Monitoring γ-glutamyl transpeptidase activity and evaluating its inhibitors by a water-soluble near-infrared fluorescent probe

γ-Glutamyltranspeptidase fluorescence lifetime response probe for precision tumor detection unveiling A549 cancer cell specificity

γ-Glutamyltranspeptidase (γ-GGT), as a key enzyme, exhibits markedly higher expression levels in tumor cells compared to normal cells. Under normal conditions, γ-GGT activity on the cell membrane is relatively low, but it undergoes a significant upregulation in cancer cells, making it a potential cancer biomarker. Particularly in A549 cells, a prominent cancer cell line, the pronounced upregulation of γ-GGT expression emphasizes its potential as a unique recognition target and a robust marker for A549 cells. This study successfully synthesized a highly selective γ-GGT fluorescent probe, the exhibits commendable sensitivity (LOD = 0.0021U/mL) and selectivity, achieving efficient detection at the cellular level and providing accurate insights into differential expression between normal and cancer cells. The alterations in fluorescence lifetime observed before and after the probe's reaction with γ-GGT serve as a crucial foundation for fluorescence lifetime imaging on living cells. The probe has become a powerful tool for precise localization of tumor cells, particularly demonstrating its capability for specific recognition in A549 cells. Overall, this research highlights the potential of γ-GGT as a target for fluorescent probes, emphasizing its prospects in specific recognition, particularly in A549 cells, with profound implications for advancing early cancer diagnosis and treatment methods.

A dual-site fluorescent probe for the detection of γ-glutamyl transpeptidase activity and its application in garlic

Developing convenient γ-glutamyl transpeptidase (GGT) activity detection methods is of great significance for soaking Laba garlic and human diseases detection. A dual-site fluorescent probe (probe 1) was developed for detection the activity of GGT. Probe 1 could recognize GGT by the enzymatic hydrolysis of peptide bond by GGT. There has a linear relationship between the fluorescence intensity of probe 1 at 416 nm and the activity of GGT. And the color of the probe solution gradually changed from colorless to blue with the increase of GGT activity under 365 nm ultraviolet light. Importantly, it has a linear relationship between the activity of GGT and the blue (B) value of probe solution photo. Therefore, probes can serve as a convenient tool for detecting GGT activity. More importantly, the probe has been successfully applied to detect of GGT activity in garlic.

γ-Glutamyltranspeptidase (GGT) Sensitive Fluorescence Probes for Cancer Diagnosis; Brief Review

Millions of deaths occur each year due to the late diagnosis of abnormal cellular growth within the body. However, the devastating impact of this can be significantly reduced if cancer metastasis is detected early through the use of enzymatic biomarkers. Among several biomarkers, γ-glutamyltranspeptidase (GGT) stands out as a member of the aminopeptidase family. It is primarily found on the surface of cancer cells such as glioma, ovarian, lung, and prostate cancer, without being overexpressed in normal cells or tissues. Recent years have witnessed significant progress in the field of cancer monitoring and imaging. Fluorescence sensing techniques have been employed, utilizing organic small molecular probes with enzyme-specific recognition sites. These probes emit a fluorescent signal upon interacting with GGT, enabling the imaging, identification, and differentiation of normal and cancerous cells, tissues, and organs. This review article presents a concise overview of recent progress in fluorescent probes developed for the selective detection of GGT, focusing on their applications in cancer imaging. It highlights the observed alterations in the fluorescence and absorption spectra of the probes before and after interaction with GGT. Additionally, the study investigates the changes in the probe molecule's structure following enzyme treatment, evaluates the sensor's detection limit, and consolidated imaging studies conducted using confocal fluorescence analysis. This comprehensive survey is expected to contribute to the advancement of sensing techniques for biomarker detection and cancer imaging, providing valuable insights for refining methodologies and inspiring future developments in this field.

Advances and Perspectives of Responsive Probes for Measuring γ-Glutamyl Transpeptidase

Gamma-glutamyltransferase (GGT) is a plasma-membrane-bound enzyme that is involved in the γ-glutamyl cycle, like metabolism of glutathione (GSH). This enzyme plays an important role in protecting cells from oxidative stress, thus being tested as a key biomarker for several medical conditions, such as liver injury, carcinogenesis, and tumor progression. For measuring GGT activity, a number of bioanalytical methods have emerged, such as chromatography, colorimetric, electrochemical, and luminescence analyses. Among these approaches, probes that can specifically respond to GGT are contributing significantly to measuring its activity in vitro and in vivo. This review thus aims to highlight the recent advances in the development of responsive probes for GGT measurement and their practical applications. Responsive probes for fluorescence analysis, including "off-on", near-infrared (NIR), two-photon, and ratiometric fluorescence response probes, are initially summarized, followed by discussing the advances in the development of other probes, such as bioluminescence, chemiluminescence, photoacoustic, Raman, magnetic resonance imaging (MRI), and positron emission tomography (PET). The practical applications of the responsive probes in cancer diagnosis and treatment monitoring and GGT inhibitor screening are then highlighted. Based on this information, the advantages, challenges, and prospects of responsive probe technology for GGT measurement are analyzed.

Near-infrared imaging of hepatocellular carcinoma and its medicinal treatment with a γ-glutamyl transpeptidase-monitoring fluorescence probe

Herein, the Near-infrared imaging of hepatocellular carcinoma (HCC) and its medicinal treatment was achieved with a γ-glutamyl transpeptidase (GGT)-monitoring fluorescence probe KYZ-GGT which consisted of the typical recognition group γ-glutamyl and the structurally modified signal reporting group hemicyanine-thioxanthene. Compared with the recently reported probes, KYZ-GGT suggested practical and steady capability for monitoring the GGT level in the cellular, xenograft, induced as well as medicinal treatment HCC models. It realized the mitochondrial targeting intracellular imaging to reflect the GGT dynamics in the induction or medicinal treatment of HCC. In the xenograft and induced model mice with multiple factors, KYZ-GGT showed stable performance for visualizing the HCC status. In the medicinal treatment of the long-period-induced HCC model mice verified by the serum indexes and histopathological analysis, KYZ-GGT successfully imaged the medicinal treatment process of HCC with two marketed drugs (Sorafenib and Lenvatinib) respectively, with an applicative penetration depth. The information here was meaningful for investigating effective medicinal strategies for overcoming HCC.

