, catalytic mechanism and substrate preference of TYR, as well as the recent advances in biochemical assays for sensing TYR activity and their biomedical applications. The design strategies of various TYR substrates, alongside with several lists of all reported biochemical assays for sensing TYR including analytical conditions and kinetic parameters, are presented for the first time. Additionally, the biomedical applications and future perspectives of these optical assays are also highlighted. The information and knowledge presented in this review offer a group of practical and reliable assays and imaging tools for sensing TYR activities in complex biological systems, which strongly facilitates high-throughput screening TYR inhibitors and further investigations on the relevance of TYR to human diseases.

Review on the recent progress in the development of fluorescent probes targeting enzymes

Enzymes are very important for biological processes in a living being, performing similar or multiple tasks in and out of cells, tissues and other organisms at a particular location. The abnormal activity of particular enzyme usually caused serious diseases such as Alzheimer's disease, Parkinson's disease, cancers, diabetes, cardiovascular diseases, arthritis etc. Hence, nondestructive and real-time visualization for certain enzyme is very important for understanding the biological issues, as well as the drug administration and drug metabolism. Fluorescent cellular probe-based enzyme detectionin vitroandin vivohas become broad interest for human disease diagnostics and therapeutics. This review highlights the recent findings and designs of highly sensitive and selective fluorescent cellular probes targeting enzymes for quantitative analysis and bioimaging.

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.

Increase of tyrosinase activity at the wound site in zebrafish imaged by a new fluorescent probe

Tyrosinase plays a pivotal role in the hyperpigmentation of wounds. Here, we develop a new fluorescent probe and with it, we reveal an increase of tyrosinase activity at the wound site in zebrafish.

Lipid Peroxidation Assay Using BODIPY-Phenylbutadiene Probes: A Methodological Overview

The assessment of reactive oxygen species has increasing importance in biomedical sciences, due to their biological role in signaling pathways and induction of cell damage at low and high concentrations, respectively. Detection of lipid peroxidation with sensing probes such as some BODIPY dyes has now wide application in studies using fluorescent microplate readers, flow cytometry, and fluorescence microscopy. Two phenylbutadiene derivatives of BODIPY are commonly used as peroxidation probes, non-oxidized probes and oxidized products giving red and green fluorescence, respectively. Peculiar features of lipoperoxidation and BODIPY dye properties make this assessment a rather complex process, not exempt of doubts and troubles. Color changes and fluorescence fading that are not due to lipid peroxidation must be taken into account to avoid misleading results. As a characteristic feature of lipoperoxidation is the propagation of peroxyl radicals, pitfalls and advantages of a delayed detection by BODIPY probes should be considered.

Detection of tyrosine and monitoring tyrosinase activity using an enzyme cascade-triggered colorimetric reaction

The aromatic amino acid tyrosine is an essential precursor for the synthesis of catecholamines, including l-DOPA, tyramine, and dopamine. A number of metabolic disorders have been linked to abnormal tyrosine levels in biological fluids. In this study, we developed an enzyme cascade-triggered colorimetric reaction for the detection of tyrosine, based on the formation of yellow pigment (betalamic acid) and red fluorometric betaxanthin. Tyrosinase converts tyrosine to l-DOPA, and DOPA-dioxygenase catalyzes oxidative cleavage of l-DOPA into betalamic acid. Response is linear for tyrosine from 5 to 100 μM, and the detection limit (LOD) is 2.74 μM. The enzyme cascade reaction was applied to monitor tyrosinase activity and tyrosinase inhibition assays. Lastly, the performance of the proposed biosensor proved successful in the analysis of urine samples without the need for pre-treatment.