Targeting gamma-glutamyl transpeptidase: A pleiotropic enzyme involved in glutathione metabolism and in the control of redox homeostasis

Gamma-glutamyl transpeptidase (GGT) is an enzyme located on the outer membrane of the cells where it regulates the metabolism of glutathione (GSH), the most abundant intracellular antioxidant thiol. GGT plays a key role in the control of redox homeostasis, by hydrolyzing extracellular GSH and providing the cell with the recovery of cysteine, which is necessary for de novo intracellular GSH and protein biosynthesis. Therefore, the upregulation of GGT confers to the cell greater resistance to oxidative stress and the advantage of growing fast. Indeed, GGT is upregulated in inflammatory conditions and in the progression of various human tumors and it is involved in many physiological disorders related to oxidative stress, such as cardiovascular disease and diabetes. Currently, increased GGT expression is considered a marker of liver damage, cancer, and low-grade chronic inflammation. This review addresses the current knowledge on the structure-function relationship of GGT, focusing on human GGT, and provides information on the pleiotropic biological role and relevance of the enzyme as a target of drugs aimed at alleviating oxidative stress-related diseases. The development of new GGT inhibitors is critically discussed, as are the advantages and disadvantages of their potential use in clinics. Considering its pleiotropic activities and evolved functions, GGT is a potential "moonlighting protein".

An activatable near-infrared molecular reporter for fluoro-photoacoustic imaging of liver fibrosis

Noninvasive and accurate detection of liver fibrosis is extremely significant for well-timed intervention and treatment to prevent or reverse its progression. Fluorescence imaging probes hold great potential for imaging of liver fibrosis, but they always encounter the inherent limitation of shallow penetration depth, which compromises their ability of in vivo detection. To overcome this issue, an activatable fluoro-photoacoustic bimodal imaging probe (IP) is herein developed for specific visualization of liver fibrosis. The probe IP is constructed on a near-infrared thioxanthene-hemicyanine dye that is caged with gamma-glutamyl transpeptidase (GGT) responsive substrate and linked with integrin-targeted peptide (cRGD). Such molecular design permits IP to effectively accumulate in the liver fibrosis region through specific recognition of cRGD towards integrin and activate its fluoro-photoacoustic signal after interaction with overexpressed GGT to precisely monitor the liver fibrosis. Thus, our study presents a potential strategy to design dual-target fluoro-photoacoustic imaging probes for noninvasive detection of early-stage liver fibrosis.

Imaging of a novel ratio γ-glutamyl transpeptidase fluorescent probe in living cells and biopsies

Herein, a novel fluorescent probe, GTP, was developed for monitoring the GGT (γ-glutamyl transpeptidase) level in living cells and biopsies. It consisted of the typical recognition group γ-Glu (γ-Glutamylcysteine) and the fluorophore (E)-4-(4-aminostyryl)-1-methylpyridin-1-ium iodide. With a ratio response between the signal intensity at 560 nm and 500 nm (R_I560/_I500), it could be important complement for the turn-on ones. With the linear range of 0-50 U/L, the limit of detection was calculated as 0.23 μM. The detection system showed the strongest response near pH 7.4, and exhibited steady fluorescence signals for at least 48 h. With high selectivity, good anti-interference and low cytotoxicity, GTP was suitable for physiological applications. By monitoring the GGT level with the ratio values in the green and blue channels, the probe GTP could distinguish cancer cells from normal cells. Furthermore, in the mouse tissues and humanization tissue samples, the probe GTP could also recognize the tumor tissues from the normal ones.

A water-soluble fluorescent probe for the determination of γ-glutamyltransferase activity and its application in tumor imaging

γ-glutamyltransferase (GGT), an important tumor marker, is highly expressed in tumor tissues, and precise detection of its activity provides a vital indicator for the diagnosis and treatment. In this work, a "lighting-on" probe (TCF-GGT) was elaborated to detect endogenous GGT with high selectivity and sensitivity. Dicyanomethyldifuranyl (TCF-OH) was employed as the fluorescence reporter and short peptide glutathione (GSH) worked as the GGT-active trigger, the introduction of which prevented the initial proton transfer of TCF-OH contributing to a blank sensing background. A bright red fluorescence could be switched on upon GGT catalytic hydrolysis, avoiding the potential interference from background. There displayed an excellent water-solubility, and little organic solvent was required during the exploration, which otherwise avoided the potential damage to enzyme and organism. TCF-GGT has been proved to be workable at cellular and organism level with highly effective imaging and a short metabolic cycle, which is expected to offer an alternative solution or reference to the early diagnosis and treatment of tumor.

γ-Glutamyltransferase-Activatable Fluoro-Photoacoustic Reporter for Highly Sensitive Diagnosis of Acute Liver Injury and Tumor

γ-Glutamyltransferase (GGT) has been recognized as an important clinical biomarker that is closely related to many diseases. Visualizing the GGT fluctuation facilitates early disease-related diagnosis and therapy. Herein, an activated probe (NIR-GGT) for the imaging of GGT activity was prepared. The probe consists of a stable NIR fluorophore with the tunable amino group decorated with the γ-glutamate group as a GGT-sensing unit linked by a self-elimination group. NIR-GGT can sensitively recognize GGT and cause a strong turn-on fluorescent and photoacoustic signal. The up-regulation of the GGT expression in acetaminophen-induced acute liver injury was imaged using NIR-GGT. The probe can track changes in the GGT level in the early stages of drug-induced acute liver injury (DIALI) and its remedy process by fluorescent and photoacoustic dual-modality imaging with a high temporal-spatial resolution. NIR-GGT can also be used to differentiate between tumor and para-carcinowa tissues in vivo. The probe may be a potential tool for the diagnosis of early-stage DIALI and accurate tumor resection in the clinical field.

Caspase-1-responsive fluorescence biosensors for monitoring endogenous inflammasome activation

The activation of inflammasome leads to secretion of inflammatory factors and cell pyroptosis that are critical in the pathogenesis of various chronic and acute inflammatory diseases. Recruitment and activation of caspase-1 is a marker of inflammasome activation. However, there is still lack of real-time and efficient methods to detect the activation of inflammasome, especially in vivo. Herein, we developed two activatable caspase-1-responsive fluorescence biosensors, WEHD-HCy and YVAD-HCy, to specifically monitor the activation of inflammasome in vivo. Our in vitro study demonstrated that WEHD-HCy and YVAD-HCy can sensitively and specifically respond to caspase-1 activation. Moreover, these biosensors can efficiency and specifically activated in the common inflammatory disease model, including inflammatory bowel disease, Salmonella infection, and acute arthritis. In particular, WEHD-HCy is more advantageous than YVAD-HCy to specifically image of caspase-1 activity both in vitro and in vivo. These caspase-1-responsive fluorescence biosensors provide an efficient, rapid, and in situ tool for monitoring inflammasome activation, and have the potential to be suitable for clinical diagnosis of various inflammatory diseases associated with inflammasome activation.

Constructing a novel fluorescence detection method for γ-glutamyltranspeptidase and application on visualizing liver injury

Liver injury is a serious threat to human health, and γ-glutamyltranspeptidase (GGT) is proven to be one of the clinical biomarkers of liver injury. The conventional detection method of GGT activity in serum suffers from the complex operation, expensive equipment, and incapability of dynamically monitoring in biological samples. Herein, in consideration of the excellent characteristics of fluorescent probes, such as simple operation, high sensitivity, low cost, and good biocompatibility, a novel fluorescence detection method for GGT based on the combination of probe Rho-GGT and glutamic acid 5-hydrazide (glutamlhydrine) was designed. This method was applied to liver injury model mice to construct the relationship between the fluorescence signal, GGT activity, and the occurrence or development stage of liver injury. The fluorescence detection method combined with clinical indexes could more accurately characterize the situation of liver fibrosis, and evaluate the efficacy of liver fibrosis drugs, which could help provide important information for accurate diagnosis and early treatment of liver injury. The successful implementation of this project would promote the accurate in situ detection of GGT in liver injury, which was expected to guide pre-clinical diagnosis and clinical practice.

A carboxylesterase-activatable near-infrared phototheranostic probe for tumor fluorescence imaging and photodynamic therapy

Phototheranostic probes have been proven to be a promising option for cancer diagnosis and treatment. However, near-infrared phototheranostic probes with specific tumor microenvironment responsiveness are still in demand. In this paper, a carboxylesterase (CES)-responsive near-infrared phototheranostic probe was developed by incorporating 6-acetamidohexanoic acid into a hemicyanine dye through an ester bond. The probe exhibits highly sensitive and selective fluorescence enhancement towards CES because CES-catalyzed cleavage of the ester bond leads to the release of the fluorophore. By virtue of its near-infrared analytical wavelengths and high sensitivity, the probe has been employed for endogenous CES activatable fluorescence imaging of tumor cells. Moreover, under 660 nm laser irradiation, the probe can generate toxic reactive oxygen species and efficiently kill tumor cells, with low cytotoxicity in dark. As far as we know, the probe was the first CES-responsive phototheranostic probe with both near-infrared analytical wavelengths and photosensitive capacity, which may be useful in the real-time and in situ imaging of CES as well as imaging-guided photodynamic therapy of tumors. Therefore, the proposed probe may have wide application prospect in cancer theranostics.

A ratiometric fluorescent probe for detection of γ-glutamyl transpeptidase in blood serum and living cells

γ-Glutamyl transpeptidase (GGT) is one of the biomarker of cancer, hepatitis, and numerous other diseases. The accurate analysis of GGT is useful for the early diagnosis of these diseases. In this work, Probe 1, a ratiometric fluorescent probe based on 2,3,5,6-tetrafluoroterephthalonitrile, was designed for GGT detection. The results indicated that Probe 1 can sensitively and selectively detect GGT in phosphate buffered solution and complex biological systems (e.g., blood serum). Furthermore, Probe 1 has been successfully applied for ratiometric imaging of GGT in cancer cells and normal cells.

A water-soluble fluorescent probe for real-time visualization of γ-glutamyl transpeptidase activity in living cells

γ-glutamyl transpeptidase (GGT) is a kind of cell-surface enzyme that is overexpressed in many cancer cells. It is of great significance to develop an ideal tool for the diagnosis of GGT-rich cancer cells. Here, we reported a simple-structured but effective imaging probe for the detection of GGT activity. In the presence of GGT, the γ-glutamyl linkage could be cleaved specifically to produce amino-substituted product, resulting in significant fluorescence enhancement at 578 nm. Moreover, we successfully employed the probe to monitor GGT activity in HepG2 cells. We envisaged that such a simple but effective imaging tool could improve the practical applications for bioimaging.