Catalysis-based specific detection and inhibition of tyrosinase and their application

Tyrosinase is an important enzyme in controlling the formation of melanin in melanosome, and plays a key role in the pigmentation of hair and skin. The abnormal expression or activation of tyrosinase is associated with several diseases such as albinism, vitiligo, melanoma and Parkinson disease. Excessive deposition of melanin could cause diseases such as freckles and brown spots in the human body, and it is also closely related to browning of fruits and vegetables and insect molting. Detecting and inhibiting the activity of tyrosinase is of extraordinary value in the progress of diagnosis and treatment of these diseases. Therefore, many selective optical detection probes and small molecular inhibitors have been developed, and have made significant contributions to the basic and clinical research on these diseases. In this paper, the detection and inhibition of tyrosinase and their application in whitening products are reviewed, with special emphasis on development of fluorescent probes and inhibitors. Hopefully, this review will help design more efficient and sensitive tyrosinase probes and inhibitors, as well as shed light on novel treatment of diseases such as melanoma.

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The abnormal expression or activation of tyrosinase is the pathogenesis of several diseases such as albinism, vitiligo, and melanoma.

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Detecting and inhibiting tyrosinase activity is of great value in the diagnosis and treatment of these diseases.

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The detection/inhibition of tyrosinase and its application in whitening products are reviewed, with special emphasis on probes/inhibitors.

Detection of Lipase Activity in Cells by a Fluorescent Probe Based on Formation of Self-Assembled Micelles

Reliable and sensitive detection of lipase activity is essential for the early diagnosis and monitoring of acute pancreatitis or progression of digestive diseases. However, the available fluorescent probes for detection of lipase activity are only implemented in a hexane-water two-phase system due to the nature of heterogeneous catalysis of lipase, thus limiting their applications in direct imaging of lipase activity in living cells and tissues. Here we designed and synthesized a "turn on" fluorescent probe CPP based on self-assembled micelles for hydrolysis of lipase. The CPP probe exhibits high selectivity and excellent sensitivity for the detection of lipase in such a homogeneous system and is successfully applied for monitoring lipase activity in pancreatic AR42J cells, tissues, and serums. Taken together, the fluorescent CPP probe not only provides a tool for diagnostic potential in pancreatic disease but also demonstrates an application potential for micelle self-assembly-based development of biological probes.

Substrate-Photocaged Enzymatic Fluorogenic Probe Enabling Sequential Activation for Light-Controllable Monitoring of Intracellular Tyrosinase Activity

Tyrosinase (TYR) is a crucial enzyme involved in melanogenesis, and its overexpression is closely associated with melanoma. To precisely monitor intracellular TYR activity, remote control of a molecule imaging tool is highly meaningful but remains to be explored. In this work, we present the first photocaged tyrosinase fluorogenic probe by caging the substrate of the enzymatic probe with a photolabile group. Because of the sequential light and enzyme-activation feature, this probe exhibits photocontrollable "turn on" response toward TYR with good selectivity and high sensitivity (detection limit: 0.08 U/mL). Fluorescence imaging results validate that the caged probe possesses the capability of visualizing intracellular endogenous tyrosinase activity in a photocontrol fashion, thus offering a promising molecule imaging tool for investigating TYR-related physiological function and pathological role. Moreover, our sequential activation strategy has great potential for developing more photocontrollable enzymatic fluorogenic probes with spatiotemporal resolution.

Ionic fluorescent sensor targeting receptor tyrosine kinases for biosystems imaging and application in flow cytometry

Fluorescent imaging of receptor tyrosine kinases in living biosystems is an important means for the early diagnosis of cancer, herein an ionic fluorescent sensor (SNB) composed of targeting unit (sunitinib) and nile blue fluorophore linked via long flexible chain has been designed and evaluated. The SNB sensor exhibits distinct fluorescence responses to receptor tyrosine kinases derived from unfolding strategy and targeting ability, which were evaluated through 2D NMR analyses, optical studies, kinase activity assays. The SNB sensor has excellent membrane fluorescent imaging by electrostatic adsorption and can selectively insert into receptor tyrosine kinases domain pocket on the membrane of cancer cell lines. The SNB sensor has been successfully applied in flow cytometry for cell sorting and fluorescence imaging with tumor mouse model in vivo. The SNB senor may help transition the technology into a widely suitable tool for flow cytometry, imaging with confocal microscopes, whole animal imaging and possibly facilitating early diagnoses and treatment of cancer.