Chemical Probes and Activity-Based Protein Profiling for Cancer Research

Chemical probes can be used to understand the complex biological nature of diseases. Due to the diversity of cancer types and dynamic regulatory pathways involved in the disease, there is a need to identify signaling pathways and associated proteins or enzymes that are traceable or detectable in tests for cancer diagnosis and treatment. Currently, fluorogenic chemical probes are widely used to detect cancer-associated proteins and their binding partners. These probes are also applicable in photodynamic therapy to determine drug efficacy and monitor regulating factors. In this review, we discuss the synthesis of chemical probes for different cancer types from 2016 to the present time and their application in monitoring the activity of transferases, hydrolases, deacetylases, oxidoreductases, and immune cells. Moreover, we elaborate on their potential roles in photodynamic therapy.

A nitroreductase-responsive near-infrared phototheranostic probe for in vivo imaging of tiny tumor and photodynamic therapy

The hypoxia-activated and nitroreductase-responsive phototheranostic probe has been developed by incorporating a nitro group into a hemicyanine fluorophore. The probe displays extremely sensitive and selective near-infrared fluorescence enhancement to nitroreductase with the detection limit of 2.10 ng/mL. The detection mechanism relies on the nitroreductase-catalyzed reduction of the nitro group to an amino group, along with the generation of the fluorophore. The availability of the probe in fluorescence imaging and photodynamic therapy was demonstrated at cellular level and in vivo. The probe can image endogenous nitroreductase and the hypoxia status of living cells. The probe also exhibits significant phototoxicity to hypoxia tumor cells under the 660 nm laser irradiation. More importantly, the probe has been successfully utilized in imaging tiny tumor (about 6 mm³) and tumor photodynamic therapy in vivo. The proposed probe integrates accurate near-infrared fluorescence imaging and photodynamic therapy into the same molecule, which probably become a promising agent in the early diagnosis and therapy of tumors.

Activity-based NIR fluorescent probes based on the versatile hemicyanine scaffold: design strategy, biomedical applications, and outlook

The discovery of a near-infrared (NIR, 650-900 nm) fluorescent chromophore hemicyanine dye with high structural tailorability is of great significance in the field of detection, bioimaging, and medical therapeutic applications. It exhibits many outstanding advantages including absorption and emission in the NIR region, tunable spectral properties, high photostability as well as a large Stokes shift. These properties are superior to those of conventional fluorogens, such as coumarin, fluorescein, naphthalimides, rhodamine, and cyanine. Researchers have made remarkable progress in developing activity-based multifunctional fluorescent probes based on hemicyanine skeletons for monitoring vital biomolecules in living systems through the output of fluorescence/photoacoustic signals, and integration of diagnosis and treatment of diseases using chemotherapy or photothermal/photodynamic therapy or combination therapy. These achievements prompted researchers to develop more smart fluorescent probes using a hemicyanine fluorogen as a template. In this review, we begin by describing the brief history of the discovery of hemicyanine dyes, synthetic approaches, and design strategies for activity-based functional fluorescent probes. Then, many selected hemicyanine-based probes that can detect ions, small biomolecules, overexpressed enzymes and diagnostic reagents for diseases are systematically highlighted. Finally, potential drawbacks and the outlook for future investigation and clinical medicine transformation of hemicyanine-based activatable functional probes are also discussed.

Synthesis and Single Crystal X-ray Study of Phenylselenyl Embedded Coumarin-Based Sensors for Selective Detection of Superoxide

Selenium-coumarin based probe 4 was synthesized from the reaction of bromo derivative of coumarin with in situ prepared sodium phenyl selenide. Esterification of probe 4 resulted in the formation of...

Optimized molecular design of PET probe for the visualization of γ-glutamyltranspeptidase activity in tumors

A fluorine-18 labeled probe [18F]JM-10 was rationally designed and optimized, which can self-assemble to generate more rigid and hydrophobic dimers upon response to γ-glutamyltranspeptidase (GGT) and glutathione (GSH) in tumor cells.

A new metal-free near-infrared fluorescent probe based on nitrofuran for the detection and bioimaging of carbon monoxide releasing molecule-2 in vivo

As a stable donor for releasing controlled amounts of CO, carbon monoxide releasing molecule-2 (CORM-2) is a new type of therapeutic drug that contributes to exploring the pathophysiological effects of CO. The accurate detection of CORM-2 in biological systems is of great significance for controlling its dosage as a therapeutic drug and elucidating the reaction mechanisms of CO, but currently there is a lack of metal-free near-infrared fluorescent probes. Herein, a new metal-free near-infrared fluorescent probe based on nitrofuran which could selectively identify CORM-2 was designed and it has been successfully applied in living cells, zebrafish and mice. After reacting with CORM-2, both the color and fluorescence signal of the solution are restored, which is ascribed to the reduction of the nitro group. The spectroscopic probe DXPN shows high sensitivity to CORM-2 with a low detection limit of 87 nM and near-infrared fluorescence emission of 712 nm. Notably, this is the first time that paper chips are being used as a carrier to detect CORM-2 through fluorescence signals instead of the traditional liquid phase detection mode of fluorescent probes. These superior properties of the probe make it a promising and reliable tool for exploring the role played by CORM-2 in biological systems.

A turn on fluorescent assay for γ-glutamyltransferase activity and its application in biological imaging

γ-glutamyltransferase (GGT) is widely presented in living cells and overexpressed in many tumor tissues. Therefore, it is generally considered as an important biomarker for the detection of tumor, especially for liver cancer. Accurate determination of its activity is helpful for early diagnosis and treatment of related diseases. In this work, a "turn on" fluorescent probe NSA-GGT for the detection of GGT activity based on glutamine bond was designed and synthesized by employing dansylamino as fluorophore. The probe shows good water solubility and can be well dispersed in aqueous buffer. After incubated with GGT in phosphate buffer, the fluorescence of NSA-GGT centered at ∼523 nm increased over 25-fold. This sensing pattern exhibits an intriguing sensing sensitivity for GGT, and has good performance on intracellular GGT staining, serving as a promising candidate for GGT measurement. Subsequent biological experiments showed that probe NSA-GGT could also be used for fluorescent imaging of GGT activity in living cells and animal tissues.