Gadolinium-based MRI contrast agent for the detection of tyrosinase

Tyrosinase is a key enzyme that has long been considered as a biomarker for melanoma as it catalyzes the oxidation of tyrosine and l-DOPA in melanogenesis. Recent studies also suggest a link between tyrosinase activity and Parkinson's disease; however, the mechanism of tyrosinase-mediated melanin formation in the brain is poorly understood. To better understand this connection, more advanced tools for the detection of tyrosinase in the brain are required. Herein, we successfully designed and synthesized a tyrosinase-targeting Gd(iii)-based MR contrast agent Tyr-GBCA 1. Tyr-GBCA 1 was synthesized by linking m-hydroxyphenyl to Gd-DOTA via a self-immolative linker. Tyr-GBCA 1 shows a 21% increase in the T1 relaxation rate (R1) in the presence of tyrosinase in artificial cerebral spinal fluid. Furthermore, Tyr-GBCA 1 is unreactive to hydrogen peroxide, which is a potential interferent in oxidation-based tyrosinase sensing systems. The reaction mechanism of the probe was studied by electrospray ionization (ESI) mass spectrometry and supports the cleavage of a reaction site.

Dual-Readout Tyrosinase Activity Assay Facilitated by a Chromo-Fluorogenic Reaction between Catechols and Naphthoresorcin

Analyte-responsive chromo-fluorogenic reactions under accessible conditions are important for designing small-molecule spectroscopic probes. We describe a series of newly constructed motifs based on the chromo-fluorogenic reaction between catechol derivatives (typically hydroxytyrosol, dopamine, and levodopa) and naphthoresorcin (NR) in aqueous solution under ambient conditions. The weakly absorptive and fluorogenic catechols/NR was converted to products having visible absorption and bright fluorescence within several minutes. The chromo-fluorophores produced from this reaction had a maximum absorbance at 458 nm and emission at 480 nm with high fluorescence quantum yields (30-84%). Inspired by the tyrosinase-catalyzed hydroxylation of monophenols to catechols, the tyrosinase-enabled chromo-fluorogenic reaction was verified by using monophenol (typically tyrosol) as the substrate. In this regard, a dual-readout tyrosinase activity assay was developed by virtue of the in situ "turn-on" optical signals. Furthermore, a test of tyrosinase inhibition, by using a common inhibitor kojic acid, demonstrated the potential of the chromo-fluorogenic reaction for developing other tyrosinase related assays and signal transduction.

Fast tyrosinase detection in early stage melanoma with nanomolar sensitivity using a naphthalimide-based fluorescent read-out probe

We report an expeditious approach for selective tyrosinase detection in early stage melanoma with nanomolar sensitivity using a napthalimide-based fluorescent probe.

Enzyme Modification and Oxidative Cross-linking Using Carboxylate-, Phenol- and Catechol-Conjugated 1,8-Naphthalimides

The ground- and excited-state interactions of β-alanine, tyrosine and l-dopa substituted 1,8 naphthalimides (NI-Ala, NI-Tyr and NI-Dopa) with lysozyme and mushroom tyrosinase were evaluated to understand the mechanism of oxidative modification. Photooxidative cross-linking of lysozyme was observed for all three conjugates. The yield was significantly reduced for NI-Tyr and NI-Dopa due to intramolecular electron transfer to the excited singlet state of the 1,8-naphthalimide. Incubation of NI-Tyr and NI-Dopa with mushroom tyrosinase resulted in an increased fluorescence from the naphthalimide, suggesting that the phenol and catechol portion of the conjugates are oxidized by the enzyme. This result demonstrates that the compounds bind in the active site of mushroom tyrosinase. The catalytic activity of mushroom tyrosinase to oxidize both tyrosine (monophenolase) and l-dopa (diphenolase) was modified by NI-Tyr and NI-Dopa. Monophenolase activity was inhibited, and the diphenolase activity was enhanced in the presence of these conjugates. Detailed Michaelis-Menten studies show that both V_max and K_m are modified, consistent with a mixed inhibition mechanism. Collectively, the results show that the compounds interact in the enzyme's active site, but also modify the distribution of the enzyme's oxidation states that are responsible for catalysis.