Tailored Biosensors for Drug Screening, Efficacy Assessment, and Toxicity Evaluation

Biosensors have been flourishing in the field of drug discovery with pronounced developments in the past few years. They facilitate the screening and discovery of innovative drugs. However, there is still a lack of critical reviews that compare the merits and shortcomings of these biosensors from a pharmaceutical point of view. This contribution presents a critical and up-to-date overview on the recent progress of tailored biosensors, including surface plasmon resonance, fluorescent, photoelectrochemical, and electrochemical systems with emphasis on their mechanisms and applications in drug screening, efficacy assessment, and toxicity evaluation. Multiple functional nanomaterials have also been incorporated into the biosensors. Representative examples of each type of biosensors are discussed in terms of design strategy, response mechanism, and potential applications. In the end, we also compare the results and summarize the major insights gained from the works, demonstrating the challenges and prospects of biosensors-assisted drug discovery.

Near-Infrared Fluorescent Probes for the Detection of Cancer-Associated Proteases

Proteases are enzymes capable of catalyzing protein breakdown, which is critical across many biological processes. There are several families of proteases, each of which perform key functions through the degradation of specific proteins. As our understanding of cancer improves, it has been demonstrated that several proteases can be overactivated during the progression of cancer and contribute to malignancy. Optical imaging systems that employ near-infrared (NIR) fluorescent probes to detect protease activity offer clinical promise, both for early detection of cancer as well as for the assessment of personalized therapy. In this Review, we review the design of NIR probes and their successful application for the detection of different cancer-associated proteases.

Biotransformation Capacity of Zebrafish (Danio rerio) Early Life Stages: Functionality of the Mercapturic Acid Pathway

Zebrafish (Danio rerio) early life stages offer a versatile model system to study the efficacy and safety of drugs or other chemicals with regard to human and environmental health. This is because, aside from the well-characterized genome of zebrafish and the availability of a broad range of experimental and computational research tools, they are exceptionally well suited for high-throughput approaches. Yet, one important pharmacokinetic aspect is thus far only poorly understood in zebrafish embryo and early larvae: their biotransformation capacity. Especially, biotransformation of electrophilic compounds is a critical pathway because they easily react with nucleophile molecules, such as DNA or proteins, potentially inducing adverse health effects. To combat such adverse effects, conjugation reactions with glutathione and further processing within the mercapturic acid pathway have evolved. We here explore the functionality of this pathway in zebrafish early life stages using a reference substrate (1-chloro-2,4-dinitrobenzene, CDNB). With this work, we show that zebrafish embryos can biotransform CDNB to the respective glutathione conjugate as early as 4 h postfertilization. At all examined life stages, the glutathione conjugate is further biotransformed to the last metabolite of the mercapturic acid pathway, the mercapturate, which is slowly excreted. Being able to biotransform electrophiles within the mercapturic acid pathway shows that zebrafish early life stages possess the potential to process xenobiotic compounds through glutathione conjugation and the formation of mercapturates. The presence of this chemical biotransformation and clearance route in zebrafish early life stages supports the application of this model in toxicology and chemical hazard assessment.

A de novo strategy to develop NIR precipitating fluorochrome for long-term in situ cell membrane bioimaging

Cell membrane-targeted bioimaging is a prerequisite for studying the roles of membrane-associated biomolecules in various physiological and pathological processes. However, long-term in situ bioimaging on the cell membrane with conventional fluorescent probes leads to diffusion into cells from the membrane surface. Therefore, we herein proposed a de novo strategy to construct an antidiffusion probe by integrating a fluorochrome characterized by strong hydrophobicity and low lipophilicity, with an enzyme substrate to meet this challenge. This precipitating fluorochrome HYPQ was designed by conjugating the traditionally strong hydrophobic solid-state fluorochrome 6-chloro-2-(2-hydroxyphenyl) quinazolin-4(3H)-one (HPQ) with a 2-(2-methyl-4H-chromen-4-ylidene) malononitrile group to obtain closer stacking to lower lipophilicity and elongate emission to the far-red to near-infrared wavelength. As proof-of-concept, the membrane-associated enzyme γ-glutamyltranspeptidase (GGT) was selected as a model enzyme to design the antidiffusion probe HYPQG. Then, benefiting from the precipitating and stable signal properties of HYPQ, in situ imaging of GGT on the membrane was successfully realized. Moreover, after HYPQG was activated by GGT, the fluorescence signal on the cell membrane remained unchanged, with incubation time even extending to 6 h, which is significant for in situ monitoring of enzymatic activity. In vivo testing subsequently showed that the tumor region could be accurately defined by this probe after long-term in situ imaging of tumor-bearing mice. The excellent performance of HYPQ indicates that it may be an ideal alternative for constructing universal antidiffusion fluorescent probes, potentially providing an efficient tool for accurate imaging-guided surgery in the future.

An Electrochemical Evaluation of Novel Ferrocene Derivatives for Glutamate and Liver Biomarker Biosensing

Here, we present an evaluation of two new monosubstituted ferrocene (Fc) derivatives, 3-(1H-pyrrol-1-yl)propanamidoferrocene and 1-hydroxy-2-[2-(thiophen-3-yl)-ethylamino]ethylferrocene, as glutamate oxidase mediators, together with their preparation and characterisation. Taking into consideration the influence of the electronic effects of substituents on the redox potentials of the Fc species, two candidates with pyrrole or thiophene moieties were proposed for investigation. Film studies involved potential sweeping in the presence of pyrrole or 3,4-ethylenedioxythiophene monomers resulting in stable electroactive films with % signal loss upon cycling ranging from 1 to 7.82% and surface coverage (Γ) 0.47-1.15 × 10-9 mol/cm2 for films formed under optimal conditions. Construction of a glutamate oxidase modified electrode resulted in second-generation biosensing with the aid of both cyclic voltammetry and hydrodynamic amperometry, resulting in glutamate sensitivity of 0.86-1.28 μA/mM and Km (app) values over the range 3.67-5.01 mM. A follow-up enzyme assay for liver biomarker γ-glutamyl transpeptidase realised unmediated and mediated measurement establishing reaction and incubation time investigations and a realising response over <100 U/L γ-glutamyl transpeptidase with a sensitivity of 5 nA/UL-1.

Hemicyanine-Based Near-Infrared Activatable Probes for Imaging and Diagnosis of Diseases

Molecular activatable probes with near-infrared (NIR) fluorescence play a critical role in in vivo imaging of biomarkers for drug screening and disease diagnosis. With structural diversity and high fluorescence quantum yields, hemicyanine dyes have emerged as a versatile scaffold for the construction of activatable optical probes. This Review presents a survey of hemicyanine-based NIR activatable probes (HNAPs) for in vivo imaging and early diagnosis of diseases. The molecular design principles of HNAPs towards activatable optical signaling against various biomarkers are discussed with a focus on their broad applications in the detection of diseases including inflammation, acute organ failure, skin diseases, intestinal diseases, and cancer. This progress not only proves the unique value of HNAPs in preclinical research but also highlights their high translational potential in clinical diagnosis.

Ultrafast fluorescent probe with near-infrared analytical wavelength for fluoride ion detection in real samples

The first ultrafast fluorescence probe with response time in seconds (10 s) for fluoride ions (F^-) has been proposed by conjugating dimethylthiophosphoryl group as a recognition unit with the near-infrared fluorophore of hemicyanine. The response mechanism is the F^--induced cleavage of the dimethylthiophosphoryl group, along with the liberation of the fluorophore, which results in a distinctly enhanced fluorescence intensity at 730 nm (λ_ex = 680 nm). The fluorescence enhancement of the probe is directly proportional to the F^- concentration in the range of 10-300 µM with the detection limit of 4.28 µM. The probe has been successfully used to determine F^- concentration in real water and toothpaste samples as well as image F^- in living cells. The simplicity and quick response of this probe endow it with the ability of detecting F^- rapidly in real samples.

A two-photon fluorescent probe for sensitive detection and imaging of γ-glutamyl transpeptidase

A GGT-activated two-photon fluorescent probe (4F-2CN-GSH) was developed based on a cascade reaction. 4F-2CN-GSH could selectivily detect GGT with low detection limit and distinguish ovarian cancer cells from normal cells using both one-photon and two-photon fluorescence imaging.

A near infrared fluorescent probe for the detection and imaging of prolyl aminopeptidase activity in living cells

Prolyl aminopeptidase (PAP) is an important exopeptidase which might be a biomarker for pathogen infection and a potential therapeutic target. However, very few fluorescent probes have been developed for detecting PAP activity. Here we report the development of the first near infrared (NIR) turn-on fluorescent probe (NIR-PAP) for detecting and imaging PAP in living cells. The probe is prepared by reacting a cysteine-proline dipeptide with an acryloylated NIR fluorophore via a facile thiol-Michael addition reaction. NIR-PAP exhibits a dynamic response toward PAP in the range of 0.02-2.5 U mL-1 with an estimated limit of detection of 0.013 U mL-1. In vitro studies also reveal that the probe displays high specificity and robust responses toward PAP under physiological pH and temperature conditions. Moreover, NIR-PAP is successfully introduced to detect and differentiate PAP activity in four different cell lines via both confocal fluorescence imaging and flow cytometry. Therefore, our probe may hold great promise in diagnosing infectious diseases caused by pathogens and screening therapeutic drugs in vivo.

Sensitive and selective SERS probe for detecting the activity of γ-glutamyl transpeptidase in serum

γ-Glutamyl transpeptidase (GGT) has attracted considerable attention for its regulatory effect on glutathione metabolism in living organisms; further, its close relationship with physiological dysfunctions such as hepatitis and liver cancers has enhanced its applicability. Therefore, the accurate detection of GGT levels is particularly important for the early diagnosis of diseases. Thus, we herein report the development of a surface-enhanced Raman spectroscopic (SERS) probe, namely bis-s,s'-((s)-4,4'-thiolphenylamide-Glu) (b-(s)-TPA-Glu), that comprises of a γ-glutamyl moiety for detection of the GGT activity. In this system, detection was achieved by observing differences in the SERS spectral profiles of the b-(s)-TPA-Glu probe and its corresponding hydrolysis product that resulted from the catalytic action of GGT. This SERS probe system exhibited a high selectivity toward GGT due to a combination of its specific catalytic action and the distinctive spectroscopic fingerprint of the SERS technique. The developed SERS approach was also found to be approximately linear in the range of 0.2-200 U/L, and a limit of detection of 0.09 U/L was determined. Furthermore, the proposed SERS method was suitable for detection of the GGT activity of clinical serum samples and also for evaluation of the inhibitors of GGT. Consequently, this approach is considered to be a promising diagnostic and drug screening tool for GGT-associated diseases.