Fluorogenic probes for disease-relevant enzymes

Traditional biochemical methods for enzyme detection are mainly based on antibody-based immunoassays, which lack the ability to monitor the spatiotemporal distribution and, in particular, the in situ activity of enzymes in live cells and in vivo. In this review, we comprehensively summarize recent progress that has been made in the development of small-molecule as well as material-based fluorogenic probes for sensitive detection of the activities of enzymes that are related to a number of human diseases. The principles utilized to design these probes as well as their applications are reviewed. Specific attention is given to fluorogenic probes that have been developed for analysis of the activities of enzymes including oxidases and reductases, those that act on biomacromolecules including DNAs, proteins/peptides/amino acids, carbohydrates and lipids, and those that are responsible for translational modifications. We envision that this review will serve as an ideal reference for practitioners as well as beginners in relevant research fields.

A near-infrared benzoquinone-coupled BODIPY: Synthesis, spectroscopic and electrochemical properties

A near-infrared absorbing boron-dipyrromethene (BODIPY) chromophore coupled with two benzoquinone moieties at its 3,5-positions, 3, was prepared via Knoevenagel condensation of 1,3,5,7-tetramethyl-8-(4[Formula: see text]-benzonitrile) BODIPY 1 with 3,5-di-tert-butyl-4-hydroxybenzaldehyde to afford 1,7-dimethyl-3,5-di-(4[Formula: see text]-hydroxy-3[Formula: see text],5[Formula: see text]-di-tert-butyl styryl)-8-(4[Formula: see text]-benzonitrile) BODIPY 2, followed by oxidization with Ag2O in good yield (91%). The UV-vis-NIR absorption spectrum of 3 exhibits two major bands at 795 and 895 nm in the near-IR region, while 2shows maximum absorbance at 661 nm and strong fluorescence at 692 nm ([Formula: see text] 0.59). The cyclic voltammetry of 3 consists of two pairs of reversible one-electron reductions at -0.61 V and -0.88 V and two pairs of one-electron oxidation waves at 0.26 V and 0.54 V. Compared with the redox potentials of 2([Formula: see text] V and [Formula: see text] 0.25 V), the first reduction of 3 is anodically shifted for 710 mV, whereas the first oxidation potential is close. Theoretical calculation reveals that conjugation with the benzoquinone moieties on the BODIPY chromophore significantly lowers the LUMO energy level and the HOMO–LUMO energy gap, resulting in a dramatic bathochromic shift of the S0–S1 transition of 3 compared with that of 2. X-ray crystallographic analysis of 3 reveals that the whole molecule adopts a V-type twisted conformation along the delocalized [Formula: see text]-conjugated pathway.

A novel hydrosoluble near-infrared fluorescent probe for specifically monitoring tyrosinase and application in a mouse model

A novel hydrosoluble near-infrared fluorescent probe is applied to imaging and detection of endogenous tyrosinase in living cells, zebrafish and a mouse model.

Aminoluciferin 4-hydroxyphenyl amide enables bioluminescence detection of endogenous tyrosinase

Tyrosinase, a copper-containing enzyme existing widely in plants, animals and microorganisms, usually serves as an important biomarker in melanoma, and is also related to hyperpigmentation of the skin, melasma, age spots and albinism. At present, only one bioluminescent probe has been applied to image tyrosinase in cells. Thus, it's of great significance to develop a new bioluminescent probe that can detect tyrosinase in living cells and in live animals. In the current work, we report a new BL probe, TyrBP-3, which not detect tyrosinase in vitro and in living cells, but can also visualize the level of tyrosinase activity in tumors of living animals. In summary, TyrBP-3 is the first bioluminescent probe that can image tyrosinase on a cellular level. Hence, we anticipate that TyrBP-3 can be a good tool to monitor tyrosinase in complex biosystems in the future.