Ratiometric Imaging of γ-Glutamyl Transpeptidase Unperturbed by pH, Polarity, and Viscosity Changes: A Benzocoumarin-Based Two-Photon Fluorescent Probe

γ-Glutamyltransferase (GGT) is involved in maintaining the intracellular glutathione levels and, at its elevated levels, is associated with various diseases including cancer and myocardial infarction. To study this enzyme in biological systems, fluorescent probes have received significant attention recently. As fluorescence signal is sensitive to environmental fluctuations; however, it is challenging to address the signal fluctuation issue. Disclosed is the benzocoumarin-based probe that enables ratiometric imaging of GGT activity levels in cells as well as in tissues, essentially unperturbed by medium pH, viscosity, and polarity changes. Validity of the probe is demonstrated by determining the GGT activity level in HeLa cells directly through ratiometric imaging. Furthermore, the probe and its enzymatic product are two-photon absorbing, extending its applicability to tissue: an 8.5-fold higher level of GGT in cancerous tissue over the normal tissue is determined, and the GGT activity levels between different mouse organ tissues are quantitatively compared with the highest level in the kidney. The probe with practicality holds great promise for studying GGT-associated biological processes directly through ratiometric imaging by two-photon microscopy.

An Activatable Chemiluminescent Probe for Sensitive Detection of γ-Glutamyl Transpeptidase Activity in Vivo

Activatable chemiluminescent probes that show enhanced chemiluminescence upon interaction with a molecular target of interest have offered promising tools for sensing and bioimaging in terms of low background, high sensitivity, and improved penetration depth in biological tissues. Here, we reported a γ-glutamyl transpeptidase (GGT) activatable chemiluminescent probe for real-time detection of GGT activity in vitro and in living mice. The probe was designed by caging an electron-withdrawing acrylic group-substituted Schaap's phenoxy-dioxetane with a GGT-recognitive substrate (γ-Glu) and a self-immolative linker (p-aminobenzyl alcohol), which was initially chemiluminescence off. Upon interaction with GGT, strong chemiluminescence with a more than 800-fold turn-on ratio could be achieved in aqueous solution, allowing to specifically detect GGT activity with ultrahigh signal-to-background ratio and sensitivity in vitro and in live cells. We demonstrated that the probe was reliable to quantify the GGT in serum, permitting to accurately report the elevated levels of GGT in lipopolysaccharide-treated mouse serum. Moreover, through real-time chemiluminescence imaging of GGT activity, the designed probe was feasible to detect GGT-positive tumors in living mice after intravenous systemic administration. This study demonstrates the high potential of GGT-activatable chemiluminescent probe for serum assays and molecular imaging, which might find wide applications in diagnosis of GGT-related diseases.

Self-Immolative Fluorescent and Raman Probe for Real-Time Imaging and Quantification of γ-Glutamyl Transpeptidase in Living Cells

Characterizing over-expressed enzymes or biomarkers in living cells is critical for the molecular understanding of disease pathology and consequently for designing precision medicines. Herein, a "switch-on" probe is designed to selectively detect γ-glutamyl transpeptidase (GGT) in living cells via a unique ensemble of enhanced fluorescence and surface-enhanced Raman scattering (SERS). In the presence of GGT, the γ-glutamyl bond in the probe molecule is cleaved, thereby activating a fluorescent probe molecule as well as a Raman reporter molecule. Consequently, the detection of GGT is achieved based on both plasmonic fluorescent enhancement and SERS with a detection limit as low as 1.2 × 10^-3 U/L (normal range for GGT levels in the blood is 9-48 U/L). The main advantage of this platform is that on the occasion of fluorescence signal interference, especially in the presence of free metal ions in cells, the SERS signals still hold high stability as a backup. This work highlights the benefits of the marriage of two complimentary sensing techniques into one platform that can overcome the major obstacles of detection of real-time biomarkers and imaging in living cells.

Recognition Moieties of Small Molecular Fluorescent Probes for Bioimaging of Enzymes

Enzymes are a class of important substances for life, and their abnormal levels are associated with many diseases. Thus, great progress has been made in the past decade in detecting and imaging enzymes in living biosystems, and in this respect fluorescent probes combined with confocal microscopy have attracted much attention because of their high sensitivity and unrivaled spatiotemporal resolution. Fluorescent probes are usually composed of three moieties: a signal or fluorophore moiety, a recognition or labeling moiety, and an appropriate linker to connect the two aforementioned moieties. At present, however, research and reviews on enzymatic probes mostly focus on fluorophores and/or linkers, whereas those on the recognition moiety are relatively few. Moreover, current enzymatic probes with some recognition moieties have drawbacks such as poor selectivity, high background fluorescence, or/and low sensitivity and are unsatisfactory for practical applications. Thus, developing new recognition moieties with higher specificity or/and sensitivity to the enzyme of interest is very desirable but still challenging. In this Account, we introduce the recognition moieties of fluorescent probes for several enzymes, including tyrosinase, monoamine oxidase A (MAO-A), nitroreductase (NTR), and aminopeptidases. Highlights are given on how new specific recognition moieties of tyrosinase and MAO-A were designed to eliminate the interference by reactive oxygen species (ROS) and MAO-B, respectively. Here we present four recent examples in which designed fluorescent probes are employed to image enzymes in living biosystems. The first example shows that 3-hydroxyphenyl can serve as a new and more specific recognition moiety than the traditional 4-hydroxyphenyl group for tyrosinase, enabling the development of a highly selective fluorescent probe for imaging of tyrosinase without interference by ROS. The second presents a general design strategy for fluorescent probes specific for an enzyme, which involves combining the characteristic structure of an inhibitor of the target enzyme along with its traditional reactive group as a new recognition moiety, and successfully demonstrates it by selective detection of MAO-A in the presence of its isomeric MAO-B. The third mainly illustrates that 5-nitrothiophen-2-yl alcohol with a stronger electron-donating S atom is a better fluorescence quenching and recognition moiety than 5-nitrofuran-2-yl alcohol for NTR, leading to the development of a highly sensitive method for NTR assay. Lastly, on the basis of known observations, we show that besides the specific interaction with the target, another function of some recognition moieties may be responsible for tuning the fluorescence signal, which is exemplified by the linking of several aminopeptidases' recognition moieties to the free hydroxyl or amino group of different fluorophores. It is our wish that this Account will promote the appearance of more specific recognition moieties and fluorescent probes with excellent properties and that new biofunctions of the enzymes will be uncovered.

Point-of-care Ratiometric Fluorescence Imaging of Tissue for the Diagnosis of Ovarian Cancer

During a minimally invasive tumor resection procedure, it is still a challenge to rapidly and accurately trace tiny malignant tumors in real time. Fluorescent molecular imaging is considered an efficient method of localizing tumors during surgery due to its high sensitivity and biosafety. On the basis of the fact that γ-glutamyltranspeptidase (GGT) is overexpressed in ovarian cancer, we herein designed a highly sensitive ratiometric fluorescent GGT-responsive probe Py-GSH for rapid tumor detection. Methods: The GGT response probe (Py-GSH) was constructed by using GSH group as a response group and pyrionin B as a fluorescent reporter. Py-GSH was characterized for photophysical properties, response speed and selectivity of GGT and response mechanism. The anti-interference ability of ratiometric probe Py-GSH to probe concentration and excitation power was evaluated both in vitro and in tissue. The biocompatibility and toxicity of the ratiometric probe was examined using cytoxicity test. The GGT levels in different lines of cells were determined by ratiometric fluorescence imaging and cytometry analysis. Results: The obtained probe capable to rapidly monitored GGT activity in aqueous solution with 170-fold ratio change. By ratiometric fluorescence imaging, the probe Py-GSH was also successfully used to detect high GGT activity in solid tumor tissues and small peritoneal metastatic tumors (~1 mm in diameter) in a mouse model. In particular, this probe was further used to determine whether the tissue margin following clinical ovarian cancer surgery contained tumor. Conclusion: In combination of ratiometric fluorescence probes with imaging instrument, a point-of-care imaging method was developed and may be used for surgical navigation and rapid diagnosis of tumor tissue during clinical tumor resection.

A protease-responsive fluorescent probe for sensitive imaging of legumain activity in living tumor cells

Legumain, a lysosomal cysteine protease, is critical for pathological progression and has been found to play an important role in the occurrence and development of several cancers. However, its biological functions remain few recognized. To further understand the role of legumain activity in tumor progression, a legumain protease-responsive fluorescent probe was developed in the present study. The probe 1 was synthesized by conjugating an aminoluciferin fluorophore with an alanine-alanine-asparagine (AAN) peptide sequence. The successful synthesis of probe 1 was validated by NMR and MS spectra as well as HPLC analysis. The probe 1 was non-toxic and exhibited great stability in the physiological solutions. More importantly, compared with the aminoluciferin fluorophore, the peptide conjugation may dramatically increase the targeting specificity. Probe 1 was able to effectively detect the legumain activity in living HCT116 cells through fluorescence imaging. All these results implied that probe 1 could act as a promising fluorescent probe specialized for the monitoring of legumain activity in living cells.

Preparation of a Near-Infrared Fluorescent Probe Based on IR-780 for Highly Selective and Sensitive Detection of Bisulfite-Sulfite in Food, Living Cells, and Mice

A near-infrared fluorescent probe was designed and synthesized for the detection of bisulfite-sulfite in food, living HeLa cells, and mice. The probe is a stable hemicyanine skeleton based on IR-780, which produces a distinct color and fluorescence change before and after reaction with bisulfite and shows excellent analytical performance, such as high sensitivity with a detection limit of 0.37 μM and high selectivity toward bisulfite over other substances. More importantly, the probe determined bisulfite in real food samples (crystal sugar and red wine) with superior recoveries (96.7-106.1%) and has been successfully used to monitor bisulfite in living HeLa cells. Notably, the probe was applied to image bisulfite in BALB/c mice in vivo for the first time. Owing to its biocompatibility, this probe provides a desired method to study the metabolism of bisulfite in cells and shows substantial potential usage in other biosystems and in vivo imaging studies.

Real-time imaging of senescence in tumors with DNA damage

Detection of cellular senescence is important not only in the study of senescence in various biological systems, but also in various practical applications such as image-guided surgical removal of senescent cells, as well as the monitoring of drug-responsiveness during cancer therapies. Due to the lack of suitable imaging probes for senescence detection, particularly in living subjects, we have developed an activatable near-infrared (NIR) molecular probe with far-red excitation, NIR emission, and high "turn-on" ratio upon senescence-associated β-galactosidase (SABG) activation. We present here the first successful demonstration of NIR imaging of DNA damage-induced senescence both in vitro and in human tumor xenograft models.

A novel mitochondrial-targeting near-infrared fluorescent probe for imaging γ-glutamyl transpeptidase activity in living cells

γ-Glutamyl transpeptidase (GGT) plays an essential role in regulating cellular glutathione and cysteine homeostasis, and its abnormal elevation is associated with different diseases including cancers. Here a novel mitochondrial-targeting near-infrared fluorescent probe was designed for GGT by conjugating glutamate acid to a newly synthesized amine hemicyanine fluorophore. The fluorescent probe was initially non-emissive due to the formation of an amide bond which destroyed the electronic-donating ability of the amine moiety and disrupted the push-pull structure. GGT-mediated cleavage of the γ-glutamyl bond regenerated the initial fluorophore with distinct intramolecular charge transfer (ICT) and activated the fluorescence signal. The fluorescent probe displayed a linear relationship to the concentration of GGT in the range of 1.0-90 U L-1, with an estimated limit of detection (LOD) of 0.4 U L-1. Its ability to target and image mitochondrial GGT activity was demonstrated in living cells with high specificity and fast response. We believe our near-infrared fluorescent probe could have great potential in imaging mitochondrial GGT activity and elucidating GGT-associated pathological consequences in living cells and even small animal models.