Polymethyldopa Nanoparticles-Based Fluorescent Sensor for Detection of Tyrosinase Activity

Being a typical copper-containing oxidase, tyrosinase plays critical roles in biological activity, and its aberrant expression might cause diverse skin diseases. Herein, we, for the first time, have found an interesting green fluorogenic reaction between methyldopa and ethanolamine. By combining transmission electron microscopy, UV-vis absorption spectroscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and MALDI-TOF mass spectrum analysis, we have confirmed that there is a reliable method for preparing the bright green fluorescent polymethyldopa nanoparticles (PMNPs) by simply mixing methyldopa and ethanolamine at room temperature. Inspired by such a simple and convenient fluorogenic reaction, a novel polymethyldopa nanoparticles-based fluorescent sensor for detection of tyrosinase activity was developed by using the commercially available metyrosine as a substrate, accompanied by the tyrosinase-catalyzed specific conversion of metyrosine into methyldopa. According to the intrinsic sensitivity/selectivity of fluorescence technology and unambiguous response mechanism, our fluorescent sensor exhibits excellent sensing performance and can be utilized in the determination of the tyrosinase activity in real biological samples and inhibitor screening.

Melanosome-Targeting Near-Infrared Fluorescent Probe with Large Stokes Shift for in Situ Quantification of Tyrosinase Activity and Assessing Drug Effects on Differently Invasive Melanoma Cells

Tyrosinase (TYR) plays a vital role in melanin biosynthesis and is widely regarded as a relatively specific marker for melanocytic lesions which involve vitiligo, malignant cutaneous melanoma, Parkinson's disease (PD), etc. However, the detection of TYR in living cells with fluorescent probes is usually interfered by diverse endogenous reactive oxygen species (ROS) and reactive nitrogen species (RNS). Herein, we synthesized a melanosome-targeting near-infrared (NIR) fluorescent probe (HB-NP) with a large Stokes shift (195 nm), achieving a highly sensitive and selective in situ detection for intracellular TYR, by incorporating a m-hydroxybenzyl moiety that recognizes TYR specifically and the morpholine unit which facilitates the probe accumulating in the melanosome into a salicyladazine skeleton. When treated with TYR, the probe itself with weak fluorescence is lit up via an inhibited photoinduced electron-transfer (PET) effect and HB-NP shows a strong fluorescence signal (nearly 48-fold enhancement) with a low detection limit of 0.5 U mL^-1. HB-NP has been successfully applied in visualizing and in situ quantification of the intracellular TYR activity. Moreover, owing to the different expression levels of TYR, two human uveal melanoma cells with different invasive behaviors are distinguished by means of bioimaging and the effects of the inhibitor, kojic acid, and the up-regulating treatment, psoralen/ultraviolet A, on TYR activity of the two melanoma cells are evaluated. HB-NP is expected to be a useful tool to monitor diseases associated with the abnormal level of melanin and screen medicines for TYR disorder more effectively.

A novel fluorescent biosensor for adrenaline detection and tyrosinase inhibitor screening

In this work, a novel simple fluorescent biosensor for the highly sensitive and selective detection of adrenaline was established. Firstly, water-soluble CuInS₂ quantum dots (QDs) capped by L-Cys were synthesized via a hydrothermal synthesis method. Then, the positively charged adrenaline was assembled on the surface of CuInS₂ QDs due to the electrostatic interactions and hydrogen bonding, which led to the formation of adrenaline-CuInS₂ QD (Adr-CuInS₂ QD) electrostatic complexes. Tyrosinase (TYR) can catalyze adrenaline to generate H₂O₂, and additionally oxidize the adrenaline to adrenaline quinone. Both the H₂O₂ and the adrenaline quinone can quench the fluorescence of the CuInS₂ QDs through the electron transfer (ET) process. Thus, the determination of adrenaline could be facilely achieved by taking advantage of the fluorescence "turn off" feature of CuInS₂ QDs. Under the optimum conditions, the fluorescence quenching ratio I_f/I_f0 (I_f and I_f0 were the fluorescence intensity of Adr-CuInS₂ QDs in the presence and absence of TYR, respectively) was proportional to the logarithm of adrenaline concentration in the range of 1 × 10^-8-1 × 10^-4 mol L^-1 with the detection limit of 3.6 nmol L^-1. The feasibility of the proposed biosensor in real sample assay was also studied and satisfactory results were obtained. Significantly, the proposed fluorescent biosensor can also be utilized to screen TYR inhibitors. Graphical abstract Schematic illustration of the fluorescent biosensor for adrenaline detection (A) and tyrosinase inhibitor screening (B).

A BODIPY-based two-photon fluorescent probe validates tyrosinase activity in live cells

Herein, we report rational design, synthesis, and application of a two-photon fluorescent probe (Tyro-1) for tracking intracellular tyrosinase activity. The chemoselective detection of tyrosinase is precluded from interference of other competitive omnipresent oxidizing entities in cellular milieu. The probe showed 12.5-fold fluorescence enhancement at λ_em = 450 nm in the presence of tyrosinase. The nontoxic probe Tyro-1 provides information about H₂O₂-mediated upregulation of tyrosinase through cellular imaging. Its two-photon imaging ability makes it a noninvasive tool for validating the expression of tyrosinase in the live cells.

A novel photoswitchable enzyme cascade for powerful signal amplification in versatile bioassays

This report outlines the construction of an advanced, exquisite photoswitchable enzyme cascade on the basis that tyrosinase (TYR) catalyzes the generation of dihydroxyphenylalanine (DOPA) coordinated TiO₂ nanoparticles (NPs) to form a light responsive nano-trigger that subsequently photoactivates the enzymatic activity of horseradish peroxidase (HRP). This photoswitchable enzyme cascade has a powerful signal transduction/amplification ability in TYR-based bioassays, and holds great promise to be applied in versatile applications.

Design and Synthesis of a BODIPY-Tetrazole Based "Off-On" in-Cell Fluorescence Reporter of Hydrogen Peroxide

BODIPY-linked bithiophene-tetrazoles were designed and synthesized for bioorthogonal photoclick reactions in vitro and in vivo. The reactivity of these tetrazoles toward dimethyl fumarate was found to depend on the BODIPY attachment site, with the meta-linked BODIPY-tetrazole being the most reactive. The resulting pyrazoline cycloadduct showed drastically reduced BODIPY fluorescence. However, BODIPY fluorescence recovered after treatment with hydrogen peroxide. This turn-on effect was attributed to conversion from the pyrazoline to a pyrazole. Finally, we showed that this unique BODIPY-tetrazole off-on fluorescence probe can be used to detect hydrogen peroxide inside HeLa cells.

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.

Synthesis and Characterization of the Hemi-Salen Ligands and Their Triboron Complexes: Spectroscopy and Examination of Anticancer Properties

The synthesis, spectroscopic properties, and in vitro cytotoxicity activity of a series of various salen-based triboron complexes have been designed and prepared from hemi-salen (L₁ H₃  - L₄ H₃ ) ligands and BF₃ ·Et₂ O or BPh₃ under simple reaction conditions. The hemi-salen (L₁ H₃  - L₄ H₃ ) ligands and their BF₂ or BPh₂ chelating triboron complexes were characterized by means of NMR (¹ H, ¹³ C, ¹⁹ F, and ¹¹ B) spectra, FT-IR spectra, UV/VIS spectra, fluorescence spectra, mass spectra, melting point, as well as elemental analysis. The triboron [L_(1 - 4) (BF₂ )₃ ] and [L_(1 - 4) (BPh₂ )₃ ] complexes were investigated for their absorption and emission properties, and these complexes are also good chelates towards boron(III) fragments such as BF₂ or BPh₂ quantum yield in solution reaching up to 38%. The hemi-salen (L₁ H₃  - L₄ H₃ ) ligands and their BF₂ or BPh₂ chelating triboron complexes were tested for the in vitro anticancer activity against various cancer and normal cells (HeLa, DLD-1, ECC-1, PC-3, PNT-1A, and CRL-4010), and it was found that the cell viability of cancer cells was decreased while most of the healthy cells could still be viable. Also, the cytotoxicity studies showed that anticancer activity of hemi-salen (L₁ H₃  - L₄ H₃ ) ligands is higher than that of triboron [L_(1 - 4) (BF₂ )₃ ] and [L_(1 - 4) (BPh₂ )₃ ] complexes. The hemi-salen (L₁ H₃  - L₄ H₃ ) ligands showing the strongest cytotoxic effect in PC-3 cells were found to exhibit anticancer activity with apoptosis by increasing the level of ROS in the PC-3 cells.

Determining mushroom tyrosinase inhibition by imidazolium ionic liquids: A spectroscopic and molecular docking study

The inhibition effects of imidazolium ionic liquids (ILs) on the enzyme kinetics of mushroom tyrosinase is reported. A simple UV-VIS spectrophotometric assay was used to measure the reaction kinetics of the reaction between mushroom tyrosinase and L-dopa. Seven different imidazolium ILs, comprised of 1-alkyl-3-methylimidazolium ([Im_n1⁺], n=2, 4, 6) cations paired with several anions that included Cl^-, [NO₃^-], methanesulfonate ([MeSO₃^-]), trifluoromethanesulfonate (or triflate, [TFMS^-]), and bis(trifluoromethylsulfonyl)imide ([Tf₂N^-]). Lineweaver-Burk plots were generated from the recovered k_cat and K_m parameters using four to six substrate concentrations per measurement. The results show that mushroom tyrosinase activity was consistently inhibited by all of the ILs and that the type of inhibition was non-competitive in nearly all cases. Only the data for [Im₂₁⁺][Tf₂N^-] suggested that the inhibition mechanism was competitive with the substrate. Molecular docking simulations were performed using AutoDock4.2 and AutoDock Vina and revealed that all cations docked in the L-dopa active site. Anions showed varied results that included locations both within and outside of the active site.

In Situ Fluorogenic and Chromogenic Reactions for the Sensitive Dual-Readout Assay of Tyrosinase Activity

As a well-known copper-containing oxidase, tyrosinase has been anticipated to serve as the biomarker of skin diseases. We describe here an exquisite label-free fluorescent and colorimetric dual-readout assay of its activity, inspired by the specific oxidation ability of monophenolamine substrates to catecholamines and a unique fluorogenic reaction between resorcinol and catecholamines. By employing commercially available tyramine as the model substrate (dopamine as the product), it is found that the tyrosinase-incubated tyramine solution exhibits obvious pale yellow with intense blue fluorescence in the presence of resorcinol and O₂, where the absorbance and fluorescence intensity are directly related to the concentration of added tyrosinase (i.e., the amount of conversion of tyramine to dopamine). The overall process of sensing tyrosinase activity takes less than 100 min at ambient temperature and pressure conditions with exceedingly simple operation procedure, explicit response mechanism, and formation of fluorophore with high quantum yield from scratch. Furthermore, such a convenient, rapid, cost-effective, and highly sensitive dual-readout assay exhibits promising prospect for the tyrosinase activity in extensive bioassays and clinic research as well as in screening potential tyrosinase inhibitors.